Showing papers on "Tetragonal crystal system published in 1985"

Dielectric properties of fine‐grained barium titanate ceramics

Abstract: Dielectric properties, lattice‐ and microstructure of ceramic BaTiO3 showing grain sizes of 0.3–100 μm were studied. At grain sizes <10 μm the width of ferroelectric 90° domains decreases proportionally to the square root of the grain diameter. The decreasing width of the domains can be theoretically explained by the equilibrium of elastic field energy and domain wall energy. The smaller the grains, the more the dielectric and the elastic constants are determined by the contribution of 90° domain walls. The permittivity below the Curie point shows a pronounced maximum er ≊5000 at grain sizes 0.8–1 μm. At grain sizes <0.7 μm the permittivity strongly decreases and the lattice gradually changes from tetragonal to pseudocubic.

Transformation of Yttria‐Doped Tetragonal ZrO2 Polycrystals by Annealing in Water

Abstract: Phase changes and the microstructure resulting from low-temperature annealing of yttria-doped tetragonal ZrO2 polycrystals in water were investigated at 65° to 120°C. Tetragonal ZrO2 on the surface of the sintered body transformed to the monoclinic phase, accompanied by microcracking. The transformation rate in water, which was much greater than that in air, was first order with respect to surface concentration of tetragonal ZrO2. Nonaqueous solvents with a molecular structure containing a lone-pair electron orbital opposite a proton donor site also greatly enhanced the transformation.

Structure and Ionic Mobility of Zirconia at High Temperature

Abstract: The high-temperature structure of zirconia was studied by powder neutron diffraction up to 2400/sup 0/C. The boundaries of the domain of the nonstoichiometric tetragonal form are defined. They are consistent with a tetragonal-cubic transition at 2350/sup 0/C for stoichiometric zirconia. The changes in the structural parameters of the tetragonal form (unit-cell, positional parameters and thermal B factors) with temperature give evidence of medium zirconium and oxygen mobilities. The oxygen ions are, however, always more mobile than the zirconium ions. An enhancement with temperature of the structural anisotropy tends to weaken the weaker of the two distinct Zr-O bonds of the tetragonal zirconia. It results in the transformation into the cubic form which is accompanied by a change in unit-cell volume; this change becomes sharper as the composition tends toward stoichiometry. This transition is probably followed by an increase of the ionic mobility.

Pressure‐Temperature Phase Diagram of Zirconia

Abstract: The pressure-temperature phase diagram of zirconia was determined by optical microscopy and X-ray diffraction techniques using a diamond anvil pressure cell. At room temperature, monoclinic ZrO/sub 2/ transforms to a tetragonal phase (tII) which is related to the high-temperature tetragonal structure (tI). The transformation pressure exhibits hysteresis and is cycle dependent. At room temperature, the initial transformation pressure for the monoclinic-tII transition on a virgin monoclinic crystal can be as high as 4.4 GPa; on subsequent cycling the transition pressure ultimately lowers to 3.29 + or 0.06 GPa. The pressure for the reverse transition is essentially constant at 2.75 + or - 0.06 GPa. At pressures > 16.6 GPa, the tII form transforms to the orthorhombic cotunnite (PbCl/sub 2/) structure. With increasing temperature, the tII form transforms to the high-temperature tetragonal phase. For increasing P and T, the monoclinic-tI-tII triple point is located at T = 596/sup 0/ + or - 18/sup 0/C and P = 2.26 + or - 0.28 GPa, whereas for decreasing P and T, the triple point is found at T = 535/sup 0/ + or - 25/sup 0/C and P = 1.7 + or - 0.28 GPa.

Thermodynamics of the tetragonal to monoclinic phase transformation in constrained zirconia microcrystals

Abstract: End-point thermodynamic analyses were made of the tetragonal to monoclinic transformation (t→m) occurring in ZrO2 precipitates in a Ca-PSZ alloy and particles in Al2O3-ZrO2 composites. Calculated plots of the reciprocal critical size for transformation temperature were in excellent agreement with experimental data for both systems. Contributions to the total free energy change included bulk chemical, dilatational and residual shear strain energies and also interfacial energies. The latter term consisted of contributions from the change in the chemical surface free energy, the presence of twin boundaries in the precipitate (particle)-matrix interfacial energy. The major impediment to the transformation was the shear strain energy which could not be reduced sufficiently by twinning alone. The t → m reaction proceeded spontaneously when the energy barrier was reduced by the response of the particle-matrix interface. The response comprised loss of coherency and grain boundary microcracking for the Ca-PSZ and Al2O3-ZrO2 alloys, respectively. These results are in accord with recent suggestions that either a stress-free strain or a free surface is a necessary conditions for the initiation of a martensitic transformation.

Phase Characterization of Partially Stabilized Zirconia by Raman Spectroscopy

Abstract: Raman spectroscopy was used to characterize phase transformations and transition temperatures in partially stabilized zirconia containing ≤20 wt% Y2O3. The completeness of the martensitic transition and its thermal hysteresis was followed in samples with ≤4 wt% Y2O3. Between 5 and 12 wt% the spectra indicate a tetragonal modification precipitated in a disordered cubic fluorite matrix. Above 15 wt% a “density of states” spectrum prevails that is characteristic of the fully stabilized disordered cubic phase.

Transformation-toughened zirconia ceramics

Abstract: An assessment has been made of the current interest and potential future of zirconia ceramics for structural applications. Interest in the mechanical properties of ceramics based on, or containing, ZrO2 centres on the manipulation of the tetragonal to monoclinic martensitic phase transformation. It will be demonstrated that the maintenance of a metastable tetragonal phase is one of the most important factors in the optimum design of a strong and tough ceramic. There are several microstructural approaches that can be taken to achieve toughening based on either precipitate-hardened or particulate-dispersed systems, and a number of different toughening mechanisms can operate, either singly or together, to produce different combinations of properties. The potential engineering future of three ZrO2 ceramics: partially stabilized zirconia, tetragonal zirconia polycrystals, and zirconia-toughened alumina, is examined in this context. The important relationship between microstructure and mechanical proper...

Structural Dependence on Sintering Temperature of Lead Zirconate‐Titanate Solid Solutions

Abstract: Pb(Zr /SUB 0.525/ Ti /SUB 0.475/ )O3 piezoceramics, both unmodified and doped with 2 wt% Bi2O3 or Nb2O5, were prepared by the usual techniques, using sintering temperatures from 900 to 1250C. The microstructural data showed that the sintering temperature which produces minimum porosity is altered by the oxide additions. X-ray diffraction demonstrated the coexistence of both ferroelectric phases. The lattice parameter measurements showed that the tetragonal and rhombohedral unit cells of the two ferroelectric phases depend on the sintering temperature.

The microstructure and mechanical properties of yttria-stabilized zirconia prepared by arc-melting

Abstract: The microstructure of ZrO2-Y2O3 alloys prepared by arc-melting was examined mainly by electron microscopy. It was found that the microstructure changed markedly with yttria content between 0 and 8·7 mol%. Pure zirconia was a single monoclinic phase, while ZrO2-8·7 mol% Y2O3 alloy was single cubic phase as expected from ZrO2-Y2O3 phase diagram. Tetragonal phase was found in alloys with 1 to 6 mol% Y2O3 together with monoclinic or cubic phase. The tetragonal phase found in present alloys normally had a lenticular shape with a length 1 to 5μm and a width 0.1 to 0.3μm, which is much larger than that formed by annealing. The phase with a herring-bone appearance was found in alloys with Y2O3 between 2 and 3 mol%, which was recognized to be a metastable rhombohedral phase. The structure of the present alloys is likely to be formed by martensitic or bainitic transformation during fairly rapid cooling from the melt temperature. The change in hardness and toughness with yttria content of the alloys is discussed on the basis of microstructural observations.

Microstructural Evolution in a ZrO2 Wt% Y2O3 Ceramic

Abstract: Several unusual microstructural features, i.e., 90 /sup 0/ tetragonal ZrO/sub 2/ twins containing antiphase domain boundaries, tetragonal ZrO/sub 2/ precipitates in a colony morphology, and precipitate free zones at the perimeter of cubic ZrO/sub 2/ grains containing fine tetragonal ZrO/sub 2/ precipitates, were observed in a single ZrO/sub 2/. 12 wt% Y/sub 2/O/sub 3/ ceramic annealed at 1550/sup 0/, 1400/sup 0/, and 1250/sup 0/C, respectively. The type of phase transformation responsible for each microstructural feature is described.

The crystal structure and the equation of state of cerium metal in the pressure range 0–46 Gpa

Abstract: High-pressure X-ray diffraction studies have been performed on cerium powder up to 46 GPa using a diamond anvil cell and synchrotron radiation. The results confirm the following transitions: γ(fcc)−α(fcc) at 0.8 GPa, α(fcc)−α″ (monoclinic b.c.) at 5 GPa and α″ (monoclinic bc)-tetragonal bc at 12 GPa. All high-pressure structures can be described as distorted fcc. No clear evidence has been found for the α'-phase with α-uranium structure. It is found that the bulk modulus is 21 GPa for γ-Ce and 20 GPa for the combined compression data of α−Ce, α″-Ce and tetragonal Ce in good agreement with other experimental and theoretical data. The experimental V(P) data are compared with a theoretical equation of state obtained by the linear muffin-tin method.

II-I phase transformation of melt-crystallized oriented lamellae of polybutene-1 by shear deformation

Abstract: Modification II-I transformation of polybutene-1 was investigated on oriented lamellae, which were melt-crystallized in the temperature gradient. WAXS photograph of as-grown specimen showed prefered orientation of [110] to the lamellar axis, and after transformation it showed “twelve point pattern” due to dual orientation of hexagonal lattice. By shear deformation parallel or perpendicular to lamellar axis, WAXS photograph showed the appearance of “six point pattern” due to single orientation of hexagonal lattice, and also the enhancement of transformation rate. These were explained by the growth of hexagonal nuclei formed by the slip in the tetragonal crystal.

Crystal and magnetic structure of Pr2Fe14B and Dy2Fe14B

Abstract: Rietveld analyses of neutron powder diffraction data on Pr2Fe14B and Dy2Fe14B are reported. Both phases form the Nd2Fe14B tetragonal crystal structure (P42/mnm). At 77 and 293 K our analyses indicate that all magnetic moments in each compound are collinear with the c axis. The rare‐earth moments in Pr2Fe14B are parallel to the Fe moments (ferromagnetic) and are antiparallel in Dy2Fe14B (ferrimagnetic).

Molecular beam epitaxial growth and transmission electron microscopy studies of thin GaAs/InAs(100) multiple quantum well structures

Abstract: GaAs/InAs(100) multiple interface structures involving 7.4 percent lattice mismatch have been fabricated via molecular beam epitaxy and examined via transmission electron microscopy. It is found that high-quality, dislocation-free interfaces involving such high lattice mismatch can indeed be experimentally realized for very thin layers provided proper care is given to achieve a balance between the growth kinetics and the thermodynamics leading to the equilibrium ground state of the strained layer. The compressive strain is homogeneously accommodated and a tetragonal distortion is induced in the InAs layer with a perpendicular lattice constant in close agreement with that expected on the basis of the continuum theory and elastic constants of bulk InAs.

Growth of crystalline zirconium dioxide films on silicon

Abstract: Zirconium dioxide (ZrO2) films have been grown on Si(100), Si(111), and SiO2/Si substrates heated at the range from room temperature to 800 °C by vacuum evaporation. X‐ray diffraction and reflection high‐energy electron diffraction observations reveal the epitaxial growth of tetragonal ZrO2(200) films on Si(100) substrates at 800 °C. The epitaxial imperfection is caused by preferentially oriented tetragonal ZrO2(002) grains. The crystalline system of ZrO2 films depends on the substrate temperature. The crystalline perfection and the orientation of tetragonal ZrO2 films grown at 800 °C depend on the substrate orientation.

Single crystal measurements of anisotropy constants of R2Fe14B (R=y, Ce, Pr, Nd, Gd, Tb, Dy and Ho)

Abstract: The first order anisotropy constants Ku1 of the recently found tetragonal rare earth-iron borides R2Fe14B (P42/mnm) have been measured for R=Y, Ce, Pr, Nd, Gd, Tb, Dy and Ho in a magnetic field up to 1600 kA/m from 4.2 K to about 600 K on single crystal samples.

Structure and phase transitions in poly(diethylsiloxane)

Abstract: The structure changes accompanying phase transitions in poly(diethylsiloxane) (PDES) have been studied by WAXS and SAXS techniques using oriented and isotropic samples. PDES may exist in two low-temperature modifications (the monoclinic α1-form and presumably the “tetragonal” β1-form) and two high-temperature modifications (the monoclinic α2-form and the “tetragonal” β2-form). In linear PDES the crystal - crystal transitions α1–α2 and β1–β2 occur near 214 and 206 K, respectively. At higher temperatures α2 (280 K) and β2 (290 K) forms transform into the mesomorphic phase αm that gradually melts at 280–300 K giving an amorphous phase. According to x-ray and density data, αm phase is also characterized by monoclinic structure slightly different from hexagonal packing.

Highly heat-resistant Nd-Fe-Co-B system permanent magnets

Abstract: The magnetic properties and crystal structures of sintered Nd-Fe-Co-B magnets have been investigated over a wide range of chemical composition. Magnetic hardening is performed for samples by a post sintering heat treatment at 500\sim1000\deg C. The curie temperature of these samples are significantly improved with the increase of cobalt content. The sample with the composition of Nd 16 Fe 66 Co 11 B 7 shows remarkable magnetic properties, such as the maximum energy product of 42 MGOe and the reversible temperature coefficient of residual magnetization of 0.02 %/°C, Electron probe microanalysis and x-ray data suggests that the highly heat resistant magnet is produced when cobalt distributes only in the tetragonal matrix phase.

Transformation of Yttria‐Doped Tetragonal ZrO2 Poly crystals by Annealing under Controlled Humidity Conditions

Abstract: Phase changes on the surface resulting from low-temperature annealing of yttria-doped tetragonal ZrO2 polycrystals under controlled humidity conditions were investigated at 0.1 to 7350 Pa of water vapor pressure and 100° to 600°C for 1 to 50 h. The tetragonal-to-monoclinic phase transformation was greatly accelerated by increasing water vapor pressure; the transformation rate was first order with respect to the concentration of tetragonal ZrO2 on the surface of the sample.

Magnetic behaviour of tetrakis[(2-diethylaminoethanolato)isocyanatocopper(II)], a complex with an antiferromagnetic ground state; the crystal and molecular structure of the triclinic modification

Abstract: The crystal and molecular structure and magnetic behaviour of [{Cu(NCO)(OCH2CH2NEt2)}4](B) from benzene have been determined. The crystal structure determination was carried out by single-crystal X-ray diffraction methods and refined to R= 0.066 for 4 379 independent reflections: triclinic, space group P, with unit-cell dimensions a= 13.478(3), b= 12.873(3), c= 11.542(3)A, α= 84.020(5), β= 80.263(5), and γ= 86.863(5)°. Each copper atom is five-co-ordinated by three oxygen and two nitrogen atoms. The magnetic susceptibility measured over the temperature range 5.0–302.5 K shows a maximum at 94 K and thus indicates that antiferromagnetic spin coupling is dominant. The magnetic behaviour can be explained on the basis of the isotropic Heisenberg–Dirac–van Vleck model. The results are compared with the structural and magnetic properties of the tetragonal form (A) of this complex. No phase transition between the two modifications could be found up to the melting point.

A calculation of elastic constants of ferromagnetic iron at finite temperatures

Abstract: The temperature dependence of the elastic constants C'=1/2(C11-C12),C44 and B of BCC iron is calculated within the tight-binding approximation using the single-site spin-fluctuation theory of band magnetism. The theory reproduces the observed anomalous 'softening' of the tetragonal shear constant C', relating it to the contribution of the eg orbitals. The negative slope with temperature of the trigonal shear constant C44 and bulk modulus B is shown to arise from the lattice expansion. The influence of magnetic moments on the elastic constants of ferromagnetic and antiferromagnetic, BCC transition metals is discussed in general.