R.G. Geyer

National Institute of Standards and Technology, Maryland, United States

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Publications (48)42.68 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A review of the most common methods for nondestructive permittivity and permeability measurements is presented. Transmission-line techniques, coaxial apertures, open resonators, surface-waves, and dielectric resonator methods are examined. Measurements on bulk, thin materials, and thin films are addressed. Measurement fixtures that can be used as sensors are highlighted. The frequency range of applicability and typical uncertainties associated with each method are addressed.
    Research in Nondestructive Evaluation 04/2009; 7(2):117-136. · 0.68 Impact Factor
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    ABSTRACT: In this paper, different densities within a ceramic are used to provide a wide continuous range of dielectric constants for high-frequency applications. Cofiring different ceramic materials together to make a single unified structure to obtain different dielectric constant combinations is quite difficult due to phase stability issues and shrinkage mismatches. However, using various levels of porosity in order to alter the effective dielectric constant in the same material allows patterning different dielectric constants into a single unit. Since the structure is made from a single material, the varying porosity regions can be made compatible. Glassy-carbon-assisted and microcellular-structure-based porous titania allow for an extremely wide range of dielectric constants, ranging from 12 to 90, while maintaining a low loss tangent. Highly anisotropic materials are demonstrated herein to achieve a dielectric constant contrast of 90/9.6 using large-range aligned microcellular structure. Dielectric-resonator antennas are shown as an application of adjusting the bandwidth between 0.5% and 2.5% by tailoring the ceramic dielectric constant. A stratified-medium-loaded cavity resonator and a buried dielectric ring resonator internal to a microcellular substrate are used to demonstrate both the cofiring and variable dielectric constant capabilities of structured porosity.
    IEEE Transactions on Microwave Theory and Techniques 12/2005; · 2.23 Impact Factor
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    ABSTRACT: Microwave dielectric properties of single-crystal incipient quantum ferroelectrics, KTaO3 and SrTiO3, have been measured at cryogenic temperatures. Cylindrical specimens were used as TE0n1-mode and quasi-TE011-mode dielectric resonators at temperatures ranging from 4 to 300 K. Conductive losses of the measurement resonant structures were taken into account, both as a function of frequency and temperature, so that uncertainties in the evaluated dielectric losses were ±5%. The real permittivity was measured with an accuracy of ±0.5%. The evaluated real permittivities of KTaO3 and SrTiO3 exhibit no ferroelectric transition, and remain paraelectric down to 5 K, consistent with soft-mode stabilization. Dielectric loss tangent values of KTaO3 at 3 GHz were 4.2×10−5 at 5.4 K, 8.9×10−5 at 77 K, and 1.4×10−4 at 300 K, while those of SrTiO3 were 3.4×10−3 at 5.4 K, 2.4×10−4 at 77 K, and 3.8×10−4 at 300 K. Results of the complex permittivity measurements are compared with theoretical predictions from a modified Devonshire phenomenological model.
    Journal of Applied Physics 05/2005; 97(10):104111-104111-6. · 2.21 Impact Factor
  • R.G. Geyer, J. Baker-Jarvis, J. Krupka
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    ABSTRACT: The microwave dielectric properties of single-crystal LiF, CaF<sub>2</sub>, MgF<sub>2</sub>, BaF<sub>2</sub>, and SrF<sub>2</sub>, synthesized by Stockbarger melt-growth techniques, are measured using cylindrical specimens as TE<sub>01δ</sub> dielectric resonators enclosed in a cylindrical cavity. Single-crystal permittivity and dielectric loss tangent were evaluated at fixed frequencies between 7 and 9 GHz and over a temperature range from -75 to 150°C. The real permittivities of the measured fluorides increase quasi-linearly with temperature, permitting evaluation of the thermal coefficients of permittivity. The dielectric loss tangents increase approximately linearly with frequency, so that Qf (GHz) products at room temperature for BaF<sub>2</sub>, SrF<sub>2</sub>, CaF<sub>2</sub>, LiF, and MgF<sub>2</sub> (normal to c-axis) are 57600, 73000, 92000, 192400, and 458600, respectively. The dielectric data supports existing ion polarizabilities that are used with molar volumes and molecular additivity rules to estimate the permittivities of more complex fluorides whose values have not been experimentally determined.
    Electrical Insulation and Dielectric Phenomena, 2004. CEIDP '04. 2004 Annual Report Conference on; 11/2004
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    ABSTRACT: The crystal structure of Ba11FeTi27O66.5 was determined using single-crystal and powder X-ray diffraction methods. This phase crystallizes in the monoclinic space group C2/m (No. 12) (a = 23.324(1) Å, b = 11.388(1) Å, c = 9.8499(3) Å, β = 90.104(3)°; Z = 2; ρcalcd. = 4.98 g/cm3), and exhibits a 10-layer structure built from close-packed [O,(Ba,O)] layers with a stacking sequence (cchhc)2. Octahedral sites are occupied by a mixture of Fe3+ and Ti4+, with some preferential ordering suggested by analysis of bond valence sums. The structure features vertex-, edge-, and face-sharing of the [Ti(Fe)O6] octahedra. Indexed X-ray powder diffraction data for a polycrystalline specimen are given. Ba11FeTi27O66.5 and the 8-layer phase Ba4Fe2Ti10O27 are built from the same types of polyhedral layers, some of which feature vacant sites between two Ba ions, which substitute for three oxygens in a row. The single-crystal results suggest that the basic structural formula of the phase is A11B28O66+x, with the value of x (and hence the Fe/Ti ratio) determined by partial occupancy of one of these vacant sites. Variation of this occupancy factor with synthesis temperature may account for apparent slight differences in the stoichiometry of this phase in polycrystalline and single-crystal form. However, solid solution formation was not observed for polycrystalline specimens. A comparison of the crystal structure obtained for Ba11FeTi27O66.5 with that previously proposed for “Zr4+-stabilized Ba2Ti5O12” indicates that the phase “Ba2Ti5O12” is actually a ternary compound which forms upon addition (either deliberately or inadvertently) of a trivalent ion such as Fe3+ or Al3+. The specimens Ba11Al2Ti26O66, Ba11Al2Ti24Sn2O66, and Ba11Al2Ti24Zr2O66 were also prepared and were found to form the A11B28O66+x-type phase. Ba11FeTi27O66.5 exhibits paramagnetic behavior that deviates somewhat from the Curie−Weiss Law below 75 K. Application of this formalism to the 1/χ vs. T data above 75 K yields an effective moment consistent with the presence of high-spin Fe3+ (S = 5/2), and a negative Weiss constant (about −25 K) indicating weak cooperative magnetic interactions that are overall antiferromagnetic. The relative permittivity and dielectric loss tangent of a sintered polycrystalline disk were measured at 5.33 GHz, yielding values (corrected for theoretical density) of 55 and 7.7(±0.3) × 10−4, respectively. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)
    ChemInform 06/2004; 35(35).
  • R.G. Geyer, P. Kabos, J. Baker-Jarvis
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    ABSTRACT: Closed-form analytical solutions are derived for accurate microwave dielectric characterization of rod test specimens inserted into dielectric sleeve resonators placed centrally in a metal cavity. Low-loss sleeve resonators can be used advantageously for multiple frequency measurements of the same specimen and may be employed for accurate dielectric characterization of high-permittivity specimens having dielectric loss factors greater than 0.001. Uncertainty relations for permittivity and dielectric loss are also shown, which demonstrate that when sample electric energy filling factors are greater than 0.4, relative uncertainties in measured permittivity and dielectric loss tangent are less than 1% and 4%, even for relative permittivities greater than 600. Example measurements are given that illustrate how this dielectric resonator system can be employed for dielectric characterization of ferroelectric materials at temperatures both near or far from their Curie temperatures
    IEEE Transactions on Instrumentation and Measurement 05/2002; · 1.71 Impact Factor
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    ABSTRACT: The dielectric resonator technique is frequently used for surface resistance measurements of superconducting films. Generally, an effective surface resistance has been measured which neglects the finite thickness of superconducting films. The thickness dependence of the surface impedance of superconducting films, whose thickness is comparable to the London penetration depth, is presented.
    Microwaves, Radar and Wireless Communications, 2002. MIKON-2002. 14th International Conference on; 02/2002
  • R.G. Geyer, P. Kabos, J. Baker-Jarvis
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    ABSTRACT: Low-loss sleeve resonators can be used for accurate microwave dielectric characterization of rod-shaped test specimens. The test specimen is inserted into the dielectric sleeve resonator and placed centrally in a metal cavity. With the use of additional sleeve resonators having differing external diameters or permittivities, a single specimen can be characterized at multiple frequencies. Sleeve resonators can also be employed for accurate dielectric characterization of high-permittivity specimens having dielectric loss factors greater than 0.001. Closed-form solutions for TE<sub>0np</sub> resonant mode structure are given. Uncertainty relations for permittivity and dielectric loss are also shown, which demonstrate that when sample electric energy filling factors are greater than 0.4, relative uncertainties in measured permittivity and dielectric loss tangent are less than 1% and 4%, even for relative permittivities greater than 600. Example measurements are given that illustrate how this dielectric resonator system can be employed for dielectric characterization of ferroelectric materials at temperatures both near or far from their Curie temperatures
    Microwave Symposium Digest, 2002 IEEE MTT-S International; 02/2002
  • Richard G. Geyer, Pavel Kabos, James Baker-Jarvis
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    ABSTRACT: Closed-form analytical solutions are derived for accurate microwave dielectric characterization of rod test specimens inserted into dielectric sleeve resonators placed centrally in a metal cavity. Low-loss sleeve resonators can be used advantageously for multiple frequency measurements of the same specimen and may be employed for accurate dielectric characterization of high-permittivity specimens having dielectric loss factors greater than 0.001. Uncertainty relations for permittivity and dielectric loss are also shown, which demonstrate that when sample electric energy filling factors are greater than 0.4, relative uncertainties in measured permittivity and dielectric loss tangent are less than 1% and 4%, even for relative permittivities greater than 600. Example measurements are given that illustrate how this dielectric resonator system can be employed for dielectric characterization of ferroelectric materials at temperatures both near or far from their Curie temperatures
    IEEE Transactions on Instrumentation and Measurement 01/2002; 51:383-392. · 1.71 Impact Factor
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    ABSTRACT: In the present work the effects of cation ordering on dielectric properties were isolated by investigating three polymorphs of Ca(Ca1/3Nb2/3)O3 that feature different arrangements of the Ca2+ and Nb5+ cations on the B-sites of the perovskite structure. Dielectric measurements at frequencies above 1 GHz revealed a systematic dependence of the properties on the type of cation ordering. In particular, the structure with 2:1 ordering exhibited a lower dielectric constant and a significantly more negative temperature coefficient of resonance frequency than the structures with 1:1 and newly described k=[1 1 1]*c ordering. Rietveld refinements of structural models for the three Ca4Nb2O9 polymorphs were conducted using X-ray and neutron powder diffraction data to elucidate structural details that could be correlated with the changes in dielectic properties. In all three polymorphs the cation ordering was combined with the same b−b−c+ octahedral tilt system, and the structural refinements yielded similar magnitudes of the tilting angles. The most significant crystal-chemical difference between the three polymorphs was in the coordination environment of Nb5+. Analysis of the refined bond distances indicated increasing average distortion of the Nb nearest-neighbor environment in going from the 1:1 to the k=[1 1 1]*c to the 2:1 ordered structure. The increased fraction of strongly compressed Nb–O bonds in the 2:1 structure associated with the large distortion was correlated with the decrease in dielectic constant and more negative value of temperature coefficient of the resonant frequency obtained for this polymorph. Raman spectra obtained for the three polymorphs exhibited differences that were consistent with the observed structural chemistry.
    Journal of Solid State Chemistry 01/2001; 156(1):122-134. · 2.04 Impact Factor
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    J. Krupka, J. Baker-Jarvis, R.G. Geyer
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    ABSTRACT: The permittivity and dielectric loss tangent of bulk strontium titanate were measured from 4 to 300 K, at frequencies from 400 MHz to 3.8 GHz, using a dielectric rod-resonator technique. Single-crystal and ceramic SrTiO<sub>3</sub> samples were both investigated. Significant differences in permittivity and losses were observed between single-crystal and ceramic materials at cryogenic temperatures. When measured at liquid nitrogen temperatures, the single-crystal sample dielectric loss tangent was approximately 10<sup>-4</sup>
    Microwaves, Radar and Wireless Communications. 2000. MIKON-2000. 13th International Conference on; 02/2000
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    ABSTRACT: Subsolidus phase equilibria in the SrO–Al2O3–Nb2O5 system were determined by synthesis of 75 compositions in air in the temperature range 1200–1600°C. Phase assemblages were determined by X-ray powder diffraction at room temperature. Two new ternary compounds, Sr4AlNbO8 and Sr5.7Al0.7Nb9.3O30, were observed to form in addition to the known double perovskite, Sr2AlNbO6 (, a=7.7791(1) Å). Sr4AlNbO8 crystallizes with a monoclinic unit cell (P21/c; a=7.1728(2), b=5.8024(2), c=19.733(1) Å; β=97.332(3)°) determined by electron diffraction studies; the lattice parameters were refined using X-ray powder diffraction data, which are given. This compound decomposes above 1525°C; attempts to grow single crystals from neat partial melts, or using a strontium borate flux, were unsuccessful. The phase Sr5.7Al0.7Nb9.3O30 (Sr6−xAl1−xNb9+xO30, x=0.3) forms with the tetragonal tungsten bronze structure (P4bm; a=12.374(1), c=3.8785(1) Å), melts incongruently near 1425°C, and occurs essentially as a point compound, with little or no range of x-values; indexed X-ray powder diffraction data are given. The tungsten bronze structure exhibits a narrow region of stability in the SrO–Al2O3–Nb2O5 system, which is probably related to the small size of Al3+. The existence of an extensive cryolite-type solid solution, Sr3(Sr1+xNb2−x)O9−3/2x, occurring between Sr4Nb2O9 (x=0) and Sr6Nb2O11 (x=0.5), was confirmed, with cubic lattice parameters ranging from 8.268(2) to 8.303(1) Å, respectively. The dielectric properties of the three ternary compounds occurring in the system were measured using the specimen as a TE011 or TE0γδ dielectric resonator: Sr2AlNbO6: εr=25, τf=−3 ppm/°C, tan δ=1.9×10−3 (7.7 GHz); Sr4AlNbO8: εr=27, tan δ=2.8×10−3 (10.5 GHz); Sr5.7Al0.7Nb9.3O30: εr=168, tan δ=3.8×10−2 (3.1 GHz). Sr2AlNbO6, when sintered in 1 atm oxygen, exhibited a reduced permittivity (εr=21) and a significantly improved dielectric loss tangent (tan δ=5.2×10−4, 8.3 GHz), resulting in a four-fold increase in Q×f as compared to the specimen sintered in air.
    International Journal of Inorganic Materials 02/2000; 2(1):107–114.
  • Richard Geyer, Jerzy Krupka, Michael Tobar
    Ceramic Transactions 01/2000; 106:219.
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    ABSTRACT: Subsolidus phase equilibria in the CaO:Al2O3:Nb2O5 system at 1325°C in air have been determined. One ternary phase forms, Ca2AlNbO6, which exhibits a perovskite-related structure with 1:1 or NaCl-type ordering of Al3+ and Nb5+ on the B sites. Indexed X-ray powder diffraction data for this monoclinic compound are given (P21/n (No. 11); a=5.3780(1), b=5.4154(1), c=7.6248(2) Å, β=89.968(2)°). The subsystem CaO–Nb2O5 was reexamined at CaO contents above 70 mol% to clarify inconsistencies in the literature. Two phases were confirmed to form in this region: the polymorphic-ordered perovskite Ca4Nb2O9, with solid solution ranging from approximately 17 to 20.5 mol% Nb2O5, and the compound referred to as Ca3Nb2O8, which was shown here to occur as essentially a point compound at the composition 75.25:24.75 CaO:Nb2O5. The perovskite-related structure of the Ca3Nb2O8-type phase was shown to be noncubic, and further studies are in progress. Capacitance methods at 1 MHz were used to determine the dielectric constants and associated temperature coefficients for eleven compounds in the CaO:Al2O3:Nb2O5 system. Ca2AlNbO6 and Ca3Nb2O8 coexist in equilibria and were found to exhibit temperature coefficients of permittivity with opposite signs. Five compositions in the xCa2AlNbO6:(1−x)Ca3Nb2O8 system were prepared and their dielectric properties measured by dielectric resonator methods at 5–7 GHz. The relative permittivities and temperature coefficients of resonant frequency obtained for the endmembers Ca2AlNbO6 and Ca3Nb2O8 were 30, −88 ppm/°C, and 45, +113 ppm/°C, respectively. Temperature compensation of the resonant frequency was obtained near x=0.67 with a permittivity of 32; no solid solution was detected by X-ray powder diffraction.
    Journal of Solid State Chemistry 01/2000; 155(1):78-85. · 2.04 Impact Factor
  • Jerzy Krupka, Richard G. Geyer
    Wiley Encyclopedia of Electrical and Electronics Engineering, 12/1999; , ISBN: 9780471346081
  • Richard G. Geyer, Pavel Kabos
    Wiley Encyclopedia of Electrical and Electronics Engineering, 12/1999; , ISBN: 9780471346081
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    ABSTRACT: A new method of compensating the frequency-temperature dependence of high-and monolithic sapphire dielectric resonators near liquid nitrogen temperature is presented. This is achieved by doping monocrystalline sapphire with Ti<sup>3+</sup> ions. This technique offers significant advantages over other methods
    IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control 08/1999; 46(4):993-1000. · 1.82 Impact Factor
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    ABSTRACT: Whispering-gallery modes are used for very accurate permittivity, dielectric loss, and temperature coefficient of permittivity measurements for both isotropic and uniaxially anisotropic dielectric materials. The relationship between resonant frequencies, dimensions of the resonant structure, and permittivity of the sample under test is calculated with a radial mode-matching technique. The relative accuracy of these computations is better then 10<sup>-4</sup>. The influence of conductor losses on dielectric loss tangent determination is treated for both whispering-gallery-mode and TE<sub>01δ</sub>-mode dielectric-resonator techniques. Two permittivity tensor components of sapphire and their temperature dependence were measured from 4.2 to 300 K. The total uncertainty in permittivity when use is made of whispering-gallery modes was estimated to be less than 0.05%. The uncertainty was limited principally by uncertainty in sample dimensions. Experimental and calculated resonant frequencies of several whispering-gallery modes differed by no more than 0.01%. The dielectric loss tangent of sapphire parallel and perpendicular to its anisotropy axis was calculated to be less than 10<sup>-9</sup> at 4.2 K. The permittivity and dielectric loss tangent of a commercially available low-loss high-permittivity ceramic material has also been measured at S- and C-band frequencies using a large number of whispering-gallery modes
    IEEE Transactions on Microwave Theory and Techniques 07/1999; · 2.23 Impact Factor
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    ABSTRACT: Single crystals of Ba 6Fe 45Ti 17O 106and BaFe 11Ti 3O 23were obtained as major and minor coproducts, respectively, by slow-cooling an off-stoichiometric BaO:Fe 2O 3:TiO 2melt. The former compound exhibits variable stoichiometry, Ba 6Fe 48- xTi 14- xO 106, with the Fe:Ti ratio dependent upon the partial pressure of oxygen. The value of xcorresponds to the equivalents of reduction that occur to maintain electroneutrality as the Ti-content increases. When prepared in air, this phase occurs at x=3 with the stoichiometry Ba 6Fe 45Ti 17O 106, while in 100% oxygen the x-value approaches zero with the resulting stoichiometry Ba 6Fe 48Ti 14O 106(all Fe 3+and Ti 4+). The structures of Ba 6Fe 45Ti 17O 106and BaFe 11Ti 3O 23were solved using single-crystal X-ray diffraction methods. Ba 6Fe 45Ti 17O 106was prepared in polycrystalline form, and further structural details, including accurate Fe/Ti occupancy factors, were determined by a combined refinement using neutron and synchrotron powder diffraction data. (Ba 6Fe 45Ti 17O 106: Space group C2/ m(No. 12); a=19.390(1) Å, b=20.260(1) Å, c=10.076(1) Å, β=105.27(1)°; V=3818.5(3)Å 3; Z=2; ρcalc=5.08 g/cm 3. Ba 6Fe 11Ti 3O 23= Space group C2/c (No. 15); a=19.561(1) Å, b=8.6614(7) Å, c=10.120(1) Å, β=105.62(1)°; V=1651.1(3) Å 3; Z=4; ρcalc=5.08 g/cm 3.) Both compounds adopt eight-layer close-packed structures built from alternating ccpand hcp[O, (Ba, O)] layers stacked along the a-direction with a ( ch) 4repeat sequence. Both structures feature octahedral sites occupied by a mixture of Fe and Ti as well as tetrahedral sites occupied by Fe 3+the structural formulas are XIIBa 6IVFe 6VI(Fe 39Ti 17)O 106and XIIBa IVFe 2VI(Fe 9Ti 3)O 23. Both compounds are partially reduced; the former contains 3 moles of Fe 2+(or Ti 3+) per formula unit, and the latter contains 1mole. The formation of Fe 2+is considered more likely than Ti 3+, but could not be experimentally confirmed. BaFe 11Ti 3O 23is apparently metastable in air when cooled from above the solidus and could not be prepared as a polycrystalline sample. Indexed experimental X-ray powder diffraction data for Ba 6Fe 45Ti 17O 106are given. Polycrystalline samples of this compound were used to measure its magnetic and electrical properties. The magnetic behavior of Ba 6Fe 45Ti 17O 106above room temperature up to 1073 K was found to obey the Curie-Weiss law, which indicated a small effective magnetic moment (34 μBper mole Ba 6Fe 45Ti 17O 106) and a large negative temperature intercept (-806 K). Electrical resistivity measurements between room temperature and 120 K revealed nonmetallic behavior with an activation energy on the order of 0.17eV. At 347 MHz under ambient conditions, Ba 6Fe 45Ti 17O 106exhibited a relative permittivity of 24 and a dielectric loss tangent of 0.10.
    Journal of Solid State Chemistry 03/1999; 143(2):182-197. · 2.04 Impact Factor
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    ABSTRACT: Whispering gallery modes were used for very accurate permittivity and dielectric loss tangent measurements for low loss isotropic and uniaxially anisotropic materials. We present the measurements of several specimens including sapphire, YAG, quartz, rutile and SrLaAlO<sub>4</sub>. The total absolute uncertainty in real part of the permittivity tensor was estimated to be less than 0.1% and was limited by the uncertainty in the dimensions of the samples. Imaginary parts of the permittivity tensor were measured to about 10% accuracy, limited by the accuracy of Q-factor measurements in whispering gallery modes. The anisotropy ratio of the measured materials varied from 1 (isotropic YAG) to 2.2 (rutile). All anisotropic materials exhibited anisotropy in the imaginary part of the permittivity tensor as well as the real part. For most crystals dielectric losses can be approximated by a power function of absolute temperature in only a limited temperature range. At very low temperatures (4-50 K) properties of both the real and imaginary permittivity tensor are often affected by impurities which are always present in real crystals
    Frequency and Time Forum, 1999 and the IEEE International Frequency Control Symposium, 1999., Proceedings of the 1999 Joint Meeting of the European; 02/1999

Publication Stats

620 Citations
42.68 Total Impact Points

Institutions

  • 1994–2009
    • National Institute of Standards and Technology
      • Materials Science and Engineering Division
      Maryland, United States
  • 1999
    • University of Western Australia
      • School of Physics
      Perth, Western Australia, Australia
  • 1998–1999
    • Warsaw University of Technology
      • Institute of Microelectronics and Optoelectronics
      Warsaw, Masovian Voivodeship, Poland