Frédéric Challita
email: frederic.challita@univ-lille.fr

Pierre Laly

Martine Liénard

Emmeric Tanghe

Wout Joseph

Davy P Gaillot

Hybrid Virtual Polarimetric Massive MIMO Measurements at 1.35 GHz

The polarimetric massive MIMO radio channel of an indoor Line-Of-Sight (LOS) scenario is investigated at 1.35 GHz using a real-time radio channel sounder. The 8 × 12 massive MIMO transmitter is constructed using a hybrid architecture including a vertical uniform linear array (V-ULA) translated at different horizontal positions forming a virtual, yet realistic, uniform rectangular array. The performance of the system is evaluated with 6 users distributed in the room for different polarization schemes and receiver orientations using propagation channel-based metrics (such as receiver spatial correlation, Rician factor, etc.) and systemoriented metrics such as sum-rate capacity and signal to interference and noise ratio. The results show a clear dependence of the performance to the polarization schemes and receiver orientation. Furthermore, it is concluded that the additional degree of freedom brought by the polarization diversity can contribute to improve spectral efficiency, paving the way for further capacity enhancements in massive MIMO systems.

Introduction

New radio requirements and challenges are arising in the near future for modern telecommunication systems [START_REF] Pirinen | A brief overview of 5g research activities[END_REF]. These requirements include increased capacity, improved data rate, decreased latency, and better quality of service [START_REF] Gupta | A survey of 5g network: Architecture and emerging technologies[END_REF] for scenarios and use-cases outlined in [START_REF] Osseiran | Scenarios for 5g mobile and wireless communications: the vision of the metis project[END_REF]. To meet these demands, drastic improvements need to be made in the cellular network architecture. Massive MIMO is a multi-user MIMO (MU-MIMO) system using spatial multiplexing over a wireless channel and is considered as one of the emerging 5G technologies capable of filling the gap between current telecommunication standards and these future requirements for next generation wireless systems. This concept was first introduced in [START_REF] Marzetta | Noncooperative cellular wireless with unlimited numbers of base station antennas[END_REF] where the author states that as the base station (BS) antennas approach infinity, individual user channels become spatially decorrelated and pair wise orthogonal. An overview of benefits, opportunities and challenges for massive MIMO are presented in [START_REF] Rusek | Scaling up mimo: Opportunities and challenges with very large arrays[END_REF][START_REF] Lu | An overview of massive mimo: Benefits and challenges[END_REF][START_REF] Larsson | Massive mimo for next generation wireless systems[END_REF]. Different precoding techniques are studied in [START_REF] Gao | Linear pre-coding performance in measured very-large mimo channels[END_REF] and it was concluded that, in the studied residential-area environment, users can be uncorrelated using reasonably large number of antennas combined with simple linear pre-coders. A comparison between the performance of the different pre-coders is presented for massive MU-MIMO systems in [START_REF] Ju | Comparison of multiuser mimo systems with mf, zf and mmse receivers[END_REF].

However, characterizing a full physical massive MIMO radio channel can be a complex and costly approach. Large-scale MIMO channel sounding and system evaluation have only been tested in some labs [START_REF] Malkowsky | The world's first real-time testbed for massive mimo: Design, implementation, and validation[END_REF][START_REF] Harris | Serving 22 users in real-time with a 128-antenna massive mimo testbed[END_REF][START_REF] Vieira | A flexible 100-antenna testbed for massive mimo[END_REF][START_REF] Titan | [END_REF][START_REF] Shepard | Argos: Practical many-antenna base stations[END_REF] and a few field trials were reported such as in [START_REF] Samsung | Samsung and Sprint Conduct Real-World Massive MIMO Testing at Mobile World Congress Fall 2017[END_REF][16][START_REF] Telkomsel | Telkomsel and zte complete fdd-lte massive mimo field trial[END_REF]. The largest gains in a massive MIMO setup come from spatial multiplexing of many users per cell, thus these gains can only be harvested when there are many users simultaneously requesting data. Recently, the Russian telecom operator MTS has deployed more than 40 state-of-the-art LTE sites with massive MIMO functionality in seven cities where the 2018 FIFA World Cup took place [START_REF] Ericsson | Ericsson and mts to deliver superior mobile broadband experiences for football fans at tournament in russia[END_REF]. Another popular sounding solution is virtual measurements of massive MIMO wherein one antenna moves over several positions to simulate a full system. However, in a virtual setting approach, mutual coupling between different antennas is overlooked as well as RF chains imperfections and power consumption issues. In addition, since the measurement time can well exceed the coherence time due to the displacement of the antenna, only static radio channels can be investigated. In [START_REF] Challita | Line-of-sight massive mimo channel characteristics in an indoor scenario at 94 ghz[END_REF], a LOS scenario was studied for an indoor environment at 94 GHz with a focus on spatial correlation and phase variations. In [START_REF] Gao | Massive mimo performance evaluation based on measured propagation data[END_REF], a virtual ULA of 128 positions (spanning 7.4 m) was used for outdoor measurements at 2.6 GHz with vertically polarized antennas. In [START_REF] Kataoka | Basic performance of massive mimo in indoor scenario at 20-ghz band[END_REF], a virtual circular array has been investigated in an actual indoor environment at 20 GHz and channel measurements with statistical modeling of virtual 2D massive MIMO UWB (ultra wideband) channels are reported for two typical indoor environments using virtual elements were reported in [START_REF] Zahedi | Characterization of massive mimo uwb channel for indoor environments[END_REF]. In [START_REF] Zheng | Propagation characteristics of massive mimo measurements in a uma scenario at 3.5 6 ghz with 100 200 mhz bandwidth[END_REF], propagation characteristics of a virtual massive MIMO (256 × 16) in an urban macro cell scenario were investigated at 3.5 GHz with 100 & 200 MHz bandwidths and 6 GHz with 200 MHz bandwidth. Other indoor measurements were also conducted at 6 GHz [START_REF] Li | Directional analysis of indoor massive mimo channels at 6 ghz using sage[END_REF], 13-17 GHz [START_REF] Chen | Measurement-based massive mimo channel modeling in 13 -17 ghz for indoor hall scenarios[END_REF] and some mobile measurements for cluster variation were reported in [START_REF] Wang | The spatial evolution of clusters in massive mimo mobile measurement at 3.5 ghz[END_REF]. Other outdoor measurements exist also in the literature at 15 GHz [27] and 3.33 GHz [START_REF] Fei | Massive mimo channel measurements and analysis at 3.33 ghz[END_REF]. All these measurements evaluate the massive channel using metrics such as eigenvalue distribution, condition number, Rician factor, delay spread and other system metrics such as sum-rate capacity.

Nonetheless, there is a clear lack in the literature regarding polarimetric massive MIMO measurements and the potential of using polarization diversity as an additional degree of freedom to improve orthogonality between the different channels. This possibility depends on the channel characteristics underlining the importance of exploiting channel parameters such as the Rician factor and the cross-polarization discrimination factor.

This work falls within this context and aims at experimentally evaluating the polarimetric 96 × 6 massive MIMO channel in an indoor scenario at 1.35 GHz where all 6 users are equipped with a single antenna. As such, this configuration can alternatively be categorized under the MU-MISO type (multiple-input single-output) which is expected to be deployed in practice. The massive MIMO transmitter is a hybrid virtual setup and consists in a real V-ULA translated at different horizontal positions. This is a trade-off between a full virtual setup and a full physical array. This approach is rather original for massive MIMO systems as far as the authors are concerned and its benefits are discussed later. The massive multi-user channel is extracted for different user orientations using the real-time channel sounder MIMOSA (Multiple-Input Multiple-Output Sounding Acquisition) [START_REF] Laly | Sondeur de canal mimo temps rÃl'el et applications[END_REF]. System performance for different polarization schemes (co-and cross-polarization) is evaluated using propagation and system metrics. Zero-forcing (ZF) is used for pre-coding and the iterative waterfilling technique for power allocation. This combination is emerging as a popular approach for massive MIMO systems [START_REF] Jin | Zero-forcing beamforming in massive mimo systems with time-shifted pilots[END_REF][START_REF] Nguyen | Energy efficiency in cellfree massive mimo with zero-forcing precoding design[END_REF][START_REF] Papazafeiropoulos | Performance of massive MIMO uplink with zero-forcing receivers under delayed channels[END_REF][START_REF] Qiao | Spectral efficiency for massive mimo zero-forcing receiver with low-resolution adc[END_REF][START_REF] Zhang | Performance analysis for massive MIMO downlink with low complexity approximate zero-forcing precoding[END_REF]. In particular, the capability of a massive MIMO system to spatially separate a given number of users and the sum-rate capacity in the studied indoor environment are addressed for the different setups. The rest of the paper is organized as follows: the system model and pre-coding techniques are outlined in section II. Section III presents system performance metrics and propagation channel evaluation tools. In section IV, the scenario, measurement setup and the channel sounder are presented. Before concluding, the different results are presented in Section V.

Notations

In this paper, E{.} denotes the mathematical expectation, (.) H represents the Hermitian matrix (conjugate transpose), |.| is the absolute value (magnitude), . the norm of a vector and . F denotes the Frobenius norm of a matrix. det is the determinant of a matrix and T r its trace. Lower case letters are used to denote scalars, upper case boldface letters represent matrices and boldface is used to denote vectors. ∼ CN (µ, σ 2 ) denotes a complex circular symmetric Gaussian distribution with mean µ and variance σ 2 . I.i.d. denotes the independent and identically elements distribution.

2

System model

System Model

Here, for post-processing the results, only the downlink (DL) channel of a massive MU-MISO system is considered. The BS is equipped with M antennas serving simultaneously and in the same time-frequency resource K single-antenna users distributed in the scenario. The channel can be described as flat-band at each subcarrier or frequency point. The received signal at user k is given by:

y k = h k w k s k + K i=1, i =k h k w i s i + n k , (1)
where y k is the received DL signal for user k, H is the composite K × M channel matrix and h k ∈ C 1×M is the k th complex raw vector of H from the M BS antennas towards user k and n the white-noise vector with i.i.d. circularly-symmetric complex Gaussian CN (0, σ 2 n ) elements. In Eq. 1, W denotes the M × K pre-coding matrix formed by the beamforming vectors for each user

W = [w 1 , w 2 , ..., w K ] . s = √ Ps, (2)
where s is the K × 1 data symbol raw vector with unit energy (E(ss H ) = 1). In this case, E(s s H ) = P T where P is the diagonal K × K power allocation matrix with diagonal elements p k designating power allocated to the k th user subject to the total available power P T constraint:

K k=1 p k ≤ P T . (3)
It should be noted that if both W and n have unit power (given that s has also unit energy ), p k can be interpreted as transmit signal to noise ratio (SN R) after the normalization. In this investigation, the well-known water-filling iterative algorithm is used for power distribution. The water-filling approach has been widely used to optimize power allocation amongst users in massive MU-MISO schemes [START_REF] Jindal | Sum power iterative water-filling for multi-antenna gaussian broadcast channels[END_REF][START_REF] Oestges | MIMO Wireless Communications: From Real-World Propagation to Space-Time Code Design[END_REF][START_REF] Börnson | Optimal resource allocation in coordinated multi-cell systems[END_REF].

Pre-coding techniques

Pre-coding techniques are designed for separating spatial data streams while minimizing inter-user interference as much as possible. Massive MIMO benefits derive from the use of linear pre-coders [START_REF] Marzetta | Noncooperative cellular wireless with unlimited numbers of base station antennas[END_REF] such as MRT, ZF, and MMSE (or regularized ZF). For DL, the base station pre-filters signals to the intended users. The performance of the different pre-coders depends on the considered scenario, the generated channels, the number and distribution of users and other factors.

In this investigation, only ZF will be evaluated and is given by Eq. 4:

W = H H (HH H ) -1 f or ZF (4)
The normalized version of each pre-coding vector is given by w k w k .

System Performance and Propagation Channel Evaluation

System Performance

In order to evaluate the system performance, the sum-rate capacity with water-filling is used. Indeed, sum-rate capacity provides an upper bound on the achievable rates and spectral efficiency aspect. Moreover, it is considered that the computation of the optimal sum capacity is a particularly complex task since the power allocation needs to be numerically optimized. This clearly shows the benefit of linear schemes alongside heuristic strategies such as water-filling in practice. This is specifically true for large-scale MIMO systems for which channel hardening [START_REF] Martìnez | Massive mimo properties based on measured channels: Channel hardening, user decorrelation and channel sparsity[END_REF] is more pronounced compared to classical MIMO systems. Taking into account Eq. 1, the corresponding signal to interference plus noise ratio (SIN R) for user k is given by:

SIN R k = p k |h k w k | 2 K i=1, i =k p i |h k w i | 2 + σ 2 n , (5)
It should be noted that ρ is included in the water-filling optimization so its value is underlying in p k . The corresponding rate for user k is:

C k = log 2 (1 + SIN R k ), in bits/s/Hz (6)
and the sum-rate in the considered cell or scenario will be:

C = K k=1 C k . (7)

Propagation Channel Characteristics

In order to characterize the behavior of the different polarization schemes XY (where X is the polarization at Tx (H or V) and Y is the polarization at the Rx (H or V)), the average received power P(ψ) is first computed in the bandwidth Bw for each Tx-Rx link at each snapshot N and for every polarization link ψ. ψ could be either co-polar (X = Y ) or cross-polar (X = Y ). The power per link and snapshot is expressed as:

P(ψ) = 1 M f M f i=1 (|h(ψ, i)| 2 ), (8)
where h(ψ, i) represents the complex transfer function at frequency point i for a given polarization link ψ and M f the number of frequency points. In this polarimetric investigation, it is critical to characterize the channel depolarization effects to understand the different mechanisms impacting the system performance. Crosspolarization discrimination (XPD) is defined as the ratio between average received power in co-polarization mode to the average received power in cross-polarization mode and can be expressed as :

XP D(dB) = 10 log 10 P co-polar P cross-polar .

Also, the Rician factor K, defined as the ratio between the LOS and MPC (multipath components without the LOS component) power (Eq. 10), a classical parameter in wireless communication systems, is here studied. Its influence on spatial correlation (defined below) and system performance is evaluated for LOS environments where users separation (and pre-coding weights) is more complex than in rich-scattered Rayleigh environment.

K(dB) = 10log 10 P LOS N i=1 P i N LOS , ( 10
)
where P LOS is the LOS power, P i N LOS is the power of the i th NLOS MPC, and N the number of computed NLOS MPC.

Spatial Channel Correlation

The Rx spatial correlation properties highlight the capability of a massive MU-MISO system to simultaneously serve a number of users. It evaluates the orthogonality between channel vectors and thus reflects if the propagation is favorable or not. The propagation is said to be favorable when users are mutually orthogonal (thus an environment with rich scattering). In Massive MIMO, it is more adequate to talk about joint correlation of multiple channel vectors (a channel vector is a 1 × M vector containing the channels for the M element array to a given user). First, the receiving correlation matrix R Rx can be expressed as:

R Rx = E{HH H }, (11)
and then normalized with the corresponding variances of the channel vectors. In order to represent the result of the operation in Eq. 11, a scalar value for ρ Rx should be derived. A macroscopic Rx correlation coefficient ρ Rx (or intercorrelation) is derived from R Rx by averaging the off-diagonal upper triangular part of R Rx (because of Hermitian symmetry). The canonical model in the Massive MIMO literature is i.i.d. Rayleigh fading. In this case the vector is a circularly symmetric complex Gaussian random variable. With i.i.d. Rayleigh fading, the channel gain h 2 follows an Erlang distribution (scaled χ 2 distribution) where the channel direction h/ h is uniformly distributed over the unit sphere in C M . This is a spatially uncorrelated channel model that will be used throughout the investigation as a reference model.

Measurement Setup and Channel Sounder

Measurement setup and scenario

The static indoor office scenario is located on the University of Lille campus in the building hosting the IEMN-TELICE research group. The office dimension is 7.92 m x 6.82 m x 3 m (W x L x h). The room is surrounded by concrete walls on all sides and furnished with wooden tables disposed as shown in the panoramic view (Fig. 1) from the transmitter array point of view. 6 Rx users with single (bi-polar) antenna each are considered. The entrance is through a wooden door and the schematic in Fig. 2 gives a detailed view of the Rx distribution in the room. The location of the Rx users was chosen randomly but some are close users (i.e. spatially correlated which are presumably harder to separate). The metallic radiator protection covers the whole wall along D under four windows. The channel sounder was in the room while the measurements were performed. As it can be seen, a LOS always exists between Tx and Rx to simulate a strong Rician environment.

Radio Channel Sounder and Hybrid Setup

The MIMOSA radio channel sounder used to measure the radio channel transfer functions has been developed jointly by the TELICE group of the University of Lille and the WAVES group of the University of Ghent. This sounder allows the simultaneous measurement of 16 outputs and 16 inputs over an 80 MHz bandwidth at 1.35 GHz 2) the number of desired positions for the V-ULA, a hybrid massive array setup can be configured. This is a simple and practical solution to test a massive MU-MISO system that does not require the full number of radio frequency chains. It is also a faster approach than a full virtual system with one displaced element along x and y direction and for which coupling effects and RF impairments are totally neglected. The link between the Lab-View program and the motor is done using optical cables such that the computer can be located outside of the room. The full simulated 96 elements array is shown as in Fig. 2. The first V-ULA position is shown on the right and the 11 other positions are represented with faded colors. Identical dual-polarized patch antennas are used for Tx and Rx. These antennas operate at 1.35 GHz with 80 MHz bandwidth, 80 degrees beamwidth at -3 dB (in azimuth and elevation), 7 dBi gain and typical nominal VSWR ≤ 2 in the band of interest [START_REF] Cheng | Polarimetric distance-dependent models for large hall scenarios[END_REF]. In these polarimetric measurements, each Tx-Rx link will have four channels formed by the combination of 4 different polarization schemes : VV and HH for co-polarization, VH and HV for cross polarization. Note that in optimal conditions, the Tx inter-element spacing for the ULA should be equal to the horizontal displacement. However, the specified step for the positioning program is in hexadecimal format and a precise value could not be reached resulting in a negligible a 3.7 % error between vertical and horizontal spacing. Main features of the channel sounder as well as the hybrid setup parameters of the Tx array are listed in Table 1.

Four different orientations O were selected for the measurements as depicted in Fig. 2 : (O1) facing the board (A side), (O2) facing the entrance door (B), (O3) facing the back wall (C) and (O4) facing the windows (D). For the following analysis, all antennas representing user equipments have the same orientation.

Results

Propagation Channel Characteristics

The empirical cumulative distribution function (ecdf) of the average received power (see Eq. 8) for each orientation O and for all four possible polarization links ψ is presented in Fig. 3 to illustrate the power variation between the different scenarios. First, it can be seen that O1 and O2 display the highest average received power because they are facing (for all users except Rx3 in O1) the transmitting array. O3 and O4 display ≈ 3 dB less received power. It can also be concluded that V and H polarization both propagate equally well in this scenario with no significant difference. Hence, only the Vpolarization for reception will be considered in order to simplify the analysis but both polarizations at Tx are considered. This assumption derives from a practical point of view where the complexity of switching polarizations is at the Tx side. In addition, Fig. 4 presents the XPD (V V /HV ) for all four orientations. The corresponding values of median values for average received power for the considered scenarios are shown in Table 2. The XPD values vary between -3 and 20 dB for O1 and -6 and 16 dB for O3. Two close families of curves appear for O1/O2 and the other for O3/O4, respectively. These differences justify the analysis as a function of different orientations to inject diversity in the observed channels for the same indoor scenario.

Rice Factor

The ecdf of K, averaged over different snapshots and users, is shown in Fig. 5. From this figure, it can be seen that K varies between ≈ 3.5 and 12.3 dB for 50% of the values underlining a considerable difference between the users channels for different orientations and links. Also, O1 and O2 experience a stronger Rician environment than O3 and O4 suggesting more correlation between users and specifically for co-polarization schemes could be reached. For O1 and O2, the gap between co and cross-polarized curves (≈ 3 -4 dB) is greater than for O3 and O4 for which VV and HV schemes display similar values. For O3 and O4 presenting smaller values of K (weaker LOS), polarization diversity does not affect the Rician factor. Hence, the dependence of polarization diversity to spatial separation between users might be expected to less pronounced than O1 and O2.

Spatial Receiver Correlation

ρ Rx (Eq. 11) is presented in Fig. 6 as a function of the number of active Tx antennas for all orientations and for VV and HV polarization. The results indicate that ρ Rx strongly depends on the user orientation O. In addition, for a given O, ρ Rx strongly depends on ψ. For O3 and O4, values of ρ Rx are slightly lower for co-polarization schemes and more importantly drop faster. This means that less Tx antennas are needed to decorrelate the users in the scenario with proper pre-coding. Moreover, for O3, the correlation curve for the cross-polarized scheme exhibits higher values than O4. This might be due to the fact the Rx antenna for O3 is facing the back wall in some positions (Rx 1,2 and 6) causing strong reflections contributing to the channel correlation and decreasing the Rician factor K as observed in Fig. 5. The highest observed values of ρ Rx correspond to the co-polar schemes in O1 and O2 which display a LOS with higher average received power compared to O3 and O4 (Fig. 3) and higher Rician factors (Fig. 5). From this point of view, better propagation conditions for massive MIMO are obtained for O3 and O4. However, for O1 and O2, the lowest values of ρ Rx are observed for the crosspolarization links. Hence, it follows a better spatial separation is obtained to enhance the system performance even under strong LOS conditions using only polarization diversity. It is noteworthy that the number of Tx antennas might be large for such indoor scenarios at the chosen frequency range. However, due to the simplicity and practicality of the hybrid setup, the selected number of Tx antennas is not an issue and consequently may allow verifying the asymptotic behavior of massive MU-MISO channels with respect to the canonical model. Reducing the number of radio frequency chains (provided that antenna selection strategies are deployed at system level), is a critical topic. Here, and for a desired correlation value threshold, the correlation results indicate that the number of active antennas can be optimized for a given orientation and polarization scheme. For example, it can be observed for ρ Rx = 0.3 that no more than 40 antennas are needed for the worst case (upper curve) and 12 antennas for the best case. However, to reach smaller values such as 0.2, polarization diversity is clearly observed as an additional degree of freedom to reduce the number of needed active antennas (30 for HV vs. 75 for VV in O1). It is also observed from Fig. 6 that the difference between two polarization schemes for a given orientation O depends on the Rician factor K and the XPD factor illustrated in Fig. 5 and4, respectively. For instance, the larger the difference in K values between co and cross-polarization schemes (see O1 and O2 in Fig. 5), the larger the gap between the corresponding configurations for ρ Rx . This suggests a clear relationship between the contribution of the MPC (strong K variations) and the evolution of ρ Rx for different schemes ψ.

Sum Rate Capacity

In this section, the aim is to evaluate the sum-rate capacity of the system and address the key points of this investigation : 1) the influence of polarization diversity on the performance of the system, 2) the ZF performance for different polarization schemes and 3) the trade-off between low correlation values and high channel gains. Indeed, a full polarimetric modeling of the massive MU-MISO channel is still missing and the purpose of this study is to demonstrate the necessity to develop such models including polarization diversity.

The results of sum-rate capacity defined in Eq. 7 for ZF with waterfilling power allocation are discussed in this section with σ 2 n = 1. First, it has to be mentioned that the presented results include averaging over either SNR or number of Tx antennas and also depend on several parameters such as the power allocated, actual channel gain, spatial receiver correlation, orientation and polarization schemes. In Fig. 7, the sum-rate capacity is plotted as a function of the number of the number of Tx antennas and averaged over the SNR range ([-5 30 dB]) after the normalization process.

Similarly to the correlation results, the data clearly indicate that a better performance is obtained with O3 and O4 compared to O1 and O2. Indeed, as shown previously, O3 and O4 exhibit favorable propagation conditions despite a lower received power on average. Also, O4 performs better than O3 and O1 better than O2 which presents the strongest LOS component because it is facing the array for all users. It is important to underline an analogy between Fig. 7 and Fig. 6. For O3 and O4, VV polarization works better than HV. For O1 and O2, HV polarization presents higher values of capacity. This shows a relationship between spatial correlation and sum-rate capacity for ZF. In strong Rician environments (high channel gains) such as in O1 and O2, using cross-polarization will not only cause a drop in the users channel vectors correlation, but will also yield to an increase in the sum-rate capacity and overall spectral efficiency improvement.

This fact establishes a trade-off between low correlation values and high channel gains. In Fig. 8, the sum-rate capacity is plotted as a function of the SNR averaged over the range of Tx antennas. It can be seen that sum-rate capacity varies linearly with SNR, especially for large SNR values and reaches very good values compared to the i.i.d. curve. The order of the curves is similar to that in Fig. 7 and the same conclusions stand. For instance, in Fig. 8, the curves are close to each other and schemes with lower correlation values result into better sum-rate capacity. However, the gap trends in Fig. 5 and6 are not observed for sum-rate capacity results. This is due to the application of the waterfilling technique for which power is not evenly distributed among users and, therefore, affects the overall sum-rate capacity. Under this umbrella, equal power allocation was considered to evaluate the effect of polarization diversity for different orientations (Fig. 9). Evidently, the obtained values are lower because this allocation strategy is not optimal (especially for low SNRs). However, the gaps between curves are in agreement with those observed for the Rician factor and spatial correlation values. For O3 and O4, wherein a lower average K is experienced and a narrower difference between co and cross-polarized schemes is reached, polarization diversity effects are not well pronounced. For O1 and O2, the wider gap in K between co and cross-polarization is shown to result in lower spatial correlation values as discussed earlier. Thus, the overall sum-rate capacity for these configurations is improved.

Finally, it is noteworthy that the overall performance is not solely dependent on K as the average received power and XPD also contribute. However, it can be safely concluded from the presented results the importance of full polarimetric modeling of the massive MU-MISO channel if benefits are to be harvested from polarization diversity.

Conclusion

In this contribution, polarimetric massive MU-MISO measurements were performed with a real-time channel sounder at 1.35 GHz over a 80 MHz bandwidth with a large 8 × 12 URA for Tx and 6 Rx users under four different orientations in an indoor scenario. The massive channel was evaluated using propagation characteristics such as average received power, XPD and Rician factor K. Moreover, the spatial correlation was evaluated for all orientations and polarization schemes. It was observed that the correlation varies as a function of the orientation and the polarization schemes. Cross-polarized schemes have better spatial separation than co-polarized schemes for orientations where a strong LOS component is present. On the other side, system performance was evaluated via sum-rate capacity using ZF linear pre-coder and compared with the spatially uncorrelated i.i.d. canonical model. From this analysis, cross-polarized schemes were shown to perform better with ZF and waterfilling when spatial correlation is correspondingly lower. The additional degrees of freedom brought by polarization diversity alongside spatial multiplexing were demonstrated to enhance the system capacity and improve spectral efficiency. Future works include polarimetric massive MU-MISO measurements in outdoor and industrial scenarios to validate the conclusions of this work.

Fig. 1 :

 1 Fig. 1: Panoramic view of the indoor room from the HV-MMIMO transmitter.

Fig. 2 :Fig. 3 :

 23 Fig. 2: Schematic top-view of the indoor room and the Tx setup

Fig. 4 :

 4 Fig. 4: ECDF of the XPD for all four orientations.

Fig. 5 :

 5 Fig. 5: ECDF of Rician factor K for all four orientations.

Fig. 6 :

 6 Fig. 6: Receiver spatial correlation for all four orientations and HV/VV polarizations.

Fig. 7 :

 7 Fig. 7: Sum-rate capacity = f(M) for ZF with waterfilling.

Fig. 8 :

 8 Fig. 8: Sum-rate capacity = f(SNR) for ZF.

Fig. 9 :

 9 Fig. 9: Sum-rate capacity = f(SNR) for ZF and equal power allocation.

Table 1

 1 Radio channel sounder main features and System Parameters

	MIMOSA Main Features	Tx array parameters
	Frequency	1.35 GHz	# of ULA elements	8
	Span BW	80 MHz	# of horizontal positions	12
	Resolution (sampling step) 12.5 ns (3.75 m) # of Tx antennas M (hybrid setup)	96
	Dynamic Range	> 100 dB	# of Rx users K	6
	Frequency points	819	Antenna spacing for ULA	10.8 cm (0.486 λ)
	Subcarrier spacing	97.7 kHz	Horizontal Step	10.368 cm (0.468 λ )
	Wavelength	22.22 cm
	Snapshot time	243.84 µs	Height of first Tx element	1.01 m
	Number of snapshots	10	Height of last Tx element	1.765 m
	Tx output power	0.01 to 1 W	Height of Rx antenna	1.65 m
	Number of snapshots	10	Number of orientations	4

Table 2

 2 Median received power for different selected configurations

	Orientation ψ	P(ψ)(dB)
	O1	VV HV	-43.11 -50.6
	O2	VV HV	-43.77 -50.61
	O3	VV HV	-47.81 -53.46
	O4	VV HV	-47.62 -52.17

ACKNOWLEDGEMENT

This work was funded through the OS4 SMARTIES research program by the ELSAT2020 project co-financed by the European Union with the European Regional Development Fund, the French state and the Hauts de France Region Council.


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■ INTRODUCTION

The design of nanoporous materials with the simultaneous control of the particle size, morphology and porosity remains one of the key elements in the context of heterogeneous catalysis applications in solution. [START_REF] Tsai | Rational Catalyst Design: A Multifunctional Mesoporous Silica Catalyst for Shifting the Reaction Equilibrium by Removal of Byproduct[END_REF][START_REF] Carrillo | Microwave-assisted catalysis by iron oxide nanoparticles on MCM-41: Effect of the support morphology[END_REF] Indeed, the resistance to mass transfer of solutes is often large enough to impede the accessibility to the surface reactive sites occurring by a diffusion process at the solid-surface interface. [START_REF] Liu | Interplay between Anomalous Transport and Catalytic Reaction Kinetics in Single-File Nanoporous Systems[END_REF] Mesoporous silica nanoparticles (MSN) display great potential advantages in liquid-phase catalytic processes, because particle size and shape can noticeably modify the transport of the solutes from the bulk solution to the internal pore surface. It is thus expected that morphology be of great importance in the development of more efficient heterogeneous catalysts. [START_REF] Chen | Phosphotungstic acid containing ionic liquid immobilized on magnetic mesoporous silica rod catalyst for the oxidation of dibenzothiophene with H 2 O 2[END_REF][START_REF] Yin | Influence of the structure and morphology of CuO supports on the amount and properties of copper-cerium interfacial sites in inverse CeO 2 /CuO catalysts[END_REF][START_REF] Gao | Synthesis of NiMo catalysts supported on mesoporous Al-SBA-15 with different morphologies and their catalytic performance of DBT HDS[END_REF] Although, rodshaped mesoporous silica nanoparticles maintaining a lengthwise alignment of uniform pore channels are of high current interest in the context of their potential applications in catalysis, [START_REF] Tsai | Rational Catalyst Design: A Multifunctional Mesoporous Silica Catalyst for Shifting the Reaction Equilibrium by Removal of Byproduct[END_REF][START_REF] Carrillo | Introducing catalytic activity in helical nanostructures: microwave assisted oxathioacetalisation catalysed by Al-containing helical mesoporous silicas[END_REF][START_REF] Carrillo | Helical Al-and Ce-MCM-41 materials as novel catalyst for acid and redox processes[END_REF] separation, [START_REF] Minakuchi | Octadecylsilylated Porous Silica Rods as Separation Media for Reversed-Phase Liquid Chromatography[END_REF] sensoring, [START_REF] Rambaud | One-Pot Synthesis of Functional Helicoidal Hybrid Organic-Inorganic Nanofibers with Periodically Organized Mesoporosity[END_REF][START_REF] Trewyn | Mesoporous silica nanoparticle based controlled release, drug delivery, and biosensor systems[END_REF] and nanomedicine, [START_REF] Huang | The effect of the shape of mesoporous silica nanoparticles on cellular uptake and cell function[END_REF][START_REF] Huang | The Shape Effect of Mesoporous Silica Nanoparticles on Biodistribution, Clearance, and Biocompatibility in Vivo[END_REF][START_REF] Xu | Rod-like mesoporous silica nanoparticles with rough surfaces for enhanced cellular delivery[END_REF][START_REF] Yang | Functionalized mesoporous silica materials for controlled drug delivery[END_REF][START_REF] Meng | Aspect Ratio Determines the Quantity of Mesoporous Silica Nanoparticle Uptake by a Small GTPase-Dependent Macropinocytosis Mechanism[END_REF] there are only few reports investigating the catalytic efficiency of rod-shaped MSN as supports of active catalytic centers. [START_REF] Tsai | Rational Catalyst Design: A Multifunctional Mesoporous Silica Catalyst for Shifting the Reaction Equilibrium by Removal of Byproduct[END_REF][START_REF] Carrillo | Microwave-assisted catalysis by iron oxide nanoparticles on MCM-41: Effect of the support morphology[END_REF][START_REF] Carrillo | Helical Al-and Ce-MCM-41 materials as novel catalyst for acid and redox processes[END_REF][START_REF] Fernandes | Highly enantioselective olefin epoxidation controlled by helical confined environments[END_REF] It was recently reported that accessibility to acid sites in Al-mesoporous silica nanoparticles could be correlated with the particle size rather than with their morphology. [START_REF] Carrillo | Helical Al-and Ce-MCM-41 materials as novel catalyst for acid and redox processes[END_REF] Compared to mesoporous silica spheres of several hundred nanometers, rod-shaped MSN possessed in fact diameters below hundred nanometers, which resulted in a high accessibly to the active sites.

Moreover, another key focus of MSN applications as catalysts was the introduction of either chemical functionalities or heteroatoms into the silica framework to improve the weak reactivity of silica surfaces. [START_REF] Beŕube | A generalized method toward high dispersion of transition metals in large pore mesoporous metal oxide/silica hybrids[END_REF][START_REF] Kargol | Copper-and Silver-Containing Monolithic Silica-Supported Preparations for Selective Propene-Propane Adsorption from the Gas Phase[END_REF][START_REF] Velu | Characterization of MCM-41 mesoporous molecular sieves containing copper and zinc and their catalytic performance in the selective oxidation of alcohols to aldehydes[END_REF][START_REF] Rascon | Molecular nature of support effects in single-site heterogeneous catalysts: silicavs.alumina[END_REF][START_REF] Perego | Porous materials in catalysis: challenges for mesoporous materials[END_REF] Among the various strategies for the synthesis of small mesoporous silica particles, [START_REF] Taguchi | Ordered mesoporous materials in catalysis[END_REF] the preparation of nanosized mesoporous silica spheres with a controlled size and morphology, as well as regular pore ordering includes the use of base-mediated conditions (i.e., modified Stober route), [START_REF] Valtchev | Porous Nanosized Particles: Preparation, Properties, and Applications[END_REF][START_REF] Huh | Organic Functionalization and Morphology Control of Mesoporous Silicas via a Co-Condensation Synthesis Method[END_REF] which are not compatible with common in situ incorporation of ionic metal precursors. Transition metal inclusion in the MSN framework can be also achieved by a postsynthesis impregnation; nevertheless, the backfilling approach risks clogging of the matrix pores, thereby reducing the pore accessibility. In view of the one-step approach strategy, a new in situ functionalization procedure has been developed in our institute based on the phenomenon of micellar solubilization. [START_REF] Nyalosaso | Aluminium-derivatized silica monodisperse nanospheres by a one-step synthesis-functionalization method and application as acid catalysts in liquid phase[END_REF][START_REF] Derrien | Copper-containing monodisperse mesoporous silica nanospheres by a smart one-step approach[END_REF] This combined synthesis and functionalization approach, depicted in Scheme 1, was for the first time, to the best of our knowledge, adapted for preparing copper-containing rod-shaped MSN with perfect conservation of pore-ordering, shape and monodispersity, and high intrapore metal dispersion. For this purpose, templating micelles of a cationic surfactant in situ loaded with a given amount of copper precursor were expected to play a double role of "soft porogens" to create nanoscopic pore structure and precursordelivery "capsules" for targeted functionalization of the future internal surfaces. Location of the copper precursor inside the micelles was conceived in a way to preclude the metal species from being inserted into the bulk of the solid during silica condensation.

It is well-known that heterogeneous catalysts supporting copper species may regioselectively catalyze the Huisgen 1,3dipolar cycloaddition of (organic) azides and terminal alkynes (CuAAC) [START_REF] Huisgen | Kinetics and reaction mechanisms: Selected examples from the experience of forty years[END_REF][START_REF] Rostovtsev | A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective "Ligation" of Azides and Terminal Alkynes[END_REF][START_REF] Tornøe | Peptidotriazoles on solid phase:[1, 2, 3]-triazoles by regiospecific copper (I)-catalyzed 1, 3dipolar cycloadditions of terminal alkynes to azides[END_REF][START_REF] Padwa | 3-Dipolar cycloaddition chemistry[END_REF][START_REF] Bock | CuI-Catalyzed Alkyne-Azide "Click" Cycloadditions from a Mechanistic and Synthetic Perspective[END_REF][START_REF] Mohammed | Recyclable clay supported Cu (II) catalyzed tandem one-pot synthesis of 1-aryl-1,2,3-triazoles[END_REF] in pure solvents [START_REF] Wang | Insights into Supported Copper(II)-Catalyzed Azide-Alkyne Cycloaddition in Water[END_REF][START_REF] Lal | CuBr(PPh3)3] for Azide-Alkyne Cycloaddition Reactions under Strict Click Conditions[END_REF] or solvent mixtures [START_REF] Soltani Rad | Copper-doped silica cuprous sulfate (CDSCS) as a highly efficient and new heterogeneous nano catalyst for [3 + 2] Huisgen cycloaddition[END_REF][START_REF] Yamada | Amphiphilic Self-Assembled Polymeric Copper Catalyst to Parts per Million Levels: Click Chemistry[END_REF] to achieve 1,2,3-triazole derivatives, an important class of bioactive compounds with, among others, antiallergenic, [START_REF] Buckle | Studies on v-triazoles. 7. Antiallergic 9-oxo-1H,9H-benzopyrano-[2,3-d]-v-triazoles[END_REF] antibacterial, [START_REF] Romero | An efficient entry to new sugar modified ketolide antibiotics[END_REF][START_REF] Genin | Substituent Effects on the Antibacterial Activity of Nitrogen-Carbon-Linked (Azolylphenyl)oxazolidinones with Expanded Activity Against the Fastidious Gram-Negative Organisms Haemophilus influenzae and Moraxella catarrhalis[END_REF][START_REF] Holla | characterization and antimicrobial activity of some substituted 1,2,3-triazoles[END_REF] or anti-HIV [START_REF] Zhou | Synthesis and Antiviral Activities of 1,2,3-triazole Functionalized Thymidines: 1,3-dipolar Cycloaddition for Efficient Regioselective Diversity Generation[END_REF][START_REF] Alvarez | Triazole-[2,5-Bis-O-(tert-butyldimethylsilyl)-.beta.-D-ribofuranosyl]-3′-spiro-5″-(4″-amino-1″,2″-oxathiole 2″,2″-dioxide) (TSAO) Analogs: Synthesis and Anti-HIV-1 Activity[END_REF] activities. In addition, immobilized copper has proven to be very efficient in catalyzing the threecomponent reactions of organic halides, sodium azide, and terminal alkynes, allowing the one-pot synthesis of triazoles. [START_REF] Beńeteau | Zeo-click synthesis: CuI-zeolite-catalyzed one-pot two-step synthesis of triazoles from halides and related compounds[END_REF][START_REF] Radatz | Recoverable Cu/SiO2 compositecatalysed click synthesis of 1,2,3-triazoles in water media[END_REF][START_REF] Lipshutz | Heterogeneous Copper-in-Charcoal-Catalyzed Click Chemistry[END_REF][START_REF] Sharghi | Copper Nanoparticles on Charcoal for Multicomponent Catalytic Synthesis of 1,2,3-Triazole Derivatives from Benzyl Halides or Alkyl Halides, Terminal Alkynes and Sodium Azide in Water as a "Green[END_REF][START_REF] Alonso | Multicomponent Synthesis of 1,2,3-Triazoles in Water Catalyzed by Copper Nanoparticles on Activated Carbon[END_REF][START_REF] Kale | Conventional and microwave-assisted multicomponent reaction of alkyne, halide and sodium azide catalyzed by copper apatite as heterogeneous base and catalyst in water[END_REF][START_REF] Nador | Copper nanoparticles supported on silica coated maghemite as versatile, magnetically recoverable and reusable catalyst for alkyne coupling and cycloaddition reactions[END_REF] This reaction pathway has the advantage of eliminating the use and handling of explosive and hazardous organic azides, as they are generated in situ via a substitution mechanism usually catalyzed by a metallic Lewis acid.

The present paper reports on the one-pot synthesis of Cudoped rod-shaped mesoporous silica nanoparticles (Cu-RMSN) with active sites located at the internal surface and thus easily accessible by the reactants, as required for specific surface reactivity in the liquid phase. The classical characterization methods were employed to determine the particle size distribution, textural properties, degree of Cu incorporation into the silica framework, and accessibility to the active sites in the resulting materials. An eco-friendly microwave-assisted multicomponent reaction in water was subsequently tested, without addition of any other cosolvent or ligand. This "green" approach succeeded in copper-mediated cycloaddition of alkynes, benzyl or alkyl halides, and sodium azide making use of Cu-RMSN as a heterogeneous and recyclable catalyst in aqueous solution, for the first time.

■ MATERIALS AND METHODS

All reagents were commercially available and used without any further purification.

Materials Characterizations. Scanning electron microscopy (SEM) and transmission electron microscopy images (TEM) were obtained with a Hitachi S-2600N (using a secondary electron scintillator as detector) and a JEOL 1200 EX microscope, respectively. For the purpose of TEM analysis, the sample particles were dispersed in ethanol and then dropped onto copper grids with porous carbon films. The specific surface area and pore structure parameters of the materials studied were determined from the measurements of nitrogen adsorption-desorption at 77 K with the use of a Micromeritics ASAP 2020 V3.00 H unit. Prior to the sorption experiment, the sample (about 40 mg) was evacuated under vacuum at 523 K for 12 h. In view of applications of the present Cu-RMSN materials as catalysts, the number of acid sites was assessed and attributed to the number of accessible copper atoms at the solid surface by assuming that pure silica displayed a very limited number of acid sites. [START_REF] Nyalosaso | Aluminium-derivatized silica monodisperse nanospheres by a one-step synthesis-functionalization method and application as acid catalysts in liquid phase[END_REF] Measurements of the two-cycle adsorption of gaseous ammonia at 373 K were achieved with the aid of a Micromeritics ASAP 2010 V2.02 and the results served to determine the number of acid sites in the material surface. The sample was outgassed prior to the measurements at 473 K for 2 h at a rate of 5 K•min -1 . Afterward, the two-cycle adsorption experiments were performed under equilibrium adsorption conditions at 373 K with gaseous NH 3 of ultrahigh purity. At the end of the first adsorption cycle, the sample was outgassed under vacuum at 373 K for 60 min and a second adsorption cycle was then performed at the same temperature. It is usually assumed that the linear section distinguished in both isotherms at higher equilibrium pressures may be ascribed to physical adsorption on nonreactive surface sites. Therefore, the two straight lines were extrapolated to zero pressure. The difference in the amount adsorbed for points at which the curves reach zero pressure was ascribed to the irreversible chemisorption of basic NH 3 at the solid-gas interface and thereby to the total number of acid sites at the solid surface per unit mass of the adsorbent.

Scheme 1. Synthesis Pathway of Metal-Rod Shaped Mesoporous Silica Nanoparticles

Small-angle powder X-ray diffraction (XRD) measurements were performed with a PANalytical X'Pert MPD (Philips 1710) diffractometer controlled by a computer. The XRD patterns were collected using a Cu Kα radiation, the 2θ diffraction angles being recorded at a rate of 0.5°min -1 in the range from 1°to 7°. For some selected samples containing copper, the XRD experiment was continued in the 2θ range up to 60°. Temperature-programmed reduction and metal dispersion experiments were also carried out in the temperature range from 35 to 800 °C (heating rate 2 K min -1 ) under a flow of H 2 utilizing a Micromeritics AutoChem 2910 equipped with a Dewar flask containing cold medium to trap water. [START_REF] Tsai | Rational Catalyst Design: A Multifunctional Mesoporous Silica Catalyst for Shifting the Reaction Equilibrium by Removal of Byproduct[END_REF] H NMR spectra were obtained at 297 K using an AC 300L Bruker spectrometer operating at a Larmor frequency of 69.19 MHz (15 000 signal acquisitions, recycle delay of 8 s between π/2 pulses). NMR spectra were recorded at room temperature with the appropriate deuterated solvent (CDCl 3 or DMSO-d 6 ). Chemical shifts (δ) of 1 H NMR and 13 C NMR spectra are reported in ppm relative to residual solvent signals (CHCl 3 in CDCl 3 : δ = 7.27 ppm for 1 H and CDCl 3 : δ = 77.04 ppm for [START_REF] Huang | The Shape Effect of Mesoporous Silica Nanoparticles on Biodistribution, Clearance, and Biocompatibility in Vivo[END_REF] C NMR); data are reported as br = broad, s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet; J values are given in Hz. [START_REF] Tsai | Rational Catalyst Design: A Multifunctional Mesoporous Silica Catalyst for Shifting the Reaction Equilibrium by Removal of Byproduct[END_REF] H and [START_REF] Huang | The Shape Effect of Mesoporous Silica Nanoparticles on Biodistribution, Clearance, and Biocompatibility in Vivo[END_REF] C NMR spectra were registered at 300 or 400 MHz. The nature of analytically pure final products was assessed by comparison of their 1 H NMR data previously described in the literature and by their fragmentation in the LC/MS equipment. Conversions and yields were calculated by adding CH 2 Br 2 (0.25 mmol; 20 μL) as an internal standard. Analytical high performance liquid chromatography (HPLC) was performed using a CHROMOLITH RP18 column (50 × 4,6 mm), variable wavelength diode, flow of 5 mL min -1 , linear gradient CH 3 CN in water 0-100% (+0.1% TFA) in 4.5 min. The LC-MS analysis was performed by means of HPLC Waters Alliance 2695 (UV Waters 2489), column Onyx C18 (25 × 4.6 mm), flow of 3 mL min -1 , linear gradient CH 3 CN in water 0-100% (+0.1% HCO 2 H) in 2.5 min. Mass spectra (electrospray ionization mode, ESI-MS) were recorded on a Micromass (Manchester, UK) Q-TOF quadrupole mass spectrometer fitted with an electrospray interface. The mass spectrometer was calibrated in the positive-and negative-ion ESI modes. Samples were dissolved in a H 2 O/CH 3 CN (50/50 v/v) mixture. Microwave-assisted reactions were performed in sealed vessels with a Biotage Initiator 60 EXP instrument. The temperature was measured with an IR sensor at the outer surface of the reaction vial. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out on a SETARAM Labsys apparatus in order to study changes in the chemical properties of the pristine samples and those containing Cu species as a function of increasing temperature. A given mass of solid sample was placed in a 80 μL alumina crucible. The TGA and DTA curves were obtained up to 873 K in flowing nitrogen using a constant heating rate of 10°min -1 . Ultraviolet-visible (UVvis) absorption spectra were collected with an UV-visible Jasco V-670 spectrophotometer. Leaching of Cu from Cu-RMSN was assessed by inductive coupled plasma (ICP-MS, Agilent 7700x) analysis of the aqueous phase; the instrument detection limit for copper was below 30 ppt.

Catalyst Preparation. The Cu-doped mesoporous silica rodshaped nanoparticles (Cu-RMSN) were prepared by a one-step synthesis-functionalization procedure, based on the co-condensation of (3-aminopropyl)triethoxysilane (APTES) and tetraethyl orthosilicate (TEOS) as a silicon source. In a typical synthesis, 0.05 g of hexadecyltrimethylammonium bromide (CTAB) was dissolved in 50 mL of deionized water under continuous stirring at room temperature. Then, Cu(acac) 2 , previously dissolved in 200 μL of chloroform, was added to the above-mentioned mixture under vigorous stirring at 343 K. This step corresponded to the solubilization of the copper precursor in micellar aggregates. 575 μL TEOS and 80 μL APTES were subsequently added to the solution at 313 K under intensive stirring. Hydrolysis and polycondensation steps were carried out at pH adjusted to 12 with sodium hydroxide. The molar composition of the final solution was as follows: (i) CTAB:NaOH:TEOS:APTES:Cu-(acac) 2 = 1:9.7 × 10 -4 :1.9 × 10 -2 :2.5 × 10 -4 ; (ii) X (X = 2.0 × 10 -5 ; 2.7 × 10 -5 ; 3.4 × 10 -5 ). The colloidal mixture was stirred for 2 additional hours. Afterward, the resulting white precipitate was recovered by centrifugation and dried in air at 323 K. The template was eliminated from Cu-RMSN by air calcinations at 723 K for 8 h with a heated rate of 1 K min -1 . A fine slightly blue powder of Cu-RMSN was finally obtained. The three samples were designated Cu-RMSN-X with X corresponding to the initial Si:Cu ratio (i.e., 190, 112, 84) in the gel synthesis. The pure siliceous material designated as RMSN was prepared using the previous procedure except that the Cu source was excluded from the gel synthesis.

General Procedure for the One-Pot Synthesis of 1,4-Disubstituted-1,2,3-triazoles under Microwave Irradiation. Cu-RMSN catalyst (35 mg, 1.25 mol %) was charged in a microwave Pyrex vial and activated throughout gassing under vacuum at 473 K for 8 h. Then, the appropriate alkyl or benzyl halide (0.2 mmol), alkyne (0.2 mmol), sodium azide (0.2 mmol), and sodium ascorbate (0.2 mmol) were added, together with about 2 mL of water. The reaction mixture was heated under microwave irradiation [START_REF] Baghbanzadeh | Microwave-Assisted Synthesis of Colloidal Inorganic Nanocrystals[END_REF] with constant stirring (900 rpm) at 333 K for the specified time. At the end of the reaction, CH 2 Cl 2 /Et 2 O (1:4 v/v, 5 mL × 3) was added into the vial and the heterogeneous system was again stirred for 1 min. Then, the upper organic layers were recovered by decantation, dried on MgSO 4 , filtered, and evaporated under vacuum to collect the corresponding 1,4-disubstituted-1,2,3-triazole as a white solid.

General Procedure for the Cu-RMSN Catalyst Recycling. To get rid of all water-soluble species from the catalyst surface, the catalyst was washed three times with 3 × 5 mL of water added under constant stirring to a given Pyrex vial containing the catalyst issued from the previous run. The aqueous phase was removed each time by centrifugation and at least the catalyst was lyophilized. The resulting solid catalyst was reactivated as described previously, before being reused in a subsequent run following the same operating procedure.

■ RESULTS AND DISCUSSION

As described previously, [START_REF] Derrien | Copper-containing monodisperse mesoporous silica nanospheres by a smart one-step approach[END_REF] the one-pot synthesis-functionalization approach allowing in situ metal incorporation in mesoporous silica led to Cu-containing nanosized silica objects having different morphologies. Figure 1 shows SEM micrographs of the resulting particles, which are either rod-like or spherical, depending on the amount of metal incorporation dissolved in chloroform. Apparently, the rod-to-sphere shape transition was favored by higher copper contents. Irrespective of their shape, the particles appeared quite homogeneous in size. The appropriate image processing gave the following size parameters: purely siliceous particles were 450-500 nm in length and 160-190 nm in width; rod-like particles with the smallest Cu content were 160-200 nm in length and 80-100 nm in width; spherical particles were in the 75-110 and 75-100 nm diameter range with a Si:Cu ratio of 112 and 84, respectively. This indicated that the increased copper incorporation resulted in the concomitant decrease in the particle size, the effect being less pronounced within the spherical shape category.

Changes in the pore arrangement and porosity parameters could be followed by the adsorption-desorption isotherms analysis of gaseous nitrogen at 77 K, powder XRD patterns, and BJH pore size distributions as reported in Figure 2. The shape of the nitrogen adsorption curves was characteristic of silica materials possessing intra particle tubular mesopores at the lower end of the mesoporous regime, some residual inter particle porosity (Cu-containing spherical particles in particular), and a significant external surface area. [START_REF] Nyalosaso | Aluminium-derivatized silica monodisperse nanospheres by a one-step synthesis-functionalization method and application as acid catalysts in liquid phase[END_REF][START_REF] Kruk | Gas Adsorption Characterization of Ordered Organic-Inorganic Nanocomposite Materials[END_REF][START_REF] Roziere | Effect of surfactant type, substitution by aluminium and additives on direct liquid crystal templated monolithic silica[END_REF][START_REF] Tompsett | Hysteresis and Scanning Behavior of Mesoporous Molecular Sieves[END_REF] A gradual increase in the amount of gaseous nitrogen adsorbed in the capillary condensation region indicated the intra particle mesopores were somehow heterogeneous, especially for the sample, which contained the highest copper amount. This heterogeneity was illustrated by the width of the BJH distribution curves. An additional characterization procedure based on the α S -method was performed to determine the "average" pore structure (see Figure S1 in the Supporting Information), thus providing the following parameters: the micropore volume, V mic , the area of the mesopore walls, S mes , the external surface, S ext , and the volume of mesopores, V mes , as well as the mean pore diameter, d m , calculated assuming a cylindrical pore shape. [START_REF] Roziere | Effect of surfactant type, substitution by aluminium and additives on direct liquid crystal templated monolithic silica[END_REF][START_REF] Gregg | Surface Area, and Porosity[END_REF] The X-ray diffraction patterns clearly showed the first-order scattering peak (d 100 ), whereas the low intensity (110) and (200) peaks were still distinguishable from the instrumental noise.

These XRD results corresponded to a 2D hexagonal mesostructure with a p6 mm space group. Therefore, the porosity of the four silica samples might be qualified of the MCM-41 type. All textural parameters inferred from the analysis of XRD patterns and N 2 adsorption-desorption isotherms are reported in Table 1. It was observed that copper incorporation during the one-pot synthesis had a positive effect on promoting porosity formation. In any case, the pore size and the specific surface area increased, compared to the corresponding textural parameters characterizing the pristine sample containing no metal at all. These trends depended on both the overall metal content and the particle morphology. The greatest values of the surface area and pore volume parameters were obtained for the Si:Cu = 112 sample. The Cucontaining rod-like particles (i.e., Si:Cu = 190) had the largest porosity, providing easier access to the internal porous space. Obviously, the selection of materials for applications in heterogeneous catalysis cannot be limited only to textural parameters. Such other characteristics like nature, distribution, and accessibility of the surface reactive sites are also of great significance. Further catalytic tests in aqueous medium revealed the superior performance of the Si:Cu = 190 sample. Therefore, the surface properties of this material are described in more details in the following section.

Surface Properties of the Cu-containing Rod-like Particles. Further confirmation of the quasi-hexagonal order in the arrangement of tubular pores on the surface of silica particles could be obtained from the analysis of TEM micrographs (Figure 3). Different regions showing more or less marked alignment of pore channels with a uniform crosssectional diameter were visible (Figure 3). For numerous rods orientated horizontally, pore channels parallel to each other were extended along the entire length of the particle. A few particles had adopted a vertical orientation on the TEM holder and a quasi-hexagonal alignment of uniform channels could be seen from the top. In some cases, partial "finger-print" structures were also observed indicating some disorders in the channel alignment. Moreover, no metal containing clusters could be revealed (Figure 3), which points out a good dispersion of the copper species in the silica matrix.

More information about the nature and location of the copper species was obtained from the diffuse reflectance UVvis spectrum, powder XRD pattern in the wide-range region, H 2 -TPR profile, as well as TGA/TDA profiles acquired from the Si:Cu = 190 sample (Figure 4). The UV-vis spectrum displayed features similar to those observed previously: [START_REF] Derrien | Copper-containing monodisperse mesoporous silica nanospheres by a smart one-step approach[END_REF] a main band representing a right-skewed peak with a maximum at about 225 nm, a weak and very broad band at high wavelength values beyond 600 nm (Figure 4a). The main band could be considered as indicative of the ligand-to-metal charge transfer between Cu(II) ion and oxygen in mononuclear species located at the internal surface. [START_REF] Derrien | Copper-containing monodisperse mesoporous silica nanospheres by a smart one-step approach[END_REF][START_REF] Chmielarz | Catalytic performance of various mesoporous silicas modified with copper or iron oxides introduced by different ways in the selective reduction of NO by ammonia[END_REF] The XRD pattern showed no peak in the 2θ range between 30°and 60°(Figure 4b), which argues against the presence of extra-framework metal oxide crystalline aggregates within the material. [START_REF] Trejda | Niobium rich SBA-15 materialspreparation, characterisation and catalytic activity[END_REF] The TGA profile of the calcined Cu-RMSN displayed no weight loss that could be ascribed to the decomposition of remaining surfactant (Figure 4d). The weight loss that occurred at temperatures close to 373 K could be assigned to the release of physisorbed water. The TPR profile showed a broad and asymmetric peak with a temperature at maximum hydrogen consumption T m around 418 K (Figure 4c), usually attributed to the reduction of Cu(II) or finely dispersed CuO to copper metal in the channels of MCM-41 materials. [START_REF] Kargol | Copper-and Silver-Containing Monolithic Silica-Supported Preparations for Selective Propene-Propane Adsorption from the Gas Phase[END_REF][START_REF] Velu | Characterization of MCM-41 mesoporous molecular sieves containing copper and zinc and their catalytic performance in the selective oxidation of alcohols to aldehydes[END_REF] Because the temperature of reduction was lower than that previously recorded for the spherical silica particles achieved by the same synthesis-functionalization procedure (i.e., 485 K 27 ), it appeared that, in the present case, copper oxide was located in readily accessible positions well dispersed at the pore surface. Moreover, on the basis of the results of EDX and those of two-cycle ammonia adsorption onto Cu-RMSN (Si:Cu = 190) (Figure 5), the accessibility of copper surface sites was evaluated by assuming that the ratio between the number of Cu atoms calculated from the number of acid sites, N acid , and the theoretical number of Cu atoms calculated from the EDX results corresponded to the number of Cu atoms accessible to gaseous NH 3 molecules. In the case of RMSN (Si:Cu = 190), the N acid value was of 70 μmol g -1 (i.e., 4.4 mg of Cu per gram of catalyst). When ascribed to the number of accessible copper atoms, this gave 85% accessibility with respect to the total number of copper atoms.

Catalytic Performance of the Cu-containing Rod-like Particles. The rod-like mesoporous silica nanoparticles with copper functionalities at the pore surface (Cu-RMSN), achieved in a one-step approach, were evaluated as nanocatalyst in the eco-friendly version of the Huisgen 1,3-dipolar cycloaddition (click reaction) leading to 1,4-disubstituted-1H-1,2,3-triazoles. The regioselectivity was studied by making use of organic halides and terminal alkynes with sodium azide in water under microwave irradiation. This multicomponent Because the ascending part of the adsorption isotherm and the hysteresis loop at relative pressures close to unity do not allow these parameters to be determined unambiguously by applying the α S -plot procedure, the values reported in the table should be treated with caution and only general trends are reliable. a As indicated by the molar Si:Cu ratio. reaction followed a two reactions cascade where the Cu-RMSN promoted first the nucleophilic substitution for the in situ formation of the organic azide and then catalyzed cycloaddition. [START_REF] Beńeteau | Zeo-click synthesis: CuI-zeolite-catalyzed one-pot two-step synthesis of triazoles from halides and related compounds[END_REF][START_REF] Lee | Copper-in-Charcoal-Catalyzed, Tandem One-Pot Diazo Transfer-Click Reactions[END_REF] This avoided the isolation of organic azides (generally stable either in water or in the presence of oxygen under most reaction conditions), some lower alkyl azides decomposing rapidly with a risk of explosion. [START_REF] Alvarez | A practical procedure for the synthesis of alkyl azides at ambient temperature in dimethyl sulfoxide in high purity and yield[END_REF] As representative substrates, benzyl bromide and phenyl acetylene were added in stoichiometric amounts to optimize the reaction conditions with NaN 3 under microwave irradiation in aqueous medium (Table 2). Because the reaction had to be performed in the absence of catalyst, no triazole product was observed (Table 2, entry 1). As expected, [START_REF] Beńeteau | Zeo-click synthesis: CuI-zeolite-catalyzed one-pot two-step synthesis of triazoles from halides and related compounds[END_REF] the solvolysis product benzyl alcohol was formed. By irradiating the mixture for 30 min at 60 °C, in the presence of Cu-RMSN catalyst (1.25 mol %), the corresponding 1,4-disubstituted-1H-1,2,3-triazole was regioselectively obtained with a high yield (97%, entry 2). The Cu-RMSN catalyst played a double role: it catalyzed the reaction [START_REF] Radatz | Recoverable Cu/SiO2 compositecatalysed click synthesis of 1,2,3-triazoles in water media[END_REF][START_REF] Veerakumar | Highly dispersed silica-supported nanocopper as an efficient heterogeneous catalyst: application in the synthesis of 1, 2, 3-triazoles and thioethers[END_REF] and avoided the formation of 1,5-disubstituted triazolyl regioisomer that had never been observed in the crude mixture, [START_REF] Rostovtsev | A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective "Ligation" of Azides and Terminal Alkynes[END_REF][START_REF] Himo | Copper(I)-Catalyzed Synthesis of Azoles. DFT Study Predicts Unprecedented Reactivity and Intermediates[END_REF] as previously reported. However, shortening the reaction time to 15 min decreased the yield (entry 3). In the absence of sodium ascorbate [Cu(II) reducing agent], the reaction did not proceed as previously reported. [START_REF] Rostovtsev | A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective "Ligation" of Azides and Terminal Alkynes[END_REF] This result ruled out the presence of Cu(I) entities dispersed in the silica matrix [START_REF] Wang | Insights into Supported Copper(II)-Catalyzed Azide-Alkyne Cycloaddition in Water[END_REF] during the oxidative homocoupling of alkynes leading to the diynes formation throughout the Glaser reaction, [START_REF] Siemsen | Acetylenic coupling: a powerful tool in molecular construction[END_REF] which had never been observed in the present study. For comparison, the reaction was also carried out with the conventional catalyst CuBr salt at 7 and 49 mol % of Cu content (entries 4 and 5) under the same reaction conditions, leading respectively to 29% and 96% yield of triazole 1. This different behavior at low Cu concentrations compared to the conventional Huisgen copper-catalyzed reaction (CuAAC) in DMF solvent [START_REF] Ellanki | Solvent effect on coppercatalyzed azide-alkyne cycloaddition (CuAAC): Synthesis of novel triazolyl substituted quinolines as potential anticancer agents[END_REF][START_REF] Glasnov | Microwave-Assisted Click Chemistry for the Preparation of 3-and 4-Triazolyl-2(1H)-quinolones as Potential Fluorescent Probes[END_REF] under microwave irradiation came from the biphasic character of the reactive medium. Indeed, copper ions were water-soluble, whereas the organic substrates were too hydrophobic to be soluble in aqueous solutions. As a result, the contact between the organic substrates and the copper species was limited and consequently the product yield was quite low. When increasing the amount of CuBr to 49% (Cu content), the solubility limit was reached with the opportunity of substrate adsorption on the active CuBr particles dispersed in the aqueous solution. As a consequence, in order to compare the action of the Cu-RMSN catalyst with that of the conventional CuBr at 49 mol %, the selected reaction conditions were fixed at 1 h and 60 °C in aqueous solution, under microwave irradiation (entry 5).

The possibility of reacting phenyl acetylene with variation in the halide substrates compatible with the standard reaction conditions was subsequently explored. Aliphatic hex-1-yne reacted similarly to phenyl acetylene, yielding the corresponding triazoles with mostly the same yields and regioselectively (entry 7). The "click reaction" with 2-iodo-benzyl bromide was also efficient and not affected by the steric and electronic effects regarding the substitution on the ortho-position (entry 9) or by using functionalized alkyl bromides (entries 6 and 8). Primary or secondary alkyl bromides (entries 10 vs 11) reacted similarly, although the yields were slightly decreased. In all our study, Cu-RMSN showed a higher efficiency and performance than CuBr. Indeed, in all cases the yield of reactions performed with Cu-RMSN was higher compared to that using CuBr. This may be explained by the higher copper dispersion at the pore surface, accounting for the higher accessibility of the active sites, as inferred from the chemisorption measurements (Figure 5). Indeed, CuBr was dissolved in the aqueous phase and was not in direct contact with the nonaqueous miscible substrates. In the case of Cu-RMSN, the substrates were adsorbed at the pore surface of the mesoporous nanoparticles in the vicinity of the Cu catalytic active sites.

Reusability of the Cu-RMSN Catalyst. Considering the great economic and environmental added values of heterogeneous catalysts, it is important to examine their ease of separation, recoverability and reusability. In order to evaluate the reuse of the mesoporous Cu-RMSN catalyst we investigated the "click synthesis" of 1-benzyl-4-phenyl-1H-1,2,3-triazole 1 (Table 2, entry 5) and we adjusted the experimental conditions of the present system. After each cycle, the reaction was followed by extraction of the products and unreacted substrates, and the catalyst was washed with water, lyophilized, and then reactivated before performing the next run, without any further purification. The performance of the recycled catalyst was tested up to ten successive runs without significant yield loss of the reaction products (Figure 6). These results pointed out the catalytic efficiency of the recovered mesoporous Cu-RMSN, retaining the same activity throughout all reaction cycles, thereby indicating the high stability of the copper species at the pore surface. Moreover, at the end of the first cycle, the reaction mixture contained only 523 ppb of Cu, as indicated by the ICP-MS analysis. These copper species in solution were released during the reaction and originated from leaching. This amount corresponded to 0.16% molar of the copper initially present in Cu-RMSN and did not disturb catalysis of the reaction, since 7% of the Cu content was globally ineffective (Table 2, entry 4). The copper amount after the second cycle was of 61 ppb and only of 9 ppb after 10 successive cycles, indicating the high stability of the Cu-RMSN catalyst (Figure 6). It seems that the copper species localized at the mesopore surface in the rod-like MSN help to prevent the metal leaching and to preserve the high efficiency of the catalyst recycling, while keeping the regioselectivity of the reactions.

■ CONCLUSIONS

The insertion of Cu into the mesopores of rod-shaped mesoporous silica nanoparticles has been achieved with good control of morphological and textural properties. The TPR analysis of Cu-RMSN showed that the well-dispersed Cu species were confined inside the mesopores. The Cu-RMSN sample with a low catalyst loading (1.25%) was used to prepare regioselectively 1,4-disubstituted-1H-1,2,3-triazoles with a high yield via an eco-friendly multicomponent approach, in aqueous media and using microwave heating. The intrinsic and optimal atom economy for the CuAAC reaction, the use of water as a sustainable solvent, the lower catalytic charge compared to the previous reports with similar systems, the solvent economy related to the recovery of the final pure product, together with the ability to reuse the catalyst for at least ten subsequent runs without a marked loss of activity, contributed to the "green" character of the present catalytic process. In the future, preparation of enlarged mesopores represents a challenge to be considered to generalize the catalytic applications of such rod-like mesoporous silica nanoparticles to other reagents and substrates.

■ ASSOCIATED CONTENT

* S Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00661.

Textural properties of Cu-RMSN and analytical data (PDF).

■ AUTHOR INFORMATION

Corresponding Authors *C. Charnay. E-mail: clarence.charnay@umontpellier.fr. *E. Colacino. E-mail: evelina.colacino@umontpellier.fr.

Notes

The authors declare no competing financial interest.

■ REFERENCES

Figure 1 .

 1 Figure 1. Scanning electron microscope images of nanosized silica objects prepared by a one-pot synthesis-functionalization method and containing different amounts of copper, as defined by the following molar Si:Cu ratio: (a) purely siliceous sample, (b) Si:Cu = 190, (c) Si:Cu = 112, and (d) Si:Cu = 84.

Figure 2 .

 2 Figure 2. Adsorption-desorption isotherms of gaseous nitrogen at 77 K (4 left panels), powder XRD patterns (upper-right panel), and BJH pore size distributions based on the desorption branch of N 2 adsorption isotherms (lower-right panel) for silica materials containing different amounts of copper, as defined by the following molar Si:Cu ratio: (a) purely siliceous sample, (b) Si:Cu = 190, (c) Si:Cu = 112, and (d) Si:Cu = 84.

Figure 3 .

 3 Figure 3. Transmission electron micrographs of nanosized rod-like silica objects containing copper (Si:Cu = 190). The white arrow indicates a region where a particle is oriented vertically thus showing a quasi-hexagonal alignment of pore channels.

Figure 4 .

 4 Figure 4. (a) Diffuse reflectance UV-vis spectrum; (b) wide-angle range X-ray powder diffraction pattern; (c) temperature-programmed desorption (H 2 -TPR) profile; (d) results of thermogravimetric analysis for the rod-like silica particles containing copper (Si:Cu = 190).

Figure 5 .

 5 Figure 5. First-cycle (•) and second-cycle (○) adsorption isotherms for gaseous ammonia onto R-CuMSN (Si:Cu = 190) at 373 K.

Table 1 .

 1 Surface Area and Pore Structure Parameters of the Calcined Silica Particles Containing Various Initial Amounts of Copper a

	Si:Cu ratio	no Cu	190	112	84
	d 100 spacing (nm)	3.5	4.10	4.01	3.87
	S BET (m 2 g -1 )	410	485	570	420
	d BJH (nm)	2.06	2.90	2.61	2.69
	V mic (cm 3 g -1 )	0.02	0.03	0.05	0.02
	S ext (m 2 g -1 )	1 5	4 7 a	40 a	54 a
	S mes (m 2 g -1 )	275	326 a	395 a	294 a
	V mes (cm 3 g -1 )	0.20	0.30 a	0.32 a	0.26 a
	d m (nm)	3	3.6 a	3.3 a	3.6 a
	a

DOI: 10.1021/acssuschemeng.5b00661 ACS Sustainable Chem. Eng. 2015, 3, 2516-2525


Ahlam Belgroune
email: ahlam.belgroune@gmail.com

Linda Aissani
email: blindaaissani2004@yahoo.fr

Faiza Salhi
email: cfaiza.salhi@gmail.comd

Corrine Nouveau
email: corinne.ouveau@ensam.fr

Akram Alhussein
email: eakram.alhussein@utt.fr

Mechanical and Tribological Behaviors of Nanocomposite Titanium Nitrides Coatings

Keywords: TiN, Nanocomposite coating, Mechanical property, Friction, Wear

TiN coatings with a thickness of 2 μm were deposited using the magnetron sputtering has developed rapidly over the last decade in such a way that it has become an established process of choice for the deposition of a wide range in various applications for different domains as it gives excellent performance in many aspects. In view of this, we have deposited the TiN coatings by magnetron sputtering using Ti target at different nitrogen content to study the influence of the nitrogen content on the mechanical properties and tribological behaviors of the TiN coatings were systematically investigated using nanoindentation and a pin-on-disk tribometer. Nanoindentation results shows that the hardness and Young's modulus of the TiN coatings increase with increasing N content in the coatings. Wear test results indicate that the wear rate and friction coefficient of the XC100 steel substrate were significantly reduced by deposition of the TiN coatings, and the tribological behaviors of the TiN coatings are strongly dependent on the nitrogen content in the coatings.

Introduction

The fast growing surface engineering research today could be explained by the high industrial demands for desirable mechanical properties as well as tribological performance; to prevent tools deterioration under severe conditions, such as drilling, milling or cutting where protective coatings are the best solution fond [START_REF] Chauhan | A review paper on tribological and mechanical properties of ternary nitride based coatings[END_REF][START_REF] Constantin | The influence of nitrogen Content on the mechanical Properties of tinxthin films Prepared by reactive magnetron Sputtering[END_REF].

Transition metal based nitrides like TiN, ZrN, TaN and CrN coatings are applied on of metallic substrates used in numerous applications; automobile parts also as wear resistant coatings for cutting tools coatings, bio-implants, and decorative coating [START_REF] Chauhan | A review paper on tribological and mechanical properties of ternary nitride based coatings[END_REF][START_REF] Domínguez-Crespo | Effect of deposition parameters on structural, mechanical and electrochemical properties in Ti/TiN thin films on AISI 316L substrates produced by r. f. magnetron sputtering[END_REF].

TiN received a great attention in the resent years, where researches confirmed that titanium nitride can improve corrosion and wear resistant, and lifetime of the materials up to 400% [START_REF] Domínguez-Crespo | Effect of deposition parameters on structural, mechanical and electrochemical properties in Ti/TiN thin films on AISI 316L substrates produced by r. f. magnetron sputtering[END_REF] thanks to its high hardness, high thermal stability, low electrical resistivity, high wear and excellent corrosion resistance chemical stability and biocompatibility, adding the interesting optical properties [START_REF] Hosseinnejad | Structural, morphological and mechanical hardness properties of titanium nitride thin films deposited by DC magnetron sputtering: Effect of substrate temperature[END_REF][START_REF] Chavdaa | Tribological Characterization of TiN Coatings prepared by Sputtering[END_REF].

The physical and tribological properties of the sputter-deposited films was found to have a strong correlations with the deposition parameters [6] for example; the substrate temperature has strong effects on the lattice constant of TiN grown in Ar-N2 mixed gas. As the substrate temperature increases, there is an increase in the adatomes mobility of the sputtered species on the substrate and increasing in the crystal size [START_REF] Hosseinnejad | Structural, morphological and mechanical hardness properties of titanium nitride thin films deposited by DC magnetron sputtering: Effect of substrate temperature[END_REF].

There are several techniques to prepare TiN thin films such as physical vapor deposition (PVD) [START_REF] Ming | Morphology and Mechanical Properties of TiN Coatings Prepared with Different PVD Methods[END_REF], chemical vapor deposition (CVD) [START_REF] Piippo | Electrochemical characterization of TiN coatings[END_REF], thermal spraying [START_REF] Şerban | Deposition of titanium nitride layers by electric arc reactive plasma spraying method[END_REF] and ion implantation [START_REF] Huang | Deformation behavior of titanium nitride film prepared by plasma immersion ion implantation and deposition[END_REF]. Among them, magnetron sputtering has specific advantages, such as low deposition temperature, large deposition area, low impurity level, high deposition rates, and superior bonding strength [START_REF] Zhang | Effects of sputtering gas on microstructure and tribological properties of titanium nitride films[END_REF][START_REF] Banerjee | Influence of the sputtering gas on the preferred orientation of nanocrystalline titanium nitride thin films[END_REF]. D.G. Constantin et al. [START_REF] Constantin | Influence of Nitrogen content on The mechanical properties of TiNx thin films prepared by reactive magnetron Sputtering[END_REF] deposited by direct current magnetron sputtering TiNx thin films onto high-speed steel, silicon and stainless steel substrates, from a pure Ti target, varying the flow rate of the reactive gas, a crystalline structure with fcc phase and strong (111) orientation of the peaks, obtained using the lowest nitrogen gas flow of 4.5 sccm and the most performing TiNx film was that one with small sized of the grains (around 8 nm), a higher value of hardness (18.4 GPa) and a good thickness of 1.5 μm were obtained also.

Using a gas mixture ratio Ar-N2 10 and 12% of N2 and power density from 7.4 to 10.8 W cm -2 by pulsed dc magnetron sputtering, C. I. Enriquez-Flores et al. studied the microstructural and mechanical properties of titanium nitride (TiN) films. It was reported that the hardness increases when the Φe increases where a significant increment in the (110) orientation growing plane obtained by the TiN films with high hardness and the smallest grain size. The films exhibit a nanostructure of elastic domains, being the principal elastic domain of 270 GPa [START_REF] Enriquez-Flores | Relation betweenwork function, microstructural and mechanical properties of TiN-films[END_REF].

More recently M.A. DomínguezCrespo et al. deposited by R.f. magnetron sputtering on AISI 316L SS different Ti/TiN coatings in order to study the effect of sputtering conditions on tribological and corrosion properties. The cross section of scanning electron microscopy and profilometer measurements indicate an average film thickness between 320 and 860 nm in dependence of the sputtered parameters, the largest increase in hardness was from ~5 to ~20 GPa with a friction coefficient (μ) that varies from 0.012 to 1.3. Power and substrate temperature played the most important role in the final properties of Ti/TiNbilayered coatings [START_REF] Domínguez-Crespo | Effect of deposition parameters on structural, mechanical and electrochemical properties in Ti/TiN thin films on AISI 316L substrates produced by r. f. magnetron sputtering[END_REF]. E. Ajenifuja et al, sputtered with different thicknesses of TiNx thin film with varying time from 5 to 20 min, using DC reactive magnetron sputtering technique. While other process parameters were kept constant during the deposition, the deposition.The optimum value of the Vickers hardness was found to be 267.9 for the film deposited at 20 min. The inter-planar spacing was found to be from 2.0362Å to 2.0890Å, while corresponding lattice parameter was calculated to be from 4.0724Å to 4.1780Å. Scanning electron microscopy observation of the films gave an indication of fine-grained microstructure, which confirms the good adhesion and hardness properties of titanium nitride layer on the LCS substrate where the changes in crystal orientations of the TiNx films with deposition time. Ion beam (RBS) results showed increase in amount of nitrogen contents (47-64%) in TiNx films with thickness and deposition time. [START_REF] Ajenifuja | Thickness dependent chemical and microstructural properties of DC reactive magnetron sputtered titanium nitride thin films on low carbon steel cross-section[END_REF].

In this paper we have therefore studied the mechanical and tribological behaviors titanium of nitrides coatings deposited on XC100 steel by magnetron sputtering at different nitrogen partial pressures. The structures of TiN films are analyzed by (XRD), the surface morphology of the coatings isobserved using a SEM and AFM for surface roughness. The friction and wear performances of the as-deposited films are testedas well the mechanical properties are measured for different mixture gaz Ar-N2.

I. Experimental

The TiN films were deposited on XC100substrates by the magnetron sputtering technique (NORDIKO type 3500). TiN films were obtained by sputtering of titanium Target (99, 8 % of purity) tin an argon and nitrogen atmosphere .The deposit parameters have been summarized in Table I.

The substrates were all ground, polished, ultrasonic cleaned and dried by air before put into the vacuum chamber. The phase and crystal structure of the as-deposited films are identified by X-raydiffraction (Diffractometer SIEMENS type D500/501» in the continuous scanning mode with Cu-Kα radiation (λ=0.15406nm), for the surface morphologies we used the scanning electron microscope (SEM-Joel JSM-6400F, 10kV), the film surface roughness was measured by atomic force microscopy (AFM.

APE research).

In order to To determine the value of hardness we used the Nanoindenteur MTS-XP, in the tribological behavior study we used a Scratch Tester Millenium 200, in order to trace the damage (wear) and a tribometer in plan point to determine the friction coefficient dring the sliting test.

II. Results and Discussion

A. Microstructures of the TiN coatings

From Table II, we obtain the nitride the Hexagonal phase Ti2N from the percentage of nitrogen in plasma (15% N2) corresponding to a ratio (N/Ti =0.62) also we noticed that the Ti2N is not detected by the DRX at 5 and 10 %N2 with ratio of 0.213 and 0.518 respectively On the other hand, at 20% N2, a mixture of phases (Ti2N and TiN) is formed. Indeed, the Cubic TiN Phase is crystallized at 20%N2 with ratio (N/Ti=1.01). We can induce that there is a difference between the ratios (N/Ti) for the formation of various phases: like the Ti2N phase, which appears at only 15% N2 (corresponding to N/Ti = 0.62) instead of 10 % N2 (corresponding to N/Ti = 0.518), this can be explained by the high oxygen concentration in the deposited coatings which interferes with the incorporation of nitrogen into Ti. Other hand, we observe a progressive increase of residual stresses with increasing nitrogen content. And we can said that the low residual stress in the case of Ti rich layers: (%N2 ≤ 5) (-0.4 GPa) and close to those of pure Ti.

At 15% N2 corresponding to the formation of Ti2N, the stresses increase and become (σ = -1 GPa), and finally these stresses reach their maximum (σ =-1.1 GPa), when the nitrogen percentage reaches 20% corresponding to the predominant nitride TiN with a ratio N/Ti≈1. To show the morphology of these nitrides, Figure 2 presents a (SEM and AFM) image of TiN film (at 20% N2). According to this figure, the surface presents a rough and compact consisting of acute agglomerations of various sizes (RMS = 70 nm). A cross-sectional image presents a compact and columnar structure.

B. Mechanical properties of Titanium nitrides

Figure 3 presents the evolution of hardness and the Young's modulus curves of titanium nitrides as a function of the nitrogen percentage in plasma. Between (5 to 10 %N2), we observed that the hardness increases from 6 to 13 GPa; on the other hand, the Young's modulus remains constant over this range about 270 GPa because of the rapid incorporation of nitrogen into Ti before stoichiometry. At (15% N2) a decrease in hardness and Young's modulus (8 GPa and 250 GPa) was observed. And from 20% N2, the low elasticity of the TiN layers can be explained by the surface roughness, which influences the measurements by nanoindentation.

C. Tribologicalbehaviourlor

We have shown that in figure 4, the evolution of the friction coefficient of the coatings (Ti, TiN and Ti2N) as a function of the sliding distance. During the test of the pure Ti film, we note that the recorded coefficient of friction stabilizes around 0.72 with a noise of 0.134 of width. This predicts a high wear which can be explained by the existence of an excess of defects in the Ti film. For the TiN film, we notice during the test a noise that was heard up to 2 m of wear track and the coefficient of friction stabilizes at a very low value of 0.50at 20% of N2and then increases rapidly to 0.72.

For the Ti-N films deposited at 10 and 20 % of N2 flow rate, it can be seen that the wear resistance of the hard Ti-N coatings was higher than that of the other coatings. The best wear resistance of Ti-N coating with high nitrogen content is closely related not only to the N/Ti ratio, but also to the morphology and structure of TiN films. A dense and smooth surface has less asperity or crevices as compared to porous coatings which result in better interlocking and adhesion. On the other hand, the coating deposited at 20 % of N2 was found to achieve the desired combination of hardness (15 GPa). However, for the films deposited at higher nitrogen flow rate (10 %), there was more debris along the edges of the track, due to the weak substrate-film cohesion (Fig. 5 a).

Conclusion

TiNx thin films were produced by magnetron sputtering. The thin films were deposited onto XC100 substrates, from a pure Ti target, varying the mixture gaz Ar-N2, We systematically clarify the effects of sputtering gas on the mechanical and tribological behaviors of TiN thin films. We have shown that the formation of the cubic TiN phase corresponds to N/Ti = 1 for 20% N2 and has compressive stresses of -1.07 GPa, for Ti2N phase of the ratio N/ Ti= 0.62 for 15 %N2 has a compressive stresses of -1GPa with contaminated coatings, moreover the TiN layers have a columnar structure with a rough and less dense surface, also it have the superior mechanical and tribological properties compared with those of the Ti2N film.

[6] N.Jianga, H.J. Zhanga, S.N. Baoa, Y.G. Shenb, Z.F. Zhou, XPS study for reactively sputtered titanium nitride thin films deposited under different substrate bias. Physica B 352 (2004) 118-126.

Fig. 1 .

 1 Fig. 1. Evolution of film thickness, stresses and N / Ti ratio of the Ti-N layers as a function of the percentage of nitrogen in the plasma.

Fig. 2 .

 2 Fig. 2. SEM (a) and AFM (b) surface morphology of TiN thin film deposited with the Ar/N2 ration~ 1:1.

Fig. 3 .

 3 Fig. 3. Hardness and Young's modulus evolution for different percentage nitrogen in plasma.

Fig. 4 .

 4 Fig. 4. Evolution of friction coefficient of the Ti-N films as a function of the slip distance.

Fig. 5 .

 5 Fig. 5. Micrographs of scratch tracks performed on the Ti-N coatings deposited at: a) 10 %N2, b) 20 %N2

Table 1 .

 1 Operating conditions for the deposit of tin obtained by sputtering magnetron R. F.

	(Ar, N2) Pourcentage	(95%,5%), (90%,10%), (85%,15%),
		(80%,20%), (75%, 25 %)
	Voltage	-900 [V]
	Target distance to substrat	80 [mm]
	Applied Power	550 [W]
	Time	120 [mn]
	Pressure	0.4 [Pa]
	Target pickling and substrate:	5 [mn]
	XC100

Table 2 .

 2 Operating conditions for the deposit of tin obtained by sputtering magnetron R. F.

	%N2	N/Ti	Thickness [nm] Phases
	5	0.213 960	Ti
	10	0.518 950	Ti
	15	0.621 945	Ti2N
	20	1.010 930	Ti2N+ TiN
	25	1.171 920	Ti N