Véronique Cerezo

Zoltan Rado
email: zoltanr@avsafetech.com

Malal Kane
email: malal.kane@ifsttar.fr

Véronique Cerezo

Corresponding Author IFSTTAR Veronique Cerezo
email: veronique.cerezo@ifsttar.fr

Encouraging Results of International Friction Measurement Harmonization through Quality Assurance and Calibration

Keywords: friction, harmonization, macrotexture, round robin tests

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INTRODUCTION

The variety and number of friction measurement devices in use all around the world for the assessment of pavement surface characteristics is quite large. These devices are using different principles of measurement and can highly differ regarding their design or operating conditions.

Regarding the range of variation of these operating conditions (load, slip ratio, pressure, etc.), it is very difficult to obtain a robust correlation between the measured friction coefficients. The scale correction requested to use more advanced modelisation.

In the past twenty years, some research projects tried to tackle this issue (Hermes, PIARC International Experiment (1), Tyrosafe, Rosanne). These projects highlighted the need of quality controlled, reliable and accurate calibration/checking process of the devices prior to round robin tests and proposed several methodologies for harmonization processes. Additionaly, a skid resistance index (SRI) was proposed to harmonize friction coefficient measurements at the European scale. Despite the progress, work remains to confirm the robustness and the effectiveness of the common scale.

Therefore, IFSTTAR (France) organized the 1 st annual European Pavement Friction Workshop hosted at the IFSTTAR facility in Nantes, France. The program included improved checking processes, new unified component level calibration processes of the participating devices and a well-controlled comparison trial on the IFSTTAR test surfaces. The results were analysed both according to the harmonization method developed in the framework of FP7 European Research project ROSANNE (2013-2016) and according the method used in the United States (International Friction Index "two steps approach"). The first methodology uses an average friction value from all measurement devices as the reference of the common scale whereas the US method uses a two-step approach utilizing a laboratory scale reference surface and a pair of unique laboratory and field useable devices to establish time stable universal check standards for friction harmonization.

This paper describes the first European Pavement Friction Workshop and the application of the two harmonization processes. After describing the common scale and its calibration, the results obtained by the two approaches are discussed.

EXPERIMENTS Devices

The 1st European Pavement Friction Workshop incorporated high-speed friction measuring devices (both at low and high speeds) and stationary friction measuring devices.

Longitudinal Friction measuring devices

Ten devices measuring Longitudinal Friction Coefficient (LFC) attended the trials. These devices operated on the principle of a vertically loaded wheel rotating at a lower speed than the forward vehicle speed. The slip ratio of the devices ranged from 10 to 25%. The static vertical load ranged from 105 to 180 kg. Three different smooth tires were used depending on the device: PIARC (2), ASTM E1551-16 (3) and ASTM E1844-96 (4). The theoretical water film thickness was 0.5 or 1 mm depending the device.

Side force friction measuring devices

Five devices measuring Sideway-Force Friction Coefficiet (SFC) attended the trials. These devices operated on the principle of a sideway-force generated by a free rotating wheel, which is set at an angle with respect to the driving direction. All the five SFC devices had the same measurement wheel angle of 20°, which generated a slip ratio of 34%. The static vertical load was 200 kg and water film thickness spread on the surface was 0.5 mm.

Stationary measurement devices

Five Dynamic Friction Tester (DFT) stationary measurement devices attended the trials. The DFT device is composed of a measuring unit and a control unit. The measuring unit operates a motor driven horizontal disc. Three rubber sliders are attached to the disc and a watering system allow maintaining a constant waterflow during the measurement process. Once the set speed of the rotating disc is reached, the electric motor is switched off and the disc with the measurement pads is lowered into contact with the surface with a constant vertical load. The speed of the pads decreases to a full stop due to the generated friction. Friction values at different speeds (mostly 40 and 60 km/h) are extracted from the recorded braking curve. For the tests, all the devices were equipped with new rubber pads from the same batch provided by the machine supplier. Tests were conducted according to ASTM standard E1911-09ae1 [START_REF]Standard Test Method for Measuring Paved Surface Frictional Properties Using the Dynamic Friction Tester[END_REF].

Macrotexture measurements

Macro-texture measurements were performed with the Circular Track-texture Meter (CTM) according to the ASTM E2157-15 (6) standard and the ISO EN 13473-1 (7) standard for Mean Profile Depth (MPD) calculation. The CTM device uses a laser sensor attached to a rotating arm, measuring texture profile along a circle of 142 mm radius. The obtained profile is composed of 1024 data points spaced at 0.87 mm intervals, meaning a total profile length of 892 mm [START_REF] Hanson | Evaluation of Circular Track Meter for measuring surface texture of Pavements[END_REF]. CTM measures profile in the exact same circular track where DFT measures friction.

Tested surfaces

The IFSTTAR test track located in Nantes (France) is composed of straight lane parallel test track surfaces incorporating various pavement surfaces arranged in several longitudinal tracks (Figure 1).

FIGURE 1 Test track (Ifsttar, France)

These test surfaces cover a wide range of micro-texture and macro-texture levels (Table 1).

Regular pavement surface monitoring is done on test track (at least every six months). Macrotexture is assessed using longitudinal laser profilometer measurements. The MPD values were calculated on each test track following ISO EN 13473-1 [START_REF]Characterisation of pavement texture by use of surface profiles --Part 1: Determination of Mean Profile Depth[END_REF]. Micro-texture was estimated through British Pendulum measurements according to EN 13036-4 (9).

Test program

Tests were performed during one week and were divided into two phases [START_REF] Cerezo | 1 st European Pavement Friction Workshopexperimental program, 1st European Pavement Friction Workshop[END_REF]. In the first phase, a calibration exercise was conducted to ensure that devices are working properly. In a second phase,

FIGURE 2 Marks on the test sections

Then, static friction measurements were performed with Dynamic Friction Tester on the same test tracks. For each test surface, measurement points were defined every twenty meters. For each location, two measurements were obtained and average values were calculated to characterize each test section. Three test speeds were used for the analysis: 20, 40 and 60 km/h. Static macrotexture values with CTM were also collected on the same locations as the DFT measurements.

Thus, a database containing 2250 average friction values for high-speed devices, 1230 DFT values and 41 macro-texture values obtained with CTM was built for the analysis.

CALIBRATION EXERCICE

Checking process was conducted only on dynamic friction measuring devices. The DFT devices were directly calibrated by the manufacturer at the beginning of the friction workshop to ensure they were working properly.

Checking process of dynamic devices focused on those main parameters, that can have an influence on friction measurements:

 Tire (pressure, cleanness, wear, Shore hardness),

 Measuring system (static vertical load, wheel angle),  Speed,  Water flow (nominal magnitude, film thickness distribution under the tire).

Participants were also requested to have a calibration certificate of the force sensor and to use a new or honed tire, without visible defects.

Tires checking

Five different tires were used during the trials depending on the device: ASTM 1551, Piarc smooth, Trelleborg T49, Avon and SKM. Checking were based on European, American, national specifications or existing standards.

TABLE 2 Checking results for the tires

Note: Tire pressures were set to comply with European, American, national specifications or existing standards. The measured Shore hardness values, stayed within the required range of acceptable values.

Angles

Five devices were working on the SFC friction measurement principle. The theoretical side-ways angle value was 20° ± 1° regarding European standards. Two measurements were conducted and the test wheel angle value was determined to be the average of these two measured values. Figure 3 shows that all the devices complied with the requirements.

FIGURE 3 Measured wheel angle

Speed

The measurement speed of each device was checked by a calibrated radar. The radar was set up on the side of the test track. Devices drove at constant speed on straight line along the test track and the radar recorded value was compared to the on-board speed measured on the friction devices.

The speeds 40, 60 and 80 km/h were tested. The results shall be within ± 5% of the reference speed.

Figure 4 presents the results obtained for the fifteen vehicles. Only, one value at 60 km/h was outside the resquested range. Further analysis showed that this lower speed value can be explained by a small braking manoeuver during the checking process. Sref -5%

Vertical load

The static vertical load was checked by using a calibrated weigh pad, readable and accurate to 0.5 kg. The test vehicle was positioned on a level surface such that the test wheel could be lowered, when required, on to the weigh pad. The weigh pad was constructed to ensure that a device's measurement wheel when lowered to the weigh padpositioned at it's normal measurement place.

During the calibration the measured and reference load values were used to ensure compliance to the standards EN TS 15901-1 to 15 (11) (12) (13) [START_REF]Road and airfield surface characteristics -Part 15: Procedure for determining the skid resistance of a pavement surface using a device with longitudinal controlled slip (LFCI): The IMAG[END_REF] or existing national procedures. Two measurements were performed and the static load was determined to be the average of the two measured values. Table 3 shows that static load values complied with European, American or national specifications or existing standards.

TABLE 3 Checking results for the vertical load

Water film thickness

Water film thickness was checked using a special aluminium tray composed of small channels.

The platform was embedded in the pavement such that the top of the platform was level with the pavement surface. The vehicles drove over the platform at 40, 60 and 80 km/h in normal measurement mode. The amount of water spread in the channels of the platform was measured using digital high-resolution pictures taken after the vehicles have passed. A special purpose software allowed a semi-automatic analysis of each photo. Two criteria were analysed: a) the homogeneity and width of the water distribution in the tubes; b) the average nominal water film thickness under the tire.

FIGURE 5 Grid (a) and method used to analyze pictures (b)

Table 4 presents the results of the water film thickness checking. The total width of water dispersal under the tire was generally sufficient, but when only the width of the homogeneous area was considereded results were different. In half of the cases, the water-film thickness under the tire was not uniform and not provided the required niminal water depth.

Even though the average water-film thickness for most of the devices was close to the required, under the tire in the homogeneous area for the majority of devices, it was significantly smaller. This fact can be explained by the calibration process employed for numerous devices, which is based on the weighing the amount of water delivered during a given time.

Only the average water-film thickness measurement was considered to accept devices (distribution and below tire water-film thicknesses were not included), due to the fact that standards and specifications generally consider an average water-film thickness value in their requirements. All the devices were compliant with these specifications.

Three devices (2 side-forces devices and 1 longitudinal friction device) were not able to perform this check due to the shape of their water delivery system. However, these devices contained electronic closed-loop controlled water delivery systems to ensure a regular and correct water flow.

Homogeneous area

Total area for calculus

METHODOLOGY FOR ROUND ROBIN TESTS ANALYSIS

Two different harmonization methodologies were examined using the friction workshop's database.

The first one was based on the Skid Resistance Index (CEN/TS 13036-2) (15) which was improved during FP7 EU ROSANNE project funded between 2013 and 2016 (16) ( 17) [START_REF] Greene | Deliverable D1.2: Analysis of data from the first round of tests and initial development of the common scale, Collaborative Project FP7-ROlling resistance, Skid resistance, ANd Noise Emission measurement standards for road surfaces[END_REF]. The second one according to the methodology developed in the US used a two-step method utilizing a laboratory scale reference surface and a pair of unique laboratory and field useable devices (DFT and CTM)

to establish time stable check standards for friction harmonization [START_REF] Astm E | Standard Practice for Calculating International Friction Index of a Pavement Surface[END_REF].

Common scale principle (SRI)

The concept is to transform measured friction values provided by an individual device under its particular test conditions (speed, load, tire) to equivalent friction values under reference conditions.

This transformation is based on the following formula defining the SRI (Skid Resistance Index):

𝑆𝑅𝐼 = 𝐵 • 𝐹 • 𝑒 𝑆-𝑆 𝑅𝑒𝑓 𝑆 0 (1) 𝑆 0 = 𝑎 • 𝑀𝑃𝐷 𝑏 (2)
Where SRI is the skid resistance value reported against the common scale a, b, and B are device-specific calibration parameters to be determined for sideways-force friction). Data scattering for each group on each surface was checked with statistical tests to detect outliers, outliers were excluded from the calculation of the reference values (20) [START_REF]Application of statistics: accuracy (trueness and precision) of measurements methods and resultspart 5: Alternative methods for the determination of the precision of a standard measurement method[END_REF]. When the reference value is defined, parameters a, b and B are adjusted with least squares method.

Check-standard IFI "two-steps approach"

The process embodies the use of a pair of macro-texture and friction measurement equipment that can be used both in the laboratory and on field and have good reproducibility and high precision.

It also uses a set of small highly reproducible manufactured standard surfaces for dynamic laboratory calibration. Thus, the calibration and harmonization of any high-speed continuous friction measurement equipment or locked wheel tester can be accomplished.

The implementation of check-standard IFI method requires the following steps:

 Calibration of DFT and CTM according to manufacturer's specifications: static calibration of each individual part of the devices and dynamical tests on two laboratory reference surfaces manufactured from special aluminium alloy and highly wear resistant ceramic (Figure 6).

 Use the calibrated DFT and CTM devices on the test sections to establish the "check standard" friction and macro-texture values for each of the test surfaces.

The F60 and Sp values of each surface were thus calculated using:

RESULTS

Application of common scale (SRI)

The database was split into two sub-bases, one for longitudinal friction devices and the other for transversal friction devices. Each sub-database was analyzed by using 60 km/h as a reference speed.

The set of parameters (a, b, B) was calculated for each operating device.

Figures 7 and8 present examples of results for both types of devices with raw data and calibrated data when the SRI approach is used. The SRI approach effectively reduces the distribution range of friction values. The same trend was observed for all the speeds and both groups of devices. The repeatability and reproducibility of the devices were also estimated before and after SRI implementation. Regarding longitudinal friction devices (Table 5), the standard deviation of repeatability of raw data was 0.020, is considered very good considering the variety of devices included in the analysis and the SRI correction did not improve this. By contrast, standard deviation of reproducibility was reduced by a factor of two (0.039 vs 0.078). This level is comparable with results obtained within a family of the same devices (between 0.030 and 0.040 obtained during round robin tests), which demonstrates the benefits of the SRI approach when applied to a group of different devices.

Repeatability

Reproducability

For side-force friction devices, both repeatability and reproducability were reduced to 50%

of the original values by applying the full SRI correction (Table 6). The results are better than the one obtained in ROSANNE project [START_REF] Greene | Deliverable 1.3: Analysis of data from the second round of tests and further development of the common scale, Collaborative Project FP7-ROlling resistance[END_REF].

TABLE 7 Disagreement of Measurement Devices after SRI implementation

The SRI approach was completed by calculating the error between the harmonized values of friction and the reference values for all the devices (Table 7). Very good agreement between the SRI harmonized values of all the side-force devices can be observed. Considering longitudinal friction devices, the agreement remains reasonable except on painted surfaces, which exhibited some in-homogeneities.

Additionaly, effect of temperature was explored. Friction measurements were correted by using linear temperature correction laws. It appears that no significant improvement was observed since the range of air and water temperatures in this study was small. Thus, the friction coefficient variation was inferior to the repeatability of the devices.

Application of the two steps approach (Check-standard IFI)

As observed on Figures 7 and8, raw data are widely scattered for the various surfaces. As this two-steps approach consider DFT and CTM values as references, all the participating devices (i.e.

longitudinal and transversal friction devices) were calibrated with an identical procedure, as described in the section "Methodology". Figure 9 presents results obtained for all the devices after harmonization process. The data scatter among different measurement equipment was reduced and the agreement between the different devices and the established check standard had been moved to a very good agreement with the line of equality.

FIGURE 9 Harmonized Friction Measurement of All Devices by IFI approach

This approach was completed by calculating the error between the harmonized values of friction and the reference for all the devices (Table 8). Good agreement between harmonized friction values on the various test sections is observed. Moreover, the results are comparable to the ones obtained with the "common scale (SRI)" approach.

TABLE 8 Disagreement of Measurement

  the round robin tests were performed by all the participating devices. Each device provided five valid measurements on ten different pavement surfaces at three speeds (40, 60 and 80 km/h). A measurement was considered as valid when it was realized in the 0.50 m centre strip of the test lane. An orange mark at the beginning of each test track indicated the valid measurement strip. Within each test track white points every 20m indicated the valid measurement strip. In parallel, continuous macrotexture measurements were performed with laser profilometers in order to provide MPD (Mean Profile Depth) values on the same test tracks according to ISO EN 13473-1.

FIGURE 4

 4 FIGURE 4 Measured speeds

F

  is the measured skid resistance value S is the vehicle operating speed (in km/h) SRef is the reference speed at which SRI values are reported (in km/h) S0 represents speed gradient of skid resistance values related to the surface texture MPD is the Mean Profile Depth (in mm) To determine the device-specific calibration factors (a, b and B), the value of the common scale for each test surface must be fixed. Based on former research work, the reference value is set to be the average value of friction coefficients measured on a given surface by a group of devices operating on the same measuring principle (i.e. 10 devices for longitudinal friction and 5 devices

FIGURE 6

 6 FIGURE 6 Small-scale laboratory standard friction surfaces

FIGURE 7

 7 FIGURE 7 Raw data measured at 60 km/h and SRI correction (Ref. speed = 60 km/h) for longitudinal friction measuring devices (22)

TABLE 1 Pavement surfaces characteristics on test tracks Surface Pavement mix Grains size MPD (mm) DFT20

 1

	A	Porous asphalt concrete	0/6	2.90	0.29
	C1	Asphalt concrete	0/10	0.35	0.61
	E1	Semi -coarse asphalt concrete	0/10	0.66	0.34
	E3	Stone Mastic Asphalt	0/10	1.19	0.40
	L2	Sand asphalt	0/4	0.50	0.38
	M2	Very thin asphalt concrete	0/6	1.10	0.30
	G0	Asphalt concrete	-	0.83	0.34
	G1	Painted surface (low microtexture)	-	0.60	0.25
	G2	Painted surface with glass balls	-	0.59	0.26
	G22	Painted surface with glass balls	-	0.67	0.35

TABLE 4 Checking results for the water film thickness

 4

	1

/h FIGURE 8 Raw data measured at 40 km/h and SRI correction (Ref. speed = 60 km/h) for transversal friction measuring devices (22)

	1,20
	data -40km
	Original
	Porous Asphalt	Asphalt concrete	New Semi-coarse	Stone Mastic	Asphalt Concrete Painted Asphalt	Painted Asphalt	Painted Asphalt	Sand Asphalt	Very Thin Asphalt
		0/10	Asphalt Concrete	Asphalt	Concrete (no	Concrete (low-	Concrete (lhigh-	Concrete	Concrete
					inclusion)	volume glass	volume glass
						beads)	beads)

TABLE 5 Repeatability and reproducability for SRI correction (longitudinal friction devices) TABLE 6 Repeatability and reproducability for SRI correction (transversal friction devices)

 5

			1,20
	(using 60km/h as	speed) -40km/h	0,70 0,80 0,90 1,00 1,10
	Full SRI correction	reference vehicle	0,20 0,30 0,40 0,50 0,60
			0,10
			0,00
			Porous Asphalt Asphalt concrete	New Semi-coarse	Stone Mastic	Asphalt Concrete Painted Asphalt	Painted Asphalt	Painted Asphalt	Sand Asphalt	Very Thin Asphalt
			0/10	Asphalt Concrete	Asphalt	Concrete (no	Concrete (low-	Concrete (lhigh-	Concrete	Concrete
						inclusion)	volume glass	volume glass
							beads)	beads)

The authors confirm contribution to the paper as follows: study conception and design: V. Cerezo, Z. Rado, M. Kane; data collection: V. Cerezo, Z. Rado; analysis and interpretation of results: V. Cerezo, Z. Rado; draft manuscript preparation: V. Cerezo, Z. Rado, M. Kane. All authors reviewed the results and approved the final version of the manuscript.

CONCLUSION

The friction workshop was aimed at testing and improving the harmonization processes for devices measuring longitudinal and sideways-force friction coefficients. During the workshop, both the SRI common scale developed in Europe and the two-steps harmonization process (IFI) used in the United States were implemented, to check the robustness and applicability of these methodologies.

Calibration is always of paramount importance for friction measuring devices since friction strongly depends on operating conditions (load, slip ratio, speed, pressure), on surface characteristics (micro and macrotexture) and on environmental conditions (water film thickness, cleanness of the surface). That is the reason why the trials were preceded by checking/calibration of the devices based on the experience gained during ROSANNE project and Penn State Friction workshops. This checking exercise lead to the conclusion that the devices were fulfilling the existing European and international standards requirements or applicable national requirements.

The round robin test analysis demonstrated that both harmonization procedures led to an improvement of the results and to a good agreement of the friction values measured by very different devices.

The SRI approach in fact achieved a standard deviation value of repeatability around 0.0017 for side-force devices and 0.0020 for longitudinal friction devices, which proved to be very satisfaying. The standard deviation values of reproducability calculated with SRI approach were 0.0024 for side-force devices and 0.0039 for longitudinal devices. These results were even better than the ones obtained during the ROSANNE project's trials.

ACKNOWLEDGEMENT

The authors want to thanks members of Laboratory EASE at IFSTTAR who helped in the organization and the realization of the 1 st European Pavement Friction Workshop.

AUTHOR CONTRIBUTION STATEMENT


Ioana Manolescu
email: ioana.manolescu@inria.fr

Mirjana Mazuran
email: mirjana.mazuran@inria.fr

Speeding up RDF aggregate discovery through sampling

 pioneered this approach, however its is quite inefficient, in particular due to the need to evaluate numerous, expensive aggregation queries. In this work, we describe Dagger + , which builds upon Dagger and leverages sampling to speed up the evaluation of potentially interesting aggregates. We show that Dagger + achieves very significant execution time reductions, while reaching results very close to those of the original, less efficient system.

INTRODUCTION

RDF graphs are oftentimes large and complex; first-time users have a hard time to understand them. Exploration methods investigated in the past are based on keyword search, or on RDF summaries, which give users a first idea of a graph's content and structure. A different method of exploring RDF graphs was introduced in Dagger [START_REF] Diao | Dagger: Digging for Interesting Aggregates in RDF Graphs[END_REF], based on aggregation. Starting from an RDF graph, a set of agregate queries are automatically identified and evaluated, the most interesting ones (in a sense to be outlined shortly below) are chosen, and shown to human users as two-dimensional plots, in particular, bar charts. Figure 1 shows two examples. Above, we see the distribution of the number of ingredients appearing in food recipes in an RDF version of the foodista.com Web site, going from 1 to 25 with a peak at 8; we consider this distribution interesting as it has a clearly identified peak and quite a narrow interval. The graph below shows the distribution of Nobel prizes between female (left) and male (right) recipients; we find this interesting as it is very unbalanced.

As these examples illustrate, in Dagger + , we consider an aggregate query interesting if its result set has a high variance (second statistical moment). Other criteria can be considered, e.g., we have experimented also with the third statistical moment (skewness); more generally, while an RDF graph may have many interesting features, we seek to find aggregates over the graph having the highest variance. The running time of Dagger was quite high: it took hours to identify the 10 most interesting aggregates on a dataset of 20 million triples. In this paper, we show how to speed it up, in particular through novel sampling techniques.

The remainder of this article is organized as follows. Section 2 introduces the state of the art. Section 3 describes the main concepts and algorithms of Dagger [START_REF] Diao | Dagger: Digging for Interesting Aggregates in RDF Graphs[END_REF], together with a set of improvements we brought through re-engineering. Then, Section 4 presents our performance-enhancing sampling technique.

STATE OF THE ART

The problem of data exploration [START_REF] Idreos | Overview of Data Exploration Techniques[END_REF] has received much attention; the automatic extraction of interesting aggregates is just one among many proposed techniques. Multidimensional data is particularly interesting in this respect, however, most works assume a fixed relational schema, which is not available for RDF graphs. More recent works [START_REF] Bleco | Using entropy metrics for pruning very large graph cubes[END_REF], consider graphs, but (unlike Dagger) assume a very regular and simple structure.

In [START_REF] Tang | Extracting Top-K Insights from Multi-dimensional Data[END_REF] the authors show how to automatically extract the top-k insights from multi-dimensional relational data, with fixed dimensions and measures. An insight is an observation derived from aggregation in multiple steps; it is considered interesting when it is remarkably different from others, or it exhibits a rising or falling trend. SeeDB [START_REF] Vartak | SEEDB: Efficient Data-Driven Visualization Recommendations to Support Visual Analytics[END_REF] recommends visualizations in high-dimensional relational data by means of a phased execution framework. The focus is to detect the visualizations with a large deviation with respect to a reference (e.g. another dataset, historical data or the rest of the data) from a database with a snowflake schema. In [START_REF] Fagin | Multi-structural databases[END_REF], multi-structural databases are proposed; their schema (i.e. possible dimensions) is known and three analytical operations are defined to: (i) determine how data is distributed across a particular set of dimensions, (ii) compare two sets of data with respect to given dimensions and (iii) separate the data into cohesive groups with respect to the known dimensions.

In contrast, Dagger (and Dagger + ) start directly from RDF, and, lacking schema information, automatically derives dimensions and measures that are good candidates to produce insights.

There is no universally accepted definition of interestingness; frequently, something unexpected (which differs from a reference) is considered interesting. The reference might be known a-priori, come from historical data, or be the average behavior. So far we have experimented with variance, skewness and kurtosis; we are also working to use the entropy. Many different RDF visualizations techniques can be used, e.g., [START_REF] Benedetti | LODeX: A Tool for Visual Querying Linked Open Data[END_REF].

DAGGER OVERVIEW

We consider three pairwise disjoint sets: the set of URIs U, the set of literals L, and the set of blank nodes B. An RDF graph G is a finite set of triples of the form (s, p, o), called subject, property and object, such that s ∈ (U ∪ B), p ∈ U and o ∈ (U ∪ B ∪ L). The RDF property rdf:type is used to attach types (i.e. classes) to an RDF resource, which can have zero, one or several types. G can have an ontology stating relationships between its classes and properties, e.g., any UndegraduateStudent is a Student; its presence may lead to implicit triples, which are part of G even if they may not appear explicitly. A graph containing all the triples which may be derived from it is said to be saturated. Without loss of generality, we consider that our input graph is saturated.

Dagger identifies unidimensional aggregate queries to be evaluated in an RDF graph G. However, unlike a traditional relational data warehouse, an RDF graph comes without identified set of facts; nor are dimensions and measures known in advance. Therefore, Dagger must enumerate candidates for each of these roles.

A set of candidate facts (c f , in short) is a set of G resources which are deemed interesting for the analysis. Dagger considers as c f (i) all the resources of a given class C, e.g., all the Students; (ii) all the resources having a certain set P of properties, e.g., all those having title and author. A simple way to pick such a property set is to compute the support of the properties in G and select property sets whose support is above a certain threshold.

A candidate dimension (denoted d) is used for grouping the candidate facts. Dagger supports as candidate dimensions (i) properties present on at least a certain fraction t t hr es of resources from c f ; (ii) derived properties, computed by Dagger in order to find potentially interesting insights. The derived properties supported in [START_REF] Diao | Dagger: Digging for Interesting Aggregates in RDF Graphs[END_REF] are count(p), where p is a property that some c f resources have. For instance, if resources of type Student are stated to takeCourse, Dagger derives the property takeCourse#. Further, Dagger will consider a candidate dimension only the number of distinct values of this property on the c f resources is smaller than t dist × |c f | for a certain ratio 0 < t dist < 1.

A candidate measure (denoted m) is something to be evaluated or measured for each candidate fact. Dagger considers candidate measures among the (original or derived) properties of candidate facts whose support is above t t hr esh . Dimension-measure combinations such that one is a property a and the other is the count a# of this property are excluded.

An aggregation function ⊕ is chosen among min, max, avg, sum, count; the first four are considered only if the measure is numeric. Given that RDF data is often untyped or only partially typed, Dagger implements a type detection mechanism by trying to convert the property values to different types.

A Dagger aggregate aдд is a tuple (c f , d, m, ⊕). To specify how interesting an aggregate is, let f (V ) be a function which inputs a set of numerical values V and returns a number. Given f , the interestingness of aдд is computed as:

• let d 1 , d 2 , . . . be the distinct values that d may take for a resource in c f ; • for each d i , let c f i be set of c f resources for which d takes the value d i ; observe that the c f i sets may overlap, as a resource with more than one value for d belongs to several such sets. For instance, students can be grouped by the courses they take, and each student takes many courses; • for each c f i , let M i be the set of m values of c f i resources, and

m i = ⊕(M i ); • the interestingness of aдд is f ({m 1 , m 2 , . . .}).
The problem considered by Dagger is: given a graph G, a set of value thresholds, function f and an integer k, find the k most interesting aggregates. Architecture. Dagger leverages the robust query evaluation capabilities of an RDBMS to store the RDF graph, enumerate and evaluate candidate aggregates. SQL queries are used to: determine the resources part of a candidate fact sets; evaluate the suitability of their properties as dimensions, respectively, measures; compute and aggregate the group measures m i . The remaining operations, e.g., the computation of the interestingness score function, are done in Java. Aggregate recommendation cost. The most expensive computation steps are: (i) finding the candidate dimensions and (ii) evaluating the aggregation operations. Indeed, when looking for candidate dimensions, several queries are issued over c f (e.g., find all distinct properties, count the number of subject that have each property, find the values of the derived properties, etc.). Moreover, many candidate aggregates are generated, also leading to a high number of potentially expensive SQL queries.

DAGGER + : SPEEDING UP DAGGER

A set of re-engineering changes were brought to Dagger since the original demonstration. In particular, it has been ported on top of OntoSQL (https://ontosql.inria.fr), a Java-based platform developed at Inria, providing efficient RDF storage, saturation, and query processing algorithms [START_REF] Bursztyn | Optimizing Reformulationbased Query Answering in RDF[END_REF][START_REF] Bursztyn | Teaching an RDBMS about ontological constraints[END_REF]. OntoSQL encodes spaceconsuming URIs and literals into compact integers, together with a dictionary table which allows going from one to the other. For a given class c, all triples of the form x type c are stored in a single-column table t c holding the codes of the subjects x; for each property p other than type, a table t p stores (s code, o code) pairs for each (s, p, o) triple in G. This re-engineering has lead to a very significant reduction in Dagger's running time, e.g., on a 20 million triples graph, from several hours to 20 minutes.

Further, we adapted an optimization previously proposed in [START_REF] Vartak | SEEDB: Efficient Data-Driven Visualization Recommendations to Support Visual Analytics[END_REF]: we evaluate all candidate aggregates that share both dimension and measure by a single SQL query. In a relational context, aggregates can be evaluated together as soon as they have the same dimension (even if the measures are different) because one relational tuple usually contains all the attributes used in the different measures. In contrast, RDF candidate facts need to be joined with the t d table corresponding to the property d chosen as dimension, and with the t m table for the property chosen as measure; distinct measures require distinct joins, thus sharing is more limited. This is not due to the storage model, but to RDF heterogeneity: the candidate facts having the measure property m 1 may be different from those having property m 2 . This forces evaluating (d, m 1 ) aggregates separately from (d, m 2 ) ones.

Below, we focus on two novel optimization techniques we applied subsequently: using an RDF graph summary to speed up candidate dimension enumeration (Section 4.1), and using sampling to accelerate candidate dimension enumeration, aggregate evaluation and ranking (Section 4.2).

Summary-based dimension enumeration

RDFQuotient [START_REF] Goasdoué | Incremental structural summarization of RDF graphs (demo)[END_REF] is a structural RDF graph summarization tool based on graph quotients. Given an equivalence relation over graph nodes, the summary contains one node for each equivalence class in the original graph. Moreover, each edge n a -→ m in the original graph leads to an edge rep(n) a -→ rep(m) in the summary, where rep(n) is the summary representative of node n. A particular equivalence relation groups the nodes by their set of types. Dagger + uses it to find: (i) all the types, (ii) for each type, the number of resources of that type, and (iii) the set of possible properties these resources might have. These questions can be answered exactly directly from the RDFQuotient reducing dimension enumeration time.

Sampling-based aggregate selection

We introduced two sampling strategies to trade some accuracy in aggregate selection for running time:

• CFSampling: the c f is sampled (draw n 1 samples of size n 2 ), and candidate dimensions and measures are found for each sample independently. For each of the n 1 samples, candidate aggregates are generated, and their interestingness is evaluated on the sample. • ESampling: candidate dimensions and measures are computed on the whole c f as in Dagger. Then, n 1 samples of size n 2 are drawn from the c f , and the candidate aggregates are evaluated on the samples.

With CFSampling, the aggregates recommended for different sample may be different, e.g., certain properties might be found to be frequent in some sample but not in all of them. After the evaluation, a ranked list of aggregates is obtained for each sample. In ESampling, instead, the set of aggregates is unique as it is derived directly from the original data. However, given that all the aggregates are evaluated on samples, it also yields a ranked list of aggregates for each sample.

To be able to compare the results found without sampling with those found through sampling, we need to reconcile the results found on different samples into a single ranked list. We can do this by taking (i) the union or (ii) the intersection of the lists obtained from all the samples. Then, we re-rank the aggregates according to a estimation of their global interestingness measure (based on their interestingness on each sample), e.g., through pooled variance; at the end of this process, we obtain a unique ranked list of candidate aggregates.

EXPERIMENTAL RESULTS

We measure the run time performances and to test the accuracy of the results obtained with sampling techniques. We used a 2,7 GHz Intel Core i7 with 16 GB of RAM. We describe experiments on the articles from the DBLP dataset 1 ; we use 20.132.491 triples describing information about 888.183 distinct articles. Dagger + without sampling. First, we evaluate how much the use of the summary, and the shared evaluation, improve performance. Figure 2 shows the two main components (finding dimensions and aggregate evaluation), as well as a third, very small bar representing the other operations. Note that the property analysis which allows to select candidate dimensions also enables to select candidate measures (thus candidate measure selection time is negligible and wrapped under "Other").

Without any optimization ("NoSummary, NoShared" bar in Figure 2), 21% of the time is spent looking for dimensions, while 78% of the time goes to evaluating aggregates. Summary use introduces a modest performance improvement, while shared aggregate evaluation has a much more significant result (see the "NoSummary, Shared" and "Summary, Shared" bars). Here, 1 http://www.rdfhdt.org/datasets/ Dagger + generates a total of 371 candidate aggregates; without shared evaluation, each of them is evaluated as one query, however, with shared evaluation, only 131 queries are executed as this is the number of distinct pairs of dimension and measure.

Sampling. We measure the running time and the accuracy of the results by varying 3 parameters: (i) the sampling strategy (CFSampling or ESampling), (ii) the number of samples (2, 3, 4 or 5); and (iii) the sample size, as a ratio of the candidate fact set size (from 1% to 10% of the number of distinct CF resources). Figure 3 shows the running times: one plot for each number of samples (from top to bottom, 2, 3, 4, 5), each of which varies the strategy and the sample size. Ten runs are averaged for each parameter combinations (different samples are drawn in each run).

Figure 3 shows that ESampling is slower than CFSampling, as the former looks for candidate dimensions on the whole candidate fact set, whereas the latter does this on smaller-size samples. ESampling enumerates all the candidate aggregates also found without sampling, while CFSampling might enumerate a different set of aggregates: some properties that are overall frequent (infrequent) might be found infrequent (frequent) in a specific sample, and thus not be considered (considered) as candidate dimensions and measures. However, even though ESampling enumerates all the no-sampling candidate aggregates, it evaluates them on samples, and may end up considering them interesting (uninteresting) differently from the way they are on the complete CF.

To evaluate the accuracy of the results obtained through sampling, we compare the Top-5, Top-10 and Top-20 aggregates found without sampling, with those found through sampling. The ranked list of aggregates found with sampling is built by taking the union of all those found across the samples and reranking them according to the interestingness measure (in our experiments we use the pooled variance to rank all the results). Aggregates whose interestingness is zero are not considered. Notice that we do not break ties, that is: if we search e.g. the Top-5 most interesting aggregates, but find that the sixth element in the list has the same interestingness value as the fifth, we also include it in the Top-5, as we did not find a meaningful way to break such ties. When no sampling is used, Top-5 returned 5 aggregates, respectively, Top-10 returned 11 while the Top-20 returned 20. We compute the accuracy as the percentage of aggregates in the sampling result, that are also in the result without sampling. Table 1 shows the precision of CFSampling while Table 2 shows that of ESampling. In 57% of the cases the accuracy obtained using ESampling is higher; on average, the two accuracy values differ by 6%. The accuracy is quite low when searching for the the Top-5 aggregates; in contrast, both Top-10 and Top-20 can be well approximated (accuracy above 80% for the Top-20 even with few samples). In general, the bigger the samples, the better the accuracy, however, results show situations where the Top-10 and Top-20 have better accuracy with a lower number of samples; there is a 10% difference in accuracy on average, i.e. the top-Ks differ by 2/3 aggregates. This does not mean that such aggregates were not found though but they have been ranked differently. Clearly, the accuracy increases with K (the amount of top aggregates) and with the size of the samples. The increase in the number of samples does not significantly improve accuracy.

CONCLUSION

Automatic selection of interesting aggregates in an RDF graph is challenging, as candidate dimensions and measures must be "guessed" from the data, and candidate aggregates must be evaluated to assess their interestingness. After re-engineering Dagger [START_REF] Diao | Dagger: Digging for Interesting Aggregates in RDF Graphs[END_REF] to exploit an existing efficient RDF platform, we have shown that sharing work and (to a lesser extent) using a graph summary can reduce its running time by a factor of 2. Our main focus has been on sampling, which, for Top-10 and Top-20 search, achieves good accuracy (above 70%) while reducing running time by another factor of 2. Overall, CFSampling appears the most interesting strategy. Our current work stretches in several directions: (i) generalizing Dagger to more complex dimension and measure selection, (ii) adopt more interestingness metrics, (iii) introduce new types of derived properties, e.g., extract meaningful keywords from textual properties to be used as potential dimensions and/or measures.

Figure 1 :

 1 Figure 1: Sample interesting aggregates.

Figure 2 :

 2 Figure 2: Impact of summary and sharing.

Figure 3 :

 3 Figure 3: Running times with sampling.

Figure 4 :

 4 Figure 4: Accuracy of results (%) with respect to running times (sec).

Figure 4

 4 plots the accuracy with respect to the running times. Each line of graphs represents the Top-5, Top-10 and Top-20 for a different number of samples; red x's indicate CFsampling, while green +'s indicate ESampling, for different sample dimensions.

Table 1 :

 1 Accuracy of results with CFSampling.

	#samples Top-K 1%	3%	5%	7%	9% 10%
		Top5	40% 36% 24% 32% 28% 32%
	2	Top10 49% 55% 63% 57% 63% 71%
		Top20 65% 69% 81% 84% 89% 93%
		Top5	28% 28% 32% 24% 16% 28%
	3	Top10 45% 68% 71% 60% 59% 72%
		Top20 67% 76% 75% 83% 83% 87%
		Top5	36% 16% 12% 24% 24% 20%
	4	Top10 49% 55% 27% 41% 60% 53%
		Top20 70% 71% 66% 73% 87% 84%
		Top5	36% 20% 12% 24% 20% 24%
	5	Top10 50% 49% 38% 45% 52% 60%
		Top20 68% 63% 68% 77% 80% 88%

Table 2 :

 2 Accuracy of results with ESampling.

* M. Mazuran is supported by the H2020 research program under grant nr. 800192.


Surface Diffusivity. The calculation of the ratio Dk/De using the experimental values allows an estimation of the contribution of Knudsen diffusivity in the internal transport. These values are included in Table 4 and show that the Knudsen diffusivity has only a slight importance and that intraparticle transport is mainly concerned by surface diffusion. Indeed the mean pore radius of zeolite is very small, close to the molecule size, and involves transport of compounds in the adsorbate state. Surface diffusivity (Ds) was calculated by means of eq 9 with the assumption that surface tortuosity and pore tortuosity were the same and equal to 4 [START_REF] Post | Proceedings of the 6th International Zeolite Conference[END_REF]. The values are of the same order as the surface diffusivity obtained for linear hydrocarbon on activated carbon (9) and NaX zeolite [START_REF] Satterfield | Heterogeneous Catalysis in Practice[END_REF]. N number of increments q moles of adsorbate adsorbed per unit mass of adsorbent (mol kg -1 ) q max maximum adsorbed phase concentration (mol kg -1 ) q s concentration adsorbed on the surface of adsorbent (mol kg

C

  gas concentration (mol m -3 ) C s gas concentration at the surface of the pellet (mol m -3 ) C e gas concentration at equilibrium (mol m -3 ) C 0 initial concentration (mol m -3 ) D fluid flow (m 3 h -1 ) D c micropore diffusivity (m 2 s -1 ) D col column diameter (m) D e effective diffusivity (m 2 s -1 ) D e,av average effective diffusivity (m 2 s -1 ) mass-transfer coefficient (m s -1 ) k p intrapellet mass-transfer coefficient (s -1 ) L′ characteristic length (m) M molecular weight (g mol -1 )


Alina-Cristina Bunea

Dan Neculoiu
email: dan.neculoiu@imt.ro

Alexandru Takacs
email: atakacs@laas.fr

Pierre Calmon
email: calmon@laas.fr

Design and Characterization of a Micromachined Receiver for W Band Applications

Keywords: direct detection receivers, slot antennas, W band, zero bias detectors. I

This paper presents for the first time the design, electromagnetic modeling and simulation of a W-band micromachined receiver with a double folded slot antenna. The antenna was processed on a thin dielectric membrane obtained through wet micromachining of a 400 µm thick, high-resistivity silicon substrate. A stand-alone antenna structure was fabricated and characterized in terms of reflection losses, showing a working band (defined for |S11|< -10 dB) between 88.3 -105.8 GHz, in good agreement with simulations. The maximum simulated directivity at 94 GHz is 6.3 dBi. The direct conversion receiver uses a hybrid integrated GaAs zero bias detector diode. Two new parameters based on the incident isotropic power are introduced: isotropic voltage sensitivity and isotropic tangential signal sensitivity. The measured isotropic tangential sensitivity is lower than -40 dBm between 80 -105 GHz, for a 10 kHz video bandwidth.

INTRODUCTION

The W-band is becoming the range of choice for numerous applications, from high-data rate communications to automotive radar systems and most of all, passive/active millimeter wave imaging (MMWI) applications. But most solutions make use of bulky horn antennas and complicated waveguide transitions in order to collect the electromagnetic radiation from the target scene. A more cost-efficient approach consists in monolithically or hybrid integrating an on-chip antenna with a detector diode and possibly a low-noise amplifier. This (quasi-)planar approach can reduce the losses, reduce the overall receiver size and simplify the packaging.

A receiver with two slots, connected by two microstrip sections and hybrid integrated with a Schottky diode was first described by Kerr et al. [START_REF] Kerr | A Simple Quasi-Optical Mixer for 100-120 GHz[END_REF] in the late '70s. The approach was considered innovative at the time and tried to solve some of the issues related to quasi-optical mixers, like the energy transfer from the local oscillator to the diode, which required complicated waveguide couplers. The new approach generated the required coupling through the two lobes of the antenna.

Based on the idea of placing a detector between two radiating slots, researchers strived, in the '90s, to fabricate highly compact and performant millimeter wave receivers [START_REF] Filipovic | Double-Slot Antennas on Extended Hemispherical and Elliptical Silicon Dielectric Lenses[END_REF]- [START_REF] Mollenkopf | A low-cost 20-22 GHz MIC active receiver/radiometer[END_REF].

One of the first papers describing a double folded slot antenna radiometer is [START_REF] Mollenkopf | A low-cost 20-22 GHz MIC active receiver/radiometer[END_REF]. The receiver was fabricated on a GaAs substrate and hybrid integrated with a Schottky diode and a low-noise amplifier (LNA). The working bandwidth was 20 -22 GHz. Some of the main issues in obtaining a fully integrated on-chip front end in modern SiGe BiCMOS technologies, are related to the low performances and large sizes of antennas [START_REF] Cheema | The Last Barrier: On-Chip Antennas[END_REF]. This is mainly due to the lossy substrates. One possible solution to this problem consists of removing the lossy silicon from under the radiating structure. Such an approach was presented in [START_REF] Seyyed-Esfahlan | SiGe Process Integrated On-Chip Dipole Antenna on Finite-Size Ground Plane[END_REF], for a dipole antenna working in the 83 -91 GHz frequency range, with a maximum gain of 4 dBi at 85 GHz.

In this paper we present a novel topology of a W band hybrid integrated receiver consisting of a membrane supported double folded slot antenna, followed by a GaAs beam lead detector diode. First, a standalone antenna test structure was designed, fabricated and characterized in terms of reflection losses and radiation behavior. The next step consisted of integrating the antenna with a matching network, a set of diode pads and a low-pass filter. The diode was hybrid integrated using conductive epoxy. The front-end was then characterized in terms of isotropic voltage sensitivity and isotropic tangential signal sensitivity (TSS).

II. MICROMACHINED ANTENNA

A fully parameterized 3D model was developed using the full-wave finite-difference time-domain electromagnetic simulation tool CST Microwave Studio (MWS), Fig. 1. The antenna structure was designed for anisotropic wet micromachining of a 400 µm thick high resistivity (>5kΩ•cm) <100> silicon wafer. This results in a sloped transition (at an angle of 54.7°) from the released membrane to the silicon bulk (see Fig. 1), which was included in the 3D model.

The membrane consists of a 0.8 µm silicon oxide layer, obtained through thermal oxidation, followed by a 0.6 µm silicon nitride layer, obtained through chemical vapor deposition. The metallization consists of a 2 µm thick gold layer, obtained through electrochemical deposition.

The membrane supported antenna consists of two folded slots, connected by a coplanar waveguide (CPW) transmission line of half free-space wavelength (λ 0 /2 ≈ 1595 µm at 94 GHz). A shorted stub of length Ls (close to λ 0 /4) is added to help tune the matching characteristic and symmetrize the layout. Extensive parametric studies with the main objective of determining the best trade-off between matching and radiation behavior were run in order to determine the layout parameters. The final geometrical sizes are as follows: Lg=2200 µm, Ld=1550 µm, Lc=1370 µm, Ls=710 µm, Lf=1320 µm, Lt=282.8 µm (Lt is computed for the transition angle of 54.7° and substrate thickness hSi=400 µm). Fig. 2a shows the electric field distribution in the slots at 94 GHz. Since the E-field is in phase in both folded slots, the energy is radiated normal to the antenna plane. Because the antenna is suspended on a very thin dielectric membrane, it operates as if suspended in air and has an almost symmetrical radiation characteristic, with two main lobes (Fig. 2b). The maximum simulated directivity at 94 GHz is 6.3 dBi, with a 3dB beam-width of 58.6° in the E plane (parallel to the feed line) and 80° in the H plane.

The 3D electromagnetic model was validated through on wafer measurements. The antenna was placed on a PCB, over an opening, in order to ensure free space conditions for the second lobe. Reflection loss measurements were performed with a Vector Network Analyzer (Anritsu 37397D) with OML extension modules. A comparison between the measured and simulated results is shown in Fig. 3. The measured bandwidth (|S11| < -10 dB) is between 88.3 -105.8 GHz, with |S11| = -29.7 dB at 94 GHz.

A standard gain horn antenna was connected to the second port of the VNA and suspended at a distance of 100 mm over the antenna under test. The measured transmission characteristic is presented in Fig. 4 (left axis, the maximum of the curve was set to 0 dB reference level). On the same plot, the simulated antenna gain is presented (right axis) for comparison. The trend of the two curves is similar, thus proving good radiation behavior of the antenna.

III. RECEIVER TOPOLOGY AND MEASUREMENTS

The receiver module is a direct conversion (video) receiver and a photo of the fabricated structure is presented in Fig. 5. One block is the double folded slot antenna supported on the thin dielectric membrane. The signal is then converted to a DC/low-frequency signal by the nonlinear characteristics of the detecting diode. The detecting diode is placed in series between the matching network connected to the antenna and the lowpass filter. The matching network has a stub-line configuration, with a CPW symmetrical shorted stub. The low-pass filter consists of a cascade of CPW low-impedance/high-impedance sections. This filter extracts the DC/low-frequency signal, removes high order spectral components and reflects the W band signal back to the detector diode. The overall receiver structure is designed to provide the correct DC return for the detecting diode. The output of the low-pass filter is connected to the low-frequency (video) output pad. The video signal is then picked up from the chip through bond wires and a coaxial cable. With the help of microwave simulation software AWR Design Environment and CST MWS, the circuit blocks (antenna, matching network, low-pass filter and diode) are linked together and the receiver layout is optimized.

The Aeroflex/Metelics MZBD-9161 is a GaAs beam lead detector diode and it is intended for zero bias detecting applications at frequencies up to 110 GHz. The data sheet main electrical specifications are: junction capacitance C j = 0.035 pF and video resistance at zero bias R V = 2.5 -7.5 kΩ. The diode is mounted using conductive epoxy on two rectangular pads (inset in Fig. 5). In the electromagnetic design, the device is inserted in the model using the discrete port feature of the CST MWS software. For this detecting application the diode was modeled with its small signal equivalent circuit.

The micromachined structure was mounted on a printed circuit board (PCB) above an opening that allows the quasifree space operation for the antenna. The opening was covered with W band absorber, in order to suppress the influence of the second lobe of the micromachined antenna. The experimental set-up is presented in Fig. 6.

The receiver (1 in Fig. 6) is illuminated by a horn antenna (2) connected to a millimeter (mm) -wave source through a W band attenuator Millitech LSA-10 (3). The signal source was a PSG Analog Signal Generator (Agilent E8257C) going up to 110 GHz with a S10MS-AG extension module (4). The standard horn antenna QWH-WPRR00 has a mid-band gain of 24 dBi. The transmitted signal was modulated in amplitude by a 1 kHz square wave with a modulation index of 100% and the detected waveform was amplified by a SR560 LNA (5) and displayed on a Agilent Tech. DSO1024A oscilloscope [START_REF] Seyyed-Esfahlan | SiGe Process Integrated On-Chip Dipole Antenna on Finite-Size Ground Plane[END_REF]. The PCB holder for the receiver was placed at a distance of 100 mm from the horn antenna (far field conditions for W band). Since the circuit combines the function of the antenna and a detector, the isotropic voltage sensitivity β iso parameter is used to characterize the efficiency of the receiver. β iso is defined as the ratio between the detected output voltage and the incident isotropic power (similar to the isotropic conversion loss defined in [START_REF] Hwang | Quasi-optical integrated antenna and receiver front end[END_REF]).

The incident isotropic power at the membrane supported antenna plane is calculated with (1). ,

where: λ is the free-space wavelength; R is the distance between the horn antenna and the double folded slot antenna (R = 100 mm in this experiment); G Horn is the gain of the horn antenna (24 dBi in this case) and P gen is the power level at the output of the rectangular waveguide connecting the horn antenna.

P gen was accurately measured with a W band power-meter in the W band. The detected voltage amplitude was measured on the oscilloscope and the β iso is calculated and presented in Fig. 7 (left axis). These results can be improved by ensuring a better matching of the junction resistance of the diode (which is in the range of a few kΩ) to the standard characteristic impedance of 50 Ω.

Despite of its subjective nature, tangential signal sensitivity (TSS) remains a practical technique to estimate the noise equivalent power (NEP) for a detector [START_REF] Bahl | Microwave Solid State Circuit Design[END_REF]. In the present work a new parameter, called isotropic TSS (TSS iso ), is defined, using the input power (P iso ) equation from [START_REF] Kerr | A Simple Quasi-Optical Mixer for 100-120 GHz[END_REF]. It gives precious information about the minimum detectable signal and makes the comparison between different receivers with integrated antennas possible. Using a variable W band attenuator connected between the generator and the horn antenna (Fig. 6) the emitted power level was reduced to fulfill the TSS measurement conditions and P gen was measured. The measured value for a video bandwidth of 1 Hz -10 kHz is shown in Fig. 7 (right axis). The peak value for the isotropic voltage sensitivity is around 1150 mV/mW at 78 GHz, 81 GHz and 88 GHz and can be considered state of the art, since it also includes the effect of the on-chip antenna. The measured isotropic TSS (taking into account the effect of the on-chip antenna) is below -40 dBm between 80 -105 GHz, for a 10 kHz video bandwidth.

For comparison, [START_REF] Richter | A broadband low cost millimeterwave slotline detector on low permittivity substrate[END_REF] describes a broadband millimeter wave slotline detector with a peak voltage sensitivity of 1660 mV/mW at 77 GHz, without any antenna and with a DC biased diode. A TSS around -46 dBm at 77 GHz is achieved for a video bandwidth of 1 kHz [START_REF] Richter | A broadband low cost millimeterwave slotline detector on low permittivity substrate[END_REF]. It should be noted that the difference in TSS is 5 dB for an increase of the video bandwidth from 1 kHz to 10 kHz [START_REF] Bahl | Microwave Solid State Circuit Design[END_REF].

IV. CONCLUSION

A hybrid integrated receiver consisting of a membrane supported double folded slot antenna, followed by a GaAs beam lead detector diode for W band applications was presented. The standalone antenna test structure showed a wide working band (defined for |S11|< -10 dB) between 88.3 -105.8 GHz, with |S11| = -29.7 dB at 94 GHz. With the help of a reference horn antenna, a good radiation behavior was demonstrated. The experimental results are in good agreement with the simulated reflection loss and antenna gain.

The voltage sensitivity and the tangential signal sensitivity (TSS) were redefined for systems with on chip antennas. The measured isotropic tangential sensitivity is lower than -40 dBm between 80 -105 GHz, for a 10 kHz video bandwidth. These are promising results for passive imaging applications and can be improved by increasing the antenna gain.

Fig. 1 .

 1 Fig. 1. Layout of the membrane suported double folded slot antenna

Fig. 2 .

 2 Fig. 2. (a) Electric field distribution and (b) 3D radiation characteristic at 94 GHz

Fig. 3 .Fig. 4 .

 34 Fig. 3. Measured (solid line) and simulated (dotted line) reflection losses for the antenna

Fig. 5 .

 5 Fig. 5. Photo of the hybrid integrated receiver structure with a detail of the GaAs beam lead zero bias detector diode

Fig. 6 .

 6 Fig. 6. Measurement setup for the receiver characterization (see text for details)

Fig. 7 .

 7 Fig. 7. Measured isotropic voltage sensitivity (left axis) and isotropic tangential signal sensitivity (right axis)

ACKNOWLEDGMENT

The authors wish to thank dr. Tero Kiuru from the Technical Research Center of Finland, VTT, for assisting with the sensitivity measurements.


Zhening Yang
email: zyang@laas.fr

Alexandru Takacs
email: atakacs@laas.fr

Samuel Charlot
email: scharlot@laas.fr

Daniela Dragomirescu
email: daniela@laas.fr

Design of Kapton based passive circuits at microwave frequencies

Keywords: Flexible substrate technology, Kapton RF characterization, Passive circuits

This paper presents a technology based on thin flexible polyimide substrate (Kapton) to develop passive circuits at microwave frequencies. In the first step, we characterize the Kapton polyimide (relative permittivity and loss tangent) by using a ring resonator method. Then a 60 GHz patch antenna is designed, fabricated, and measured to validate our technology. Finally, a design of cross slot dipole antenna with a large bandwidth at 60 GHz is proposed.

I. INTRODUCTION

There is an increasing demand of high efficiency antennas and passives for applications where high frequency operation, lightweight, and conforming to a curved surface are required. Flexible electronics has spanned impressively over the past years, and flexible substrate became a strong competitor versus his rigid substrate counterparts because they are typically more rugged, lighter, portable and less expensive to manufacture [START_REF] Wong | Flexible Electronics: Materials and Applications[END_REF]. The choice of flexible substrate for the applications at microwave frequencies is a challenge, because the substrate must exhibits good electrical (e.g. low losses) mechanical and chemical properties. Moreover, the substrate should keep these electrical properties over a wide range of frequencies and under external conditions such as temperature and pressure etc.

Finally, substrates used in RF devices should be able to withstand the rigors (e.g. humidity, gasing) of its operating environment and of the fabrication processes used in its packaging [START_REF] Vyas | Liquid Crystal Polymer (LCP): The ultimate solution for low-cost RF flexible electronics and antennas[END_REF].

Several flexible substrates used for printed circuit packaging are listed in TABLE I. One can see the characterizations for polyethylene teraphthalate (PET) [START_REF] Hettak | Flexible plastic-based inkjet printed CPW fed dipole antenna for 60 GHz ISM applications[END_REF], polyethylene naphthalate (PEN) [START_REF] Bisognin | Inkjet Coplanar Square Monopole on Flexible Substrate for 60-GHz Applications[END_REF], Kapton [6], liquid crystal polymers (LCP) [START_REF] Vyas | Liquid Crystal Polymer (LCP): The ultimate solution for low-cost RF flexible electronics and antennas[END_REF] and paper-based substrate [START_REF] Shaker | Inkjet Printing of Ultrawideband (UWB) Antennas on Paper-Based Substrates[END_REF]. LCP has a nearly constant relative permittivity of 3.1 and low loss (tan δ = 0.002 ~ 0.005) up to millimeter wave frequency range, nearhermetic nature (water absorption < 0.04%) makes it suitable for high frequency designs. The paper is a good candidate for its low cost, but it has limitations issues related to high frequency, absorption, and humidity. Finally, we selected Kapton as substrate for the development of our passive circuits due to its good RF and thermal properties, very good flexibility over a wide temperature range (-73 °C to +400 °C) and its resistance to many chemical solvents.

The most widespread technologies used in microfabrication are photolithography, screen printing, and inkjet printing. (Fig. 1). Inkjet printing is a direct write technology which presents a low cost advantage because the design pattern is transferred directly to the substrate and does not require masks. However, using this technology for millimeter wave applications could be a challenging task because of the highly required accuracy. Since both screen printing and inkjet printing can only offer highest resolution around 20 µm [START_REF] Singh | Inkjet printing-process and its applications[END_REF], the traditional photolithography remains most suitable method for microfabrication at this frequency range. This paper is organized as follows. The second chapter presents a description of the fabrication process we have developed, followed in the third chapter by a study of Kapton dielectric properties using a ring resonator method. Then, the design of a grounded coplanar waveguide feeding patch antenna on a 127-µm-thick Kapton in the 60 GHz band is described. Its main features in impedance and radiation are discussed. Finally, a design of cross slot dipole antenna with a large bandwidth at 60 GHz is proposed.

II. TECHNOLOGICAL PROCESS FOR FLEXIBLE SUBSTRATE

INTEGRATION

To manufacture our circuits onto a flexible substrate, we have chosen the traditional photolithography, the mainly difficulty during the fabrication process lies on the flexible film handling and its use in micro-technology equipment, in order to overcome this difficulty, a 4-inch silicon wafer is used as a host carrier. One of the critical obstacles consists of finding a way to adhere the polyimide film on the Si support. This adhesion has to be compatible with the various technological stages (vacuum, solvent and temperature) and allows after manufacture a peeling without any physical or chemical constraint.

A well matured fabrication process is used to spin coating a polymer for the adhesion of the polyimide on the holding wafer. Then the Kapton polyimide is patterned on the substrates by lamination. A resin spin coating is then realized followed by a photolithography process for the metallization layer [START_REF] Jatlaoui | 3D heterogeneous integration of wireless communicating nano-sensors on flexible substrate[END_REF].

III. RF CHARACTERIZATION OF KAPTON

To start the design of any high-frequency structure by using numerical simulation, the knowledge of dielectric properties of the substrate becomes necessary. A commercially 127-µm Kapton (type 500HN) was chosen for our substrate, the value of thickness is in fact imposed by an effective patch antenna design in V-band. The antenna is composed by a rectangular patch on the top side of the substrate and a ground plane on the back side. Thus a minimum thickness must be respected to avoid undesired capacitive effect and to ensure a high-efficient radiation mechanism. A study of influence of thickness versus maximum antenna gain (simulation HFSS [11]) is given in Fig. 2. One can see that thicker the substrate, a better antenna gain can be obtained regardless the type of metallization.

The dielectric properties of Kapton: the relative permittivity ε r and the loss tangent tan δ were extracted from S-parameters measurement of ring resonator, which is composed by a ring with mean radius 4.265 mm, the width of microstrips on the Kapton surface is 306 µm to make sure that the characteristic impedance of the microstrips is 50 Ω. There are two 80 µm gaps at the end of the ring to couple the resonator to the measurement system, which provides us sufficiently coupling to measure the resonators without loading too much the test equipment.

IV. DESIGN AND MEASEREMENT OF KAPTON BASED PASSIVE CIRCUITS

A. Patch antenna

A 60 GHz grounded coplanar waveguide (GCPW) feeding rectangular patch antenna (see Fig. 5(a)) was designed, fabricated, and measured on a flexible 127-µm-thick polyimide substrate (Kapton). The GCPW-to-microstrip transition was optimized with the help of HFSS electromagnetic (EM) software in order to minimize the impedance mismatch. The antenna was then characterized in terms of return loss, gain, and radiation pattern.

The simulated and the measured return losses of the patch antenna versus frequency are presented in Fig. 6. The agreement between experiments and simulations is quite good: a relative frequency shift of only 1.4% is observed between the simulated (HFSS) and measured results. It could be due to uncertainty in the substrate permittivity value and under/over etching of the conductive patterns. The measured bandwidth, defined by return loss less than -10 dB, is from 62.4 GHz to 64.4 GHz. The far-field radiation pattern and gain were measured with a probe based antenna measurement setup at LAAS-CNRS (Fig. 5(c)). The antenna is fed through a 150-µm Ground-Signal-Ground (GSG) probe, and this probe is directly connected to a 65 GHz VNA (Anritsu 37397D) with a flexible V-cable. Dielectric foam (Rohacell) was used on the bottom to prevent reflections from the metallic part of the setup. The antenna under test (AUT) was illumined by a field generated by a calibrated horn antenna in the 60GHz band with a gain around 25 dBi.

Under these conditions, a maximum gain of 6.31 dBi was found in the perpendicular direction of the patch antenna around 63 GHz. The simulated and measured radiation patterns in the E-and H-plan are given in Fig. 7. The E-plan radiation pattern has a restricted range due to architecture of the measurement setup. Pronounced ripples are observed in the Eplane due to reflection and diffraction effects on the metallic micro-positioner and the probe. The 3-dB beam width for the E-plan is 64 ° and 52 ° for the H-plan.

The results demonstrate the quality of fabrication on flexible polyimide substrate and the accuracy of measurement setup for microwave antennas implementation. The geometry of the proposed cross slot dipole antenna fed by a CPW is depicted in Fig. 8, the antenna is designed on a 127-μm thick Kapton with a dielectric constant 3.2 and a loss tangent 0.002. CPW feed dimensions of S = 170 μm and G = 12 μm were selected corresponding to the 50 Ω GSG probe. The slot length L is fixed to 2.5 mm (quarter wavelength for 60 GHz) for the simulation, the slot width w, the stub length d and the angle between slots α were set to 40 μm, 0.69 mm and 45 ° as default respectively. Fig. 9(a) shows the simulated return loss. The center frequency is 60.2 GHz, and the -10 dB bandwidth is from 58.2 GHz to 62.05 GHz. The antenna radiation patterns were also presented in Fig. 9(b), a peak antenna gain of 4.02 dB is obtained at 60.2 GHz. A parametric study for the angle between slots is shown in TABLE II. It is seen that when the angle α increase, the peak antenna gain decrease along with a larger half power beamwitdh (HPBW) on H-plan due to the more important coupling effect when two slots approach to each other.

V. CONCLUSION

In this work, we demonstrate the feasibility of manufacturing microwave passive circuits at high frequency with very good RF performances through a traditional photolithography on a commercially available polyimide substrate. A quite good agreement between the measurements and simulations has been obtained. Future measurement for the cross slot dipole antenna will be carry on. This work is one step further towards a heterogeneous integration on flexible substrate of different components for wireless sensor nodes.

Fig. 1 .

 1 Fig. 1. Comparison of the processing steps involved in subtractive microfabrication, screen printing and inkjet printing [9].

Fig. 2 .Fig. 3 .

 23 Fig. 2. Maximum antenna gain versus substrate thickness.

Fig. 4 .

 4 Fig. 4. Simulated and Measured Insertion Loss of the ring resonator.

Fig. 5 .

 5 Fig. 5. (a) 60GHz GCPW feeding rectangular patch antenna, (b) Demonstration of the flexibility of patch antenna on Kapton., (c) Antenna radiation measurement setup.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Simulated and Measured Return Loss of the patch antenna.

Fig. 8 .Fig. 9 .

 89 Fig. 8 . 60 GHz CPW fed cross slot dipole antenna.

TABLE I .

 I Comparative properties of state-of-art flexible substrates[START_REF]Printing Technology for Flexible Substrates[END_REF]

	Material	PET	PEN	Kapton	LCP	Paper
	Mechanical	Good	Good	Excellent	Good	-
	properties
	Heat	Low	Very good	Excellent	Good	-
	resistance
	Chemical	Good	Good	Good	Excellent	-
	resistance
	Electrical	Good	Good	Good	Good	Good
	properties

TABLE II .

 II Study of angle α effect on radiation

	α (°) fc (GHz) Return Loss (dB) Maximum Gain HPBW (°)
	40	59.7	-21.09	4.14	82
	45	60.2	-22.76	4.02	86
	50	61.15	-27.48	4.08	90


P Monneyron

S Mathe ´1

M.-H Manero

J.-N Foussard

REGENERATION OF HIGH SILICA ZEOLITES VIA ADVANCED OXIDATION PROCESSESÐA PRELIMINARY STUDY ABOUT ADSORBENT REACTIVITY TOWARD OZONE

Keywords: high silica zeolites, adsorption, regeneration, advanced oxidation processes (AOP), ozone, catalytic decomposition

A iming at regenerating adsorbents, the reactivity toward ozone of two high silica zeolites (HSZ), a dealuminated faujasite Y (Fau Y) and a silicalite (Sil Z), was investigated. In case of Fau Y, no physical adsorption occurred but a total degradation of ozone. This phenomenon was attributed to an active sites-role of silanol groups (SiOH), mainly developed by the dealumination step. In contrast, the ozone adsorption in the smaller channels of Sil Z was important (about 17 mmol g 1 ), but when a certain local concentration of ozone was reached, the ozone degradation became signi cant and increased up to 100%. The ozonated zeolites were regenerated in an oven. Whereas the regeneration was total at 773K, a treatment at 500K seemed insuf cient since a co-adsorption phenomenon was observed with suspected nitrous oxides (NO 2 ) as a product of ambient nitrogen oxidation. Whereas the Fau Y was not affected at all by ozonation, the adsorption capacities of Sil Z zeolite were enhanced, and it exhibited a higher selectivity for polar compound. This was attributed to a surface modi cation, by highly reactive species generated during molecular ozone decomposition.

INTRODUCTION

In the treatment of industrial polluted air streams, adsorption on a porous media, such as activated carbon or hydrophobic zeolites, is commonly adopted (Le Cloirec, 1998). Instead of being land lled and replaced, activated carbon is mainly regenerated via pressure or thermal desorption, aiming at restoring the adsorption activity with as little damage as possible to the original pore structure. Advanced oxidation processes (AOP) were investigated as an alternative regeneration method for activated carbon (Masuda et al., 2001), highlighting its scavenger effect toward ozone (Dusenbury and Canon, 1996), but also in case of zeolites (Leichsenring et al., 1996a;Zlotopol' sky and Smolenskaya, 1996), notably when coked in catalytic petrochemistry (Querini, 2000;Mariey et al., 1996).

In parallel, AOP are investigated as direct technologies for water or gas streams treatment (Shen and Ku, 1999) and, interestingly, adsorbents can provide a good environment for oxidation reaction (Vidic et al., 1997), as shown in previous studies of zeolite regeneration by photocatalysis (Monneyron et al., 2003a) or V-UV radiation combining the effect of photolysis and the creation of ozone from ambient oxygen (Monneyron et al., 2003b).

To use ozonation as a regeneration technique for organic compounds adsorbed on high silica zeolites (HSZ), it is essential to establish the interaction between ozone and the adsorbent. Moreover, as shown by Chiang et al. (2002) for activated carbon, any modi cation of adsorption properties of the solid is to be determined.

METHODS AND MATERIALS

Adsorbents

Two commercial HSZ from TOSOH Corp. were used in this work, a dealuminated faujasite Y (Fichtner-Schmittler et al., 1990) and a silicalite (Olson et al., 1980). The main characteristics of adsorbents are given in Table 1. Prior to experimentation, the zeolites were heated at 230 ¯C (or 500 ¯C when speci cally mentioned) overnight, and kept in a desiccator to prevent ambient humidity to interfere with experiments.

Ozonation Experimental Setup

A commercial ozone generator from air (model 5LO from Trailigaz) was used here, and since the ozone production is function of air pressure ( p ˆ0.7 bar), ow (F ˆ0.25 m 3 h ¡1 ), and power applied, P (W), the ozone inlet concentration was reproduced with an uncertainty of §0.5 g m ¡3 . The reactor, regulated in temperature at 298K with an external water ow circulating in a double glass wall, was 20 cm high and 8 cm in internal diameter. At the inlet, outlet and each 5 cm, an intermediate gaseous sample could be taken and ozone concentration determined with a BMT 964 Ozone Analyser. At these ve points, the temperature was followed at the same time, as can be seen in Figure 1. Whereas the inlet and outlet enabled the gas temperature to be followed, the inner-reactor measurements reported the solid pellets surface temperatures.

Adsorpti on Equilibria Measurements

The investigation of ozonation in uence on adsorption capacity of HSZ toward model VOCs was performed using batch experiments via a standard volumetric method described elsewhere (Monneyron et al., 2003c).

In the same way, the selectivity of adsorbent (Z 1=2 ), calculated using equation ( 1), where X and Y are respectively the adsorbed and gas phase molar fraction, was determined in competitiveadsorption experiments (Monneyron et al., 2003c).

Z 1=2 ˆX1 ¢ Y 2 X 2 ¢ Y 1 …1 †

RESULTS AND ANALYSIS Ozonated Air Flow on HSZ Fixed Bed

In this set of experiments, the inlet concentrations (C IN ) of ozone were equal to 9.5, 18.5, and 26.5 g m ¡3 respectively for P ˆ50, 100 and 150 W. The mass of adsorbent was adjusted to practical requirements. The results obtained on the activated Fau Y exhibited two different parts, the rst being an instantaneous breakthrough of ozone up to C OUT ˆ65%¢C IN , immediately followed by a decrease of the concentration, as shown in Figure 2. After reaching a minimum, the ozone concentration increased to C OUT ˆCIN . This evolution does not involved any physical adsorption phenomenon, but it has rather to be seen as an evolution of ozone decomposition on the Fau Y surface. There have been very few studies on ozone decomposition mechanisms but, as reported by Tsyganenko et al. (Mariey et al., 1997;Tsyganenko et al., 2001), the molecular ozone exhibits some Lewis basic characteristics, notably on silanol groups (SiOH), that were reported by Dhandapani and Oyama (1997) to be well-known catalytic sites for ozone decomposition in indoor air treatment patents. The supplier of the commercial Fau Y used in this study kept undisclosed the chemical nature of the clay binder together with details of dealumination technique. But whatever conditions may have been applied to obtain the high Si=Al ratio, a signicant number of SiOH groups were formed, typically 10=100 framework atoms (Meininghaus and Prins, 2000;Kawai and Tsutsumi, 1998), and are suspected to be responsible for the initiation of the decomposition complex mechanism of ozone on Fau Y.

After the regeneration step, the Fau Y had the same global behavior toward gaseous ozone, except that the decomposition was total from the very beginning of the experiment. The sampling of temperature at different points of the adsorbent bed (5, 10 and 15 cm) enabled us to explain the two different behaviors observed until 400 min. Whereas during the second use of Fau Y sample, the full decomposition of ozone was taking place almost completely at the beginning of the bed, shifting to the higher parts with time, in the rst experiment the reactions took place in the whole bed and much more slowly, i.e. with a smaller heat emission notably visible at the 5 cm-DT measurement. It is therefore suspected that the contact with ozone increased the number of decomposition active sites by modifying the surface. The ozone decomposition slowly ended, the previously mentioned sites being deactivated with time.

The observed behaviour of Sil Z toward ozonation was very dissimilar from Fau Y. As shown in Figure 3 for the (P ˆ50 W)-experiment, a classical adsorption breakthrough curve was obtained. Moreover, it is clear that physical adsorption of ozone took place on Sil Z since the breakthrough time decreased, and its slope was higher as the ozone concentration increased.

In our case, the maximum outlet concentration was dependent on the inlet concentration (C OUTmax =C IN ˆ100, 75 and 20%, respectively, for P ˆ50, 100 and 150 W), meaning that some ozone was also decomposed. Whereas for the lowest concentration, the outlet concentration was stable at (C OUT ˆCIN ), on the second part of the other curves, the ozone concentration decreased to a steady state of 100% decomposition.

The temperature pro les, shown in Figure 4, obtained during these experiments are another indicator of the two different phenomena happening consecutively. Indeed in the two experiments where ozone decomposition occurred, for P ˆ100 and 150 W, the temperature elevations were quite different from the P ˆ50 W where only adsorption occurred. Indeed, the second temperature increase of P ˆ100 and P ˆ150 W experiments did not exhibit the same behaviour, the respective slopes being 1.8 and 1:3 K m 3 g ¡1 O3 for adsorption and decomposition phases. As a quick approximation of the adsorption capacity of Sil Z toward ozone, the linear part of the breakthrough was used to estimate t 1=2 when (C OUT ˆCIN =2), directly or by extrapolation, assuming that at the beginning of the breakthrough no ozone decomposition occurred. The values, obtained for P ˆ50 and 100 W, are quite consistent with each other (16 and 17:5 mmol g ¡1 Sil Z ), whereas for P ˆ150 W, the adsorbed quantity seems overestimated (24 mmol g ¡1 Sil Z ). This is not surprising since, in the last case, the linear part of the curve is very short (C OUTmax C IN ˆ20%) so that the ozone decomposition could not be neglected. This surprising difference between the two HSZ was reported also in the aqueous phase by Sakoda et al. (2001), leading to a very different aqueous ozone enrichment factor (1:100). In the same way, Izumi et al. (2001), investigated the gaseous ozone enrichment on HSZ at low temperatures (down to 213 K) where no ozone decomposition occurred, but the difference in adsorption capacities of both zeolites was much less important. These results tend to show that the decomposition of ozone requires a minimum local concentration, the kinetic of decomposition depending notably on the ozone concentration, supplied by its physisorption in the framework channels of Sil Z. This decomposition is therefore much slower than on Fau Y, which is consistent with the hypothesis of a determining role of silanol groups as actives sites, the Sil Z having undergone no dealumination treatment. It may be supposed that in this case the decomposition was initiated by the hydroxyls groups of the clay binder (Alpay et al., 1996) or by local heterogeneity of the zeolite framework. Then, the ozone decomposition involving very reactive species (Dhandapani and Oyama, 1997), some complex radical chain reactions would have led to a slow spreading into the whole bed, down to a total decomposition of the inlet ozone.

Regeneration of Sil Z After Ozonation

As shown in Figure 5 by the two experiments conducted for P ˆ150 W after a 500 ¯C overnight heat treatment, the regeneration of the Sil Z sample was total, whereas in a (P ˆ100 W)-set of experiments, when the oven temperature was generally xed to 230 ¯C only, the regeneration was insuf cient as an initial partial breakthrough of ozone occurred. However, in both cases, the ozone global physical adsorption capacity decreased, indicating a modi cation of zeolite internal surface. Considering these results and as mentioned in the introduction of this paper, it was absolutely necessary to check the adsorption properties of the oxidised zeolites.

Adsorpti on Properti es of Ozonated HSZ

Adsorption equilibria measurements showed that the contact of ozonated air with Fau Y (3000 min) had no effect at all on its adsorption capacities in single component isotherms, and on its selectivity toward VOC binary mixtures. On the contrary, the Sil Z zeolite was signi cantly affected by the ozone treatment (9000 min), as shown by Figures 6 and7.

Indeed, the adsorption capacity of Sil Z toward both components increased after ozonation, which is to be linked with the hypothesis of a surface modi cation. This augmentation was much greater and earlier, in terms of minimum concentration, for TOL ( ‡25%, from Ce ˆ0.001 mmol l ¡1 ) than for DIO ( ‡8%, from Ce ˆ0.007 mmol l ¡1 ). This could be attributed to the difference of their micropore llings (52 and 87%, respectively, for TOL and DIO; Monneyron

Figure 2 .

 2 Figure 2. Ozonated air ow on Fau Y (400 g-20 cm).

Figure 3 .

 3 Figure 3. Ozonated air ow on Sil Z (200 g-6 cm).

Figure 4 .

 4 Figure 4. Temperature pro le during ozone treatment of Sil Z.

Figure 5 .

 5 Figure 5. Oven temperature in uence on Sil Z regeneration after ozone treatment.

Figure 6 .

 6 Figure 6. Ozone treatment in uence on adsorption capacity of Sil Z toward 1,4-dioxan (DIO) or toluene (TOL).

Table 1 .

 1 Main characteristics of zeolites.

		HSZ trade name
		HSZ-390HUD3C	HSZ-890HOD1C
	Type (symbol) SiO 2 =Al 2 O 3 (mol mol ¡1 )	Faujasite (Fau Y) 360	ZSM-5 (Sil Z) 1880
	Na 2 O (wt%)	0.02	<0.01
	Clay binder content (%) Bulk density (kg l ¡1 )	25 0.42	20 0.66
	Pellet diameter (mm)	3.0	1.5

Figure 1. Adsorption=ozonation experimental setup.