Thermal and dielectric properties of zirconyl phosphate compact

Experimental results are presented on the measurements of thermal expansion (up to 1500°C), thermal conductivity (up to 1000°C), dielectric constant (up to 450 °C) and tan (up to 800 °C) of zirconyl phosphate compacts obtained by sintering at 1600°C. The thermal expansion coefficient of the samples at the temperature below 1100°C was less than 1.7 × 10^–6°C^–1. The samples showed a definite shrinkage at temperatures of 1110 and 1470°C due to the phase transformations. The expansion at 1500°C was less than that at 1100°C probably because of the phase transformation. The thermal conductivity at room temperature was a very small value (0.0046 to 0.0065 cal s^–1 cm°C^–1 cm^–2). The dielectric constant was close to 9. The value of tan° (–0.0001) measured is one of the lowest values for ceramic materials.     

Electrical properties of ZrF4-BaF2-GdF3-AlF3-glass

The electrical and dielectric behaviour of a gadolinium-based zirconium fluoride glass have been studied over a wide temperature (23–275 °C) and frequency (100 Hz–1 MHz) range. The conductivity at 200 °C was of the order of 10^–6 (ohm-cm)^–1 and the activation energy was 0.75 eV. The dielectric constant measured at 1 kHz was 14 and was independent of frequency at low temperatures. The observed dielectric dispersion has been attributed to the conduction mechanism.     

Ionic conductivity of synthetic analcime,sodalite and offretite

The ionic conductivity of pressed pellets of dehydrated synthetic analcime, sodalite and offretite was determined by a.c. measurements within the range 10 Hz to 10 MHz. The ionic conductivity values of those zeolites exchanged with various monovalent cations were determined by the complex impedance plane method. The conduction activation energies range between 63 and 101 kJ mol^–1. Sodium analcime shows the best ionic conductivity, namely 1.8×10^–4–1 cm^–1 at 400° C. A comparison with the conductivity of other ion-conducting solids was made.     

Dielectric and electrical properties of ordinary Portland cement and slag cement in the early hydration period

The dielectric constant and electrical conductivity of ordinary Portland cement (OPC) with water-cement ratios (w/c) of 0.30, 0.35 and 0.40 were measured for the first 30 h hydration, using a microwave technique in the frequency range 8.2–12.4 GHz. It was found that both the dielectric constant and electrical conductivity of the cement paste are sensitive to the water-cement ratio, the higher the w/c value, the greater the dielectric constant and electrical conductivity, and the longer the hydration time. We also found that the higher the frequency the greater the electrical conductivity but the smaller the dielectric constant. The dielectric constant and electrical conductivity of high- and low-slag cement with water-solid ratio (w/s) of 0.40 were measured in the first 30 h after mixing. The changes in dielectric constant and electrical conductivity of low-slag cement with time are similar to that of OPC, but the high-slag cement shows very different dielectric and electrical properties compared with OPC and low-slag cement. The relationship between the dielectric and electrical properties of cement paste and cement hydration was also discussed.     

Interpretation of the impedance spectroscopy of cement paste via computer modelling

The d.c. conductivity, , and low-frequency relative dielectric constant, k, of Portland cement paste were monitored, using impedance spectroscopy, during cooling from room temperature down to -50 °C. Dramatic decreases in the values of and k, as great as two orders of magnitude, occurred at the initial freezing point of the aqueous phase in the macropores and larger capillary pores. This result provides strong experimental support for the dielectric amplification mechanism, proposed in Part II of this series, to explain the high measured low-frequency relative dielectric constant of hydrating Portland cement paste. Only gradual changes in the electrical properties were observed below this sudden drop, as the temperature continued to decrease. The values of and k of frozen cement paste, at a constant temperature of -40 °C, were dominated by properties of calcium-silicate-hydrate (C-S-H) and so increased with the degree of hydration of the paste, indicating a C-S-H gel percolation threshold at a volume fraction of approximately 15%–20%, in good agreement with previous predictions. Good agreement was found between experimental results and digital-image-based model computations of at -40 °C. Freeze-thaw cycling caused a drop in the dielectric constant of paste in the unfrozen state, indicating that measurements of k could be useful for monitoring microstructural changes during freeze-thaw cycling and other processes that gradually damage parts of the cement paste microstructure.     

Dielectric relaxation and electrical conduction at low field of mineral-filled epoxy

Dielectric properties and conduction of the epoxy and its composites were measured over the temperature range — 20 to 70°C and the frequency range 10^–4-10^–1 Hz. Dielectric properties were obtained by performing Fouriertransforms on the charging and discharging curves. The resulting isothermal frequency spectra of dielectric constants and dielectric loss factors were analysed using the Cole-Cole law to obtain the activation energy for each material. The activation energies were also obtained for isothermal d.c. current. Current density-electric field-temperature characteristics are obtained for field levels up to 60 kV cm^–1, with step excitation of the applied field and currents recorded after a delay time of 10 min. Current density and electric field were computed and plotted for constant temperature. The linear (ohmic) curves were obtained for fields up to about 60 kV cm^–1 for temperatures up to about 20 °C. The non-linearity at the higher fields and temperatures did not imply the occurrence of non-ionic conduction. It has been demonstrated that both electric conduction and relaxation behaviour were ionic and could be fit by the Nakajima model for the unfilled epoxy and the Taylor model for the composites.     

Fine-tuning of temperature coefficients of capacitance (TCC) and dielectric constant (TCK) of Mg2SnO4 via second phase addition

Fine-tuning of the temperature coefficients of capacitance and dielectric constant of magnesium orthostannate (Mg₂SnO₄) has been attempted by means of Sn (IV) and Zr (IV) oxide incorporation as a second phase. The additives were also employed to enhance the density and minimize or eliminate porosity at lower sintering temperatures. Phase-pure magnesium stannate powder was synthesized via conventional solid-state reaction. It was mixed with ZrO₂ and/or SnO₂ and sintered in the temperature range 1500°–1600°C for up to 6 h. Electrical measurements using an AC immittance spectroscopic technique over the temperature range 25°–300°C, on Mg₂SnO₄ compacts containing 5 wt.% of additives and sintered at 1500 °C/ 6 h, were carried out. Data analyses revealed that the capacitance and the derived dielectric constant remained invariant over more than 3 decades of frequency in the kilo to megahertz regime. It was also found that addition of ZrO₂ and SnO₂ has a benign effect on both temperature coefficient of capacitance (TCC) and temperature coefficient of dielectric constant (TCK) as it resulted in smaller dependence of capacitance and dielectric constant compared to pure Mg₂SnO₄. Typically, the TCC values were 5 and 30 ppm/°C and TCK values were 20 and 30 ppm/°C for 5 wt.% ZrO₂- and 5 wt.% SnO₂- added Mg₂SnO₄, respectively, in the temperature range 25°–300°C.     

Polymorphism and dielectric electric properties of Ba- and Sr-containing feldspars

The polymorphism and dielectric properties of Ba_1–x Sr _x Al₂Si₂O₈ (x = 0.00, 0.01, 0.05, 0.10, and 1.0) feldspars were examined. Strontium additions enhanced the formation of the monoclinic phase, decreasing heat treatment times from three days to 24 hrs. The dielectric constants of all compositions varied between 7 and 8 over a temperature range of –175 to 200°C and at 1, 10, and 100 kHz. An abrupt change (about 5%) in the dielectric constant of Sr-feldspar occurred at –70°C, confirming the existence of the triclinic low temperature polymorph.     

Effect of Zr substitution on structural and dielectric properties of Pb5EuTi3−xZrxNb7O30 (x=0, 1, 2 and 3) ferroelectric ceramics

The polycrystalline samples of Pb₅EuTi_3–x Zr _x Nb₇O₃₀ (x=0, 1, 2 and 3) were prepared by a high-temperature solid-state reaction technique. Formation of the compounds was checked by X-ray diffraction method. All the compounds were found to have orthorhombic structure at room temperature. Studies of their dielectric constant (e) as a function of temperature (25–375 °C) at 10 kHz exhibit ferroelectric phase transition of a diffuse-type. With increasing Zr concentration (x), transition temperature (T_c) as well as dielectric constant decreases. Temperature variation of d.c. resistivity/conductivity shows that these compounds exhibit negative temperature coefficient of resistance in the high temperature range.     

Dielectric properties of LiF single crystals X-ray irradiated under d.c. fields

The dielectric constant (K), loss (tan ) and a.c. conductivity () of LiF single crystals subjected to X-ray irradiation in the presence of cl.c. fields up to 16 kVcm^–1 are measured in the frequency range 10² to 10⁵ Hz and in the temperature range 30 to 400° C. The space charge and Bipolar effects observed from this study are discussed in the light of optical absorphon data.     

Ionic conductivity of electrolytes formed from PEO-LiCF3SO3 complex low molecular weight poly(ethylene glycol)

Films of poly(ethylene oxide)-LiCF₃SO₃-based complexes containing different amounts of poly(ethylone glycol) (PEG) with molecular weights ranging from 400 to 2000 were prepared by solution casting. The ionic conductivity is presented as a function of temperature, molecular weight and the PEG content used. The conductivity increases with decreasing molecular weight of PEG and with increasing PEG content. The incorporation of PEG with a molecular weight of 600 or less gives rise to a maximum conductivity value of 3 × 10^–3 Sm^–1 at 25° C. The conductivity enhancement at room temperature can be attributed to the increase in the amorphous regions responsible for the ionic conduction.     

Thermal expansion of the cubic (3C) polytype of SiC

Thermal expansion of the cubic beta or (3C) polytype of SiC was measured from 20 to 1000° C by the X-ray diffraction technique. Over that temperature range, the coefficient of thermal expansion can be expressed as the second order polynominal: ₁₁=3.19×10^–6+ 3.60×10^–9 T–1.68×10^–12 T ² (1/° C). It increases continuously from about 3.2×10^–6/° C at room temperature to 5.1×10^–6/° C at 1000° C, with an average value of 4.45 × 10^–6/° C between room temperature and 1000° C. This trend is compared with other published results and is discussed in terms of structural contributions to the thermal expansion.     

On electrical transport in CoWO4 single crystals

CoWO₄ is a p-type semiconductor with conductivity in the range 10^–8 to 10^–3 ohm^–1 cm^–1. The solid exhibits extrinsic behaviour up to 750 K and intrinsic behaviour above 750 K. In this solid conduction by hopping of small polarons seems to be dominating up to 750 K and above 750 K conduction becomes band type. The energy band gap of the solid has been found to be 2.80 eV. The values of mobility and the mean free path are estimated. The variation of dielectric constant with temperature has been attributed to changes in atomic and ionic polarization and space charge polarization of thermally generated charge carriers.     

Effect of La/Sn co-substitution for Ba/Ta on crystal structure and dielectric properties of Ba5NdTi3Ta7O30 ceramics

The effects of La/Sn co-substitution for Ba/Ta were investigated for the modification of Ba₅NdTi₃Ta₇O₃₀ ceramics. The modified ceramics (Ba_5–x La _x )NdTi₃(Ta_7–x Sn _x )O₃₀ exhibited single tetragonal tungsten bronze phase for x<1.5, while a small amount of secondary phase BaTi₄O₉ was observed for x>1.5. The lattice constants decreased with increasing La/Sn content, while the axial ratio 10^1/2c/a decreased when x was below 1.5, then slightly increased. With increasing La and Sn content, the temperature coefficient of dielectric constant (at 1 MHz) was remarkably lowered from –1560 ppm/ °C to –286 ppm/ °C, while the dielectric constant gradually reduced, and the dielectric loss slightly increased. There were some clear relationships between the temperature coefficient and bond valence, tolerance factor: the temperature coefficient of dielectric constant linearly increased when the bond valence of the ions at B sites increased, while the same effect occurred when the tolerance factor decreased. In addition, the stability of the tetragonal tungsten bronze phase is discussed in relation to electronic difference and tolerance factor.     

On the structure and transformation of the orange form of Sb2S3 layers

X-ray diffraction study showed that the mange modification of antimony trisulphide can neither be considered as amorphous material nor does it change into the black modification upon grinding. Thin vacuum-deposited layers, prepared by conventional thermal evaporation of the bulk material in 5 × 10^–6 torr on an amorphous substrate at room temperature, were crystalline. The behaviour of the electrical conductivity with temperature in the range 25 to 190° C indicated transition points at 80, 108, 135 and 175° C.     

Fabrication, characterization and sintering of glass-ceramics for low-temperature co-fired ceramic substrates

Cordierite-based glass-ceramics with non-stoichiometric composition doped with rare earth oxide (CeO₂) and heavy metal oxide (Bi₂O₃) respectively were fabricated from glass powders. After sintering and crystallization heat treatment, various physical properties, including compact density and apparent porosity, were examined to evaluate the sintering behavior of cordierite-based glass-ceramics. Results showed both that the additives heavy metal oxide and rare earth oxide promoted the sintering and lowered the phase temperature from - to -cordierite as well as affecting the dielectric properties of sintered glass-ceramics. The complete-densification temperature for samples was as low as 900 °C. This material has a low dielectric constant (5.3), a low dielectric loss (0.2%) and a low thermal expansion coefficient (2.8–3.52×10^–6 K^–1), and can be co-fired with high conductivity metals such as Au, Ag, Cu, Ag/Pd paste at low temperature (below 950 °C), which makes it a promising material for low-temperature co-fired ceramic substrates.     

Thermal properties of MoSi2 and SiC whisker-reinforced MoSi2

The heat capacity, thermal conductivity and coefficient of thermal expansion of MoSi₂ and 18 vol % SiC whisker-reinforced MoSi₂ were investigated as a function of temperature. The materials were prepared by hot isostatic pressing between 1650 and 1700 °C, the hold time at temperature being 4 h. The heat capacity of MoSi₂ showed an increase from about 0.44 Wsg^–11K^–1 at room temperature to 0.53 at 700 °C. Whisker reinforcement increased heat capacity by about 10%. Thermal conductivity exhibited a decreasing trend from 0.63 Wcm^–1 K^–1 at room temperature to 0.28 Wem^–1 K^–1 at 1400°C. Whiskers reduced conductivity by about 10%. The thermal expansion coefficient increased from 7.42 °C^–1 between room temperature and 200 °C to 9.13 °C^–1 between room temperature and 1200 °C. There was a 10% decrease resulting from the whiskers. The measured data are compared with literature values. The trends in the data and their potential implications for high-temperature aerospace applications of MoSi₂ are discussed.     

Electrical properties of high purity tin dioxide doped with antimony

The electrical conductivity of high purity tin dioxide doped with antimony was studied at temperatures of 900 to 1200° C and partial pressures of oxygen between 10^–8 and 1 atm. For specimens having a dopant concentration over 1 × 10¹⁹Sb cm^–3, the electrical conductivity decreased slightly with temperature and independent of oxygen partial pressure. The electrical conductivity of specimens having a dopant concentration under 1 × 10^–8Sb cm^–3 increased with temperature and with decreasing partial pressure of oxygen. The significance of the dopant and the thermally created defects is discussed.     

Studies of dielectric and varistor behavior of lead manganese tungstate ceramics

Polycrystalline samples of Pb(Mn_1/2W_1/2)O₃ were prepared by the high-temperature solid-state reaction technique. Preliminary crystal structure and microstructure of the compound at room temperature were studied using the X-ray diffraction (XRD) technique and scanning electron microscopy (SEM), respectively. The dielectric permittivity () and loss tangent (tan) of the compound were obtained both as a function of frequency (0.5×10³–10⁴Hz), at room temperature, and temperature (30 °C–320 °C), at 10 kHz. Both the a.c. and d.c. conductivities have been studied over a wide range of temperature. The forbidden energy gap (E_g) and activation energy (E_a) of the compound were calculated from the plot of d.c. resistivity and a.c. conductivity versus inverse absolute temperature, respectively. The current density–field strength (J–E) characteristics of the compound studied at different temperatures reveals that the compound has excellent varistor behavior. The change in magnitude of the current with time was also studied at different voltage.     

High-temperature phase transition of Tm<Subscript>2</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript>

The high-temperature (>1600°C) order—disorder phase transition of Tm₂Ti₂O₇ is shown to be irreversible. The 740°C ionic conductivity of nanocrystalline Tm₂Ti₂O₇ ceramics synthesized at 1670°C is 2 × 10^-3 S/cm and remains unchanged upon heat treatment in air at 860°C for 240 h. The conductivity of the high-temperature (disordered pyrochlore) phase of Tm₂Ti₂O₇ is independent of grain size in the range 20–30 nm.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1495–1500.Original Russian Text Copyright © 2004 by Shlyakhtina, Knotko, Larina, Borichev, Shcherbakova.