Planar Microstrip Ring Resonators for Microwave-Based Gas Sensing: Design Aspects and Initial Transducers for Humidity and Ammonia Sensing

sensors

Article Planar Microstrip Ring Resonators for Microwave-Based Gas Sensing: Design Aspects and Initial Transducers for Humidity and Ammonia Sensing

Andreas Bogner, Carsten Steiner, Stefanie Walter, Jaroslaw Kita, Gunter Hagen and Ralf Moos * Department of Functional Materials, University of Bayreuth, 95447 Bayreuth, Germany; [email protected] (A.B.); [email protected] (C.S.); [email protected] (S.W.); [email protected] (J.K.); [email protected] (G.H.) * Correspondence: [email protected]; Tel.: +49-921-55-7401

Received: 21 September 2017; Accepted: 17 October 2017; Published: 24 October 2017

Abstract: A planar microstrip ring resonator structure on alumina was developed using the commercial FEM software COMSOL. Design parameters were evaluated, eventually leading to an optimized design of a miniaturized microwave gas sensor. The sensor was covered with a zeolite ﬁlm. The device was successfully operated at around 8.5 GHz at room temperature as a humidity sensor. In the next step, an additional planar heater will be included on the reverse side of the resonator structure to allow for testing of gas-sensitive materials under sensor conditions.

Keywords: microwave cavity perturbation; resonant frequency; radio frequency based gas sensors; zeolites; in operando spectroscopy

1. Introduction Sensors for gas detection include optical gas sensors based on light absorption, metal oxide gas sensors based on a resistive effect (chemiresistors), catalytic gas sensors through adsorption and thermic reaction, gravimetric SAW detectors, gas chromatography, calorimetric devices, biochemical sensors, and many other sensors based on capacitive, amperometric, or potentiometric effects [1]. In typical chemiresistors, gas-sensitive functional ﬁlms are applied on substrates that are covered with (interdigital) electrodes, and their resistances or their complex impedances are determined [2–4]. Mostly, sensors are operated in the range of room temperature to 400 ◦C. High ohmic electrode–ﬁlm interfaces and/or very high resistivities of the sensitive materials may exclude promising sensor materials from technical application. As an alternative approach, devices based on microwave transducers have also been suggested with growing research interest in recent years [5–9]. For sensor purposes, microwaves, especially electromagnetic waves in the range from 1 to 20 GHz, are typically used with planar or hollow waveguides. The sensor effect is the analyte-dependent permittivity of the material. With respect to miniaturization and low-cost applications, planar waveguide-based chemical sensors have attracted recent attention [10]. Several microwave-based sensors have been proposed in the past: moisture detection for the food and chemical industries [11–14] or glucose detection for biomedical applications [15], to name a few. In the past few years, planar microwave transducers for NH3, toluene, CO2, or CH4 based on the highly sensitive resonant method were suggested [5–9]. Rossignol et al. used coplanar microwave transducers and developed a resonant coplanar structure for detecting NH3 and toluene in concentrations up to 500 ppm [7]. They carried out their experiments at room temperature and regenerated the sensitive phthalocyanine layer under

Sensors 2017, 17, 2422; doi:10.3390/s17102422 www.mdpi.com/journal/sensors Sensors 2017, 17, 2422 2 of 14 argon. Bailly et al. demonstrated a hematite- and a zeolite-based variant of this sensor [8,9]. A concept by Zarifi et al. involves planar resonators and an active feedback loop for a gain that yields up to five times higher quality factors and thus higher resolutions [16]. They also published a zeolite-based version of their high Q resonator for sensing of CH4 and CO2 concentrations between 1% and 50% [5]. With respect to the sensing aspect, framework materials that adsorb chemical species, like metal-organic frameworks (MOFs) [17] or zeolites [18–20], may be preferred for microwave gas sensors range since the adsorption of large amounts of polar gas species goes along with an appropriate change in the complex electrical permittivity. Very recently, Bahoumina et al. proposed a chemical gas sensor with a planar capacitive microwave transducer in the microwave range and a polymer carbon nanomaterial as the sensitive layer [21], and Zarifi et al. demonstrated their active-resonator concept with MOFs as a sensitive material to detect CO2 [22]. Another, newer publication of Bailly et al. demonstrated a microstrip spiral resonator covered with titanium dioxide nanoparticles [8]. Recently, further microwave-based sensing concepts with different sensing purposes and planar geometries were published. They include a coupled line section for dielectric sample measurements, interdigital capacitors for liquid mixture concentration measurement, and a disc-shaped tip with a zeolite for thermal mass gas sensing [23–26]. Even proteins can be detected if the resonator (or the split in a split ring resonator) is functionalized with aptamers [27]. Another biomedical application using an interdigital microstrip capacitor is reported by Rydosz et al. They developed a microwave-based sensor to detect various volatile organic compounds and confirmed that it can be used for exhaled acetone detection [28]. The previous solutions demonstrate the capability of microwave-based sensors to monitor gas concentrations and/or the analyte loading of a sensitive layer. However, research on design and application of microwave transducers for gas detection is in its infancy and needs further investigations to develop integrated sensor devices. In addition, the size of recent devices prevents small applications and dynamic fast measurements with gas sensitive framework layers like zeolites due to their room temperature application. Hence, in this work, a very small 9 GHz resonant microstrip structure with a ring geometry for NH3 detection is investigated. As the sensitive material, the ring resonator is coated with an NH3 and water adsorbing Fe-zeolite as it is used for selective catalytic NOx reduction reactions in automotive exhausts [29,30]. It is demonstrated that small humidity and ammonia concentrations can be detected. The paper discusses design aspects that were obtained by FEM simulation for planar cavity ring resonators and demonstrates their applicability by fabricating an entire setup and conducting initial measurements at room temperature. We expect that the proposed microwave transducer will open up new possibilities for material characterization and gas-sensing purposes, especially when in the second step a heater layer will be implemented.

2. Principle of Planar Microwave Transducers Due to their interaction with the analyte, the gas sensitive material changes its complex electrical properties of permittivity ε = ε’ − jε” (with j being the imaginary unit) and permeability µ = µ’ − jµ”. The varying material properties lead to a changing wave propagation. Besides these intrinsic material effects, the wave propagation depends on the material’s geometry and on the design of the used waveguide (extrinsic properties) [10]. Generally, electromagnetic waves are guided to a desired transmission mode by restricting their expansion in one or two dimensions. A widely used transmission structure is the planar microstrip line, as depicted in the AppendixA (Figure A1). Microstrip lines consist of a strip conductor and a ground plane separated by a dielectric substrate and thus only support the transversal electromagnetic mode (TEM). TEM modes can be found whenever there are two conductors in only one medium. Hence, to be more precise, the supported wave mode for microstrips is not strictly TEM since the waves propagate not only in the substrates but also in the medium above the conductor line. This causes different phase velocities and, in turn, a longitudinal component of the electric magnetic field. However, since this component is very small it can be often neglected and the supported wave mode is then called Sensors 2017, 17, 2422 3 of 14 quasi-TEM mode. In addition, the two-dimensional structure of microstrips make them well suited for miniaturization and integration with other components and, as a result of the single plane structure, they canSensors be fabricated 2017, 17, 2422 conventionally by thick or thin film technology, photolithography, photoetching,3 of 14 or laser structuring [31]. Likeas fora result microwave-based of the single plane material structure, characterization they can be fabricated methods conv basedentionally on microstrips, by thick or non-resonant thin film or resonanttechnology, methods photolithography, are applicable, photoe buttching, resonant or laser methods structuring are preferred [31]. for sensing applications Like for microwave-based material characterization methods based on microstrips, non- due to their higher sensitivity and accuracy. Well-known in this respect is the resonant perturbation resonant or resonant methods are applicable, but resonant methods are preferred for sensing method,applications which is based due to on their the resonanthigher sensitivity frequency and change accuracy. of theWell-known scattering in parameters this respect byis the introducing resonant a sampleperturbation [10]. Examples method, for such which applications is based on inthe the resonant field offrequency chemical change sensors of the are scattering the characterization parameters of catalystby powder introducing samples a sample in situ [10]. during Examples chemical for such reactions applications [32,33 in], the or thefield state of chemical monitoring sensors of are catalysts the directlycharacterization in exhaust for three-wayof catalyst catalystspowder samples (TWC) [in34 situ], selective during chemical catalytic reactions reduction [32,33], catalysts or the (SCR) state [35 ], and dieselmonitoring particulate of catalysts filters (DPF)directly [ 36in]. exhaust All are for based three-way on the catalysts fact that (TWC) the metallic [34], selective catalyst catalytic or filter housingreduction form an catalysts electromagnetic (SCR) [35], waveguide and diesel particulate cavity [37 ].filters (DPF) [36]. All are based on the fact that Typicalthe metallicplanar catalystresonance or filter structures housing form with an microstrips electromagnetic are ribbon waveguide resonators, cavity [37]. disk resonators, or Typical planar resonance structures with microstrips are ribbon resonators, disk resonators, or ring resonators [31]. Owing to their geometry, standing waves form. They are trapped between the ring resonators [31]. Owing to their geometry, standing waves form. They are trapped between the boundary conditions of the designed microstrips. Microstrip resonators must be coupled with one or boundary conditions of the designed microstrips. Microstrip resonators must be coupled with one or two feed lines for excitation. Two types of information can be obtained: the reflection parameters S , two feed lines for excitation. Two types of information can be obtained: the reflection parameters S11, 11 where onlywhere one only coupled one coupled feed line feed is required;line is required; and the and reflection/transmission the reflection/transmission type. type. Here, Here, two two feed feed lines are used,lines allowing are used, evaluation allowing ofevaluation the reflection of the parameterreflection parameterS11 as well S11 as as the well transmission as the transmission parameter S21 [31].parameter S21 [31]. The standingThe standing waves waves on the on structures the structures can can be be perturbed perturbed by by a a sample. sample. In the the case case of of gas-sensitive gas-sensitive layers,layers, the resonance the resonance condition condition is thenis then not not only only dependentdependent on on the the permittivity permittivity of the of substrates, the substrates, but also from the cover layers, like the here-used zeolites. Then, the effective permittivity εeff describes but also from the cover layers, like the here-used zeolites. Then, the effective permittivity εeff describes the combined permittivities of the substrate and gas-sensitive layer. This leads to the resonant the combined permittivities of the substrate and gas-sensitive layer. This leads to the resonant frequency fres: frequency f res: c f = c f =res √ , (1) res L , (1) Lch chεeffε eff where Lch denotes the respective characteristic resonance length. It can be obtained by the design equationswhere of HammerstadLch denotes the and respective Jensen characteristic [31]. The characteristic resonance length. resonance It can lengthbe obtained depends by the only design on the equations of Hammerstad and Jensen [31]. The characteristic resonance length depends only on the resonance geometry used. For a ring resonator, Lch = 2·π·R can be used [31]. For the meaning of resonance geometry used. For a ring resonator, Lch = 2·π·R can be used [31]. For the meaning of the the ring resonator radius, see Figure1. Besides the resonant frequency, the 3dB-bandwidth BW ring resonator radius, see Figure 1. Besides the resonant frequency, the 3dB-bandwidth BW3dB is3dB is anotheranother characteristic characteristic parameter parameter of of microwave microwave resonators.resonators. It It leads leads to to the the loaded loaded quality quality factor factor Q, Q, which canwhich denote can denote a measure a measure for the for accuracythe accuracy of theof the resonance resonance peak peak [ 31[31].].

(a) (b)

Figure 1. Microstrip ring resonators: (a) basic setup and (b) transmission-type ring resonator with two Figure 1. Microstrip ring resonators: (a) basic setup and (b) transmission-type ring resonator with two capacitively coupled feed lines for excitation. capacitively coupled feed lines for excitation. f Q= res f Q = resB W (2) (2) BW 3dB 3dB All in all,All in the all, transducing the transducing principle principle can can then then bebe easily understood. understood. With With gases gases that adsorb that adsorb in the in the sensitivesensitive layer, layer, the the overall overall complex complex effective effective permittivitypermittivity changes changes and and a aresonant resonant frequency frequency and and qualityquality factor shiftfactor occurs. shift occurs.

Sensors 2017, 17, 2422 4 of 14

3. Sensor Design Sensors 2017, 17, 2422 4 of 14 3.1. Microstrip Ring Resonator Design 3. Sensor Design In this work, a microstrip ring resonator is used. The lack of open ends decreases the radiation losses and thus3.1. Microstrip increases Ring the qualityResonator factor Design compared with open-end resonators. The main design parameters are the couplingIn this work, gap d abetween microstrip the ring ring resonator resonator is used. and the The feed lack lines; of open the ends microstrip decreases width the radiationW of the ring (bothlosses leading and to thus the characteristicincreases the quality impedance); factor compar and theed mean with open-end ring radius resonators.R, which The determines main design the base resonantparameters frequency are the (Figure coupling1). All gap these d between parameters the ring influence resonator the and electric the feed field lines; distribution the microstrip at width resonance and thusW of the the sensitivity ring (both asleading well asto thethe resolutioncharacteristic of impedance); the observed and resonance the mean peak. ring Toradius understand R, which their impact,determines a detailed the simulation base resonant study frequency was conducted. (Figure 1). All It is these discussed parameters in the influence following. the electric field distribution at resonance and thus the sensitivity as well as the resolution of the observed resonance 3.2. Simulationpeak. To understand Studies their impact, a detailed simulation study was conducted. It is discussed in the following. The simulations were implemented using the RF-module of the FEM software COMSOL Multiphysics.3.2. Simulation The Studies general structure of the model includes 50 Ω SMA connectors, two 50 Ω feed lines, andThe the simulations microstrip were ring resonator.implemented For using the firstthe RF-module models, RO4003C of the FEM (manufactured software COMSOL by Rogers Corp.)Multiphysics. specifications The were general used structure as a substrate of the model material includes (substrate 50 Ω SMA thickness connectors,h = two 0.813 50 mm; Ω feed permittivity lines, ε’ = 3.38;and the loss microstrip tangent tan ringδ resonator.= ε”/ε’ = 0.0027).For the first models, RO4003C (manufactured by Rogers Corp.) specifications were used as a substrate material (substrate thickness h = 0.813 mm; permittivity ε’ = 3.3. Influence3.38; loss oftangent the Coupling tan δ = ε Gap’’/ε’ = 0.0027). To study the influence of the coupling gap, the distance d between the ring resonator and the 3.3. Influence of the Coupling Gap feed lines of a transmission-type microstrip ring resonator was increased. Generally, it is known that a larger couplingTo study the gap influence improves of the coupling transferred gap, signalthe distance since d the between electrical the ring field resonator perturbation and the in the couplingfeed gaplines isof largera transmission-type for small coupling microstrip gaps. ring Furthermore, resonator was it increased. influences Generally, the resonant it is known frequency that and a larger coupling gap improves the transferred signal since the electrical field perturbation in the the resolution of the resonance peak (quality factor). To be more precise, a loose coupling yields a high coupling gap is larger for small coupling gaps. Furthermore, it influences the resonant frequency and qualitythe factor resolution and thusof the a resonance good accuracy peak (quality (undercoupling), factor). To be whereas more precise, a high a coupling loose coupling level isyields needed a for powerhigh transfer quality (overcoupling) factor and thus [31 a ].good The accuracy required (u levelndercoupling), of coupling whereas is complex a high and coupling simulation level studiesis of theneeded coupling for gappowerd help transfer to identify (overcoupling) the influences [31]. The of couplingrequired gaplevel variations of coupling and is to complex estimate and needed values.simulation The parameters studies of were the coupling varied withgap d the help first to identify resonant the frequency influencesf resof coupling= 4 GHz, gap microstrip variations width W = 1.86and to mm estimate, and an needed impedance values. ofThe 50 parametersΩ. The smallest were varied coupling with gapthe first was resonant set to d =frequency 0.025 mm fresand = 4 was increasedGHz, inmicrostrip 0.025 mm width steps W to= 1.86 0.300 mm, mm. and an impedance of 50 Ω. The smallest coupling gap was set Theto d = expected 0.025 mmbehavior and was increased with increasing in 0.025 mm gap steps width to 0.300 is confirmed mm. by the simulations in Figure2. The qualityThe factor expected is higherbehavior (smaller with increasing peak width) gap width for looselyis confirmed coupled by the resonators simulations and in Figure the resonant 2. The quality factor is higher (smaller peak width) for loosely coupled resonators and the resonant frequency increases as well due to the perturbation at the coupling gap. Using the reflection parameter frequency increases as well due to the perturbation at the coupling gap. Using the reflection S , an increased coupling broadens the minimum, i.e., the resonant frequency cannot be as accurately 11 parameter S11, an increased coupling broadens the minimum, i.e., the resonant frequency cannot be determined.as accurately This determined. phenomenon This has phenomenon to be taken has into to be account taken into if the account reflection if the parameter reflection parameter is used as the microwaveis used signal,as the microwave and the coupling signal, and gap thed couplinghas to be gap selected d has to differently. be selected differently.

FigureFigure 2. Scattering 2. Scattering parameters parameters of of the the coupling coupling gap simulation study study (d (=d 0.025= 0.025 mm mm to 0.300 to 0.300 mm in mm in stepssteps of 0.025 of 0.025 mm). mm).

Sensors 2017, 17, 2422 5 of 14

3.4. Miniaturization Sensors 2017, 17, 2422 5 of 14 For miniaturized devices, the microstrip resonators have to be operated at higher frequencies. However,3.4. Miniaturization then the microstrip losses also increase and the quality factor gets smaller. In addition, a small ratio W/R is critical due to curving effects that inﬂuence the resonator characteristics. Consequences For miniaturized devices, the microstrip resonators have to be operated at higher frequencies. of such small W/R-ratios may be parasitic resonances as well as higher losses. To account for this However, then the microstrip losses also increase and the quality factor gets smaller. In addition, a effect,small another ratio parameterW/R is critical study due for to different curving resonanteffects that frequencies influence fromthe resonator 1 GHz upcharacteristics. to 10 GHz was conducted.Consequences The characteristic of such small impedance W/R-ratios (ring may microstrip) be parasiticwas resonances kept to as 50 wellΩ, the as ringhigher width losses. was To set to W = 1.86account mm for, and this the effect, coupling another gap parameter to d = 0.15 study mm. for Thedifferent ring resonant radius R frequencieswas modiﬁed from between 1 GHz up 2.88 to mm and 29.210 GHz mm was in a conducted. way such thatThe resonantcharacteristic frequencies impedance between (ring microstrip) 1 GHz and was 10 GHzkept to occurred. 50 Ω, the In ring Table 1, the exactwidth ring was radius set to W values = 1.86 andmm, the and respective the coupling resonance gap to d = frequencies 0.15 mm. The are ring listed. radius R was modified between 2.88 mm and 29.2 mm in a way such that resonant frequencies between 1 GHz and 10 GHz Tableoccurred. 1. Ring In Table radius 1, Rtheto exact obtain ring resonant radius frequenciesvalues and thef res respectivebetween 1 resonance and 10 GHz. frequencies The ring are width listed.W was set to 1.86 mm. Table 1. Ring radius R to obtain resonant frequencies fres between 1 and 10 GHz. The ring width W

was fsetres/GHz to 1.86 mm. 1 2 3 4 5 6 7 8 9 10 R/mm 29.2 14.6 9.72 7.28 5.81 4.84 4.14 3.62 3.21 2.88 /GHz 1 2 3 4 5 6 7 8 9 10 R/mm 29.2 14.6 9.72 7.28 5.81 4.84 4.14 3.62 3.21 2.88 In Figure3, the results of the miniaturization study are plotted. At higher resonant frequencies, the resonanceIn Figure peaks 3, the broaden, results of i.e.,the miniaturization the quality factors study are get plotted. smaller. At higher Furthermore, resonant frequencies, the transferred signalthe increases. resonance peaks This couldbroaden, be i.e., due the to quality thesmaller factors get distances smaller. Fu thatrthermore, have to the be transferred overcome. signal Finally, the miniaturizationincreases. This studycould showsbe due the to importancethe smaller ofdist increasingances that the have quality to be factor overcome. by other Finally, parameters the to miniaturization study shows the importance of increasing the quality factor by other parameters to ensure a proper resolution for sensing. Hence, a miniaturization by using high-permittivity substrates ensure a proper resolution for sensing. Hence, a miniaturization by using high-permittivity should also be considered to improve the sensor signal. Despite the miniaturization improvements, substrates should also be considered to improve the sensor signal. Despite the miniaturization the microstripimprovements, substrate the microstrip has other substrate impacts has on other the sensor impacts device on the characteristics.sensor device characteristics. These criteria These and the here-usedcriteria substrate and the here-used are brieﬂy substrate discussed are briefly in the discussed following. in the following.

Figure 3. Scattering parameters for the miniaturization simulation study. The ring radius R was varied Figure 3. Scattering parameters for the miniaturization simulation study. The ring radius R was varied between 2.88 mm and 29.2 mm, so that the resonant frequencies increased from 1 GHz to 10 GHz. between 2.88 mm and 29.2 mm, so that the resonant frequencies increased from 1 GHz to 10 GHz. Choosing the substrate plays a key role in the design of microstrip circuits and is widely Choosingdiscussed in the literature substrate [38]. plays Since a key it is role intended in the to design heat up of microstripthe device in circuits further and works, is widely an alumina discussed in literaturesubstrate [38 with]. Since a purity it is of intended 99.6% and to gold heat as up metallization the device in is furtherused. Alumina works, also an alumina ensures a substrate very low with a purityloss oftangent 99.6% (ε’ and = 10.1 gold @ 1 GHz, as metallization tan δ = 0.00022 is @ used. 1 MHz, Alumina h = 0.635 mm; also data ensures sheeta of very CeramTec low loss Rubalit tangent (ε’ = 10.1710). @ On 1 GHzthe ,other tan δ =hand, 0.00022 concerning @ 1 MHz, theh =theory 0.635 mm;of field data distribution sheet of CeramTec in microstrips, Rubalit the 710). high On the permittivity negatively affects the sensitivity, since the field concentrates in the substrate with its other hand, concerning the theory of ﬁeld distribution in microstrips, the high permittivity negatively higher permittivity. However, a higher permittivity allows smaller devices. affects the sensitivity, since the ﬁeld concentrates in the substrate with its higher permittivity. However, a higher3.5. Final permittivity Microwave allows Gas-Sensing smaller Device devices.

3.5. Final MicrowaveGenerally, a Gas-Sensing design with Deviceonly one port is desired for gas detection due to the simplified setup. However, a two-port microwave transducer delivers more information than a one-port reflection- Generally,type resonator. a design Thus, with differences only one between portis a desiredreflection- for and gas a detection transmission-type due to the resonator simpliﬁed with setup. However, a two-port microwave transducer delivers more information than a one-port reﬂection-type

Sensors 2017, 17, 2422 6 of 14 resonator. Thus, differences between a reﬂection- and a transmission-type resonator with respect to the sensitivity were simulated. The sensitivity is here deﬁned as a change of the resonant frequency towardsSensors a change2017, 17, 2422 in the intrinsic material properties of a ring covering layer: 6 of 14

respect to the sensitivity were simulated. Thefres sensitivity∆ fres is here defined as a change of the resonant Sε = 0 . (3) frequency towards a change in the intrinsic material∆ propertiesεeff of a ring covering layer: Δf For the simulations, the mean radius was setfres to R =res 2.07 mm, the covering layer thickness (zeolite) Sε = . (3) was 0.1 mm, the coupling gaps were set to 0.15 mm,Δ andε ' the Al O substrate (ε’ = 10.1) was chosen to eff 2 3 be 0.635 mm. For the simulations, the mean radius was set to R = 2.07 mm, the covering layer thickness (zeolite) The setup of both simulated resonator types is depicted in Figure4a,b. The complex permittivity was 0.1 mm, the coupling gaps were set to 0.15 mm, and the Al2O3 substrate (ε’ = 10.1) was chosen to of the zeolite was increased in the real and the imaginary part to account for the polarization change be 0.635 mm. and a changeThe setup in intrinsic of both lossessimulated with resonator increasing types absorbed is depicted ammonia in Figure 4a,b. or water The complex [35]. A permittivity list of the study stepsof used the zeolite is depicted was increased in Table2 in. Figurethe real5 anddemonstrates the imaginary that, part for to the account selected for data,the polarization the sensitivity change of the reﬂection-typeand a change device in intrinsic is higher losses and with the increasing resonance absorbed peak isammonia less broad or water than [35]. for the A list transmission-type of the study sensor.steps Consequently, used is depicted inthe in Table following 2. Figure measurements, 5 demonstrates the that, reﬂection-type for the selected resonator data, the wassensitivity used. of the reflection-type device is higher and the resonance peak is less broad than for the transmission- Tabletype sensor. 2. Data Consequently, for the sensitivity in the following simulation. meas Theurements, complex the permittivities reflection-type were resonator roughly was estimated used. from [32]. Table 2. Data for the sensitivity simulation. The complex permittivities were roughly estimated from [32].

StepStep 11 2 2 3 3 4 4 5 5 6 6 ε’ε’ 3 + 3 j0 + j0 3.2 3.2 + + j0.1j0.1 3.4 3.4 + + j0.2 j0.2 3.6 3.6 + +j0.3 j0.3 3.8 3.8 + j0.4 + j0.4 4 + j0.5 4 + j0.5

Figure 4. Three-dimensional sensor models with gas-sensitive (zeolite) cover layer (blue) for the Figure 4. Three-dimensional sensor models with gas-sensitive (zeolite) cover layer (blue) for the sensitivitysensitivity simulation. simulation. (a) Transmission-type (a) Transmission-type and (band) reﬂection-type (b) reflection-type resonator. resonator. (c) Fabricated (c) Fabricated microwave ring transducermicrowave ring with transducer Fe-zeolite with as sensitive Fe-zeolite layer as sensitive and end-mounted layer and end-mounted SMA connector SMA (speciﬁcations:connector

Al2O(specifications:3 with 99.6%purity Al2O3 with and 99.6% a thickness purity ofand 0.635 a thickness mm, R of= 0.635 2.07 mm,mm, dR == 2.07 0.15 mm, mm, dW = 0.15= 0.22 mm, mm). W = 0.22 mm). Finally, the coupling gap, microstrip width, and first resonance were chosen based on the Finally, the coupling gap, microstrip width, and first resonance were chosen based on the performed simulations. To miniaturize the microwave transducer, the resonant frequency of the performed simulations. To miniaturize the microwave transducer, the resonant frequency of the uncovereduncovered ring resonatorring resonator was was set toset 9 GHz,to 9 GHz, which which leads leads to a ringto a radiusring radius of only of Ronly= 2.07 R = mm.2.07 However,mm. the simulationHowever, the studies simulation showed studies that showed this wouldthat this lead would to lead a smaller to a smaller accuracy accuracy of theof the resonance resonance peak. To counterpeak. To this, counter the this, ring the strip ring width strip width was setwas tosetW to W= 0.22= 0.22 mm, mm, which is is very very small small and and improves improves accuracyaccuracy as well as well as sensitivityas sensitivity due due to to the the higher higher fieldfield concentration above above the the substrate substrate for smaller for smaller stripstrip widths. widths. The The value valued = d 0.15= 0.15 mm mm was was empirically empirically determined determined by by the the preliminary preliminary fabricated fabricated ring ring resonatorsresonators and and denotes denotes a compromise a compromise between between signal intensity intensity and and resonance resonance peak peak accuracy. accuracy. Finally, Finally, the designedthe designed microwave microwave ring ring resonator resonator on on alumina alumina substrate substrate was was coveredcovered withwith the zeolite. zeolite. Due Due to to the the mechanical instability of the zeolite cover layer, a thickness measurement was difficult. mechanical instability of the zeolite cover layer, a thickness measurement was difficult. Additionally, Additionally, the cover layer is not exactly planar, which leads to different thicknesses across the ring resonator. The thickness is assumed to be between 0.5 mm and 1 mm.

Sensors 2017, 17, 2422 7 of 14 the cover layer is not exactly planar, which leads to different thicknesses across the ring resonator. The thicknessSensors 2017,is 17 assumed, 2422 to be between 0.5 mm and 1 mm. 7 of 14

Figure 5. Scattering parameters of the sensitivity simulation with changing complex permittivity from Figure 5. Scattering parameters of the sensitivity simulation with changing complex permittivity from 3 + j0 to 4 + j0.5. Two resonator configurations were simulated: a transmission-type sensor with two 3 + j0 to 4 + j0.5. Two resonator configurations were simulated: a transmission-type sensor with two excitation lines (2-port measurement, left figure) and a reflection-type sensor with only one excitation excitation lines (2-port measurement, left figure) and a reflection-type sensor with only one excitation line (1-port measurement, right figure). The mentioned sensitivities are calculated from a linear fit of line (1-portthe resonance measurement, frequency right change. figure). The mentioned sensitivities are calculated from a linear fit of the resonance frequency change. 4. Experiment and Methods 4. Experiment and Methods Al2O3 substrates were purchased from CeramTec (Rubalit 710, thickness 0.635 mm, ε’ = 10.1 @ 1 GHz, tan δ = 0.00022 @ 1 MHz) and in-house coated by evaporation with a 400nm gold layer (adhesion Al2O3 substrates were purchased from CeramTec (Rubalit 710, thickness 0.635 mm, ε’ = 10.1 @ 1 GHz, tan δlayer= 0.00022 4 nm chromium). @ 1 MHz) The and microstrip in-house ring, coated as well by as evaporation the microstrip with feed a lines, 400nm were gold structured layer (adhesionusing a frequency-tripled Nd:YAG-Laser (LPKF Microline 350L, LPKF Laser & Electronics AG, Garbsen, layer 4 nm chromium). The microstrip ring, as well as the microstrip feed lines, were structured using Germany). The laser was also used to drill holes at the ends of the feed lines. They served to fix the a frequency-tripledend launch (Southwest Nd:YAG-Laser Microwave, (LPKF Tempe, Microline AZ, USA) 350L, at the LPKF ends of Laser the substrate. & Electronics Both types—the AG, Garbsen, Germany).reflection The type laser with was one also port used and the to drilltransmission holes at type the with ends two of theports—were feed lines. fabricated They served this way. to fix the end launchThe (Southwest Fe-zeolite powder Microwave, (obtained Tempe, from AZ,an automotive USA) at the catalyst ends (PSA, of the further substrate. described Both in types—the [39])) reflectionwas mixed type withwith onea 1:11 port compound and the of transmission ethyl cellulose type and with terpineol, two ports—were which was then fabricated applied this on the way. Themicrostrip Fe-zeolite ring powderand fired (obtained at 600 °C from (layer an thickness automotive between catalyst 0.5 (PSA, mm and further 1 mm). described After this, in [39 the])) was mixedconnectors with a 1:11 were compound attached and of ethyl coaxial cellulose cables connected and terpineol,. The entire which reflection-type was then applied sensor on is theshown microstrip in ring andFigure fired 4c. This at 600 type◦ Cof (layerzeolite thicknesswas used because between we 0.5know mm its andelectrical 1 mm). properties After in this, the themicrowave connectors wererange attached from and microwave-based coaxial cables measurments connected. in The exhaust entire gas reflection-type catalysts [39]. sensor is shown in Figure4c. The experimental setup included a glass sample chamber containing the microwave transducer, This type of zeolite was used because we know its electrical properties in the microwave range from a vector network analyzer (VNA: Anritsu MS2820B), a data acquisition system (Laptop), and a gas microwave-basedsupply unit connected measurments to the sample in exhaust chamber. gas catalystsThe latter [ 39consisted]. of three mass flow controllers to Thecontrol experimental the flows of setup N2, NH included3, and the a glass water-saturated sample chamber N2. The containing total gas theflow microwave rate was set transducer, to 0.5 a vectormL/min. network The analyzer network (VNA: analyzer Anritsu with connected MS2820B), coaxia a datal cables acquisition was calibrated system using (Laptop), a SOLT and calibration a gas supply unit connectedkit (Anritsu toSMA the Calibration sample chamber. Kit 3650). The The latter setup consistedis depicted ofin threeFigure mass 6. flow controllers to control the flowsAs of the N2 sensor, NH3 ,response, and the the water-saturated reflection parameter N2. TheS11 was total used. gas The flow complex rate wasreflection set to parameter 0.5 mL/min. The networkS11 was measured analyzer in witha frequency connected range coaxial from 8 GHz cables to 10 was GHz calibrated every 10 to using 20 seconds. a SOLT Then, calibration before kit (Anritsuthe resonant SMA Calibration frequency Kitand 3650). the absolute The setup value is of depicted the reflection in Figure coefficient6. (|S11|) were determined, the reflection spectrum around the operating resonant frequency was filtered using a Lorentz fit to As the sensor response, the reflection parameter S11 was used. The complex reflection parameter remove noise and parasitic reflections. S was measured in a frequency range from 8 GHz to 10 GHz every 10 to 20 seconds. Then, before the 11 Generally, after each adsorption measurement, the zeolite had to be thermally regenerated in an resonantexternal frequency furnace. andTherefore, the absolute the coated value microwav of thee reflectiontransducer coefficientwithout the (|SMAS11 |)connector were determined, was heated the reflectionup to spectrum120 °C over around 2 min and the kept operating at 120 resonant°C for 10 min, frequency then heated was filteredup to 600 using °C over a Lorentz 25 min and fit to kept remove noiseat and 600 parasitic°C for 10 min. reflections. Finally, the transducer was cooled down to room temperature over 25 min (21 Generally,°C). Furthermore, after each after adsorptionstoring in ambient measurement, air, the microwave the zeolite transducer had to bewas thermally purged in regeneratedN2 for several in an externalminutes furnace. to desorb Therefore, stored thewater coated on the microwave zeolite (see transduceralso next section). without the SMA connector was heated up to 120 ◦C over 2 min and kept at 120 ◦C for 10 min, then heated up to 600 ◦C over 25 min and kept at 600 ◦C for 10 min. Finally, the transducer was cooled down to room temperature over 25 min ◦ (≈21 C). Furthermore, after storing in ambient air, the microwave transducer was purged in N2 for several minutes to desorb stored water on the zeolite (see also next section).

Sensors 2017, 17, 2422 8 of 14 Sensors 2017, 17, 2422 8 of 14

Figure 6. Experimental set-up with glass sample chamber containing the microwave transducer, a Figure 6. Experimental set-up with glass sample chamber containing the microwave transducer, vector network analyzer (VNA, Anritsu MS2820B), and a gas supply unit connected to the sample a vector network analyzer (VNA, Anritsu MS2820B), and a gas supply unit connected to the chamber. sample chamber. 5. Results and Discussion 5. Results and Discussion 5.1. Initial Tests: Proof of Concept 5.1. Initial Tests: Proof of Concept Before the measurements in the above-described setup started, initial tests were performed to Beforeverify the the basic measurements concept and the in therepeatability above-described of the microwave-based setup started, adsorption initial tests monitoring were performed of the to verifyzeolites the basic at room concept temperature. and the For repeatability that purpose, of the the microwave microwave-based sensor was adsorption connected to monitoring the VNA and of the zeolitesplaced at room in different temperature. atmospheric For thatconditions. purpose, Apart the fr microwaveom ambient sensor air, a rolled-neck was connected glass topartly the filled VNA and placedwith in an different ammonia-water atmospheric solution conditions. (25 m% NH Apart3 in H2O) from was ambient used (setup air, in a Figure rolled-neck A2). The glass evaporation partly filled of ammonia into the gas phase established an NH3 partial pressure in the headspace of the liquid with an ammonia-water solution (25 m% NH in H O) was used (setup in Figure A2). The evaporation inside the glass, which was used for the first3 tests.2 Finally, four different cases were distinguished of ammoniawith this intoexperiment: the gas phase established an NH3 partial pressure in the headspace of the liquid inside the glass, which was used for the first tests. Finally, four different cases were distinguished with this experiment:1. Sensor outside the glass (i.e., in air) without zeolite layer. 2. Sensor inside the glass (ammonia-in-air ambience) without zeolite layer. 1. 3.Sensor Sensor outside outside the the glass glass (i.e., (i.e., in in air) air) without with zeolite zeolite layer. layer. 2. 4.Sensor Sensor inside inside the the glass glass (ammonia-in-air (ammonia-in-air ambience)ambience) with without zeolite zeolite layer. layer. 3. SensorIf the outside sensor the is placed glass (i.e.,inside in the air) glass with (in zeolite the headspace layer. of the liquid), both gaseous ammonia 4. andSensor water inside should the adsorb glass (ammonia-in-air in the zeolite and ambience) shift the sensors’ with zeolite resonant layer. frequency towards lower frequencies since the effective permittivity increases. For reference purposes, the experiment was also Ifrepeated the sensor without is placed a zeolite inside cover thelayer. glass In addition, (in the headspacethe initial test of cases the liquid),were simulated both gaseous with Comsol ammonia and waterMultiphysics. should The adsorb geometry in the for zeolitethe models and was shift set the to be sensors’ the same resonant as the fabricated frequency ring towards resonator lower frequenciessensor (specifications: since the effective substrate permittivity thickness increases.of 0.635 mm, For mean reference ring radius purposes, R = 2.07 the mm, experiment coupling wasgap also repeatedd = 0.15 without mm, ring a zeolite microstrip cover width layer. W In = addition, 0.22 mm, thezeolite initial thickness test cases 0.75 weremm). simulatedThe Al2O3 substrate with Comsol Multiphysics.properties Thewere geometrychosen to be for ε’ = the 9.87 models and tanwas δ = 0.0003, set to which be the is sameclose to as the the corresponding fabricated ring datasheet resonator sensorvalues. (specifications: Furthermore, substrate there were thickness no data ofvalues 0.635 fo mm,r the meancomplex ring permittivity radius R =available 2.07 mm, for couplingthe used gap Fe-zeolite and the ammonia-saturated Fe-zeolite. Thus, the complex permittivities of the zeolites were d = 0.15 mm, ring microstrip width W = 0.22 mm, zeolite thickness 0.75 mm). The Al2O3 substrate chosen in a way that they fit the measurement data best (ε’zeolite,air = 3.35, tan δzeolite,air = 0.07 and properties were chosen to be ε’ = 9.87 and tan δ = 0.0003, which is close to the corresponding datasheet ε’zeolite,NH3 = 3.97, tan δzeolite,NH3 = 0.095). These values can be thought of as a first guess of the actual values.permittivity Furthermore, of the therezeolite were and nothe datasensitivity. values Roughly for the they complex agree permittivitywith data from available [40] for zeolites. for the used Fe-zeoliteFinally, and considering the ammonia-saturated the attenuation Fe-zeolite.of transmission Thus, lines the in complex the measurement permittivities setup, of an the attenuation zeolites were chosenof 0.55 in a dB way was that added they to the fit theComsol measurement simulation data. data best (ε’zeolite,air = 3.35, tan δzeolite,air = 0.07 and ε’zeolite,NH3Figure= 3.97, 7 depicts tan δzeolite,NH3 the measured= 0.095). reflection These spectra values for can the be four thought cases. ofAs as expected, a first guess all resonance of the actual permittivitypeaks differ of thefrom zeolite each other. and The the resonant sensitivity. frequencies Roughly without they agreea zeolite with layer data in air from and in [40 ammonia-] for zeolites. Finally,in-air considering are identical the (① attenuation and ②). ofThis transmission was expected, lines since in thethe measurementgas phase above setup, the sensor an attenuation hardly of 0.55 dBcontributes was added to the to resonant the Comsol frequency simulation due to data.their low permittivity. As soon as the microstrip ring is covered with a zeolite layer, the permittivity of the zeolite forces the fundamental resonant frequency Figure7 depicts the measured reflection spectra for the four cases. As expected, all resonance peaks to shift to lower frequencies (③). Furthermore, by exposing the zeolite-covered sensor to the differ from each other. The resonant frequencies without a zeolite layer in air and in ammonia-in-air are identical ( 1 and 2 ). This was expected, since the gas phase above the sensor hardly contributes to the resonant frequency due to their low permittivity. As soon as the microstrip ring is covered with a zeolite layer, the permittivity of the zeolite forces the fundamental resonant frequency to shift to lower Sensors 2017, 17, 2422 9 of 14 Sensors 2017, 17, 2422 9 of 14 Sensors 2017, 17, 2422 9 of 14

ammonia-in-air ambience, a further decrease to lower frequencies can be observed, which can be frequenciesammonia-in-air ( 3 ). Furthermore, ambience, a byfurthe exposingr decrease the zeolite-coveredto lower frequencies sensor can to thebe ammonia-in-airobserved, which ambience, can be attributed to the permittivity increase when ammonia and/or water adsorbs in the zeolite [41–43]. a furtherattributed decrease to the topermittivity lower frequencies increase canwhen be ammo observed,nia and/or which water can beadsorbs attributed in the to zeolite the permittivity [41–43]. From in situ experiments with zeolites applying the cavity perturbation method, the general relation increaseFrom in when situ experiments ammonia and/or with zeolites water adsorbsapplying in th thee cavity zeolite perturbation [41–43]. From method, in situ the experimentsgeneral relation with between ammonia adsorption and the electromagnetic material properties have already been studied between ammonia adsorption and the electromagnetic material properties have already been studied zeolites[32,35]. applying the cavity perturbation method, the general relation between ammonia adsorption and[32,35]. the electromagnetic material properties have already been studied [32,35].

FigureFigure 7. 7. Initial tests: absolute value of the reflection parameter |SS11| of the microwave sensor for Figure 7.Initial Initial tests: tests: absolute absolute value value of of the thereflection reflection parameterparameter | S1111|| of of the the microwave microwave sensor sensor for for different atmospheres: solid line = measurement; circle line = simulation. ① Sensor outside the glass differentdifferent atmospheres: atmospheres: solid solid line line = = measurement;measurement; circle line = = simulation. simulation. ① 1 SensorSensor outside outside the the glass glass (i.e., in air) without zeolite layer (black dashed); ② Sensor inside the glass (ammonia in air ambience) (i.e.,(i.e., in in air) air) without without zeolite zeolite layer layer (black (black dashed);dashed); ②2 Sensor inside the the glass glass (ammonia (ammonia inin air air ambience) ambience) without zeolite layer (green); ③ Sensor outside the glass (i.e., in air) with zeolite layer (red); ④ Sensor withoutwithout zeolite zeolite layer layer (green); (green); 3 ③Sensor Sensor outside outsidethe the glassglass (i.e., in in air) air) with with zeolite zeolite layer layer (red); (red); ④ 4SensorSensor inside the glass (ammonia in air ambience) with zeolite layer (blue). insideinside the the glass glass (ammonia (ammonia in in air air ambience) ambience) with with zeolitezeolite layerlayer (blue). The zeolites were regenerated and the experiment was repeated another two times. For each run, The zeolites were regenerated and the experiment was repeated another two times. For each run, theThe resonant zeolites frequency were regenerated for the unloaded and the case experiment (after regeneration was repeated in air), another fully twoammonia-loaded times. For each case run, the resonant frequency for the unloaded case (after regeneration in air), fully ammonia-loaded case the(ammonia-in-air resonant frequency ambience), for the and unloaded now also case the partially (after regeneration loaded case in(in air), air after fully exposing ammonia-loaded to ammonia) case (ammonia-in-air ambience), and now also the partially loaded case (in air after exposing to ammonia) (ammonia-in-airwere recorded ambience),and compared. and nowFor the also latter, the partially the fully loaded loaded case zeolites (in air were after again exposing exposed to ammonia) to air. were recorded and compared. For the latter, the fully loaded zeolites were again exposed to air. wereThereby, recorded ammonia and compared. (defined Foras weakly the latter, bonded the fully ammo loadednia zeolites[44]) desorbs were againuntil a exposed new equilibrium to air. Thereby, is Thereby, ammonia (defined as weakly bonded ammonia [44]) desorbs until a new equilibrium is ammoniaestablished (defined and only as weakly strongly bonded bonded ammonia ammonia [44 remains]) desorbs in untilthe zeolite. a new equilibriumThe respective is establishedresonant established and only strongly bonded ammonia remains in the zeolite. The respective resonant andfrequencies only strongly are illustrated bonded ammonia in Figure remains8. It should in the be zeolite.noted that The the respective experiments resonant were carried frequencies out at are frequencies are illustrated in Figure 8. It should be noted that the experiments were carried out at different days and thus at different room temperatures as well as humidity. Nevertheless, in Figure illustrateddifferent indays Figure and 8thus. It shouldat different be notedroom temperatur that the experimentses as well as werehumidity. carried Nevertheless, out at different in Figure days and8, thusthe resonant at different frequencies room temperatures at the time as t well0 for asthe humidity. empty zeolite Nevertheless, coincide. in Additionally, Figure8, the resonantfor all 8, the resonant frequencies at the time t0 for the empty zeolite coincide. Additionally, for all experimental runs the same trends can be seen in the frequency shift. The quantitative differences frequenciesexperimental at the runs time thet 0samefor thetrends empty can zeolitebe seen coincide. in the frequency Additionally, shift. The for allquantitative experimental differences runs the may be explained by the varying experimental differences in humidity and room temperature. samemay trends be explained can be seen by inthe the varying frequency experimental shift. The d quantitativeifferences in differenceshumidity and may room be explained temperature. by the Furthermore, the unknown ammonia and water concentrations make an explanation difficult. varyingFurthermore, experimental the unknown differences ammonia in humidity and water and roomconcentrations temperature. make Furthermore, an explanation the difficult. unknown Therefore, for the subsequent tests, the above-described sample chamber with the gas supply unit ammoniaTherefore, and for water the subsequent concentrations tests, make the above-de an explanationscribed difficult.sample chamber Therefore, with for the the gas subsequent supply unit tests, and defined concentrations was used. theand above-described defined concentrations sample chamberwas used. with the gas supply unit and defined concentrations was used.

FigureFigure 8. 8.Initial Initial tests: tests: resonant resonant frequencies frequencies ofof thethe microwavemicrowave sensor sensor for for the the three three experimental experimental runs runs Figure 8. Initial tests: resonant frequencies of the microwave sensor for the three experimental runs withwith different different loading loading conditions conditions ofof thethe zeolite:zeolite: empty, partly loaded, loaded, und und fully fully loaded loaded (for (for further further with different loading conditions of the zeolite: empty, partly loaded, und fully loaded (for further detailsdetails see see text). text). details see text).

SensorsSensors 20172017,, 1717,, 24222422 1010 ofof 1414 Sensors 2017, 17, 2422 10 of 14

5.2. Tests under Defined Gas Exposure 5.2.5.2. TestsTests under under Deﬁned Defined Gas Gas Exposure Exposure SinceSince humidityhumidity inin airair shouldshould affectaffect thethe resonantresonant frequency,frequency, magnitudemagnitude andand timelytimely responseresponse behaviorSince were humidity observed in airduring should water affect desorption. the resonant A sensor frequency, that was magnitude stored in air and was timely installed response in the behaviorbehavior were were observed observed during during water water desorption. desorption. A A sensor sensor that that was was stored stored in in air air was was installed installed in in thethe test rig as described in Figure 6 and was then purged with pure N2. The respective reflection spectra test rig as described in Figure 6 and was then purged with pure N2. The respective reflection spectra testaround rig asresonant described frequency in Figure are6 and shown was in then Figure purged 9. The with blue pure curve N 2. denotes The respective a spectrum reﬂection after spectrastorage aroundaround resonant resonant frequency frequency are are shown shown in in Figure Figure9. The9. The blue blue curve curve denotes denotes a spectrum a spectrum after after storage storage in in air. The black and the red curve were recorded after purging with N2. One can clearly see that both in air. The black and the red curve were recorded after purging with N2. One can clearly see that both air. The black and the red curve were recorded after purging with N2. One can clearly see that both fres and |S11| are affected due to the incorporation of water into the zeolite during storage in ambient fres and |S11| are affected due to the incorporation of water into the zeolite during storage in ambient f res and |S11| are affected due to the incorporation of water into the zeolite during storage in ambient air.air. SinceSince waterwater increasesincreases thethe permittivitypermittivity ofof thethe zeolites,zeolites, thethe resonantresonant frequencyfrequency increasesincreases afterafter thethe air.desorption Since water of water. increases The results the permittivity also show ofthat the af zeolites,ter 10 min the most resonant of the frequency water has increasesalready desorbed, after the desorptiondesorption ofof water.water. TheThe resultsresults alsoalso showshow thatthat afterafter 1010 minmin mostmost ofof thethe waterwater hashas alreadyalready desorbed,desorbed, eveneven atat roomroom temperaturetemperature (Figure(Figure 9b).9b). OnOn thethe otherother hand,hand, itit isis clearclear thatthat beforebefore eacheach measurement,measurement, eventhe sensor at room has temperature to be purged (Figure for several9b). On mi thenutes other to hand,obtain it reproducible is clear that beforeresults. each measurement, the sensorthe sensor has tohas be to purged be purged for several for several minutes minutes to obtain to obtain reproducible reproducible results. results.

(a)(a) (b)(b)

Figure 9.9. AbsoluteAbsolute value value of of the the reflection reﬂection parameter parameter | |SS1111| |of ofthe the microwave microwave sensor sensor when when stored stored in Figure 9. Absolute value of the reflection parameter |S11| of the microwave sensor when stored in in ambient air and when purged with nitrogen: (a) overall response from 8 to 9 GHz (b) near ambientambient air air and and when when pu purgedrged with with nitrogen: nitrogen: ( (aa)) overall overall response response from from 8 8 to to 9 9 GHz GHz ( (bb)) near near resonance resonance resonance response. response.response.

Then,Then, thethethe inﬂuenceinfluenceinfluence ofofof deﬁneddefineddefined water waterwater content contentcontent was waswas investigated. investigated.investigated. FigureFigure 101010 is isis a aa time-dependent time-dependenttime-dependent plot of fres and |S11|@fres for an admixture of 1 vol % and 2 vol % of water into nitrogen. One can clearly plotplot ofoff fresres andand | |SS1111|@|@fresf forres foran admixture an admixture of 1 ofvol 1 % vol and % and2 vol 2 % vol of water % of water into nitrogen. into nitrogen. One can One clearly can see that the device responds to water. This excludes O2 or CO2 (both from air) from being responsible clearlysee that see the that device the responds device responds to water. to This water. excludes This excludesO2 or CO2 O (both2 orCO from2 (both air) from from being air) fromresponsible being responsibleforfor thethe sensorsensor for effecteffect the sensor inin FigureFigure effect 9.9. Again, inAgain, Figure adsorbedadsorbed9. Again, waterwater adsorbed cancan bebe water desorbeddesorbed can in bein water-freewater-free desorbed innitrogennitrogen water-free eveneven nitrogenatat roomroom eventemperature,temperature, at room temperature,butbut thethe processprocess but requires therequires process aa longerlonger requires time.time. a longer Here,Here, time. higherhigher Here, temperaturestemperatures higher temperatures wouldwould bebe wouldfavorable.favorable. be favorable.

FigureFigure 10. 10.10. Resonant ResonantResonant frequency frequencyfrequency( a ((a)a) and) andand reﬂection reflectionreflection parameter parameterparameter at atat resonant resonantresonant frequency frequencyfrequency (b ) ((b whenb)) whenwhen exposed exposedexposed to to pure N2 and to 1 vol % water or to 2 vol % water in N2. pureto pure N2 andN2 and to1 to vol 1 vol % water % water or toor 2 to vol 2 vol % water % water in N in2 .N2.

In further experiments, defined NH3 concentrations were admixed. The sensor was exposed to InIn further further experiments, experiments, deﬁned defined NH NH3 concentrations3 concentrations were were admixed. admixed. The The sensor sensor was was exposed exposed to to a a stepwise-changing NH3 concentration from 0 ppm to 500 ppm (t1) and from 500 ppm to 1000 ppm stepwise-changinga stepwise-changing NH NH3 concentration3 concentration from from 0 0 ppm ppm to to 500 500 ppm ppm (t (1t)1) and and from from 500 500 ppm ppm to to 1000 1000 ppm ppm NH3 (t2) without regenerating the zeolite in between. Again, the obtained results in Figure 11 show NHNH33 ((tt22)) withoutwithout regeneratingregenerating thethe zeolitezeolite inin between.between. Again,Again, thethe obtainedobtained resultsresults inin FigureFigure 11 11 show show different resonant frequency shifts during the loading with 500 ppm and 1000 ppm NH3. In the first differentdifferent resonant resonant frequency frequency shifts shifts during during the the loading loading with with 500 500 ppm ppm and and 1000 1000 ppm ppm NH NH3.3. In In the the ﬁrst first measurement step between t1 and t2, when loading with 500 ppm NH3, it can be clearly seen that the measurementmeasurement step step between betweent 1t1and andt 2t2,, whenwhen loadingloading withwith 500500 ppmppm NHNH33,, itit cancan bebe clearlyclearly seenseen thatthat the the sensorsensor signal signal is is accumulating accumulating unti untill saturation saturation with with ammonia ammonia is is reac reached.hed. Furthermore, Furthermore, increasing increasing the the

Sensors 2017, 17, 2422 11 of 14

Sensors 2017, 17, 2422 11 of 14 sensor signal is accumulating until saturation with ammonia is reached. Furthermore, increasing the ammonia concentration to 1000 ppm also leads to a distinguishable but smaller resonant frequency shift. TheThe differencesdifferences in in the the sensor sensor signal signal between between the 500 the ppm 500 saturation ppm saturation and the 1000and ppmthe 1000 saturation ppm issaturation due to the is due higher to the partial higher pressure partial pressure of NH3 that of NH causes3 that a causes shift towards a shift towards the adsorbed the adsorbed phase phase in the adsorptionin the adsorption isotherm. isotherm. The feed The withfeed 1000with ppm1000 ppm ammonia ammonia is ﬁnally is finally stopped stopped and theand gasthe sensorgas sensor test chambertest chamber is purged is purged with with pure pure N2 (t 3N).2 Consequently,(t3). Consequently, the resonant the resonant frequency frequency shift decreases, shift decreases, but not but to thenot initialto the valueinitial asvalue probably as probably expected expected from water from desorption. water desorption. It is assumed It is assumed that only that so-called only so-called weakly bondedweakly NHbonded3 can NH desorb3 can at desorb room temperature.at room temperature. Strongly bonded Strongly ammonia bonded remains ammonia on remains the zeolite on even the inzeolite a pure even nitrogen in a pure atmosphere nitrogen at atmosphere this low temperature. at this low These temperature. results agree These well results with data agree obtained well with for >200data obtained◦C for an for H-ZSM5 >200 °C zeolite for an obtained H-ZSM5 in zeolite an electrical obtained in situin an cavity electrical resonator in situ [40 cavity], if one resonator extrapolates [40], theirif one data extrapolates to room temperature. their data However, to room with temper respectature. to gas-sensingHowever, applications,with respect it isto cleargas-sensing that one needsapplications, a higher it is temperature clear that one at whichneeds a the higher desorption temperature is fast at enough which tothe guarantee desorption a reversibleis fast enough sensor to behavior.guarantee Therefore, a reversible it issensor mandatory behavior. to establish Therefore, a heater it is mandatory layer on the to sensor establis substrate.h a heater This layer will on be the nextsensor step substrate. during theThis sensor will be development. the next step during the sensor development.

Figure 11. Time dependencydependency ofof thetheresonant resonant frequency frequency of of the the microwave microwave sensor sensor according according to to Figure Figure4c

when4c when 500 500 ppm ppm (t1 )(t or1) or 1000 1000 ppm ppm NH NH3 (t32 ()t are2) are added added to to N 2N.2 At. Att3 t,3 NH, NH3 3was was turned turned off. off.

6. Conclusions and Outlook This work investigated the design parameters of a microstrip ring resonator with an Fe-zeolite as a gas-sensitive layer. Based on FEM simulationsimulation results and commonly used microstrip design equations, a planar microstripmicrostrip ringring resonatorresonator structurestructure on on alumina alumina was was developed developed and and covered covered with with a Fe-zeolitea Fe-zeolite film. film. Finally, Finally, the the device device was was successfully successfully operated operated at around at around 8.5 GHz 8.5 asGHz a humidity as a humidity sensor andsensor for and ammonia-loading for ammonia-loading detection detection at room at temperature. room temperature. The amount The ofamount stored of water stored and water ammonia and inammonia the zeolite in the is mirroredzeolite is mirrored by both the by absoluteboth the absolu value ofte value the reflection of the reflection coefficient coefficient at resonance at resonance and by theand resonantby the resonant frequency frequency itself. Due itself. to the Due better to the resolution, better resolution, we conclude we thatconc thelude resonant that the frequency resonant isfrequency a suitable is measure a suitable for ammoniameasure for and ammonia water detection and water with thisdetection planar with device. this Furthermore, planar device. the concentration-dependentFurthermore, the concentration-dependent frequency shifts during freq loadinguency illustrate shifts theduring accumulating loading characteristics illustrate the of theaccumulating sensor signal characteristics and make it of conceivable the sensor tosignal measure and make small it amounts conceivable of gas to concentrationsmeasure small byamounts using theof gas device concentrations operating inby the using so-called the device dosimeter-mode operating in [ 45the]. so-called dosimeter-mode [45]. In future work, work, an an integrated integrated heating heating element element will will be be added added to to the the device. device. Then Then it may it may work work as asa fast a fast and and reversible reversible gas gas sensor. sensor. In addition, In addition, it may it may help help to determine to determine adsorption adsorption isotherms. isotherms. In Incombination combination with with in insitu situ DRIFT DRIFT spectroscopy, spectroscopy, the the effe effectct of of distinct distinct adsorbed adsorbed species species on on the the dielectric dielectric behavior of the gas sensitive material can alsoalso be studied in a widewide temperaturetemperature regime.regime.

Acknowledgments: The authors would like to thankthank M. Dietrich and D. Rauch for the constructive discussions and some technical assistance. The publication of this paper was funded by the German Research Foundation (DFG) and the University ofof Bayreuth in thethe fundingfunding programprogram “Open“Open AccessAccess Publishing”.Publishing”.

Author Contributions: A.B., C.S., S.W., G.H., and R.M. conceived the experiments. J.K. manufactured the ceramic transducers of the devices in thin- and thick-film technology. A.B. performed the experiments. All together analyzed the data, evaluated the results, and wrote the paper.

Conflicts of Interest: The authors declare no conflict of interest.

Sensors 2017, 17, 2422 12 of 14

Author Contributions: A.B., C.S., S.W., G.H., and R.M. conceived the experiments. J.K. manufactured the ceramic transducers of the devices in thin- and thick-film technology. A.B. performed the experiments. All together analyzed the data, evaluated the results, and wrote the paper. Sensors 2017, 17, 2422 12 of 14 SensorsConflicts 2017 of, 17 Interest:, 2422 The authors declare no conflict of interest. 12 of 14

Appendix A Field distribution in a microstrip waveguide.waveguide

Figure A1. Microstrip waveguide: (a) cross section with ground plane and strip conductor; (b) electric Figure A1. Microstrip waveguide: (a) cross section with ground plane and strip conductor; (b) electric (red)Figure and A1. magnetic Microstrip (blue) waveguide: ﬁeld distribution (a) cross section with sample. with ground plane and strip conductor; (b) electric (red) and magnetic (blue) field distribution with sample.

SimplifiedSimpliﬁed setup for the initial measurements.measurements

Figure A2.A2. SchematicSchematicSchematic illustrationillustration illustration ofof of thethe the simplifiedsimplified simpliﬁed exex experimentalperimental setup setup for for basic verification veriﬁcation of adsorption and desorption monitoring with thethe microwavemicrowave sensorsensor andand aa VNA.VNA.

References

1. Comini, E.; Faglia, G.; Sberveglieri, G. G. Solid State Gas Sensing;; SpringerSpringer InternationalInternational PublishingPublishing AG:AG: AG: Heidelberg, Germany, 2009.2009; doi:10.1007/978-0-387-09665-0.

2. Bănică,, F.-G. F.-G. ChemicalChemical SensorsSensors andand Biosensors:Biosensors: Fundamentals Fundamentals and and Applications Applications;;; JohnJohn John WileyWiley Wiley && & Sons,Sons, Ltd.:Ltd.: Ltd.: Chichester, UK, 2012; pp.pp. 246–257.246–257, [doi:10.1002/9781118354162.CrossRef] 3. Yamazoe, N.; Shimanoe, K. FundamentalsFundamentals of SemiconductorSemiconductor Gas Sensors.Sensors. InIn Semiconductor Gas Gas Sensors ;;; Jaaniso, R., Tan, O.K.,O.K., Eds.;Eds.; WoodheadWoodheadWoodhead Publishing PublishingPublishing Ltd.: Ltd.:Ltd.: Cambridge,Cambridge, UK,UK, 2013;2013; pp.pp. 3–34.3–34. 4. Barsan, N.; Koziej, D.; Weimar, U.U. Metal Oxide-Based GasGas Sensor Research:Research: How to? Sens. Actuators Actuators B B Chem. Chem. 2007,, 121,, 18–35, 18–35.18–35, doi:10.1016/j.snb.2006.09.047.doi:10.1016/j.snb.2006.09.047. [CrossRef] 5. Zarifi,Zarifi, M.H.; Shariaty, P.; Hashisho,Hashisho, Z.;Z.; Daneshmand,Daneshmand, M.M. A Non-Contact Microwave Sensor for Monitoring the Interaction of Zeolite 13X with CO2 and CH4 in Gaseous Streams. Sens. Actuators B Chem. 2017, 238, the Interaction of Zeolite 13X withwith COCO22 and CH4 in Gaseous Streams. Sens. Actuators Actuators B B Chem. 20172017,, 238238,, 1240–1247.1240–1247, doi:10.1016/j.snb.2016.09.047. [CrossRef] 6. Zarifi,Zarifi, M.H.; Sohrabi, A.; Shaibani Shaibani,,, P.M.;P.M.; Daneshmand,Daneshmand, M.;M.; Thundat,Thundat, Thundat, T.T. T. DetectionDetection ofof VolatileVolatile Organic Compounds Using Microwave Sensors. IEEE Sens. J. 2015,, 15,, 248–254, 248–254.248–254, doi:10.1109/JSEN.2014.2345477.doi:10.1109/JSEN.2014.2345477. [CrossRef] 7. Rossignol, J.; Barochi, G.; de Fonseca, B.; B.; Brunet, Brunet, J.; J.; Bouvet, Bouvet, M.; M.; Pauly, Pauly, A.; Markey, Markey, L. Microwave-based gas sensor withwith phthalocyanine phthalocyanine film film at roomat room temperature. temperature.Sens. ActuatorsSens. Actuators B Chem. B2013 Chem., 189 ,2013 213–216.,, 189,, [ CrossRef213–216,213–216,] doi:10.1016/j.snb.2013.03.092. 8. Bailly, G.; Harrabi, A.; Rossignol, J.; Stuerga, D.; Pribetich, P. Microwave Gas Sensing with a Microstrip Interdigital Capacitor: Detection of NH3 with TiO2 nanoparticles. Sens. Actuators B Chem. 2016, 236, 554– Interdigital Capacitor: Detection of NH3 with TiO2 nanoparticles. Sens. Actuators B Chem. 2016, 236, 554– 564, doi:10.1016/j.snb.2016.06.048. 9. Bailly, G.; Rossignol, J.; de Fonseca, B.; Pribetich, P.; Stuerga, D. Microwave Gas Sensing with Hematite: Shape Effect on Ammonia Detection Using Pseudocubic, Rhombohedral, and Spindlelike Particles. ACS Sens. 2016,, 1,, 656–662,656–662, doi:10.1021/acssensors.6b00297.doi:10.1021/acssensors.6b00297.

Sensors 2017, 17, 2422 13 of 14

8. Bailly, G.; Harrabi, A.; Rossignol, J.; Stuerga, D.; Pribetich, P. Microwave Gas Sensing with a Microstrip

Interdigital Capacitor: Detection of NH3 with TiO2 nanoparticles. Sens. Actuators B Chem. 2016, 236, 554–564. [CrossRef] 9. Bailly, G.; Rossignol, J.; de Fonseca, B.; Pribetich, P.; Stuerga, D. Microwave Gas Sensing with Hematite: Shape Effect on Ammonia Detection Using Pseudocubic, Rhombohedral, and Spindlelike Particles. ACS Sens. 2016, 1, 656–662. [CrossRef] 10. Chen, L.F.; Ong, C.K.; Neo, C.P.; Varadan, V.V.; Varadan, V.K. Microwave Electronics: Measurement and Materials Characterization; John Wiley & Sons, Ltd.: Chichester, UK, 2004. 11. Yeow, Y.; Abbas, Z.; Khalid, K. Application of Microwave Moisture Sensor for Determination of Oil Palm Fruit Ripeness. Meas. Sci. Rev. 2010, 10, 714. [CrossRef] 12. Yogi, R.A.; Parolia, R.S.; Karekar, R.N.; Aiyer, R.C. Microwave Microstrip Ring Resonator as a Paper Moisture Sensor: Study with Different Grammage. Meas. Sci. Technol. 2002, 13, 1558–1562. [CrossRef] 13. Sarabandi, K.; Li, E.S. Microstrip Ring Resonator for Soil Moisture Measurements. IEEE Trans. Geosci. Remote Sens. 1997, 35, 1223–1231. [CrossRef] 14. Abegaonkar, M.P.; Karekar, R.N.; Aiyer, R.C. A Microwave Microstrip Ring Resonator as a Moisture Sensor for Biomaterials: Application to Wheat Grains. Meas. Sci. Technol. 1999, 10, 195–200. [CrossRef] 15. Schwerthoeffer, U.; Weigel, R.; Kissinger, D. A Highly Sensitive Glucose Biosensor Based on a Microstrip Ring Resonator. In Proceedings of the IEEE MTT-S International Microwave Workshop Series on RF and Wireless Technologies for Biomedical and Healthcare Applications (IMWS-BIO), Singapore, 9–11 December 2013; pp. 1–3. [CrossRef] 16. Zarifi, M.H.; Thundat, T.; Daneshmand, M. High resolution microwave microstrip resonator for sensing applications. Sens. Actuators A Phys. 2015, 233, 224–230. [CrossRef] 17. Yi, F.-Y.; Chen, D.; Wu, M.-K.; Han, L.; Jiang, H.-L. Chemical Sensors Based on Metal–Organic Frameworks. ChemPlusChem 2016, 81, 675–690. [CrossRef] 18. Marr, I.; Reiß, S.; Hagen, G.; Moos, R. Planar Zeolite film-Based Potentiometric Gas Sensors Manufactured by a Combined Thick-film and Electroplating Technique. Sensors 2011, 11, 7736–7748. [CrossRef][PubMed] 19. Chen, P.; Schönebaum, S.; Simons, T.; Rauch, D.; Dietrich, M.; Moos, R.; Simon, U. Correlating the Integral Sensing Properties of Zeolites with Molecular Processes by Combining Broadband Impedance and DRIFT Spectroscopy—A New Approach for Bridging the Scales. Sensors 2015, 15, 28915–28941. [CrossRef][PubMed] 20. Xu, X.; Wang, J.; Long, Y. Zeolite-based Materials for Gas Sensors. Sensors 2006, 6, 1751–1764. [CrossRef] 21. Bahoumina, P.; Hallil, H.; Lachaud, J.L.; Rebiere, D.; Dejous, C.; Abdelghani, A.; Frigui, K.; Bila, S.; Baillargeat, D.; Zhang, Q.; et al. Chemical Gas Sensor Based on a Novel Capacitive Microwave Flexible Transducer and Composite Polymer Carbon Nanomaterials. In Proceedings of the Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), Cannes, France, 1–4 April 2017. [CrossRef] 22. Zarifi, M.H.; Gholidoustb, A.; Abdolrazzaghic, M.; Shariatyb, P.; Hashisho, Z.; Daneshmand, M. Sensitivity

Enhancement in Planar Microwave Active-Resonator Using Metal Organic Framework for CO2 Detection. Sens. Actuators B Chem. 2017, in press. [CrossRef] 23. Bailly, G.; Harrabi, A.; Rossignol, J.; Michel, M.; Stuerga, D.; Pribetich, P. Microstrip Spiral Resonator For Microwave-Based Gas Sensing. IEEE Sens. Lett. 2017, 1.[CrossRef] 24. Piekarz, I.; Socki, J.; Wincza, K.; Gruszczynski, S. Microwave Sensors for Dielectric Sample Measurement Based on Coupled-Line Section. IEEE Trans. Microw. Theory Tech. 2017, 65, 1615–1631. [CrossRef] 25. Yakhlef, Y.; Benhabiles, M.T.; Benkhaoua, L.; Riabi, M.L. Compact Miniature Sensors Based on Tapered Lines Coupled Metamaterial Resonators. In Proceedings of the First IEEE MTT-S International Microwave Bio Conference (IMBIOC), Gothenburg, Sweden, 15–17 May 2017; pp. 1–4. 26. Denoual, M.; Pouliquen, M.; Jorel, C.; Radu, C.; Robbes, D.; Harnois, M.; de Sagazan, O.; Grand, J.; Awala, H.; Mintova, S.; et al. Zeolite-Based Thermal Mass Gas Sensor with Self-Identiﬁcation Algorithm. In Proceedings of the Symposium on Design, Test, Integration and Packaging of MEMS/MOEMS (DTIP), Gothenburg, Sweden, 19–21 September 2017; pp. 1–4. 27. Reinecke, T.; Walter, J.-G.; Kobelt, T.; Ahrens, A.; Scheper, T.; Zimmermann, S. Biosensor Based on a Split-Ring Resonator. In Proceedings of the SENSOR 2017, Nuremberg, Germany, 30 May–1 June 2017; pp. 78–83. [CrossRef] 28. Rydosz, A.; Maciak, E.; Wincza, K.; Gruszczynski, S. Microwave-Based Sensors with Phthalocyanine Films for Acetone, Ethanol and Methanol Detection. Sens. Actuators B Chem. 2016, 237, 876–886. [CrossRef] Sensors 2017, 17, 2422 14 of 14

29. Kröcher, O.; Devadas, M.; Elsener, M.; Wokaun, A.; Söger, N.; Pfeifer, M.; Demel, Y.; Mussmann, L.

Investigation of the Selective Catalytic Reduction of NO by NH3 on Fe-ZSM5 Monolith Catalysts. Appl. Catal. B Environ. 2006, 66, 208–216. [CrossRef] 30. Reiß, S.; Schönauer, D.; Hagen, G.; Fischerauer, G.; Moos, R. Monitoring the Ammonia Loading of Zeolite-Based Ammonia SCR Catalysts by a Microwave Method. Chem. Eng. Technol. 2011, 34, 791–796. [CrossRef] 31. Pozar, D.M. Microwave Engineering, 4th ed.; John Wiley & Sons, Ltd.: Hoboken, NJ, USA, 2012. 32. Dietrich, M.; Rauch, D.; Porch, A.; Moos, R. A Laboratory Test Setup for in Situ Measurements of the Dielectric Properties of Catalyst Powder Samples under Reaction Conditions by Microwave Cavity Perturbation: Set up and Initial Tests. Sensors 2014, 14, 16856–16868. [CrossRef][PubMed]

33. Heine, C.; Girgsdies, F.; Truschke, A.; Schlögl, R.; Eichelbaum, M. The Model Oxidation Catalyst α-V2O5: Insights from Contactless in Situ Microwave Permittivity and Conductivity Measurements. Appl. Phys. A 2013, 112, 289–296. [CrossRef] 34. Beulertz, G.; Votsmeier, M.; Moos, R. Effect of Propene, Propane, and Methane on Conversion and Oxidation State of Three-Way Catalysts: A Microwave Cavity Perturbation Study. Appl. Catal. B 2015, 165, 369–377. [CrossRef] 35. Dietrich, M.; Rauch, D.; Simon, U.; Porch, A.; Moos, R. Ammonia Storage Studies on H-ZSM-5 Zeolites by Microwave Cavity Perturbation: Correlation of Dielectric Properties with Ammonia Storage. J. Sens. Sens. Syst. 2015, 4, 263–269. [CrossRef] 36. Feulner, M.; Seufert, F.; Müller, A.; Hagen, G.; Moos, R. Inﬂuencing Parameters on the Microwave-Based Soot Load Determination of Diesel Particulate Filters. Top. Catal. 2017, 60, 374–380. [CrossRef] 37. Moos, R. Microwave-Based Catalyst State Diagnosis—State of the Art and Future Perspectives. SAE Int. J. Engines 2015, 8, 1240–1245. [CrossRef] 38. Hoffmann, R.K. Integrierte Mikrowellenschaltungen; Springer: Berlin/Heidelberg, Germany, 1983. 39. Dietrich, M.; Steiner, C.; Hagen, G.; Moos, R. Radio-Frequency-Based Urea Dosing Control for Diesel Engines with Ammonia SCR Catalysts. SAE Int. J. Engines 2017, 10, 1638–1645. [CrossRef] 40. Rauch, D.; Dietrich, M.; Simons, T.; Simon, U.; Porch, A.; Moos, R. Microwave Cavity Perturbation Studies on H-form and Cu Ion-Exchanged SCR Catalyst Materials: Correlation of Ammonia Storage and Dielectric Properties. Top. Catal. 2017, 60, 243–249. [CrossRef]

41. Simon, U.; Flesch, U.; Maunz, W.; Müller, R.; Plog, C. The Effect of NH3 on the Ionic Conductivity of Dehydrated Zeolites Na Beta and H Beta. Microporous Mesoporous Mater. 1998, 21, 111–116. [CrossRef] 42. Anderson, P.A.; Armstrong, A.R.; Porch, A.; Edwards, P.P.; Woodall, L.J. Structure and Electronic Properties of Potassium-Loaded Zeolite L. J. Phys. Chem. B 1997, 101, 9892–9900. [CrossRef] 43. Kraus, M.; Kopinke, F.-D.; Roland, U. Inﬂuence of Moisture Content and Temperature on the Dielectric Permittivity of Zeolite NaY. Phys. Chem. Chem. Phys. 2011, 13, 4119–4125. [CrossRef][PubMed] 44. Niwa, M.; Iwamoto, M.; Segawa, K. Temperature-Programmed Desorption of Ammonia on Zeolites. Inﬂuence of the Experimental Conditions on the Acidity Measurement. BCSJ 1986, 59, 3735–3739. [CrossRef] 45. Marr, I.; Groß, A.; Moos, R. Overview on Conductometric Solid-State Gas Dosimeters. J. Sens. Sens. Syst. 2014, 3, 29–46. [CrossRef]

© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).