Dielectric dispersion and its temperature variation in calcite single crystals

The variation of dielectric constant and loss of calcite single crystals with frequency and temperature over wide ranges (10² to 2.4×10¹⁰c/s and ?193 to 450 °C) in two orientations of the crystal, namely, the electric field (i) parallel to the optic axis and (ii) perpendicular to the optic axis, has been studied and the results reported. The dielectric constant at 25 °C in both orientations (E‖, 7.8 andE⊥, 8.2) is frequency-independent. The dielectric loss shows a minimum in the 10⁵ to 10⁸ c/s region. Dielectric constant as a function of temperature exhibits two distinct regions: frequency-independent (?193 to 50 °C) and frequency-dependent (50 to 450 °C). The temperature variation of specific conductivity can be represented by a sum of two exponentials, e.g., \(\sigma = Ae^{ - E_1 /kT} + Be^{ - E_2 /kT}\) with the usual notation; the constants have different values in the two orientations. The intrinsic conductivity with larger activation energy (E‖ 0.91 eV andE⊥ 1.05 eV) is attributed to the movement of lattice vacancies in the crystal. The 15% larger value for the intrinsic activation energy alongE⊥ optic axis over the other orientation may be due to a probably large electrostatic interaction between the charge carriers and the lattice ions in this orientation.     

Ferroelectric and paramagnetic properties of Li2Gd4 (MoO4)7 single crystals

Thermal behavior of such fundamental physical properties as polarization, pyroelectric current, dielectric constant and paramagnetic susceptibility are reported for dilithium heptamolybdotetragadolinate, Li₂Gd₄ (MoO₄)₇. The ferroelectric transition point has been determined by various methods and the results compared. The most reliable value of the Curie point has been obtained by the measurement of differential magnetic susceptibility as a function of temperature and is found to be 52±2°C. The room temperature values for the relative dielectric constant and paramagnetic susceptibility are 51.5 and 59.8 x 10^-6 cm³. g^-1, respectively. From the susceptibility measurements the values obtained for the Curie constant, C, and the paramagnetic Curie point, θp, are 1.79 x 10^-2 cm³ . g^-1 . deg and 247°K, respectively. It is believed that Li₂Gd₄ (MoO₄)₇ could be antiferromagnetic between 273 and 325°K.     

Dielectric relaxation of KBr doped with KOH and KOD

The complex dielectric constant of KBr single crystals doped with KOH and with KOD has been measured in the temperature range from 0.32°K to 300°K using a small a. c. signal. A relaxation timeτ ₀ was determined by means of Cole-Cole plots. ForT<4°K the temperature dependence ofτ ₀ can be approximated byAT ^?n , wheren is between 1.2 and 1.3 for the dilute samples, and between 0.8 and 1.0 for two samples with large hydroxyl concentrations.n has the same value for OH^? and OD^? dipoles. The constantA roughly doubles upon substitution ofH byD. The relaxation behavior was found to be independent of concentration in the range between 3×10¹⁸ cm^?3 dipoles and 10¹⁹ dipoles cm^?3.     

COMBINED EFFECTS OF SUBCOOLING AND SURFACE ORIENTATION ON POOL BOILING OF HFE-7100 FROM A SIMULATED ELECTRONIC CHIP

The effects of orientation and subcooling on pool boiling of the HFE-7100 dielectric liquid near atmospheric pressure (0.085 MPa) from a 10 × 10 mm smooth copper surface are investigated experimentally. Results are obtained for inclination angles θ = 0° (upward-facing), 30°, 60°, 90°, 120°, 150°, and 180° (downward-facing) and liquid subcoolings ΔT_sub = 0, 10, 20, and 30 K. Increasing θ decreases the saturation nucleate boiling heat flux at high surface superheats (ΔT_sat > 20 K), but increases it only slightly at lower surface superheats. The critical heat flux (CHF) decreases slowly with increasing θ from 0° to 90°, and then deceases faster with increasing θ to 180°. CHF increases linearly with increased subcooling, but the rate increases from 0.016 K^?1 at 0° to 0.048 K^?1 at 180°. At θ = 0° and ΔT_sub = 30 K, CHF is ～ 36 W/cm² and 24.45 W/cm² for saturation boiling, while at θ = 180° CHF = 10.85 W/cm² at ΔT_sub = 30 K and only 4.30 W/cm² at saturation. The developed correlation for CHF of HFE-7100, as a function of θ and ΔT_sub, is within ±10% of the present data. The recorded still photographs of the boiling surface in the experiments illustrate the effects of liquid subcooling and surface orientation at different nucleate boiling heat fluxes and surface superheats on vapor bubble accumulation and/or induced mixing at the surface.     

Indices of refraction of ZnS,ZnSe, ZnTe,CdS, and CdTe in the far infrared

The indices of refraction of pure ZnS, ZnSe and ZnTe at 2, 80, and 300°K have been measured in the spectral region from 10 to 100 cm^-1. The indices of refraction can be described by the simple dispersion formula for an undamped harmonic oscillator. The static dielectric constants of these materials are obtained from the best fit parameters using a simple dispersion equation. The average indices of refraction of pure CdS and CdTe at 5°K in the spectral region from 10 to 100 cm^-1 have also been measured.     

Dielectric relaxation in vanadium phosphate glasses

An asymmetric distribution of relaxation times has been inferred from an increase in the Cole-Cole distribution parameter α with increasing values of ωτ in 62% v₂O₅–38% P₂O₂ glass. The conventional Debye type relaxation loss peaks in the frequency range 10²–10⁵ Hz are observed in this sample above 85°K. The extrapolated values of dielectric constant and relaxation time below 100°K seem unexpectedly large while the high temperature extrapolated values of ?' are close to ?_∞ as expected. Probably the conventional dielectric loss peaks are observed only above a critical temperature at which the carriers gain sufficient energy to be excited to the conduction band edge. Below this temperature hopping of carriers within kT of the Fermi level may dominate and conventional Debye type dielectric loss peaks may lose their significance as envisaged in the models of frequency dependent ac conductivity.     

Dielectric responses of composite films based on P(VDF-TrFE) copolymer with TGS inclusions

The dielectric characteristics of film samples of P(VDF-TrFE) + TGS composite in the frequency range of 10³–10⁷ Hz are studied. The values of the real and imaginary parts of the complex dielectric constant in the temperature range of–40 to 140°C, including the points of the polymer matrix’s transition to the glassy state when T_g ≈–25°C and the ferroelectric phase transition. An analysis of the ferroelectric crystal inclusion effect on the dielectric response of P(VDF-TrFE) copolymer matrix is carried out.     

Impact ionization in silicon dioxide

The dielectric strength of silicon dioxide is related to the breakdown field at which electron-hole pairs can be generated by inverse Auger scattering of hot electrons. This field is calculated to be about 3 × 10⁷ V/cm for a 300°K ambient oxide.     

Electronic polarisabilities of ions in alkali halide crystals

Tessman, Kahn and Shockley calculated the electronic polarisabilities of ions in alkali halide crystals using the long wavelength limiting values of the visible light dielectric constants. We have recalculated these widely used polarisabilities using the more accurate room-temperature dielectric constant data of Lowndes and Martin and a better minimisation procedure of Pirenne and Kartheuser. We have also calculated for the first time the low temperature values of these polarisabilities. The computed values of the polarisability in ų are Li⁺ 0·029, Na⁺ 0·285, K⁺ 1·149, Rb⁺ 1·707, Cs⁺ 2·789, F^? 0·876, Cl^? 3·005, Br^? 4·168, I^? 6·294 at 300°K and Li⁺ 0·029, Na⁺ 0·290, K⁺ 1·133, Rb⁺ 1·679, Cs⁺ 2·743, F^? 0·858, Cl^? 2·947, Br^? 4·091, I^? 6·116 at 4°K. The relative standard deviations for all the alkali halides are 1·20 and 1·43 per cent at 300°K and 4°K respectively justifying the additive nature of the individual ion polarisabilities.     

The bulk effect of point defects on young's modulus in electron irradiated copper

Abstract

Pairs of copper samples—one for electrical resistivity, the other for Young's modulus measurements - were irradiated simultaneously at 120°K with 3 MeV electrons up to an integrated dose of 2 × 10²⁰ el/cm². The effect of dislocation pinning and the bulk effect of point defects on Young's modulus E could clearly be separated. The following relation between the bulk effect ΔE/E and the resistivity increase Δρ[Ωcm] was found: ΔE/E = ?25 × 10⁴ × Δρ. Besides strong annealing in stages II and III (180–300°K) and some annealing between 300–500°K, stage V annealing (500–600°K) also was found. In stage III the resistivity annealed more than Young's modulus. whereas the converse occurred in stage V. These measurements are discussed in connection with the electron microsopical observation of point defect clusters after electron irradiation at 120°K and heating to room temperature.

Probenpaare, bestehend aus einer Widerstandsprobe und einer Probe zur Messung des Elastizitätsmoduls, wurden gleichzeitig bei 120°K mit 3 MeV-Elektronen bis zu einer Dosis von 2 × 10²⁰ el/cm² bestrahlt. Die direkte Reein-flussung des E-Moduls durch die im Gitter verteilten Punktdefekte (Volumeneffekt) konnte getrennt von der Beeinflussung durch Versetzungsverankerung gemessen werden. Es ergab sich dabei folgende Beziehung zwischen relativer Modulanderung ΔE/E und strahlungsinduziertem Widerstand Δρ[Ωcm]: ΔE/E = ?25 × 10⁴ × Δρ. Neben starker Erholung in den Stufen II und III (180–300 °K) und schwacher Erholung zwischen 300–500°K wurde auβerdem Stufe V (500–600°K) beobachtet. In Stufe III erholte sich der Widerstand starker als der E-Modul, wahrend in Stufe V das umgekehrte der Fall war. Die Messungen werden diskutiert in Zusammenhang mit der elektronenmikroskopischen Beobachtung von Punktdefektclustern nach Elektronenbestrahlung bei 120°K und anschlieβender Erwärmung auf Raumtemperatur.     

Hyperfine fields in erbium metal

The hyperfine Splitting of the 80.6-keVγ transition in Er¹⁶⁶ has been measured in erbium metal between 4.2 °K and 40 °K using the Mössbauer effect. There is evidence for a unique magnetic field and electric fieldgradient at all nuclei in erbium metal. The magnetic field decreases from (7.55±0.20)·10⁶ Oe at 4.2 °K to (6.10±0.40)·10⁶ Oe at 40 °K. Extrapolation to 0 °K yieldsH (0 °K)=(7.60±0.20)·10⁶ Oe. The quadrupole interaction energy for the 80.6-keV state iseQ V _Z′Z′/4=(0.95±0.20)·10^?6 eV at 4.2 °K. These results are discussed and compared with other measurements.     

Electrocaloric effect in antiferroelectric PbZrO3 thin films

Antiferroelectric PbZrO₃ thin films have been deposited on Pt(111)/Ti/SiO₂/Si substrate by polymer modified sol–gel route. Temperature dependent P –E hysteresis loops have been measured at 51 MV/m within a temperature range of 40 °C to 330 °C. The maximum electrocaloric effect ～0.224 × 10^–6 K mV^–1 has been observed near the dielectric phase transition temperature (235 °C) of the thin films. The electrocaloric effect and its strong temperature dependence have been attributed to nearly first‐order phase transition. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The millimeter-wave dielectric properties of Al2O3 after irradiation to moderate neutron fluence

Abstract

Mono- and polycrystalline Al₂O₃ has been irradiated to 3.5· 10¹⁹ f.n/cm², and the increase in dielectric loss has been measured at 28–38 GHz and 144–146 GHz. Step annealing experiments have been performed between 150 °C and 1100 °C. The recovery of dielectric loss has been analyzed aiming at identifying the defect types affecting dielectric loss. A pronounced recovery step observed at 450–550 °C is explained by F-centres with strong electron-lattice coupling which contribute predominantly to dielectric loss at room temperature.     

Dielectric properties of CsBr single crystals-effect of d.c. bias

The dielectric properties of CsBr single crystals, both virgin and γ-irrddiated have been measured as a function of frequency (10³–10⁵Hz), temperature (300–575 K) and d.c. bias (0–350 V cm⁻¹). γ-irradiation produces colour centres in these crystals and consequently an increase in space charge density is observed. D.c. bias has brought in considerable changes in dielectric constant (ϵ') and loss (tanδ) of these crystals particularly at low frequencies and high temperatures. An attempt has been made to understand these data qualitatively through barrier layer formation.     

The dielectric and piezoelectric behaviour of pyrolusite (polycrystalline ore of MnO2)

The variation of dielectric constant? of pyrolusite (sintered polycrystalline ore of MnO₂) with temperature, frequency and voltage has been studied within the frequency range 200 c/s to 10 kc/s. The samples studied show a very high value of dielectric constant ≈ 10⁵. The?-temperature curve indicates that? attains a maximum value at a certain temperature. The temperature at which? attains a maximum value appears to vary from sample to sample. At higher temperatures (i.e. higher than that at which? attains a maximum value),? decreases with temperature as?=c/(T?Θ). The dielectric constant is dependent on the frequency; it has a higher value at lower frequencies. The applied voltage across the specimen has a marked influence on the dielectric constant, which increases rapidly with voltage. A capacity with pyrolusite as dielectric shows a hysteresis loop (electrical). The dielectric loss factor tanδ is found to depend on temperature, frequency and voltage. Tanδ attains a maximum value at lower range of frequencies between 300 c/s to 700 c/s, for a given voltage, and temperature. This indicates the existence of dielectric relaxation for these specimens. Tanδ also varies exponentially with temperature. Samples of pyrolusite studied show a piezo electric effect as well. The electromechanical coupling coefficient is found to vary from 2·4×10^?5 to 8·8×10^?5 e.s.u/ dyne. Slabs cut from the samples of pyrolusite mineral were found to control the frequency of a Hartley oscillatory circuit. The resonant frequency range was found to increase as the thickness of the slab is reduced.     

Dipolar dielectric relaxation in CeO2-YO1,5 solid solutions

The dielectric properties of yttrium oxide-doped cerium oxide solid solutions have been studied by using a.c. techniques. Frequency and temperature respectively range from 10 to 10⁵ Hz and from 70 to 200°C. Both a complex impedance plot (Z″ vs Z′) and a conductance plot (G at fixed frequency vs inverse of temperature T^?1) are shown to be complementary for an accurate determination of the bulk resistance and of the dielectric loss. For the three investigated concentrations of yttrium oxide (m/% YO_1,5:0.001, 0.01 and 0.1), the activation energies for conduction are equal to 0.86, 0.75 and 0.75 eV, while the activation energy for dipole reorientation is found to be concentration independent (0.75 eV; angular frequency preexponential factor: 6.0 × 10¹², 1.43 × 10¹³ and 3.05 × 10¹³ s^?1). Moreover, the dipolar relaxation rates are found to be of the same order of magnitude as that of the migration rate. Interpretation is given on the basis of Wachtman's model and of previous results obtained by Nowick et al. from ITC (ionic thermo-current) investigations. The observed dielectric peak appears to be due to the redistribution of oxyygen vacancies between charged (Y_Ce-V₀) pairs and fixed charged Y_Ce defects.     

The exciton absorption in deformed germanium

Absorption spectra at 77° K near the direct (κ = 0) exciton transition are reported for deformed and undeformed single-crystal films of n-type Ge oriented on (111); Elliott's theory is applied. The optical width of the forbidden band for this transition is found as E_{g 0} = (0.8821 ±±0.0002) eV, while the exciton binding energy is found as E_ex(0) = = (0.0016±0.0003) eV for undeformed Ge at 77 ° K. The mean temperature coefficient of E_g for κ = 0 in the range 77 °–297 ° K is (dE_g/ /dT)_p =?3.50 · 10^?4 eV/deg. The effects of thermoelastic deformation on the exciton spectrum give (dE_g/dT)_d = (?1.5±0.1) · 10^?4 eV/deg. The half-width σ ≈ 5 · 10^?4 eV of the exciton peak gives the exciton lifetime as gt ≥ 10^?12 sec.     

Über Zusammenhänge zwischen Elektronenspinresonanz,elektrischer Leitfähigkeit und Photoleitung bei reinem und dotiertem Bor

The conductivity mechanism in pure and doped, β-rhombohedral, polycrystalline boron between 1.5 °K and 900 °K is clarified by measurements of electrical conductivity, photoconductivity, electron paramagnetic resonance and thermoelectric effect. The semiconductor behaviour of boron between 1.5 and 900 °K is similar to that of doped and compensated germanium and silicon at helium temperatures concerning the temperature-independent number of carriers and the thermally activated conduction process at low and high carrier concentrations. The paramagnetic centres are nearly localized electrons at 1.5 °K and nearly free electrons at 900 °K with a continuous transition between these two extreme kinds of behaviour. Mobilities of charge carriers in carbon doped boron over a range from 10¹⁶ cm^?3 to 10²⁰ cm^?3 and 77 °K to 900 °K were measured for the first time and were found to obey an exponential law.     

Anomalous temperature dependence of the energy gap of AgGaS2

The lowest energy gap Eg of AgGaS₂ in the temperature range from 4.2 to 300° K was determined from the reflectivity, photoluminescence and absorption measurements. Below ～ 80° K the temperature coefficient of the energy gap is +6 × 10^-5eV/°K. Above ～80° K the sign of the coefficient reverses and the value is -1.8 × 10^-4eV/°K. The positive value is explained with the lattice dilation effect being the dominant mechanism for the energy gap variation at lower temperatures than ～80°K.     

Temperature dependence of recoilless fractions in tungsten

The recoilless fractions of ¹⁸²W, ¹⁸⁴ W, and ¹⁸⁶W have been simultaneously measured from 30°K to 100°K in 10°K steps. They are found to agree with previous calculations of Raj and Puri.