Dielectric properties measurement system at cryogenic temperaturas and microwave frequencies

Transcripción

1 735 ISSN X Dielectric properties measurement system at cryogenic temperaturas and microwave frequencies Molla, J. 1 Ibarra, A. 1 Margineda, J. 2 Zamarro, J.M. 2 Hernández, A. 2 1 EURATOM-CIEMAT Association. CIEMAT. Av. Complutense, Madrid. Spain. 2 Dpto. de Física Aplicada. Campus de Espinardo. Universidad de Murcia Murcia. Spain. CENTRO DE INVESTIGACIONES ENERGÉTICAS, MEDIOAMBIENTALES Y TECNOLÓGICAS MADRID, 1994

2 CLASIFICACIÓN DOE Y DESCRIPTORES: DIELECTRIC PROPERTIES PERMITTIVITY DIELECTRIC MATERIALS ALUMINIA CORUNDUM CAVITY RESONATORS

3 Toda correspondencia en relación con este trabajo debe dirigirse al Servicio de Información y Documentación, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Ciudad Universitaria, MADRID, ESPAÑA. Las solicitudes de ejemplares deben dirigirse a este mismo Servicio. Los descriptores se han seleccionado del Thesauro del DOE para describir las materias que contiene este informe con vistas a su recuperación. La catalogación se ha hecho utilizando el documento DOE/TIC-4602 (Rev. 1) Descriptive Cataloguing On- Line, y la clasificación de acuerdo con el documento DOE/TIC.4584-R7 Subject Categories and Scope publicados por el Office of Scientific and Technical Information del Departamento de Energía de los Estados Unidos. Se autoriza la reproducción de los resúmenes analíticos que aparecen en esta publicación. Este trabajo se ha recibido para su impresión en Junio de 1993, Depósito Legal n M ISBN ISSN X ÑIPO IMPRIME CIEMAT

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5 INTRODUCTION Over the last ten years considerable interest has grown in the characterization of the dielectric properties of insulator materials with a very low loss tangent. This is due to their increasing importance in different high technology fíelds ( nuclear fusión, microelectronics, materials processing, etc), and is particularly true at low temperatures and at microwave and millimeter wavelengths [1; 2; 3]. The temperature dependence of the dielectric properties is poorly known in the insulator materials, and the results available are not in agreement. The interest in a precise characterization of these properties can be illustrated by the following application. It has recently been proposed to use radiofrequency windows cooled down to K in high power gyrotrons for electrón cyclotron (EC) heating systems of fusión devices to increase the power capabilities of these machines [4; 5]. The windows must provide a minimum of reflected and absorbed power. As the electrical length of the window is determined by the permittivity (a function of temperature) and the temperature profile is determined by the power absorption (i.e. the loss tangent), a precise characterization of the temperature dependence of dielectric properties is needed for modelling the window performance and henee determining the feasibility of these designs. In addition, data on dielectric properties at low temperatures and microwave frequencies are required for oxides due to their important role as substrate materials in microelectronics and in the development of high Te superconductor films 6: 7]. From the fundamental point of view the study of the temperature dependence is also of interest. The permittivity of dielectric materials is closely related to its structure, defect types and their concentration. It depends on the frequeney of the applied field and also on the temperature [8]. Historically, the dielectric properties (permittivity and loss tangent) has received considerable attention at low and optical frequencies but it has been less deeply studied at microwaves and millimeter wavelengths, mainly due to the difficulties of the associated instrumentation. At these frequencies the dielectric properties provide information about both the dipolar and ionic response of the material, although the theoretical predictions for the expected behaviour are not clearly established, especially for the case of low loss materials. The temperature dependence for very puré materials can provide information about the different mechanisms for the phonon-photon interaction [5; [9]- The aim of this paper is to describe a cyündrical cavity system for the measurement of dielectric properties at microwave frequencies in the temperature

6 range from 80 K to 300 K. The system may be used for insulating materials of interest in fusión technology. i.e. permittivity valúes below 15 and loss tangent below 10"" 3. MEASUREMENT SYSTEM DESCRIPTION It is possible to measure the dielectric properties of insulator materials at low temperatures using different techniques. References for systems based on closed cavity resonators [10], ring resonators (ii], open Fabry-Perot resonators [12] or FTIR [2] can be found in the literature. In general, the most widely used method is based on resonant techniques in which the permittivity and loss tangent are obtained from the comparison between the resonant frequency and the quality factor of the resonator with and without a sample [13]. The main disadvantage of all these systems is that they require a very high temperature stability ( around 10 K) to avoid uncontrolled temperature gradients. In this work a new procedure will be described which avoids this problem. The selection of the resonator type (ring, Fabry-Perot, closed cylindrical, closed reentrant or others) is a compromise between the operating frequency and sample size. Taking this into account. a closed cylindrical type has been used. However most of the techniques described are useful for any of them. Closed cylinder resonators have been used extensively during the last 50 years and some very comprehensive works have been published i 13; 14). Here we will not make a detailed description of the whole measurement procedure. but only those points relevant to the temperature dependence study will be presented. The resonator is a right circular cylinder, 30 mm diameter and mm long, operating by transmission in TEQI modes (usually n=l, 2 or 3) at GHz. The cavity characteristics are determined by measuring the power transmitted through the cavity at different frequencies. The microwave oscillator ( HP8350B + HP83593 Plug-In ) generates a CW signal from 10 khz to 20 GHz. A frequency counter ( EIP 578B ) stabilizes the signal by means of a phase-lock loop with 1 Hz resolution and 10 khz steps. Transmitted power is detected and measured using a scalar network analyzer (HP8757C ). Furthermore, the oscillator and scalar network analyzer can opérate in sweep mode. This is very useful to find the transmitted resonance peak when an unknown dielectric has to be measured. The coupling between microwave Unes and the cavity is made through

7 slots 4 mm high, 0.5 mm wide through a wall thickness of 0.5 mm, with the higher dimensión parallel to the axis of the cylinder. The slots are situated on the middle of the cylinder walls at symmetric positions. This configuration, together with a small separation between the cylinder block and the end plates avoids the excitation of the TM1 ln modes [13]. A HP300 personal computer controls all the instrumentation via a standard GP-IB (IEEE-488). It manages the data flow and makes the calculations using a computer program. Transmitted power near the resonance is given by : E o ( i ) where p a =p(f & ) and Q is the loaded Q- or quality factor 115]. Experimentally about 10 valúes (p, f ) are measured at stabilized frequencies around the resonant frequency f {). The Q-factor and resonant frequency are computed by fitting the theoretical expression to the experimental data using the procedures described in 15. The whole procedure takes about 30 s. Other methods. like the one used in 116] can be used. The coupling factors ( p\ and fu ) between the transmission Unes and the cavity are needed to obtain the unloaded Q-value from the measured loaded Q t -valué, using the expression Q = Qi.(l + P 1 + P 2 ) (2) The coupling factors can be obtained from the measurements of the reflected and transmitted power 15]. If they are small enough. one may aproximate pxfjjxfu and obtain it only from the transmitted power. In our cavity ps=10" 2 and taking into account that the coupling slots have the same dimensions and are made in symmetrical positions in the cavity, it can be supposed that the error in Q, due to such an approximation, is negligible. For stable temperatures accuracies of 5 khz for the resonant frequency and 0.5 % for the Q-factor can be obtained.

8 TEMPERATURE DEPENDENT MEASUREMENT As previously mentioned, several systems to measure the temperature dependence of dielectric properties have been described. One of the main problems in these systems is to avoid the errors in the determination of the cavity characteristics induced by vanations mainly due to the temperature dependence of resonant frequency (typical valúes around 300 khz/k). Taking account the obtainable experimental accuracies, the resonant frequency determination requires temperature stability around 0.01 K. It is rather complex to obtain this stability in large resonators as used in most of the systems. Moreover, the irreproducible nature of the power deposition by the temperature controller, induces temperature instabilities that reduces the attainable precisión. As it was previously mentioned, in this work a new technique avoids the stabilization of the temperature using the following procedure. The cavity, in nitrogen atmosphere, is cooled down to 80 K by a liquid nitrogen bath (see figure 1). The temperature of both cavity end plates is measured by two chromel-alumel thermocouples. The sample must be circular with the same diameter as the cavity and must be placed on one of the end plates during the measurement. The thermocouple voltages are measured with a digital graphic register (Yokogawa LR4220). also controllable via GP-IB. The objective of the measurement is to determine f f1 and Q as a function of the temperature. This requires the characterization of the resonant peak at several temperature points, i.e. the measurement of 10 power-frequency (p-, f-) pairs for each temperature. The measurements are made during a "free heating" of the cavity during which the resonant peak shifts about 40 khz/min. Taking account that this change is larger than the accuracy in the resonant frequency determination, each one of the frequency-power (f-, p ') pairs is associated with a resonant peak with a different resonance frequency (f Q ) according with the expression (1) (3) assuming that Q and p Q do not change during the measuring time (around 30 s). This assumption can be justified taking account that Qj changes about 0.2%/min, which is well below the experimental precisión. Similar arguments can be applied top 0.

9 To avoid this problem the following argument can be applied. In general, let N pairs be measured at equal frequency steps and at a constant time interval. If the resonance frequency changes slowly with time, the instantaneous resonant frequency f () " can be approximated by: fo) (4) f 0 being the resonant frequency associated to the point (N/2), and 6 a constant. 6 is negative if the measurement is made increasing the frequency (f j+1 >f j) and positive if the measurement is made decreasing the frequency (f +1 <f )- Substituting (4) in (3) gives, rt &>- r (5) l+4qf(l-5) 2 f -1 In others words, a uniform shift of the resonant frequency during the measurement produces a data set that fits to a resonance peak with a false valué of Q L =Q,_(1± 6 ) "+" if f, >f and "-" if f, < f 1+1 (6) This error can be eliminated by doing two measurements. one with an increasing frequency sweep and another with a decreasing frequency sweep. The correct Q, is the mean valué of both measurements. This argument can be applied for the determination of the true dependence of the Q-factor of a cavity independently of the origin of the process that induces the change of the resonant frequency. In our case, it is the increasing temperature of the system. Then. using this procedure the quality factor and resonance frequency of the cavity are determined as a function of temperature with the same precisión as at constant temperature. The behaviour of the cavity and cryostat are very reproducible from one measurement to another on heating because there are no internal heat sources. The resonant frequency and the quality factor of the cavity are measured continously during the slow heating up to room temperature ( heating rate < 0.15 Kmin" 1 ), taking about 500 measurements in 24 hours. In resonant methods, equations to calcúlate permittivity and loss tangent involve, besides the resonant frequency and quality factor, the sample and cavity

10 size as vvell as the surface resistivity of the cavity walls. These parameters also change with temperature and it is necessary to know their variation. The change of the sample size with temperature ( less than 0.05% in the whole temperature range ) can be neglected in the calculations of permittivity since it introduces a correction 1000 times lower than the temperature effect. The changes of the cavity size and resistivity are obtained from the temperature dependence of the resonant frequency and quality factor of the empty cavity. The change of the cavity dimensions are described by the thermal expansión coefficient while the temperature dependence of surface resistivity is governed by a resistivity coefficient. a) Thermal expansión coefficienu a): When the cavity temperature changes, the radius (a) and length (L) change in such a way that: J_dL_]_da_,, L dt ~ a dt A change in the cavity dimensions produces a change in its resonant frequency. f. For the circular cylinder operating in TEQj n mode: (8) with f =cx /2jta. f = nc/2l. c being the velocity of light and X () the first root of the derived Bessel function j () (x). Henee, the change in resonant frequency is determined by the change in L and a. Differentiating (8) one finds and therefore: 1 -*- (9) f dt tíl =^l (10) L a LJJ, a^j and f 0 being the length, radius and resonant frequency at room temperature, T (). So the valúes of length L. and radius a, at any temperature T, can

11 be calculated from the measurements oí the resonant frequency f. b) Resistivity coefficient {y): The quality factor (Q) of the circular cylinder forteqj n mode is given by the following expression l4]: f 2 2a f 2 c L n where d is the skin depth of the material. From the Q-factor, the resistivity can be obtained by the following equation l4]: p = ;t.i () fd~ (12) where j.i.=4te xlct Henry/m is the magnetic permeability in vacuum. The temperature dependence of resistivity in metáis is linear with temperature except for very low temperatures ( put" 1 if T«8 ;). where Q D is the Debye temperature ) 171. Usually the change in resisitivity is expressed by the resistivity coefficient y. defined by the following equation i8]: p(t)=p(t () )[l+ Y (T-T () )J (13) From all the considerations discusssed above. the measurement procedure can be summarized as follows: 1) resonant frequency and Q-factor in the empty cavity as a function of temperature are measured: 2) resonant frequency and Q-factor as function of temperature with the sample inside the cavity are measured; 3) surface resistivity and cavity dimensions as a function of the temperature, are calculated; 4) finally, loss tangent and permittivity are computed as function of temperature from the measured and calculated data, using standard procedures based in equantions given in [13], and described in detail in [19]. EXPERIMENTAL RESULTS A cavity made from oxygen free copper has been used to perform measurements. The data from the empty cavity were tested calculating the thermal

12 expansión and resistivity of copper. The quality factor vanes from to and.the resonant frequency increases by about 50 MHz when the temperature changes from 300 K to 80 K. From the measured data, using the equations (9) and (11), (12) and (13), a and y have been calculated. The agreement between measured a valúes and published data (20) is excellent, as can be seen in figure 2. The resistivity calculated with equation 12 is only an effective resistivity because it includes, in addition to the contribution described by the skin effect, other cavity losses such as surface impurities, roughness or geometrical effects. However, as shown in figure 3, it fits very well to a linear equation for T>80 K. It can be interesting to evalúate the magnitude of these extra losses. Figure 3 shows the effective resistivity obtained in our cavity, the contribution of the theoretical resisitivity of copper (using data from different sources) and the estimated extra losses. It is important to notice that this contribution has a very small temperature dependence, and so the obtained resisitivity coefficient (3.43x10 3 K" 1 at room temperature) is in closé agreement with the literature data: 3.93x10~ 3 K" 1 at d.c. [21], or 4.03xl0" 3 K" 1 (calculated from data found in different sources [20:22]). Using the above procedure. the permittivity of single crystal sapphire from Union Carbide (USA) has been obtained. Figure 4 shows the temperature dependence of the permittivity for the crystal orientation with the c axis perpendicular to the electric field. Permittivity change as a function of temperature is presented instead of absolute valúes. The valué measured at room temperature is Changes of permittivity as lovv as 0.01% can be detected, althought the accuracy for absolute permittivity is. 1%. The explanation for this s that permittivity change depends only on the resonant frequency change while absolute valúes depend also on the cavity radius which is the highest error source for permittivity. The results are very similar to those obtained by other authors at low frequencies (khz range)[23]. Figure 5 shows the temperature dependence of the loss tangent of alumina 99.9% purity, Bio/AL2O3 from Friedrichsfeld (Germany). As can be seen the resolution obtained is very good down to valúes of loss tangent of 10". The optimum resolution achieved in the system is 3x10. CONCLUSIONS A low temperature dielectric properties measurement system has been developed. The system able to measure from 80 K to 300 K has a resolution of 0.01% in permittivity change and 3x10" in loss tangent. The main advantage of the system is that it does not require temperature stabilization.

13 From the measured data for an empty cavity, the thermal expansión and the resistivity for copper have been calculated as a function of temperature. Permittivity of sapphire and loss tangent of alumina have been obtained at cryogenic temperatures. Permittivity valúes of sapphire are very similar to those reported at low frequency. ACKNOWLEDGMENTS The authors wish to thank Dr. R. Heidinger (Kernforschungszentrum Karlsruhe, Germany) for kindly supplying the alumina sample used in this work and to Dr. E.R. Hodgson (CIEMAT, Spain) for critical reading of the manuscript. This work has been supported by the European Fusión Technology Programme.

14 REFERENCES 1] R. Heidinger, "Dielectric loss of Alumina between 95 K and 330 K at ECRH frequencies"', /. M/c7. Mater., vol. 173, pp , [2] M. N. Afsar, H. Chi, X. Li, "Window materials for high power gyrotrons", in Digest of the 15th. Int. Conf. on Infrared and Millimeter Waves, vol. 1514, pp , [3] V. S. Ilchenko, "Intrinsic microwave dielectric losses in a-al 2 O 3 at K", Sov. Phys.Solid State, vol. 31(7), pp , ] H.U. Nickel, A. Hofmann, P. Norajitra, "Investigation on cryogenically cooled windows for MW/cw gyrotrons" in Digest of the 15th Int. Conf. on Infrared and Millimeter Waves, vol. 1514, pp , [5] P. B. Sushilin, A. Sh. Fiks and V. V. Parshin, "Perspectives of increasing the transmisión capacity of windows for energy extracción in gyrotrons" in Gyrotrons, Gorky, U. S. S. R., Academy of Sciences of the U.S.S.R., Institute of Applied Physics pp [6] J.S. Thorp. D. Evans. M. Al-Naief, M. Akhtaruzzaman. "The dielectric properties of aluminium nitride substrates for microelectronics packaging". J. Mater. Sci. vol. 25. pp , [7] J. Konopka. I. Wolff. S.J. Lewandowski. "Dielectric properties of CaNdAlO 4 at microwave frequencies". J. Appl. Phys. vol. 72 (1). pp [8] M. Sparks, D.F. King. D.L. Mills. "Simple theory of microwave absorption in alkali halides". Phys. Rev. B. vol. 26 (12) [9] V.L. Gurevich. A.K. Tagantsev. "Intrinsic dielectric loss in crystals", Adv. Phys, vol. 40(6). pp [10] D.M. Strayer. G.I. Dick, E. Tward. " Superconductor-Sapphire cavity for an all-cryogenic SCSO ". IEEE Trans. Magn. vol. 19 (3), pp , [11] V. B. Braginsky, V. S. Ilchenko, Kh. S. Bagdassarov, "Experimental obsevation of fundamental microwave absortion in high-quality dielectric crystals", Physics Letters A., vol. 120 (6), , [12] R. Heidinger, F. Koniger, G. Link, "A Fabry-Perot resonator system for mm-wave dielectric measurements at cryogenic temperaturas" in the Digest of the 15th Int. Conf. on Infrared and Millimeter Waves, vol. 1514, pp. 10

15 , f 13J R. J. Cook, "Microwave cavity methods" in High Frecuency Dielectric Measurements. J. Chamberlain, G. W. Chantry, Eds., Guilford, England: Technol. Press, 1973, pp [14] R. E. Collin, Foundations for Microwave Engineering, McGraw Hill International Book Company, 1966, ch. 2, pp. 36. [15] M. C. Sánchez, A. Hernández, E. Martin, J. Margineda, J. M. Zamarro, "New method for the measurements of coupling coefficients of transmission cavities", IEEProc, vol. 134, pp , [ 16] U. Stumper, "Automatic measurement of the complex permittivity at millimeter wavelengths", Int. Journal of Infrared an Millimeter Waves, vol. 2, pp , [17] C. Kittel Iníroduction to Solis State Physics, New York. NY: John Wiley & Sons, Inc. 1971,4 th ed. ch. 7, pp [ 18] F. J. Tischer, Y. H. Choung, "Anomalous temperature dependence of the surface resistance of copper at 10 GHz". IEE Proc. vol. 129 (H2), pp , [ 19 J. Molla. A. Ibarra. "A system for the measurement of dielectric properties of insulator materials". Report EUR-CIEMAT 89/13 (1989) [20 D. E. Gray, ed., American lnstitute of Physics Handbook, 3 r edition, Me. Graw-Hill, 1972, ch. 9, pp (211 R- C. Weast, ed. Handbook of Chemistry and Physics 63 rd edition. CRC Press pp. E-81. [22] N. W. Ashcroft, N. D. Mermin. Salid State Physics, Holt Saunders Int. Ed ch.l,pp. 8. [23] R. Shelby, J. Fontanella. C. Andeen, "The low temperature electrical properties of some anisotropic crystals", J. Phys. Chem. Solids. vol. 41, pp , "11

16 FIGURE CAPTIONS Figure 1. Cryostat used for the low temperature measurements. Figure 2. Temperature dependence of the linear expansión coefficient of copper. ( O ) 120], ( m ) thiswork. Figure 3. Temperature dependence of copper resistivity. ( ) [20; 22], (# ) this work, ( ^ ) extra losses. Figure 4. Temperature induced permittivity change of sapphire ( E J_ c ).( O ) this work, ( % ) [23]. Figure 5. Temperature dependence of loss tangent of alumina 99.9% purity, Bio/AL2O3 from Friedrichsfeld. 12

17 Liquid Microwaves signal Nitrogen and thermocouples Nitrogen gas /Sample Liquid Nitrogen 1 1.Vacuum pump 13

18 2.5 i i * wi 1.5 h 1 h 5 h Temperature ( K ) 14

19 a (10" 5 K" 1 ) H 5-15

20 > I o o o o o 16

21 tg8 (10 -A 17

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23 CIEMAT-735 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas. Instituto de Investigación Básica. "Dielectric properties measurenient systeni at cryogenic temperatures and microwave frequencies". MOLIÁ J., II3ARRA, A., MARÜINEDA, J., ZAMARRO, J.M., HERNÁNDEZ, A. (1994) 23 pp,; 5 í'igs.; 23 reís. A systeni based on ihc resonant cavity method has been developed to measure the permittivity and loss tangent at GHz over the temperatura range 80 K to 300 K. Changos of permittivity as low as 0.01 % in the rango 1 to 30, and 3 x 10' 6 for loss tangent valúes below 10" 2, can be obtained without requiríng temperature stability. The thcrmanl expansión coefficient and resistivily factor of copper have been measured between 80 K and 300 K. Permittivity of sapphire and loss tangent of alumina of 99.9 % purity in the same temperature range are presented. DOE CLASSIFICATION AND DESCRIPTORS: Dielectric properties. Permittivity. Dielectric malcriáis. Aluminia. Corundum. Cavity Resonators. CIEMAT-735 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas. Instituto de Investigación Básica. "Dielectric properties measurenient system at cryogenic temperatures and microwave frequencies". MOLIÁ,.1., IBARRA, A., MARG1NEDA, J., ZAMARRO, J.M., HERNÁNDEZ, A. (1994) 23 pp.; 5 figs.; 23 refs. A system based on the resonant cavity method has been developed to measure the permittivity and loss tangent at GHz over the temperature range 80 K to 300 K. Changes of permittivity as low as 0.01 % in the range 1 to 30, and 3 x 10' 6 for loss tangent valúes below 10" 2, can be obtained without requiring temperature stability. The thermanl expansión coefficient and resistivity factor of copper have been measured between 80 K and 300 K. Permittivity of sapphire and loss tangent of alumina of 99.9 % purity in the same temperature range are presented. DOE CLASSIFICATION AND DESCRIPTORS: Dielectric properties. Permittivity. Dielectric malcriáis. Aluminia. Corundum. Cavity Resonators. CIEMAT-735 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas. Instituto de Investigación Básica. "Dielectric properties measureinent system at cryogenic temperatures and microwave frequencies". -4- CIEMAT-735 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas. Instituto de Investigación Básica. "Dielectric properties measurement system at cryogenic temperatures and microwave frequencies". MOLLA, J., IBARRA, A., MARGINEDA,.1., ZAMARRO, J.M., HERNÁNDEZ, A. (1994) 23 pp.; 5 figs.; 23 refs. A system based on the resonant cavity method has been developed to measure the permittivity and loss tangent at GHz over the temperature range 80 K to 300 K. Changos of permittivity as low as 0.01 % in the range 1 to 30, and 3 x 10" 6 for loss tangent valúes below 10" 2, can be obtained without requiring temperature stability. The thermanl expansión coefficient and resistivity factor of copper have been measured between 80 K and 300 K. Permittivity of sapphire and loss tangent of alumina of 99.9 % purity in the same temperature range are presented. DOE CLASSIFICATION AND DESCRIPTORS: Dielectric properties. Permittivity. Dielectric malcriáis. Aluminia. Corundum. Cavity Resonators. MOLLA, J., IBARRA, A., MARGINEDA, J., ZAMARRO, J.M., HERNÁNDEZ, A. (1994) 23 pp.; 5 figs.; 23 refs. A system based on the resonant cavity method has been developed to measure the permittivity and loss tangent at GHz over the temperature range 80 K to 300 K. Changes of permittivity as low as 0.01 % in the range 1 to 30, and 3 x 10' 6 for loss tangent valúes below 10 2, can be obtained without requiring temperature stability. The thermanl expansión coefficient and resistivity factor of copper have been measured between 80 K and 300 K. Permittivity of sapphire and loss tangent of alumina of 99.9 % purity in the same temperature range are presented. DOE CLASSIFICATION AND DESCRIPTORS: Dielectric properties. Permittivity. Dielectric materials. Aluminia. Corundum. Cavity Resonators.

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25 CIEMAT-735 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas. Instituto de Investigación Básica. "Un sistema para la medida de propiedades dieléctricas a baja temperatura y frecuencia de microondas". MOLLA, I, IBARRA, A., MARGINEDA, J., ZAMARRO, J.M., HERNÁNDEZ, A. (1994) 23 pp.;5 figs.; 23 refs. Se ha desarrollado un sistema para la medida de la permitividad y la tangente de pérdidas entre 12 y 18 GHz en el rango de temperaturas entre 80 y 300 K, basado en el método de las cavidades resonantes. Pueden medirse cambios de la permitividad de hasta un 0.01 % para valores entre 1 y 30, así como cambios de la tangente de pérdidas de hasta 3 x 10' 6 para valores menores a 10' 2, sin necesidad de estabilizar la temperatura. Se ha medido el coeficiente de expansión térmica y la resistividad del cobre entre 80 y 300 K. Se presentan datos de la dependencia con la temperatura de la permitividad del zafiro y de la tangente de pérdidas de una alumina de una pureza del 99,9 %. CLASIFICACIÓN DOE Y DESCRIPTORES: Diclcctric properties. Pcrmittivity. Dielectric malcriáis. Aluminia. Corundum. Cavity Resonators. CIEMAT-735 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas. Instituto de Investigación Básica. "Un sistema para la medida de propiedades dieléctricas a baja temperatura y frecuencia de microondas". MOLLA, J., IBARRA, A., MARGINEDA, J., ZAMARRO, J.M., HERNÁNDEZ, A. (1994) 23 pp.; 5 figs.; 23 reís. Se ha desarrollado un sistema para la medida de la permitividad y la tangente de péry 18 GHz en el rango g de temperaturas entre 80 y 300 K,, basadoo en el méto- didas entre 12 d do de d las l cavidades idd resonantes. Pueden Pd medirse cambios de d la l permitividad iiidd de hasta un 0.01 % para valores entre 1 y 30, así como cambios de la tangente de pérdidas de hasta 3 x 1(V 6 para valores menores a 10, sin necesidad de estabilizar la temperatura. Se ha medido el coeficiente de expansión térmica y la resistividad del cobre entre 80 y 300 K. Se presentan datos de la dependencia con la temperatura de la permitividad del zafiro y de la tangente de pérdidas de una alumina de una pureza del 99,9 %. CLASIFICACIÓN DOE Y DESCRIPTORES: Dielectric properties. Pcrmittivity. Dielectric materials. Aluminia. Corundum. Cavity Resonators. CIEMAT-735 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas. Instituto de Invesiigación Básica. "Un sistema para la medida de propiedades dieléctricas a baja temperatura y frecuencia de microondas". CIEMAT-735 Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas. Instituto de Investigación Básica. "Un sistema para la medida de propiedades dieléctricas a baja temperatura y frecuencia de microondas". MOLLA, J., IBARRA, A, MARGINEDA, J., ZAMARRO, J.M., HERNÁNDEZ, A. (1994) 23 pp.; 5 figs.; 23 refs. Se ha desarrollado un sistema para la medida de la permitividad y la tangente de pérdidas entre 12 y 18 GHz en el rango de temperaturas entre 80 y 300 K, basado en el método de las cavidades resonantes. Pueden medirse cambios de la permitividad de hasta un 0.01 % para valores entre 1 y 30, así como cambios de la tangente de pérdidas de hasta 3 x 10' 6 para valores menores a 10" 2, sin necesidad de estabilizar la temperatura. Se ha medido el coeficiente de expansión térmica y la resistividad del cobre entre 80 y 300 K. Se presentan datos de la dependencia con la temperatura de la permitividad del zafiro y de la tangente de pérdidas de una alumina de una pureza del 99,9 %. CLASIFICACIÓN DOE Y DESCRIPTORES: Dielectric properties. Permittivity. Dielectric materials. Aluminia. Corundum. Cavity Resonators. MOLLA, J., IBARRA, A., MARGINEDA, J., ZAMARRO, J.M., HERNÁNDEZ, A. (1994) 23 pp.; 5 figs.; 23 refs. Se ha desarrollado un sistema para la medida de la permitividad y la tangente de pérdidas entre 12 y 18 GHz en el rango de temperaturas entre 80 y 300 K, basado en el método de las cavidades resonantes, rueden medirse cambios de la permitividad de hasta un 0.01 % para valores entre 1 y 30, así como cambios de la tangente de pérdidas de hasta 3 x 10' 6 para valores menores a 10" 2, sin necesidad de estabilizar la temperatura. Se ha medido el coeficiente de expansión térmica y la resistividad del cobre entre 80 y 300 K. Se presentan datos de la dependencia con la temperatura de la permitividad del zafiro y de la tangente de pérdidas de una alumina de una pureza del 99,9 %. CLASIFICACIÓN DOE Y DESCRIPTORES: Dielectric properties. Pcrmittivity. Dielectric materials. Aluminia. Corundum. Cavity Resonators.