M Benamara
email: benamara@univ-mlv.fr

M Grzeskowiak

M Salhi

G L1ss0rgues

Esycom Ea

Improvement of HF RFID Detection for Small and Misaligned Tag

Keywords: RFID, coil, mutual inductance, detection I

A design of a reader antenna is proposed to optimize HF RFID range detection in parallel and perpendicular configuration. The reader antenna has a surface of 500 cm 2 , the tag corresponds to 0.7% of the reader surface. For passive RFID system, if the reader antenna does not provide the threshold energy to the tag, this one will not be detected. This is the case of the misalignment systems with great difference size antenna.

Another problem limiting RFID detection is the different possible tag orientations. In perpendicular configuration, the detection is weak, the tag can only be detected above the edges of the antenna. These limits are minimized in this work by using a multiple loop antenna including resonators. Electrical model is developed to calculatethe equivalent mutual inductance of the system from the impedance matrix:the measured results confirm the simulated ones. The detection measurements validate the improvement.

INTRODUCTION

RFID (Radio Frequency Identification) at LF (Low Frequency, 125 kHz) and HF (High Frequency, 13.56 MHz) is based on load modulation principle, thanks to inductive coupling effect between the reader and the tag coils. Optimizing mutual inductance depends on the distance between the reader and the tag but also on their respective shapes and spatial orientation [1] [START_REF] Babic | Validity check of mutual inductance fonnulas for circular filaments with lateral and angular misalignments[END_REF][3] [START_REF] Reinhold | Efficient antenna design of inductive coupled RFID-system with heigh power demand[END_REF]. As the size of the tag coil is usually small compared to the reader coil, the coupling effect is considered to be weak.

In the context of such near field communications, the structure of the reader coil can be modified in a multi-coils way [START_REF] Liu | Resonant coupling of a passive inductance capacitance-resistor loop in coil-based sensing system[END_REF][6] [START_REF] Grzeskowiak | Pebbles Tracking Thanks to RFID LF Multi-loops Inductively Coupled Reader[END_REF] in RFID and in WPT (Wireless Power Transfer) applications [START_REF] Zhang | Quantitative Analysis of Mutual Inductance for Optimal Wireless Power Transfer via Magnetic Resonant Coupling[END_REF] [START_REF] Hackl | A Novel Method for Determining the Mutual Inductance for 13.56MHz RFID Systems[END_REF]. Equivalent circuit of the systems is useful for detailed analysis [START_REF] Diet | Improving LF Reader Antenna volume of detection, for RFID token tag, with a combination of ICLs and in/out-of phase multiple-loops structures[END_REF] and the calculation of the mutual inductance [12][13] is a key parameter to improve the tag detection in RFID [14] and the efficiency in WPT system. In the literature, several proposed structures exhibit symmetric, and co-axial coils [START_REF] Hackl | A Novel Method for Determining the Mutual Inductance for 13.56MHz RFID Systems[END_REF][10], but a few ones are applied to asymmetric structures with perpendicular and offset arrangement of the transmitting and receiving coils [START_REF] Zhang | Quantitative Analysis of Mutual Inductance for Optimal Wireless Power Transfer via Magnetic Resonant Coupling[END_REF].

A. DIET, Y. LE BIHAN GEEPS (PIEM) UMR 8507, Univ. Paris Saclay F. 91192 Gif sur Yvet1e, France

In this paper, a method to improve detection range for misaligned small tag is proposed. Reader antenna based on dual twisted loop antenna (three sub-coils) including resonators is used to optimize misalignment tag detection. Each juxtaposed coils of the multi-loop reader antenna are fed in out-phase current. The mutual coupling between the reader multi-coils and the tag depends on the mutual inductance; the corresponding electrical model is developed in theoretical study of this paper. Herein, we focus first on a "three sub-coils prototype" including resonator in each sub-coil. The optimization and maximization of the mutual impedance value is our figure of merit. The proposed structure is produced and validated by detection measurement In parallel and perpendicular configurations.

II.

THERITICAL STUDY HF RFID system is based on magnetic coupling. The generated magnetic field by the reader coil creates a current circulation in the tag coil, hence, the tag chip is loaded. The effect of the tag chip variation on the shown impedance of the reader is modeled by:

2 M 2 Z In = Z + -"OJ=-�= Z 2 + Z1 (I)
With Z], Z2 and ZT are respectively the impedance of the reader coil, tag coil and tag chip. This is the principle of the load modulation. Then, to enhance tag detection, the mutual inductance should be increased.

To determinate theoretically the equivalent mutual inductance between the reader multiple coils including resonators and the tag, we use the electrical equivalent model in Fig. I. Each coil is modeled by an inductor Li with its serial resistance rio L1, L2 and L3 are respectively the self inductances of the sub-coils of the reader antenna. Lrb Lr2 and Lr3 are the self-inductances of the resonators and LT is the inductance of the tag coil. Cb Crb Cr2 and Cr3 are the tuning capacitors at 13.56 MHz of the reader antenna and the resonators. As the multi-coils antenna is defined by three serial inductances, the voltage can be written in the harmonic regime (at frequency w = 21fi) as:

VI = (r 1 + j OJ (L 1 + M 12 + M 13 )) J R + j OJ M I T J T V 2 = � 2 + jOJ(L 2 + + M 21 + M 23 )} R + jOJ M 21 J 1 (2) V 3 = (r3 + jOJ(L3 + M 32 + M 3d)] R + jOJ M J 3T T
Where M13 and M3J are negligible to M12 and M32 The voltage V R is the sum of the different voltages:

(rJ +r2+r3+ J V R = ] jOJ(LI+L2+L3+ 2 *MI2+ 2 *M23) R + jOJ ( M I T + M 2T + M JT h T
The mutual inductance between the reader multiple coils and the tag is then equal to:

M RT = M I T + M 2 T + M 3T
In presence of resonators the voltages becomes:

+ J (J) M + I rl T r I I l {J) 2 M M l I T a rl T [ {J) 2 ( M 2r2 � ( ) J V2= r2+ a +J{J)!"2++M21+M 2 3 I r2 R (5)
(3)

Where:

arj = _ 1_ ' --(jOJ L + 1') (Jg�.3).
OJC.

n n n

The equivalent mutual impedance between the tag and the reader including resonators can be expressed by:

Z RT OJ 2 M Il' 1 M Tr 1 j OJ M + __ -----"-LL_"'-'--""- IT + jOJ M 2T arl OJ 2 M 2 r 2 M Tr 2 + __ -2�_�� a r2 OJ 2 M 3 I' 3 M Tr 3 + jOJ M + (6)

III. SIMULTAION AND MEASURES RESULTS

A. Validation a/the concept a/multiple coils For two reader antenna: rectangular one of 50* 1 0 cm 2 and a multi-coils ( three sub-coils of 10*16.6 cm 2 ) , and a tag of 1.2 cm radius coil, the concept of the multi-coils is validated by evaluating improvement of mutual inductance in equations (4 and 7) (Fig. 2). The proposed design permits the improvement of mutual inductance in both parallel and perpendicular configurations. The maxima are above the transition zones between each two sub-coils for perpendicular configuration with an improvement of 16 nH compared to the rectangular antenna. In parallel configuration the maximum of mutual inductance is seen at the second sub-coil (improvement of the mutual inductance from 20 nR) The improvement of the mutual inductance is due to the modified distribution of the magnetic field (Fig. 3).

15 ��--�--�,-----------�--�--�--, 10
1\ I i 11 : o� ---------�\ ----------i---------- -IS : \ i : \ ! -10 ----------� -t,.

a b

Fig. 3. Generated magnetic field for rectangular and multiple coils antenna

B. Proposed prototype including resonator

Thegenerated magnetic field by the proposed design has minimum zones of the magnetic field. The addition of a resonator at the surface of each sub-coil of the reader antenna concentrates and modifies the distribution of the generated magnetic field, as it can be shown in (Fig. 4). In equation 7 the mutual inductance is depending on the mutual inductances between the reader coils and the resonators, the tag and the resonators, but also on the electrical parameters of the resonators. In this study, the resonators have the same electrical properties, their impedance are equal to 1.5+jI24. Each one of the resonators is including at the surface one sub-coil.

The prototype is produced and measured. The inductances of the antennas are l.05 flR and Ll = l.46 flR respectively for rectangular and multi-coils antenna. The resonators are inserted at the center of each sub-coil. "Fig. 5" reports the simulated and measured results in two cases: with and without resonators. For a distance 1 cm, the calculated equivalent mutual inductance has comparable values to the measures.

For lateral misalignments, the addition of the resonators increased, in parallel configuration, the equivalent mutual inductance by 30 nR (from 13 nR to 43 nH) and above each sub coil. However, the equivalent mutual inductance presents three peaks corresponding at the surface of the resonators. In perpendicular configuration; the mutual inductance is improved by added resonators, from 8 nR and 5 nH respectively at the edges of the antenna and the center of the sub-coils (edges of resonators). 5 0 ,---==----,--------------------, --� -15

-'to With resonators (measures) Without resonators (measures) - 20 ---------------

IV. EXPERIMENTAL VALIDATION

An experimental test of increasing detection volume and surface with adding resonator was made using a RFID reader from Ib Technologies. The principle of adding resonator on the co-planar surface of the reader coil was applied. The detection of parallel and perpendicular tag was evaluated for lateral misalignment and different distances between the tag and the reader antenna (Fig. 6).

Res()nator'.

" I

?;,!I� Fig. 6 proposed multiple coil antenna including resonators

The result of tag detection in parallel and perpendicular configurations without resonators (a, c), with resonators (b, d) are reported in Fig. 7. The (X, Y) axes correspond respectively to a lateral misalignment according to the length and the width of the reader antenna, the color scale defined the distance of detection.

For resonator with size corresponding to 27% of the sub coil surface, both the volume and the surface of detection for parallel and perpendicular configuration are increased (table I). The surface of the reader was increased byl8% (From 70 % without resonators to 88% with resonators), and 29 % (From The RFID detection results exhibit clearly that new detection zones appear with the addition of the resonator in the parallel and perpendicular orientations. This principle, using inductive coupling, can therefore be applied to HF and LF RFID applications to detect small tags in comparison with the global studied volume. For larger radii of readers coil, several resonators, weakly coupled to each other, can be added in different places on the reader surface therefore increasing the volume of detection.

V. CONCLUSION

The improvement of HF RFID system performances, such as surface and volume of tag detection, was obtained by association of multi-coil reader antenna including resonators. The improvement of equivalent mutual inductance was achieved thanks to the addition of a co-planar resonators.

The proposed multi-loop antenna, based on the changing phase currents between juxtapose coils, is fruitful for detection in both parallel and perpendicular orientation of the tag compared to the conventional loop but the distance and the surface of detection are not optimized. The added resonator at the surface of the sub coils of the reader antenna permits the improvement of detection.

Analytical formula for equivalent mutual inductance of the system is developed using electrical model of the structure, constituted of multi-coils reader antenna including resonators and tag. Simulations of the calculated equivalent mutual inductance with HFSS and comparisons with VNA measurementsare also performed.

In perspectives, the same principle will be also applied in wireless power transmission (WPT) to maximize the power transfer and the energy efficiency at the receiver and we will realize an optimized prototype with multiple resonating coils.

Fig. l .

 l Fig.l.Electrical model of the proposed antenna The voltages VI, V2 and V3 in the circuit satisfy the following relations (l), with Mi j =M j i being the mutual inductances between coils of the reader antenna (I <i,j<3)), Mlrl, M2r2 and M1r3 are the mutual inductances between the resonator and the corresponding sub coil. M2rl, M2r2 and M2r3 are the mutual coupling between resonators and the tag coil.

Finally

  , the equivalent mutual inductance between reader and the tag is the imaginary part of the Z R T -. 2 M M M RJ -JOJM 1 T + OJ 11' 1 Tr l Yrl With r,., + j OJM 2 T + OJ 2 M 2 r 2 MT r 2 Yr2 + J OJ M 3 T + OJ 2 M 3 r 3M Tr 3Yr3 C I. -Ul L ri ) + r ri nUl (7) the The equations are used in the following sections to evaluate the improvement of the mutual inductance with the proposed design.

- 5 Fig. 2 :

 52 Fig. 2:Mutual inductance for rectangular and multiple coils antenna in parallel (a) and perpendicular configuration (b)

Fig. 4 .

 4 Fig. 4. Generated magnetic field with resonators

  --__, ---Including reson.toTS (m •• su,.,s) -'b T'c -'b Including ,.. sonatoTS (simulation) -----, WIthout ,.son.to,.. (.'mu/atlon) WIthout re,sonators (m •• ,su,.s) ------Length of the antenna (cm) a 20 -----:----. ----:----:--------:----. ----:---L -----:

-Fig. 5 .

 5 Fig. 5. Equivalent mutual inductance with and without resonators in parallel and perpendicular configuration

Fig. 7 .

 7 Fig. 7. Detection results in: parallel configuration with (b) and without (a) resonators and perpendicular configuration with (d) and without ( c) resonators


I. CONTEXT

RFID by magnetic coupling, in LF (125-134 kHz) and HF (13,56 MHz), is a mature technology nowadays. However, the radiofrequency link based on magnetic mutual coupling between reader and tag antenna, often loops antenna, is still a complex challenge to improve the system. Detection range and areas are considered an assessment factor for RFID system performances [START_REF] Kwon | a magnetic resonnat loop to enhance the operating distance of 13.56 MHz RFID system[END_REF] [START_REF] Grzeskowiak | Pebbles Tracking Thanks to RFID LF Multi-loops Inductively Coupled Reader[END_REF]. The detection of the tag is performed when its effective surface has generated enough power to supply the RFID tag chip, by integrating the magnetic field generated by the reader loop current. This is maximized when the effective surface is perpendicular to the magnetic field. As the detection is depending on the generated magnetic field, the reader structure can present many null areas detection versus the possible orientations of the tag (fig. 1).

In this paper we focus on two configurations, parallel (θ=0°) and perpendicular (θ=90°), corresponding respectively to the best and the worst geometrical configuration between the reader and the tag loops in the case of RFID detection. Some studies were proposed in the literature optimizing the mutual coupling between the reader and the tag by changing either the sizes of antennas or theirs geometrical structures from a mono-layer loop to multi layer loops [START_REF] Goemaere | Automated Design of an HF RFID Loop AntennaBased on Parametric Geometry Modification[END_REF][4] [START_REF] Eteng | Loop Antenna Design for Lateral H-Field Uniformity in Misaligned HF-RFID Links[END_REF][6] [START_REF] Grzeskowiak | Coils for ingestible capsules: Near-field magnetic induction link[END_REF] in the parallel configuration. However such studies did not solve the question of detection with a mono-layer 2D structure with other configurations, especially perpendicular configuration. Our idea is to change the distribution and the orientation of the magnetic field generated by the reader antenna by modifying the path of the feeding current in the reader loop. Therefore, we propose a "twisted loop antenna" (TLA) that is a mono-layer 2D structure allowing a maximum of field in both edge and center of the reader, especially in the perpendicular orientation (fig. 2a). We focus on the mutual inductance, M, which is a good figure of merit for evaluating the performance of the proposed antenna in the case of weak coupling. The enhancement of detection areas and volume of the TLA, due to the maximization of M, is done by comparison with a classical rectangular loop for both parallel and perpendicular configurations.

II. CLASSICALANTENNA AND TLA DESIGN

The loop antenna structures were simulated with the electromagnetic software HFSS (High Frequency Structural Simulator) and compared with analytical calculation thanks to Matlab. Surfaces of the loops were respectively 200cm 2 and 128 cm 2 for the rectangular reference and the TLA. Their So, the objective is to maximize the mutual inductance between the reader loop (rectangular or TLA) and the tag loop while keeping constant the global size of the reader antenna. Also, the reader antennas present a relative difference inferior to 5% for the self-inductance value and less than 2% for the resistance. The repartition of the magnetic field of the TLA is similar to the one obtained with two juxtaposed circular loops separated by a distance of 5mm. The magnetic field lines (fig. 2.b) are oriented parallel to the structure in the center and on the edges, so the TLA could detect TAGs oriented perpendicularly to the reader antenna in several zones, contrary to the case of a loop antenna where the tag can only be detected on the edges.

Therefore the TLA offers more possibilities of detection thanks to the repartition and orientation of its field.In the next part of this work, we confirm the validity of the proposed TLA reader structure by calculating the mutual inductance as required comparing with HFSS simulation and experimental results.

III.

THEORETICAL STUDY We investigated the calculation of the mutual inductance by using the electrical model in fig. 3, where L 1 , M 12 in series with L 2 , M 21 represent the two sub loops of the TLA. As the twisted antenna is defined by two coupled inductances in series, the voltage can be written in the harmonic regime (at frequency ) as:

(1)

The voltage on the tag is:

(2)

Where:

The calculation of the mutual inductance under Matlab is based on the magnetic field (B r , B θ , B y ) generated by a current circulation, as defined by (3). The evaluation of M eq is achieved by a numerical discrete integration on a surface corresponding to the reader loop surface and orientation [START_REF] Grzeskowiak | Coils for ingestible capsules: Near-field magnetic induction link[END_REF][8]. (3)

Where r corresponds to the distance between the center of the loops and the functions K( ) and E( ) are the complete elliptic integrals of first and second kind respectively. After discretization of the equation3, the mutual inductance can be calculated with Matlab. In figure 4, we have presented our structures with the following parametrical studies: variations of the distance dz, of misalignment dy and orientation angle θ.

To simplify the calculus with Matlab we have considered a rectangular 10cm*20cm shape for the classical loop, while the twisted loop can be seen like two juxtaposed square loops separated by a distance of 5mm. For comment, the fabricated TLA is made of two hexagons because of the difficulty of handmade realization of circular loops while the simulated classical loop and TLA have circular shapes. The results of the analytical study are presented in the following part The TLA improves the mutual inductance, especially in perpendicular configuration (fig. 5 (a)), where the mutual inductance has three maxima: 8nH at the center and 4nH at the edges of the TLA instead of a zero in the classical loop at its center and 4nH at its edges (only two maxima). For parallel configuration (fig. 5(b)), the mutual inductance is also improved at the close surface of the antenna where we have 6nH around the center and 3nH at the edges.

IV. HFSS SIMULATION AND MEASUREMENTS

The antennas presented in figure 2 were studied using HFSS and fabricated by sticking copper ribbons on plastic sheet. Measurements were realized with a Vectorial Network Analyzer N9923A (fig. 6). The tag antenna is a circular loop with 30 mm of diameter.

A. Comparison of HFSS simulation versus measurements for both loop antenna and TLA

As explained in introduction, the performance of RFID detection is based at first glance on the evaluation of mutual inductance between the reader and the tag, as the coupling is In the case of perpendicular configuration, in both measurement and simulation, the TLA has three maxima of mutual inductance (absolute values). However, the classical loop has therefore only two maxima of mutual inductance at the edges of the antenna. The results are comparable to the analytical study in figure 5.a. Also, for parallel configuration, the TLA has 4 maxima of mutual inductance corresponding to the edges of the sub-loops and the areas near to their centers. In the case of the classical loop antenna, the mutual inductance has only two maxima corresponding to the edges of the loop (the results are comparable for the Matlab calculation in figure 5.b). In both parallel and perpendicular configurations we can see the symmetrical values at y=0mm (center of the reader antenna).

The results of simulation are fruitfully confirmed by measurements.

B. Tag loop distance variation

After validating the advantages of the TLA structure, we focused on the influence of the tag distance on the mutual inductance.

For a lateral misalignment (-150mm<y<150mm) and two different tag distances (10mm and 20mm), we plotted in figure 8 the mutual inductance variation in parallel and perpendicular configuration for the TLA, thanks to HFSS. As expected, the mutual inductance decreases with increasing distance between the reader and the tag in both parallel and perpendicular configurations. The degradation is more visible at the edges of reader loop. However, this distance variation has no influence in the position of the areas of minima and maxima of mutual inductance.

V. EXPERIMENTAL VALIDATION

Our objective is to reduce the null areas of detection in both parallel and perpendicular configurations, to later improve objects detection in any configurations. For experimental validation of the TLA, we have used a commercial RFID reader from IB technologies and a tag with 30 mm of diameter (Fig. 9). For the rectangular antenna (20*10 cm²), the detection was performed at a distance of 2cm around the edges of antenna, for parallel and perpendicular configurations. However, the detection in Fig. 10 was increased for distance and surface for both configurations, 2cm at the edges of the TLA and 4cm at the center (The color scale represents the distance of detection).

VI. CONCLUSION

In this paper we have developed and designed a new reader twisted loop antenna, TLA, for RFID applications. The TLA is based on a modified distribution of the magnetic field, as in the case of co-planar complementary loops. The TLA was compared with a classical rectangular loop with the same electrical and geometrical characteristics. The interest of the proposed structure is evaluated according to the improvement of the mutual inductance in function of the tag orientation and a lateral misalignment. Simulations results and measurements showed that the TLA structure potentially improves both distance and surface of detection by locally maximizing the mutual inductance with a tag loop.

Additionally tests of small tags detection with a commercial RFID HF reader confirm that TLA has a higher total detection area and volume than the classical rectangular loop.

In the future, the TLA will be used for detecting simultaneously many tags in different angular and distance positioning.

A 2 1

 2 Twisted Loop Antenna to Enhance HF RFID Detection for Different Tag Positioning Megdouda BENAMARA 1 , Marjorie GRZESKOWIAK 1 , Antoine DIET 2 , Gaëlle LISSORGUES 3 , Yann LE BIHAN 2 , Stéphane PROTAT 1 , Christophe CONESSA Univ Paris-Est. ESYCOM (EA 2552), UPEMLV, ESIEE-Paris, CNAM, F.77454 Marne-la-Vallée, France2 Univ. Paris Saclay, GeePs (UMR 8507),CNRS, CentraleSupelec, Univ. Paris Sud, UPMC) 91192 Gif-sur-Yvette, France 3 ESIEE-Paris, ESYCOM (EA 2552), UPEMLV, ESIEE-Paris, CNAM, Cite Descartes, BP99, 93162 Noisy le Grand Abstract-In this paper, a new LF/HF RFID reader loop antenna design is proposed and tested, in order to increase detection areas of the tags. The studied structure is a Twisted Loop Antenna (TLA) which is based on a modified distribution and orientation of the magnetic field lines. Our structure fruitfully uses the complementary antenna principle in a coplanar configuration. This offers more possibilities of tag detection whatever the tag position and orientation. The antenna performances are evaluated by optimizing the equivalent mutual inductance between reader antenna and tag antenna. Results are presented firstly in simulation (MATLAB and HFSS electromagnetic calculator) and secondly by experimental tests at different distances and misalignments, for the two possible tag orientations: parallel and perpendicular. Index terms: Mutual inductance, LF / HF RFID, loop antenna, co-planar configuration, coupling optimization

Fig. 1 :

 1 Fig.1: Different positioning of a RFID tag

  electrical and geometrical characteristics are presented in table 1 / fig 2a, where L and r are respectively proportional to the imaginary and the real part of the input impedance.

Fig. 2a :

 2a Fig.2a: Loop antenna (bottom) and TLA (top)The simulated loops are designed without substrate and with Perfect-Electrical-Conductor (PEC), while for the fabricated corresponding structures, copper ribbons (0.017mm thick) are stuck on a plastic sheet.Tab1: Electrical characteristics of studied antennas rectangularloop

Fig. 2 .

 2 Fig.2.b.Magnetic field distribution for classical loop antenna (top) and TLA (bottom) for a scale of 0 to 1 A/m

Fig. 3 .

 3 Fig.3. Electrical model for the TLA (left) and a tag (right)

Fig. 4 .Fig. 5 .

 45 Fig.4.Parametrical studies of the reference loop antenna and TLATo validate the proposed TLA structure we have to prove its interest compared to the classical loop in both perpendicular and parallel configurations as seen in figure5.

Fig. 6

 6 Fig.6 Experiment setup

  practice. For all comparisons, the distance between the loop and the tag is 10mm.

Fig 7 .

 7 Fig 7. HFSS simulations and measurements for both TLA and rectangular loop antenna in (a) perpendicular and (b) parallel configurations

Fig 8

 8 Fig 8 Mutual inductance versus tag-loop distance, (a):perpendicular configuration. (b): parallel configuration (HFSS simulations)

Fig 9 .

 9 Fig 9. Experimental setup for RFID detection tests

  Fig 10. Detection results for: (a): Loop antenna in parallel configuration (b): TLA in parallel configuration (c): Loop antenna in perpendicular configuration (d): TLA in perpendicular configuration

  [8] A .Diet, M.Grzeskowiak, Y.LeBihan, C.Conessa, Improving LF Reader Antenna volume of detection, for RFID token tag, with a combination of ICLs and in/out-of phase multiple-loops structures, IEEE RFID-TA, , 208-213, 2014.


A Diet

Y Le Bihan

C Conessa

F Alves

M Grzeskowiak

M Benamara

G Lissorgues

M Biancheri-Astier

A Pozzebon

LF RFID Chequered Loop Antenna for pebbles on the beach detection

Keywords:

This paper focus on low frequency (125 kHz) RFID by magnetic coupling, more precisely using glasstag type of tags in the context of pebble detection on the beach. The challenge is to detect over a wide area very small size tags which are highly sensitive to the orientation of the magnetic field. To improve the detection ability of the reader loop antenna, the paper proposes to exploit the principle of complementary loops. Theoretical simulations with MATLAB show the potential increase by means of mutual inductance value along a displacement of the tag. A prototype of a chequered loop structure is presented and tested with a classical low power RFID reader to demonstrate the improvement without increasing the current in LF reader loops. The detection performances reach 12,8% for a 900 cm² surface of a prototype reader loop, whatever the orientation of the glasstag.

Radio Frequency IDentification (RFID) is used in numerous applications domains (logistic, security, traceability of devices or persons…) by means of different realization technologies [START_REF] Paret | Identification Radio-Fréquences et Cartes à puce sans contact[END_REF] [START_REF] Finkenzeller | RFID handbook, fundamentals and applications in contactless cards and identifications[END_REF]. The frequency of operation, between the RFID reader/interrogator and the transponders (tags), is the discriminant factor due to multiple tradeoffs mainly concerning the detection range, data rate transfer and sensitivity to the environment (water, metal…). The RFID frequency of operation determines the electromagnetic phenomenon in nearfield (magnetic coupling) or far-field (backscattering of radiating waves) used for data transfer.

Herein, we focus on the Low Frequency band, LF (125/134 kHz), in which the communication between the tag and the reader is achieved by load modulation, i. e. based on coils/loops magnetic coupling. In LF, most of the tags does not have battery and need being powered by magnetic coupling before being able to communicate. In a second step, the tag switches its load impedance to provide amplitude modulated information towards the reader. This type of LF RFID tags, often named as passive tags, should be powered efficiently by magnetic coupling but needs a sufficient bandwidth to transfer the data towards the reader. Consequently, a tradeoff is present on the quality factor of the transfer function between the tag and the reader, which is based on the impedance value seen by the reader, including the mutual inductance [1][2][3]. This mutual inductance can be defined by the Neumann formula [START_REF] Paret | Identification Radio-Fréquences et Cartes à puce sans contact[END_REF] where M is the value, in Henry, between two closed paths Γ 1 and Γ 2 where r 12 is the distance between the two elementary vectors dl 1 and dl 2 . As it can be seen in ( 1), the mutual inductance is exclusively based on geometrical considerations: the position and orientation of the paths (loops in classical examples). RFID in LF (125/134 kHz) and HF (13,56 MHz) is widely used for traceability of animals (implanted tags for dogs, cats and fishes) or tracking of pebbles [START_REF] Benelli | An RFID-based toolbox for the study of under-and outside-water movement of pebbles on coarse-grained beaches[END_REF]. The LF is particularly used in presence of water (or/and metal) because the perturbation of such elements is proportional to the frequency.

As a consequence, LF RFID tags are dedicated technology for the study of coastal morpho-dynamics, in that it can be efficiently used to track the movements of sediments on coarse grained beaches, for both the under and outside sections of a beach. LF RFID tags can be embedded inside autochthonous pebbles turning them into tracers that can be positioned directly on the beach. These tracers can be then detected, identified and possibly recovered to study their movements and variations in terms of size and shape. These data are fundamental to evaluate the effects of coastal erosion on a beach and to plan the countermeasures to be deployed [START_REF] Benelli | An RFID-based toolbox for the study of under-and outside-water movement of pebbles on coarse-grained beaches[END_REF]. LF is the only frequency band that can be employed in such a scenario in that is the only one that is able to provide a Long Range communication channel even under sea water, allowing the detection of Smart Pebbles also in the submerged portion of a beach. Experimental tests with off-the-shelf devices have proven that 60cm communication ranges can be achieved if choosing specific types of tags [START_REF] Benelli | An analysis on the use of LF RFID for the tracking of different typologies of pebbles on beaches[END_REF] [START_REF] Benelli | An analysis of the performances of Low Frequency cylinder glass tags for the underwater tracking of pebbles on a natural beach[END_REF] with high power RFID reader (>1W) and classical single turn wide loop (almost 50 cm of diameter).

Tags on pebbles has a minimum volume and the dedicated structure is a glasstag type which is typically made of a 1-1,5 cm long ferrite rod, with diameter in the range of 1-2 mm with enameled copper wire soldered to a RFID chip, as seen in Figure 1. Moreover, the penalty of the glasstag is it high sensitivity to the orientation because the ferrite rod is equivalent to a magnetic dipole with a very small effective surface due to the rod diameter value. As the pebbles can be positioned randomly, the RFID reader loop antenna is supposed to detect them whatever their orientation. This is technologically challenging because of the glasstag (high) sensitivity to the magnetic field orientation. As it can be seen in Figure 1, the horizontal and vertical modes for RFID detection create optimum cases near the edges of the reader loop. The horizontal mode does not present a maximum at the center (coaxial case for the reader and tag loops) in this Figure 1 because we suppose in the described context that the reader loop size is higher than the tag loop size. To overcome this penalty when using glasstags with wide reader loop, one can increase the current or modify the geometry of the reader loop [START_REF] Ahmad | Multiple-bridge-loop RFID Reader Antenna for improved Positioning and localisation[END_REF][8] [START_REF] Fotopoulou | Optimum antenna coil structure for inductive powering of passive RFID tags[END_REF] in order to increase the value of M, given by ( 1) and [START_REF] Finkenzeller | RFID handbook, fundamentals and applications in contactless cards and identifications[END_REF].

Γ Γ Γ Γ 12 2 1 0 M M r dl . dl r dl . dl 4π μ r dl . dl 4π μ M b 1 2 a 1 2 b 1 a 1 1 2 (2)
As expressed in (2) one idea is to separate the reader loop path defined by Γ 1 into loop sub-paths (multi-loop or series loops) Γ 1 a and Γ 1 b which will drive to the addition of the two partial mutual inductances parts, one of which being potentially better suited for mutual coupling with the given geometry of the glasstag. This would improve the reader loop antenna structure before increasing the current, as is the classical solution proposed. As the pebbles are in the range of several centimeters, the spatial resolution is approximated to be 10x10 cm².

The idea in that paper is to improve the structure of the reader loop by modifying the ability (capacity) of mutual coupling in the case of a glasstag. The multi-loop principle is explored in that way.

Additionally, the ability to detect in both horizontal and vertical modes is more useful in the case of pebbles detection than to increase the detection range in only one mode.

II. THEORETICAL APPROACH

To improve the detection of LF RFID glasstag, we investigated three different solutions for a given area of 10x20 cm², as shown in Figure 2. The first solution (case "a") is a wide loop corresponding to current commercial reference solutions. The second and third cases are two serial loops of 10x10 cm², covering the same total area as the first loop, and for which the current is in phase (case "b") or out of phase (case "c"). The loops of case "c" are also named complementary loops [START_REF] Fotopoulou | Optimum antenna coil structure for inductive powering of passive RFID tags[END_REF][10] [START_REF] Diet | Improving LF Reader Antenna volume of detection for RFID token tag thanks to Identical Coaxial Loops (ICLs) and in/out-of phase multiple-loops structures[END_REF]. To compare the detection ability of such solutions described in Figure 2, we compute under MATLAB the evaluation of the mutual inductance "M". This value correspond to the flux induced from the reader loop onto the magnetic dipole surface, but also to the reciprocal flux induced by an equivalent magnetic dipole source onto the reader loop surface. Consequently, M is computed by means of the discrete orientated surface integral of the magnetic field generated by a turn of 2 mm equivalent diameter, with a spatial step of 5 mm², at a fixed distance of 4 cm on the Y axis, see Figure 2. The magnetic field is computed with (3).

y r R Rr k k K k E y r R y r R r y r R π Iμ B B k K k E y r R y r R r y r R π y Iμ B y θ r 2 2 2 2 2 2 2 2 2 1 2 2 0 2 2 2 2 2 2 1 2 2 0 4 2 0 2 (3)
The results of the evaluation of M are presented in Figure 3 and, respectively, Figure 4 in function of the glasstag movement along the axis parallel to the reader loop surface ("X" axis in Figure 2), for horizontal and, respectively, vertical modes.

As the goal is to detect the tag whatever its orientation during a studied movement, we propose a figure of merit called Coupling Capacity (CC) by means of the integral of M(x) absolute value along the X axis, from -20 cm to +20 cm to include two times the length of the reference reader loop surface. This criterion, defined by (4), uses the absolute value of M because the powering of the tag is proportional to M². As it can be seen in Figure 3 for the horizontal mode, the complementary loops (case "c") provides higher maximum values of |M| than cases "a" and "b", and presents also a stronger convexity of the curve M(x). It has to be noticed that the tag could not be detected at the center of the loop for case "c", unlike for cases a) and b), due to the magnetic field distribution of the complementary loops. Results of M in the vertical mode are shown in Figure 4 and show a high similarity for case "a" and "b". This is due to the fact that, in the case "b", the loops currents along the common side are in opposite phase. In case "c" the loops currents along the current axis are in phase and, consequently, generate a strong magnetic field driving to a maximum value of |M| at the position X=0. Complementary loops of case "c" present an additional maximum for |M(x)| compared to cases "a" and "b".

Additionally, the values of CC in the horizontal (see Figure 3) and vertical (see Figure 4) modes drive to the conclusion of a better capacity of the glasstag detection in the case "c".

To conclude, the complementary loops structure is theoretically a better solution to improve the total surface of detection whatever the glasstag orientation because it provides the highest CC value for both modes.

III. PRACTICAL TESTS WITH LF RFID GLASSTAGS

The theoretical part emphasizes the advantage of complementary loops for LF RFID glasstag detection. In this part, we prototype the chosen structure for a wide area and use the loops with a commercial reader (Ib technology [12]) to perform detection tests. As the pebbles need not a resolution less than 10 cm, we build the structure with complementary square loops of 10x10 cm², as studied in the theoretical part. The enameled copper wire used for the loops has a diameter of 0.015 mm and a serial loss resistance which limit the surface. We set a total surface of 30x30 cm² in this paper because too high losses can prevent the detection. This matrix of 9 complementary squares loops is called the chequered loop antenna because it corresponds to an alternating distribution of the loops, in series, in function of the current phases, as seen qualitatively in Figure 5. The chequered loop antenna is realized with enameled copper wire fixed on a plexiglas board (0.5 cm thickness + 0.5 cm nylon spacer) and we test the RFID LF tags with HITAG S glasstag and the commercial reader from Ib technology. The chequered loop has a measured inductance of 820 μH (Q=12). The setup is shown in Figure 6 in which the pebble and the tag position are noticed. The RFID reader board is a low power (less than 1W) reader and the dedicated loop used for LF glasstag is typically a multi-turn loop of 10 cm diameter (can be seen in Figure 6 near the reader board) for which the detection range is about 4 cm. As the pebble is placed on the plexiglas, the detection range tested is at least at 1 cm. In Figure 7, the detection areas are qualitatively presented for multiple positions of the pebbles in horizontal and vertical modes. For such difficult cases of mutual coupling with a glasstag (very small equivalent surface of the magnetic dipole) the total detection area is made of the sum of each green areas represented in Figure 7, that is 2x30x4-2x2x4 = 224 cm² for the vertical mode (24,8%) and 9x(10²-9²) = 171 cm² for the horizontal mode (19%). An interesting aspect of these areas are their repartition, almost uniform, along the overall surface which make possible the potential detection of a moving glasstag (displacement and random orientation). If we define the intersection of the two detection surfaces in Figure 7, we have a total of 1x30x4-1x1x4 = 116 cm² (12,8%) which correspond to an area where the glasstags are detected whatever the mode, i. e. whatever their orientation.

IV. CONCLUSION AND PERSPECTIVES

RFID LF pebble detection over a wide area is a very difficult challenge because the mutual coupling is very low due to the size of a glasstag effective surface. To overcome this problem, the typical solution is to increase the current and to use wider single turn loop antenna.

Another solution was proposed in this paper by means of a strong geometrical modification of the reader loop structure itself. The theoretical simulations showed that complementary loops are fruitful when trying to improve the mutual coupling for a given area of detection. Without increasing the current, a prototype for LF RFID glasstags detection is tested and showed that this chequered loop antenna has a total area of detection of 224+171 cm² for the horizontal and vertical modes of tag detection.

As the two modes are detected, the ability to detect a moving tag with a randomized orientation is possible without necessarily increasing the current.

Figure 1 :

 1 Figure 1: glasstag on pebbles (top left) and (right) parallel and vertical modes with mutual inductance when tag surface is small

Figure 2 :

 2 Figure 2: Reader loop area and glasstag equivalent magnetic dipole. Representation (left) of the three different studied solutions .

  Movement/distance shift in meters

Figure 3 :

 3 Figure 3: Magnetic coupling for the different solutions of reader loops structures, in function of the tag movement (X axis) in horizontal mode

Figure 4 :

 4 Figure 4: Magnetic coupling for the different solutions of reader loops structures, in function of the tag movement (X axis) in vertical mode

Figure 5 :

 5 Figure 5: (top) Twisting of the wire to build the chequered loop lines. Representation of the current phases (bottom left) corresponding to the complementary loops principle . Chequered loop (bottom right)

Figure 6 :

 6 Figure 6 : RFID detection tests setup and a position where the glasstag is detected in vertical mode at 1 cm height.

Figure 7 :

 7 Figure 7 : RFID Detection area at a distance of 1 cm (at least) in vertical (top) and horizontal (bottom) modes

b a Γ Γ

1 Γ Γ 12 2 1 0


Arnaud Regazzi

Maxime Teil

Pierre J J Dumont
email: pierre.dumont@insa-lyon.fr

Barthélémy Harthong

Robert Peyroux

Jean-Luc Putaux

Microstructural and mechanical properties of biocomposites made of native starch granules and wood fibers

Keywords: A, short-fiber composites B, Mechanical properties C, Multiscale modeling E, Powder processing

Novel biocomposites were fabricated using preforms of unmodified starch powder and wood pulp fibers. Stacks of preforms were consolidated using thermo-compression (TCM) and ultrasonic compression moldings (UCM). The characterization of the microstructure of the biocomposites showed that TCM enabled a better preservation of the crystallinity of starch granules during their welding than UCM. However, UCM allowed a significant gain in processing time. For the best set of forming and material parameters, the composites exhibited an elastoplastic response with strain hardening. Their Young's modulus, flexural strength and strain at ultimate stress reached up to approx. 6 GPa, 70 MPa, and 8%, respectively. The best properties were associated to the partial preservation of the native crystallinity of starch and lowered porosity. Bleached and fibrillated fibers with a large aspect ratio also contributed to the enhancement of composite properties. These effects were explained by a better starchfiber interface and the presence of a network of connected fibers within the composites.

Introduction

The need for sustainable materials in the field of polymer-matrix composite materials prompted the growing use of biopolymer matrices and bio-based reinforcement fibers. Several bio-based components exhibit excellent native mechanical properties and are promising candidates as raw materials for biocomposites with enhanced mechanical properties. Plant and wood fibers (elastic modulus 5 < < E 90 GPa), as well as unmodified starch granules (elastic modulus 3 GPa) exhibit interesting mechanical properties because of their nanoscale semicrystalline structure [START_REF] Schroeter | On the mechanical properties of native starch granules[END_REF][START_REF] Lourdin | Crystalline Structure in Starch[END_REF][START_REF] Regazzi | Effectiveness of thermo-compression for manufacturing native starch bulk materials[END_REF][START_REF] Dicker | Green composites: a review of material attributes and complementary applications[END_REF]. Both materials are also interesting because their extraction chains are well established and their cost is relatively low. Starch-based composites have been widely investigated. Starch is usually plasticized, i.e. thermally-processed with water and other plasticizers, and blended with other polymeric materials [START_REF] Xie | Rheology to understand and optimize processibility, structures and properties of starch polymeric materials[END_REF]. It can be associated with various fiber reinforcements, including nano-and micrometric bio-based fibers [START_REF] Avérous | Biocomposites based on plasticized starch, Biofuels, Bioprod[END_REF][START_REF] Xie | Starch-based nano-biocomposites[END_REF][START_REF] Martins | New biocomposites based on thermoplastic starch and bacterial cellulose[END_REF]. Classical composite-forming processing routes such as extrusion and injection result in a complete modification of the structure of starch, inducing a possible recrystallization, and/or degradation [START_REF] Xie | Rheology to understand and optimize processibility, structures and properties of starch polymeric materials[END_REF][START_REF] Avérous | Biocomposites based on plasticized starch, Biofuels, Bioprod[END_REF]. The mechanical properties of injectionmolded starch-based composites can range between 0.6 and 8.3 GPa for the elastic modulus, 7.6 and 51 MPa for the tensile strength, depending on the fiber content and the amount of plasticizer when, for instance, starch acetate is associated with cellulosic fibers [START_REF] Nättinen | Naturally compatible: starch acetate/cellulosic fiber composites. I. Processing and properties[END_REF][START_REF] Madsen | Short cellulosic fiber/starch acetate composites -micromechanical modeling of Young's modulus[END_REF]. The use of native starch granules is less widely spread except as fillers in polymers to reinforce plastics [START_REF] Avérous | Biocomposites based on plasticized starch, Biofuels, Bioprod[END_REF]. Recently, unmodified starch granules have been used to obtain reinforced polymer matrices by taking advantage of starch-absorbed moisture to initiate polymerization or cross-linking reactions [START_REF] Ceseracciu | Robust and biodegradable elastomers based on corn starch and polydimethylsiloxane (PDMS)[END_REF]. Starch granules have also been used to anchor and support graphene oxide (GO) nanosheets to enhance the dispersion of these nanoparticles in poly(lactic acid) PLA matrix [START_REF] Xu | Immobilized graphene oxide nanosheets as thin but strong nanointerfaces in biocomposites[END_REF]. This approach resulted in PLA/GO@starch composites with properties outperforming those of PLA/starch composites. Furthermore, the dielectric properties of starch granules can be used to control their dispersion in elastomeric matrices and obtain enhanced mechanical properties [START_REF] Hao | Mechanical behavior of starch/silicone rubber hybrid electric elastomer[END_REF]. It was shown that unmodified starch granules could also be used to fabricate parts by compression molding without using chemical additives, while controlling the amount of absorbed water and preserving their native crystalline structure [START_REF] Regazzi | Effectiveness of thermo-compression for manufacturing native starch bulk materials[END_REF]. These materials exhibit Young's moduli that are comparable to those of usual thermoplastic matrices such PVC, PS or PMMA. However, the presence of defects such as porosity or cracks, is detrimental to their strength properties.

T

In this study, the manufacturing of biocomposites was investigated using thermo-compression molding (TCM) as a long-cycle technique [START_REF] Regazzi | Effectiveness of thermo-compression for manufacturing native starch bulk materials[END_REF] and ultrasonic compression molding (UCM) as a short-cycle technique [START_REF] Kuznetsov | Sintering of polymeric powders under pressure -I. Ultrasonic analysis of the variation in contact between polymer powder particles[END_REF] without adding any chemicals. In both techniques, starch granules were mixed with wood pulp fibers using a papermaking process to enhance both the elastic and strength properties of unmodified starch matrices. The objective was to evaluate both the microstructure and mechanical properties of the resulting bio-based composites, and to assess the suitability of both techniques for preserving the microstructural properties of the biocomposite constituents.

Experimental

Materials

Starch

Two different sources of starch granules were used, differing by both their amylopectin/amylose ratio and crystalline structure: (i) a waxy maize starch C*Gel 04201 from Cargill, that contained 99% of amylopectin and exhibited an A-type crystalline structure, and (ii) an amylomaize starch N-460 from Roquette with 50% of amylopectin and a B-type crystalline structure.

Pine wood fibers

Both a chemo-thermomechanical pulp (CTMP), i.e. rich in lignin, and a bleached chemical Kraft pulp (CB), i.e. where the lignin was extracted from fibers, were used. They were obtained from pine wood. The CB pulp was refined using a PFI mill (10,000 rotations). The dewatering properties of the pulp fibers were assessed using a Schopper-Riegler index testing device [START_REF] Sampson | Materials properties of paper as influenced by its fibrous architecture[END_REF]. Besides, their morphology was evaluated using a fiber and shive analyzer MorFi from TechPap (Grenoble, France). The results are summarized in Table 1 and showed, as expected, that the CB pulp was much more fibrillated than the CTMP pulp.

Processing

Preform manufacturing

In order to disperse fibers into the starch powder matrix, semiproducts were manufactured in the form of sheets. First, the appropriate amount of pulp was weighed. This amount depended on the targeted fiber content, such that the weight of the granules/fibers mix was 10 g for all sheets. Then, the pulp was immersed in 1 L of distilled water. The preparation was mixed for 5 min in a domestic blender to disperse fibers. Then, starch powder was added and slowly stirred with fibers for 20 min to obtain a homogeneous suspension. Preform sheets of 20 cm in diameter were obtained by filtering the suspension using a Rapid Köthen sheet former and a 1 μm filter cloth. Then, sheets were oven-dried at 40 °C overnight and cut into layers matching the dimensions of the molds. Finally, the molds were filled with a stack of starch/ fibers layers to achieve a given mass (constant for each technique). Fig. 1 shows SEM images of the cross sections of several preforms. These images reveal that for both types of fibers, starch granules were well trapped and intercalated within the fiber network during the filtering step of the preform manufacturing. However, a slight sedimentation of starch granules was also observed inside the preforms. In addition, these images show that fibers mainly exhibited a planar orientation due to the deposition process. Note the apparent fibrillation of fibers that is much more pronounced for CB than CTMP pulp fibers, which agrees with the aforementioned results (Table 1). The fibrils of CB fibers coated the starch granules, showing a good affinity between both constituents. In addition, the lumens of the CTMP fibers were partially open, whereas those of CB pulp fibers where mostly closed. In these preforms, the pores were not only located inside and between the fibers, but also inside some starch granules that were hollow.

Thermo-compression molding (TCM) and ultrasonic compression molding (UCM)

TCM induces the welding of granules and fibers by conduction heating in quasi-static compression conditions. A processing temperature of 100 °C was applied to the thermo-regulated plates of the molding press while the compression stress was kept at 100 MPa for 60 min. Thermogravimetric analyses showed that this temperature was far below the degradation of starch and wood fibers (see also section 3.4). This compression stress was necessary to achieve a good cohesion in composites. Accounting for the amount of starch powder that ranged between 50 and 70 wt%, the relative part of compression stress on starch granules was roughly estimated to range between 30 and 70 MPa. These compression stresses were optimal to prepare starchonly samples with a good cohesion [START_REF] Regazzi | Effectiveness of thermo-compression for manufacturing native starch bulk materials[END_REF]. For this technique, a 80 × 10 mm 2 stainless steel rectangular mold was designed. Eighteen or nineteen rectangular layers of semi-products were stacked into the mold cavity to reach a mass of 4.80 ( ± 0.05) g. After molding, samples with a thickness of approximately 4 mm were stored at least 12 h in a climate-controlled room at 23 °C and 50 °% of relative humidity (R.H.) before characterization so as to reach an equilibrated moisture content.

The aim of UCM is to induce the welding of granules and fibers by local heating under the effect of viscous dissipation and/or inter-particle friction [START_REF] Fairbanks | Applying ultrasonics to the moulding of plastic powders[END_REF][START_REF] Matsuoka | Effects of ultrasonic vibration on the compaction molding of polymeric powders[END_REF][START_REF] Paul | Ultrasonic moulding of plastic powders[END_REF][START_REF] Regazzi | Ultrasonic welding of 100% lignocellulosic papers[END_REF]. This technique consists in simultaneously applying a constant average compression stress of 15 MPa and a forced 20 kHz vibration of 60 μm amplitude on the top of the sample during 0.70 s. During that time, the temperature of the sample should not increase over 200 °C, as measured by the authors during the ultrasonic welding of lignocellulosic materials [START_REF] Regazzi | Ultrasonic welding of 100% lignocellulosic papers[END_REF]. Once again, this temperature was below the degradation temperatures of the constituents of the composite materials. For UCM, the mold was made of stainless steel and was a 32 × 4 mm 2 oblong cavity. The filling technique was similar to that previously described for TCM, and the targeted material weight was 300 ( ± 15) mg, corresponding to a stacking of approximately 7 or 8 sheets. Again, after molding, samples with a thickness of about 1.6 mm were stored at least 12 h in a climate-controlled room at 23 °C and 50% R.H. before characterization so as to reach an equilibrated moisture content. UCM allows a significant gain in processing time compared to TCM. However, the effect of applying vibrations to starchbased composites maintained under compression is not known. This study thus allowed a comparison of the microstructure and mechanical properties of the materials prepared using both techniques.

Characterization methods

Density and bulk porosity measurements

The determination of the bulk density d bulk of the samples was based on mass and volume measurements. The samples mass m was measured using a Mettler Toledo ME204 balance. Their thickness and width were assessed using a Mitutoyo Digimatic 293-821 μm. Three measurements along the length of samples were used to define the mean value of these

Thermogravimetric analysis (TGA)

The thermal stability of samples was evaluated by TGA (STA 6000, Perkin Elmer Instruments, USA), heating from 50 °C to 450 °C at a rate of 10 °C min -1 under an air flow of 20 mL min -1 .

Bending tests

The mechanical properties of composites were determined by threepoint bending tests. An Instron 5965 testing machine was used with a 5 kN force sensor. Considering the low stress levels, the deformation of the testing machine was neglected, and strain calculations were based on the crosshead displacement. Experiments were performed using a crosshead velocity of 2 mm min⁻ 1 in accordance with the ISO178 standard. All experiments were conducted in triplicate in a climate-controlled room at 23 °C and 50% R.H.

Scanning electron microscopy (SEM)

Preform samples were cut with a razor blade, coated with Au/Pd in a Baltech MED-020 sputter coater and the cross-sections were observed in secondary electron mode with a FEI Quanta-FEG 250 microscope operating at 1 kV. The processed composite samples were fractured in liquid nitrogen. The fracture surfaces were coated with Au/Pd in an Emitech K550X sputter coater and observed with a FEI Quanta 200 microscope operating at 10 kV.

X-ray diffraction (XRD)

The XRD analysis was conducted on starch only. Native starch granules and compacted starch materials ground to a fine powder were poured into glass capillaries of 1.5 mm outer diameter. The capillaries were flame-sealed and exposed to X-rays in a vacuum chamber using a Philips PW3830 generator operating at 30 kV and 20 mA (Ni-filtered CuKα radiation, λ = 0.1542 nm). Two-dimensional diffraction patterns were recorded on Fujifilm imaging plates. The plates were read off-line using a Fujifilm BAS 1800-II bio-imaging analyzer. The resulting ring patterns were radially averaged in order to get diffraction profiles.

X-ray microtomography

X-ray microtomography imaging experiments were performed at the European Synchrotron Radiation Facility (ESRF, ID19 beamline, Grenoble, France) to record 3D images of the microstructure of the central region of the samples (X-ray energy = 19 keV, CCD detector of 2560 × 2560 pixels, voxel size 0.324 μm × 0.324 μm × 0.324 μm, scanning time < 5 min). The samples were placed vertically and their length was always orthogonal to the X-ray beam during scanning. The scans were reconstructed using the so-called Paganin procedure, which is based on the use of the phase contrast in the images [START_REF] Beltran | 2D and 3D X-ray phase retrieval of multi-material objects using a single defocus distance[END_REF][START_REF] Paganin | Simultaneous phase and amplitude extraction from a single defocused image of a homogeneous object[END_REF]. Then, the ImageJ software was used to get binary images and filter noise [START_REF] Dello | Liver volumetry plug and play: do it yourself with ImageJ[END_REF]. Binarization was applied to extract either the fibrous network or the pores from the starch matrix. This segmentation was achieved by applying a threshold on the intensity of voxels. The volume fiber content and the porosity were defined as the ratio of the number of voxels corresponding to the volume of fibers and pores, respectively, over the total number of voxels in the scanned volume. Typical images before and after treatment for the extraction of fibers are shown in Fig. 2. The volume of the 3D images was a 750 μm wide and 700 μm high cylinder. Only the CB/waxy maize starch composite obtained by UCM was used for the analysis of the volume fiber content, due to the difficulty of separating matrix and fibers when their density was close, resulting in a very low contrast between both phases.

Results

Visual aspects and handling of samples

The visual aspect of the different materials mainly depended on the nature of the reinforcement (Fig. 3). CB-reinforced composites were completely white, while CTMP-reinforced composites were light-brown due to the presence of lignin in CTMP fibers. For both processing techniques, the aspect of both sides of the samples was slightly different. The side in contact with the punch was slightly brighter than the opposite side (bottom of the mold), probably because of a denser and smoother surface. As a result, the visual aspect of the samples appeared to be controlled by the nature of the reinforcement as well as compaction heterogeneities [START_REF] Regazzi | Effectiveness of thermo-compression for manufacturing native starch bulk materials[END_REF].

Evolution of the crystallinity of starch

The crystallinity of waxy maize starch samples was assessed by XRD before and after forming. In order to avoid undesired diffraction peaks originating from fiber constituents, starch samples were fabricated in the same conditions than composite samples, except that the molds were directly filled with 100 wt% starch powder before processing. Starch samples had the same mass as composite samples, i.e. 4.8 g for TCM and 0.3 g for UCM. Fig. 4 shows the XRD profiles of as-received starch, after TCM or UCM. The diffraction profile of the as-received starch exhibited diffraction peaks typical of the A-type allomorph, with the main peaks located at about 15.1°, 17.1°, 18.1°, and 22.9°diffraction angles [START_REF] Cheetham | Variation in crystalline type with amylose content in maize starch granules: an X-ray powder diffraction study[END_REF]. The same diffraction peaks were observed for starch processed by TCM, with a slightly lower amplitude and a simultaneous increase of the amorphous background, indicating a slight decrease in the overall crystallinity of TCM-processed starch. A much more pronounced decrease of the peak intensity was observed for UCM-processed starch. Yet the diffraction peaks remained visible, indicating that UCM-processed starch was not amorphous. As a result, both TCM and UCM forming techniques produced cohesive samples (i.e. the welding of starch granules was successfully obtained) without completely modifying their crystalline structure (i.e. without completely decrystallizing them). UCM induced a higher decrease in crystallinity of the starch granules compared to TCM. Assuming that the behavior of starch during processing was similar in the presence of fibers, both techniques enabled starch granules to be welded, while partially preserving their native crystallinity, and presumably their excellent mechanical properties. Investigating the crystallinity of both phases of the composite samples would be necessary to confirm this assumption. However, this would require using advanced XRD techniques for the deconvolution of the diffractograms of both starch and wood fibers.

The SEM images of fracture surfaces of cryo-fractured samples shown in Fig. 5 confirms that both processing techniques induced a rather different evolution of the powder-like starch matrix. A clear difference between the starch matrices obtained by TCM or UCM was observed. For TCM, the starch granules were extremely deformed and compressed, but still clearly visible (Fig. 5a,c), whereas, for UCM, the starch matrix was much more homogeneous, and the boundaries between the starch granules were hardly visible, confirming a drastic evolution of starch for this forming process.

Microstructure

Porosity -Table 2 gives the evolution of the bulk porosity of composites as a function of the weight fiber content for both molding techniques. The bulk porosity of TCM-processed samples was globally lower than that of UCM-processed samples. This difference was attributed to the difference in the compression stress: 100 MPa for TCM and 15 MPa for UCM. For both forming techniques, the bulk porosity also slightly increased with the fiber content, suggesting that pores were partially located close to fibers. For a given type of starch matrix, samples containing CTMP fibers showed a higher bulk porosity than samples containing CB fibers. This increase in porosity could be attributed to the usually higher stiffness of CTMP fibers that contained lignin compared to CB fibers [START_REF] Tam Doo | Flexibility of wet pulp fibres[END_REF]. This effect could also be enhanced by the refining treatment of CB fibers that might have decreased their stiffness [START_REF] Gharehkhani | Basic effects of pulp refining on fiber properties -a review[END_REF].

The analysis of the X-ray microtomography 3D images gave quantitative data on the spatial distribution of fibers and the porosity at the core of samples (called the core porosity thereafter). The results given in Table 3 show that the mean core porosity of all specimens remained below 2%, regardless of the fiber content and the forming technique. However, the mean core porosity was lower than the bulk porosity given in Table 2. Two reasons could explain this difference. First, the volume analyzed by microtomography represented only a fraction of the volume of the sample. It means that the image analysis results could not be representative of the entire sample and it would be difficult to capture possible heterogeneities of the porosity of samples throughout its entire thickness. Second, errors could also originate from the dimensions of samples that were measured and used to calculate the bulk density, assuming perfectly planar faces. Such errors could be significant for small samples such as those made by UCM. Despite these differences, the results given in Table 3 confirm the trends that were seen from Table 2 regarding the role of the type of fibers and the nature of starch on the porosity. The results from 3D image analysis presented in Fig. 6a also revealed that the pores were predominantly located within the regions of high fiber density between the fibers. The concentration of pores in the vicinity of fibers most probably induced a weakening of the apparent fiber-fiber contact zones. This might explain why the fracture surfaces in Fig. 5 exhibit some fibers that were pulled out from the composites during breaking. Through-thickness porosity profiles were extracted from the 3D X-ray microtomography images, as shown for instance in Fig. 6b for the specimen from Fig. 6a. It reveals a non-uniform distribution of porosity through the thickness in the imaged volume of this specimen. This non-uniform distribution is certainly related to the change from one compacted preform to another.

The less porous regions corresponded to the transition zones between the various stacked layers. At a larger scale of analysis, it was also observed that the microstructure of samples fabricated by UCM depended on the distance from the horn of the ultrasonic machine. Basically, the welding of starch and fibers was improved in the vicinity of the horn. It was visible with the naked eye that the most porous zones were located at the bottom of the mold, opposite from the horn, where the attenuation of acoustic waves was maximum.

Fiber content -X-ray microtomography images also allowed studying the volume fiber content profile. For instance, a mean fiber volume content of 25 vol% was determined from Fig. 6b while the targeted weight content was 30 wt%, i.e. 29.4 vol%. This difference was attributed to the low contrast between the fibers and the starch matrix in the X-ray images because both materials had a very similar density. As a result, the measured volume fiber content was very sensitive to the threshold value used for segmenting fibers. Despite this difficulty to quantitatively assess the volume fiber content using X-ray microtomography, its variations revealed by this profile were significant and were again presumably related to the layered structure of the composite materials. In addition, as previously mentioned, preforms also exhibited a through-thickness heterogeneity. Such variations are commonly encountered for various types of industrial composite materials that are obtained by stacking several layers of preforms. For instance, parts that are fabricated using stacks of pre-impregnated short-fiber composites such as Sheet Molding Compounds (SMC) also exhibit this kind of through-thickness variations if their in-plane flow is limited [START_REF] Le | X-ray phase contrast microtomography for the analysis of the fibrous microstructure of SMC composites[END_REF].

Thermal behavior

Typical examples of TGA measurements are shown in Fig. 7 for starch/CB and starch/CTMP composites obtained using TCM. These curves reveal a 2 % weight loss from 50 to 150 °C attributed to the vaporization of water as temperature increased. Besides the degradation onset temperatures of both composites ranged between 220 and 240 °C, thus far above the temperatures reached during TCM or UCM.

Mechanical properties

Fig. 8a shows typical stress-strain curves obtained from bending experiments for two composites that were fabricated by TCM. Similar curves were obtained for composites fabricated by UCM. These curves typically show one initial quasi-linear sharp increase up to a transition stress above which the composites exhibit a more or less pronounced plastic regime with strain hardening up to a maximum stress, and then a slight decrease up to the sample breakage. Table 2 shows the Young's moduli of composites, the ultimate stress, i.e. the strength, defined as the maximum stress reached during bending tests, and the strain at the ultimate stress as a function of fiber content, for both processing techniques. For TCM (resp. UCM), the Young's moduli ranged between 4 and 6 GPa (resp. 3.8 and 5.6 GPa), whereas the strength ranged between 26.5 and 71 MPa (resp. 50 and 69 MPa), and the strain of samples at ultimate stress ranged from 0.9 to 7.6% (resp. 2.2 and 4.8%), showing an effect of the process conditions on the ultimate properties of biocomposites. Both strength and strain at ultimate stress depended more on the nature of the composite constituents for TCM than for UCM. The Young's modulus and the strength exhibited the same trend when the composite constituents varied. Indeed, for CB fibers, these mechanical properties were better with amylomaize starch matrix, whereas for CTMP fibers, they were better with waxy starch matrix. In addition, for both processing techniques, CB fibers induced an increase in the strain at ultimate stress compared with CTMP fibers.

Discussion

Role of fiber connectivity -In a previous study [START_REF] Regazzi | Effectiveness of thermo-compression for manufacturing native starch bulk materials[END_REF], the authors have shown that specimens made of pure waxy maize starch exhibited a maximum average Young's modulus of approximately 3 GPa, and average ultimate stress and strain that did not exceed 12 MPa and 0.4%, respectively, for the best set of TCM conditions. Further, the properties of specimens made of pure starch matrix that were fabricated using UCM exhibited similar properties (results not shown). Both starch matrices could be considered as particularly brittle. By comparison, adding fibers enhanced the elastic properties and the ultimate stress of starch matrices. In addition, the strain at ultimate stress of all studied composites was also greatly enhanced compared to that of the starch matrix alone. These results prove the leading role of the fibers in the composites obtained using both forming techniques. For the investigated fiber contents, the number of fiber-fiber contacts per fiber Z was roughly estimated using the statistical tube model [START_REF] Toll | Note: on the tube model for fiber suspensions[END_REF][START_REF] Martoïa | On the origins of the elasticity of cellulose nanofiber nanocomposites and nanopapers: a micromechanical approach[END_REF][START_REF] Marulier | 3D analysis of paper microstructures at the scale of fibres and bonds[END_REF]:

= + + Z r 4 2 1 f 1 2
where = r l d / is the fiber aspect ratio, f the fiber volume fraction, and

1 , 2 orientation distribution functions ( = = 2/ 1 2
for 2D planar random fibrous networks). Considering that = r 21 and 33 for CTMP and CB fibers, respectively (cf. Table 1), the number Z was estimated to range between 12 and 20 for CTMP fibers, and between 18 and 30 for CB fibers. This is in accordance with previous results for the number of contacts in paper fibrous networks [START_REF] Marulier | 3D analysis of paper microstructures at the scale of fibres and bonds[END_REF]. Therefore, the presence of a network of fibers within the composites certainly plays a crucial role on the increase in their ultimate strain. It could also explain the yielding phenomenon in the plastic regime as such behavior is typically observed for papers [START_REF] Gustafsson | Paper as an engineering material[END_REF]. Moreover, it can be assumed that after the matrix cracking, which is likely to occur for low strain (≈ 0.4%), the strength of composites is controlled by the strength of the fiber network.

Role of processing conditions -The Young's moduli were almost systematically higher when samples were fabricated by TCM instead of UCM. This effect could be interpreted as a consequence of the lower bulk porosity of TCM composites (section 5.3 and Table 2), but also from the lower starch crystallinity of the UCM composites.

Effect of constituents -Composites reinforced with CTMP fibers exhibited lower Young's moduli, ultimate stresses and strains than composites made of CB fibers. These trends can presumably be attributed to several origins: (i) the higher bulk and core porosities of CTMP reinforced composites (Tables 2 and3), (ii) the smaller aspect ratio of CTMP fibers (Table 1), which limits the connectivity of the fiber network, and (iii) the starch-fiber interfacial properties. For the latter argument, fibrillation and the associated increased specific surface area of fibers could partially explain the better properties of CB fiber composites (see also Fig. 1). The physico-chemical properties of the interface are also important, as the properties of composites reinforced with CB fibers were better when made of amylomaize starch instead of waxy maize starch. It can also be noted that these starch-based composites exhibit comparable Young's moduli and ultimate stresses than those made of starch acetate and lignocellulosic fibers [START_REF] Nättinen | Naturally compatible: starch acetate/cellulosic fiber composites. I. Processing and properties[END_REF][START_REF] Madsen | Short cellulosic fiber/starch acetate composites -micromechanical modeling of Young's modulus[END_REF] or cellulose powders [START_REF] Pintiaux | Binderless materials obtained by thermo-compressive processing of lignocellulosic fibers: a comprehensive review[END_REF].

Potential mechanical properties of composites -The potential Young's modulus E of the CB fiber composites was estimated using the Halpin- Tsai-Kardos (HTK) model, assuming a 2D planar orientation of the fibers [START_REF] Halpin | Moduli of crystalline polymers employing composite theory[END_REF]:

= + E E E 0.375 0.625 HTK L T with = + E E L l d m 1 2( / ) 1 L f L f and = + E E T m 1 2 1 T f T f where = + L E E E l d E 2( / ) f m f m and = + T E E E E 2 f m f m
The Young's modulus E m of the starch matrix was considered to be 3 GPa [START_REF] Regazzi | Effectiveness of thermo-compression for manufacturing native starch bulk materials[END_REF], while the Young's modulus E f of CB pulp fibers was rea- sonably chosen to range between 10 and 20 GPa [START_REF] Borodulina | Stress-strain curve of paper revisited[END_REF]. In addition, the aspect ratio l d / of fibers involved in the HTK model was 33 (Table 1).

The Young's modulus of these composites was also estimated using the extended Cox's model for planar fibrous structures with random orientations [START_REF] Martoïa | On the origins of the elasticity of cellulose nanofiber nanocomposites and nanopapers: a micromechanical approach[END_REF][START_REF] Cox | The elasticity and strength of paper and other fibrous materials[END_REF]:

= + E E 1/3 (1 ) Cox f f f m For = f
30% and the HTK model, the Young's modulus of the CB fiber composites ranged between 4.6 GPa and 6.2 GPa, whereas for the Cox model, the Young's modulus ranged between 3.1 GPa and 4.1 GPa. For TCM and = f 30%, the actual Young's moduli of the composites (Table 2) were slightly higher than the theoretical predictions of Cox's model and in the same order of magnitude as the HTK model, e.g. 5.7 GPa and 5.3 GPa for CB fiber composites with amylomaize and waxy maize starch, respectively. Since the experimental Young's moduli are close to the theoretical predictions, these results indicate (i) that there are good fiber-matrix interfacial properties of the composites, and (ii) the potential beneficial effect of the fiber-fiber contacts (which are not taken into account neither in the mean-field HTK model nor in Cox's model). For CB-fiber composites processed by UCM, both theoretical predictions and experiments are close, showing also good interfacial properties. However, at = f 50%, the Young's moduli of composites seem to be lowered, possibly because of the increase in the number of fiber-fiber contacts with low interfacial properties or lowered fibermatrix interfacial properties.

Comparison with polymers and other composite materials -Fig. 8b shows the mechanical performance of the obtained starch-based composites (green ellipse) in an Ashby-type materials selection chart [START_REF] Ashby | Materials Selection in Mechanical Design[END_REF]. The obtained composites exhibit interesting mechanical properties compared to various thermoplastic polymers such as high density polyethylene (HDPE), polypropylene (PP) or polyvinyl chloride (PVC) reinforced with glass fibers. Besides, the obtained composites exhibit mechanical properties that are comparable with the best properties of biocomposites using starch acetate as a matrix [START_REF] Nättinen | Naturally compatible: starch acetate/cellulosic fiber composites. I. Processing and properties[END_REF][START_REF] Madsen | Short cellulosic fiber/starch acetate composites -micromechanical modeling of Young's modulus[END_REF].

Conclusion

Bio-based composites were successfully prepared using a matrix made of waxy maize or amylomaize native starch granules, and softwood thermomechanical (CTMP) or bleached and fibrillated chemical Kraft (CB) pulp fibers. The specimens were processed using either thermo-compression (TCM) or ultrasonic compression (UCM) moldings of stacks of several preforms obtained by a papermaking-like technique. Both forming techniques produced composite samples with a good cohesion. XRD profiles showed that the native crystallinity of starch was better preserved in the case of TCM than UCM. SEM and X-ray microtomography images revealed the planar orientation of fibers within the preforms and the resulting biocomposites. Residual pores were mainly located between fibers, and more particularly in the regions with the highest fiber contents. These biocomposites exhibited an elastoplastic mechanical response. The partial preservation of the native crystallinity of starch, the lower porosity, the use of bleached and fibrillated fibers with a high aspect ratio contributed to enhance their mechanical performances. These effects were related to the presence of fiber-fiber contacts within the composites and improved starch-fiber interface properties. The mechanical properties of the biocomposites are comparable to those of other biocomposites made for instance of lignocellulosic fibers and starch acetate [START_REF] Nättinen | Naturally compatible: starch acetate/cellulosic fiber composites. I. Processing and properties[END_REF][START_REF] Madsen | Short cellulosic fiber/starch acetate composites -micromechanical modeling of Young's modulus[END_REF]. UCM does not provide as good properties as TCM. However, UCM is a fast (processing time of ca 1 s, resp. 1 h, for UCM, resp. for TCM) and energy-efficient solution to obtain the biocomposites. Further investigations are needed to better understand the influence of the type of starch and the source of pulp fibers on the properties of these new biocomposites.

  two dimensions. The length was measured using a Mitutoyo Absolute Digimatic 500-181 caliper. Finally, the volume V of samples was cal- culated using these measured dimensions making the assumption of a plane-parallel geometry. The density d bulk is the ratio m V / . A helium pycnometer (Accupyc 1330, Micromeritics Instrument Corporation, USA) was used to measure the absolute density of starch granules d starch and wood fibers d fiber . The determination of volume was based on the reduction of the gas capacity of a chamber where the sample is placed. In practice, the device measures the pressure drop when the sample chamber is connected to an expansion chamber. All experiments were conducted in triplicate. The values of d starch for waxy and amylomaize starches were 1.495 and 1.488, respectively. The values of d fiber for CTMP and CB pulps were 1.486 and 1.542, respectively. The bulk porosity of composites was evaluated from their bulk density d bulk and absolute density d abs , as follows: density of composites d abs was determined from the absolute density of its constituents (d starch and d fiber ) and the weight fiber content w, as follows:

Fig. 1 .

 1 Fig. 1. SEM images of cross-sections of preforms made of softwood pulp and waxy maize starch granules: 30 wt% CB (a), 30 wt% CTMP (b), 50 wt% CB (c), 50 wt% CTMP (d). The arrows point at starch granules (SG), wood fibrils (WF) and pulp fibers (PF).

Fig. 2 .

 2 Fig. 2. Slices of X-ray microtomography images before (a) and after (b) binarization of fibers (pictures represent a square of 750 μm side length) -3D image after binarization (c).

Fig. 3 .

 3 Fig. 3. CB/waxy (foreground) and CTMP/waxy (background) maize starch composites obtained by TCM (a) and UCM (b).

Fig. 4 .

 4 Fig. 4. XRD profiles of waxy maize starch samples before and after TCM and UCM. The profiles were shifted vertically for clarity.

Fig. 5 .

 5 Fig. 5. SEM images of cryo-fracture surfaces of 50 wt%/50 wt% softwood pulp/waxy maize starch composites: a) TCM -CTMP, b) UCM -CTMP, c) TCM -CB, d) UCM -CB.

Fig. 6 .

 6 Fig. 6. (a) Slice of an X-ray microtomography image after segmentation of fibers (white), pores (black) and starch matrix (gray). The picture represents a 750-μm diameter circle for a sample of CB/waxy maize starch composite obtained by UCM. (b) Average fiber volume content (black curve) and core porosity (gray curve) along the thickness direction (y-axis of Fig. 2) of the same composite.

Fig. 7 .

 7 Fig. 7. Typical TGA curves obtained for TCM specimens made of waxy maize starch and CTMP fibers or CB fibers.

Fig. 8 .

 8 Fig. 8. (a) Typical stress-strain curves obtained from the bending tests of TCM specimens. (b) Tensile strength as a function of Young's modulus for the starch-based biocomposites and for some commercially-available polymers and composite materials.

Table 1

 1 Morphological characteristics and Schopper-Riegler index of pine wood pulp fibers.

	CTMP	845	40.9	1.65	6.09	16
	CB	1085	33.1	0.94	3.51	58

Pulp

Mean fiber length l [μm] Mean fiber width d [μm] Fibrillation index [%] Fine content [% in area] Schopper-Riegler index

Table 2

 2 Bulk porosity and mechanical properties of pulp/starch composites.

	Process	TCM

Table 3

 3 Mean core porosity of pulp/starch composites fabricated by TCM and UCM.

	Matrix	TCM		UCM
		Waxy maize	Amylomaize	Waxy maize	Amylomaize
	CTMP	3.0%	4.0%	2.6%	1.0%
	CB	1.0%	2.1%	0.6%	1.5%

Acknowledgments

The authors acknowledge the financial support of Institut Carnot PolyNat (Investissements d'Avenir -grant agreement #ANR-11-CARN-007-01), the NanoBio-ICMG Platform (FR 2607) for granting access to the electron microscopy facility, as well as Cécile Sillard and Raphaël Passas for their technical support, Bertine Khelifi (LGP2) and Christine Lancelon-Pin (CERMAV) for electron microscopy, and Romain Léger (C2MA) for pycnometer experiments.