Dielectric discontinuities and multipolar interactions in ferroelectric perovskites

Ferroelectric perovskites, in particular KNbO₃, PbTiO₃ and BaTiO₃, are characterized by discontinuities in the dielectric constant at the tetragonal-cubic transition (T*) as well as by the abrupt disappearance of the spontaneous polarization which goes from a minimum finite value P_s* =P_s (T) to zero at T=T*. It is shown that, taking into account quadrupolar and higher order terms into the effective field acting on the elementary dipoles lattice, good fits to the observed P_s(T) in a wide temperature range below T* and to the inverse dielectric constant discontinuity (C/T_c)[ε₊^-1(T*)-ε_-^-1(T*)] at T⋍T* are obtained for these three perovskites.     

Pyroelectric properties of valine doped TGS single crystals

TGS was grown by a slow cooling technique adding several kinds of dopants (10 and 20 mole% alanine, 10% alanine + LiVO₃, 10% and 20% valine) to an aqueous solution. The temperature dependent pyroelectric coefficient, p and the spontaneous polarization, P_s, of doped-TGS crystals were measured to study the transition temperature, P_max and room temperature pyroelectric coefficient depending on the dopants and to calculate the figure of merit, (p/K), (K is a dielectric constant). Furthermore, several pieces were selected to investigate the positional inhomogeneities of the same sample. Moderate figure of merit (p/K) was obtained in valine doped TGS because of low temperature dielectric relaxation around -10°C, although valine doped TGS has a three times higher pyroelectric coefficient (p = 1.671μC/cm²K) at transition temperature (T_c) than that of alanine doped TGS.     

Field dependent temperature shift of dielectric losses peak in TGS

Dielectric constant and loss factor measurements have been performed in triglycine sulfate single crystals at the vicinity of the Curie temperature (T_c≈48.5°C) as a function of ac driving field (20≤v≤10⁶ Hz) in a wide range of field amplitude (50≥E_o≥0.15V/cm). Well defined shifts of loss factor peak (D_max), increasing monotonously with the field, not previously reported as far as we know, were observed and analyzed in terms of the energy dissipated within the vanishing hysteresis loops at T≈T_c corresponding to E_o≈E_c(T), coercive field.     

Thermoopitical study of precursor polarization in febroelectrics with diffuse phase transitions

Temperature variations of the refractive index on have been studied in single crystals of ferroelectric perovskites with diffuse phase transitions, with a different degree of ordering and also in the nonferroelectric crystals with pyrocphlore-type structure. Ferroelectric contribution δ^p to the index of refraction was obtained in ferroelectric perovskites. In all these compounds the temperature behaviour of δ(T) above T_c was found to vary in a same manner and this behaviour was interpreted in terms of local polarization in microregions with different transition temperatures. Contrary to that δ(T) around T_c behaves differently in ferroelectrics with diffuse phase transitions and in ferroelectrics with a different degree of ordering, that can be understood within the framework of percolation model.     

Electrogyration of pure and X-Irradiated TGS crystals

Dispersion of optical rotatory power δ, temperature dependence of δ and electrogyration of hysteresis loops of pure and X-irradiated TGS crystals have been measured, The relation between OA and P_s and the origin of OA in TGS crystal is discussed.     

Dielectric and pyroelectric properties of triglycine sulphate-polystyrene composites

A composite of Triglycine sulphate (TGS) and polystyrene has been fabricated by mixing different particle size TGS powders (75 μm to 400 μm) and polystyrene solution. The dielectric constant has been measured in the frequency range 0.1 to 100 KHz, and the pyroelectric current measurements were made by the Byer and Roundy direct method. Specific heat of the composite was determined by the differential them analysis (DTA) technique.

Particle size dependance of dielectric constant, pyroelectric coefficient and responsivity have been studied. Anomalies in ε' and P_y were observed even in the composite with the lowest particle size of TGS (75 μm). The surface effects have much greater influence on electrical properties but thermal properties are, not affected. This is indicated by a decrease in ε' and P^y with decreasing particle size of TGS, while specific heat of composite is almost independent of particle size.     

Preferred orientation of the grains in the ceramics of high Tc-superconductors

Highly grain oriented YBa₂Cu₃O₇ and GdBa₂Cu₃O₇ ceramics are prepared by using the controlled calcining and sintering conditions. T_c and ▵T_c on such ceramics were ∼94K and 2K respectively.     

Ferroelasticity and ferroelectricity in betaine arsenate

Single crystals of betaine arsenate, an addition compound of the amino acid betaine with H₃AsO₄, were investigated by dielectric, optical, caloric, and X-ray methods. At room temperature betaine arsenate is ferroelastic with two different monoclinic domains. This state seems to disappear at 411 K. Below T_c = 119 K the ferroelastic domains show a ferroelectric behaviour with a spontaneous polarization P_s = 2μC/cm² at T = T_c - 35 deg.     

Well behaved temperature dependence of the ferroelectric coercive field near Tc in TGS

A new and simple method is introduced to make experimental observations of the coercive field of a ferroelectric which avoids the ambiguities associated with the amplitude dependence of this quantity. The temperature dependence of the coercive field of TGS in the vecinity of T_c has been measured and found to obey a law E_c = A(T_c - T)^s, with s = 1.45±0.10, in good agreement with the theoretical prediction s = 3/2.     

On the elfctrical and thermal properties of NaKSO4 crystals

The d.c. conductivity of NaKSO₄ crystal is investigated, along various crystallographic axes in the temperature range from 300 to 500 K. Two segments are observe in the temperature dependence of electric conductivity at T_c=453 K. The spontaneous strain η_1s, η_2s and η_3s, deduced from measurements of the linear thermal expansion coefficients along various crystallographic axes a, b and c showed linear temperature dependence below the transition temperature.     

Ferroelectric Pb5-xBix (Ge.Si)3-xBxO11 single crystals as a material for IR radiation detectors

The coupled substitution of Bi³⁺ for Pb₂₊ and B³⁺ for Ge⁴⁺ was successfully achieved in ferroelectric Pb₅Ge₃O₁₁. Large single crystals of optical quality were grown from the melt. Deterioration of the crystal quality could not be observed within a period of two years under ambient conditions. The dielectric permittivity ε_r, electric conductivity p and pyroelectric coefficient γ are investigated as a function of temperature. The room temperature data (εr = 43, γ = 2.4·10^-4Cm^-2 K^-1, p = 7·10⁷ Ω, volume specific heat s = 2·10⁶ Jm^-3K^-1) show the usefulness of Pb_5-xBi_xGe_3-xB_xO₁₁ around x = 0.1 as a material for pyroelectric thermal detectors operating at room temperature. Also the difficulty of producing lead germanate silicate mixed crystals in the size and quality needed for vidicon applications is overcome by additional replacement of some Pb and Ge by Bi and B.     

Dielectric behaviour of betaine arsenate

Dielectric constant measurements of the ferroelectric material Betaine Arsenate showed a maximum value of 1.057*10⁶ (tan δ = 1.73*10^-2). The dielectric constant is compared with hysteresis loops. It is shown that close below T_c, a small signal of 1.3 V/m is sufficient to switch the spontaneous polarization.     

Microstructure and properties of various fluorine-containing SiAlON ceramics synthesized by HIPing

An attempt was made to prepare various F-doped β-, O-, X-, and -SiAlONs from a mixture of Si₃N₄, SiO₂, Al₂O₃, AlN, or Y₂O₃ using AlF₃ or topaz as the fluorine source by HIPing at 1500–1800°C and 150 MPa. The phases were identified and the z, x, and m/n values determined for β-, O-, and -SiAlONs by X-ray diffraction. When AlF₃ was used, a single phase ceramic (O-SiAlON) was produced from a mixture of -Si₃N₄ and SiO₂ at 1500°C, with a mixture of O- and β-SiAlONs formed at 1700°C. A mixture of -Si₃N₄, AlN, and Y₂O₃ with AlF₃ produced β-/Y--SiAlON ceramics at 1730°C. The use of topaz produced the β-SiAlON ceramic with a trace of mullite from a mixture of -Si₃N₄ and AlN at 1770°C and mixed phase β-/O-SiAlON ceramics from -Si₃N₄ and SiO₂ at 1700°C. Single phase X-SiAlON could not be obtained under the present conditions. The microstructures of the single phase O- and β-SiAlON ceramics and the β-/Y--SiAlON mixture showed the growth of O- and β-SiAlON and Y--SiAlON crystals with hexagonal and/or long rod-like or platy shapes in a matrix of F-containing glassy phase. The compositions of the SiAlON crystals and the glass phase were semi-quantitatively determined by EDX; the total glass phase was estimated by a quantitative Rietveld XRD powder method. The F-doped β-SiAlON ceramics showed better corrosion resistance towards NaCl vapor and lower Vickers hardnesses.     

Thermal shock resistance of ceramic materials in melt immersion tests

A melt immersion test is applied to determine the relative resistance of ceramic materials to thermal shock failure under high heat flux conditions. The testing method is demonstrated mainly for Al₂O₃ pellets, while AlN is included to represent elevated thermal shock resistance. In order to quantify the resistance to crack formation, the critical temperature difference ΔT_c between sample and metal melt is determined from the failure probability distribution of a set of pellets.

In quenching tests correspondence of ΔT_c with the thermal shock parameter R = σ(1 − μ)/E was found, if the initial surface temperature of the sample was correctly estimated. This assessment was the main concern of the evaluation work.

ΔT_c resulting from heating tests was correlated with the maximum tensile stress in the sample by modeling calculations. The stress limits determined show that the ultimate bending strength could serve as a rough approximation for the materials tested.     

PENETRATION THEORY ANALYSIS FOR NONISOTHERMAL GAS ABSORPTION ACCOMPANIED BY A SECOND-ORDER CHEMICAL REACTION

The penetration theory equations for nonisothermal gas absorption with second-order reaction were solved numerically using the Saul'yev method. Results for the enhancement factor and interfacial temperature rise were obtained for Hatta numbers between 1 and 10, It was found that the enhancement factor and interfacial temperature rise depend on four parameters; (i) the effective activation energy, ε_eff = (ε_R + ε_DA)/2 - ε_S, where ε_R, ε_DA, and ε_S are the activation energies for the reaction, diffusion, and solubility of the solute gas, respectively; (ii) the effective heat of generation, β_eff = (β_R + β_s)√Le, where β_R and β_s are the dimensionless heat of solution and reaction, respectively, and Le is the Lewis number; (iii) the product √rq where r is the ratio of the diffusivity of the liquid reactant to that of the solute gas and q is the ratio of the concentration of the liquid reactant in the bulk solution to that of the solute gas at the interface at the bulk temperature of the solution; and (iv) the activation energy for diffusion of the liquid reactant, ε_DB. This effectively reduces the number of parameters required to give a complete picture of the absorption from nine to four. The reduction is important since it reduces the number of van Krevelen-Hoftijzer plots and interfacial temperature-Hatta number plots required for the design of gas-liquid reactors.     

Effect of Microstructure on Ferroelectric and Piezoelectric Properties of A-Site and B-Site Modified PLZT

Ba modified (Pb,La,Ag)(Zr,Ti)O₃ ceramics synthesized by conventional mixed-oxide method with chemical formula [Pb_{0.983 - z}La_0.012Ag_0.005Ba_z] [(Zr_0.52Ti_0.48)_0.99825]O₃ were investigated for ferroelectric and piezoelectric properties. XRD studies indicate that increment of Ba concentration resulted in strong tetragonality in the ceramics. Grain growth (3.08 μ m) inhibited and apparent density (7.54 gm/cm³) enhanced up to 1.5 mole% Ba respectively. As Ba content increased dielectric properties showed maximum values (ε_RT = 2347, ε_Tc = 23449) while T_c constantly decreased. The remanent (P_r = 34.72 μ C/cm²) and spontaneous polarization (P_s = 41.84 μ C/cm²) were optimum in 1 mole% Ba respectively while E_c showed increasing trend throughout the series. Piezoelectric properties attained maximum values of d₃₃ = 438 pC/N in 1.5 mole% Ba while k_p = 0.533 and k_t = 0.412 in 1 mole% Ba doped (Pb,La,Ag)(Zr,Ti)O₃ ceramics respectively.     

Acoustic emission, dielectric constant, thermal expansion and domain structure of BaTiO3 crystals near the phase transition

By the acoustic emission, dielectric constant and thermal expansion methods BaTiO₃ crystals have, been investigated near the phase transition. Domain structive have been photographed on the reflected polarized light. The crystale composite etched and its properties have been recorded after every etching. The dielectric constant and the thermal expansion curves have been carried out the anomalies are accomponied by the acoustic emission. Being lied at 7-10°C below T_c, the temperature of these anomalies have been approaching to T_c as far as one etch the crystals. The quantities of these anomalies and the acoustic emission activity has been decreasing, and the domain structure has been simplefied too. It is shown, these observed anomalies lire a result of the internal stresses into the surface layers.     

Ferroelectric behavior of TAAP single crystal near Tc

Dielectric and hysteresis loop measurements in good quality telluric acid ammonium phosphate from 20 to 50°C with special attention to the behavior in the vicinity of T_c ≈44.80 °C were performed. The effective field higher order parameters characterizing the temperature dependence of the spontaneous polarization were obtained. The main features of the ferroelectric transition in this “hard” ferroelectric have been compared with those of other well known ferroelectric crystals.     

Phase transitions and polytypes in β TlInS2 ferroelectric-semiconductor

The temperature dependences of the dielectric properties, conductivity, spontaneous polarization as well as the effect of hydrostatic pressure on the anomalies of these properties have been investigated. The β-TlInS₂ crystals are shown to be often a mixture of the polytypes. Transition into a ferroelectric state for some of the β-TlInS₂ polytypes is not preceded by an incommensurats phase transition.     

Ferroelectric behavior of TAAP single crystal near Tc

Dielectric and hysteresis loop measurements in good quality telluric acid ammonium phosphate from 20 to 50°C with special attention to the behavior in the vicinity of T_c ≈ 44.80 °C were performed. The effective field higher order parameters characterizing the temperature dependence of the spontaneous polarization were obtained. The main features of the ferroelectric transition in this “hard” ferroelectric have been compared with those of other well known ferroelectric crystals.