Synthesis, crystal structure and X-ray powder diffraction data of the phosphor matrix 4SrO·7Al2O3

The white phosphor matrix 4SrO·7Al₂O₃ has been synthesized by firing the appropriate mixture of SrCO₃, Al(OH)₃ and H₃BO₃ in the molar ratios 1:3.5:0.135 at 1300°C for 4–7 h. The crystal structure of 4SrO·7Al₂O₃ has been determined as a orthorhombic Pmma space group with a=24.7451(2)Å, b=8.4735(6)Å, c=4.8808(1)Å, V=1023.41(3)ų, Z=2, and D=3.66 g cm^–3 by the Rietveld analysis. The refinement figures of merit are R_p=8.26, R_wp=11.60, R_bragg=4.44 and s=2.61 for 844 reflections with 2<119.94°. And the corresponding X-ray powder diffraction data are presented for search/match analysis.     

Compressibility measurements of β- and β″ -alumina

The compressibilities of the a- and c-axes for sodium - and -aluminas were determined up to 10 GPa from the pressure dependence of powder X-ray diffraction measured at room temperature using synchrotron radiation as an X-ray source. Powders of sodium - and -aluminas which were prepared from grinding synthesized single crystals were used as the specimens for X-ray diffraction. The compressibilities of - and -aluminas are 1.5 ± 0.2 ×10^–12 and 1.7 ± 0.2 × 10^–12 Pa^–1 for the a-axis and 2.9 ± 0.2x10^–12 and 1.6 ± 0.2 ×10^–12 Pa^–1 for the c-axis, respectively. For the c-axis, the compressibility of -alumina is larger than that of -alumina. This experimental fact is explained by the different stacking of oxygen layers and the different content in sodium ion between - and -aluminas.     

Heat conduction in a multilayer composite wedge

A new method for analytically solving a problem of steady-state heat conduction for multilayer composite wedge-shaped bodies is suggested based on a generalization of the integral Mellin transform.Notation T temperature - _rr, thermal conductivity coefficients - thickness of composite material layers (1) - N₁(), N ₂ ⁽¹⁾ (), N ₂ ⁽²⁾ () auxiliary local functions from the rapid variable =r/ - m(r, p) auxiliary function entering the core of the generalized integral Mellin transform - ₀ half of the wedge aperture angle Moscow Institute of Chemical Engineering. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 64, No. 4, pp. 487–491, April, 1993.     

The formation of NaMg2Al15O25 in an α-Al2O3 matrix and its effect on the mechanical properties of alumina

An in-situ sintering reaction was designed to produce lath-like \-alumina in an -alumina matrix in order to make alumina ceramics stronger and tougher. The reaction sequence to produce \-alumina (NaMg₂Al₁₅O₂₅) requires the formation of \-alumina (NaAl₁₁O₁₇) and spinel (MgAl₂O₄) at around 1100 dgC followed by a solid-state reaction of these two phases to give \-alumina at elevated temperatures; this reaction is complete at around 1600 °C. The in-situ sintering reaction produces near-theoretically dense alumina ceramics in which lath-like \-aluminas are homogeneously distributed. The bending strength and fracture toughness increase to 620 MPa and 5 MPam^1/2, respectively; these increases are thought to be due to the suppression of grain growth as well as the crack deflection and bridging associated with lath-like \-alumina.     

Anisotropic spin diffusion and multiple spin echoes in3He-4He solutions

We propose a ₁-t₁-₂ pulse-NMR experiment to detect the spin-diffusion anisotropy, =D-D, in degenerate spin-polarized³He-⁴He mixtures, where D and dare the transverse and longitudinal spin diffusion coefficients. In such an experiment the nonlinearity of the dynamics produces multiple spin echoes (MSE). At the ³He concentration x₃ 4% the spinrotation parameter vanishes (M 0), so that the nonlinearity of the equations of motion is entirely due to the anisotropy. In this situation, detection of MSE amounts to observation of D. For slight anisotropy, i.e. D/D 0.25, we use a perturbation scheme similiar to that developed by Einzel et al. (in that case, for small M and small demagnetizing field) to calculate the second and third echo heights. For larger anisotropy we numerically calculate the echo heights. We find that for D/D = 0.5 the heights are 2 % of the first echo, and should be detectable. The (₁, ₂) tip-angle dependence of the D echoes is different from that of the M and demagnetization echoes, and furthermore, they occur at right angles to these echoes (in spin space). Thus, even when small spin-rotation and demagnetization effects are present, the ₁-t₁-₂ experiment provides a sensitive means of detecting the anisotropy.     

Thermodynamic investigations of liquid Bi-Na and Sn-Na alloys by coulometric titration using β″-alumina

The following double galvanic cell was assembled and the thermodynamic properties of liquid Bi-Na and Sn-Na alloys, and the ion selectivity of -alumina during coulometric titration, were investigated. Mo, Na(I)¦-alumina¦M-Na(I), Mo [I] M-Na(I)¦-alumina¦Au + Au₂Na, Mo [II] (M = Bi or Sn) where M-Na(1) and Au + Au₂Na were used as the common electrode and reference electrode, respectively. Sodium was coulometrically titrated through the -alumina electrolyte of cell I both ways, and the EMFs were measured. It was found that no ion-exchange reaction occurs between the liquid alloys and the -alumina, and only Na was transferred in the -alumina during coulometric titrations. The thermodynamic properties of liquid Sn-Na and Bi-Na alloys were found to be in agreement with the literature.     

Pressureless sintering of β-SiC with Al2O3 additions

-SiC was pressureless sintered to 98% theoretical density using Al₂O₃ as a liquid-phase forming additive. The reaction between SiC and Al₂O₃ which results in gaseous products, was inhibited by using a pressurized CO gas or, alternatively, a sealed crucible. The densification behaviour and microstructural development of this material are described. The microstructure consists of fine elongated -SiC grains (maximum length 10 m and width 2–3 m) in a matrix of fine equi-axed grains (2–3 m) and plate-like grains (2–5m). The densification behaviour, composition and phases in the sintered product were studied as a function of the sintering parameters and the initial composition. Typically, 50% of the -phase was transformed to the -phase.     

High-resolution measurements of the heat capacity of MnBr2·4H2O near its Néel temperature

The heat capacityC _P of the antiferromagnet MnBr₂ ·4H₂O has been measured for polycrystalline and single-crystal samples nearT _N(2.123 K) with temperature resolution of 1×10^–6 K. Similar rounding of the lambda anomaly is found in both cases. For |1 –T/T _N| 10^–1 all data can be well fitted by assuming the samples to consist of many independent subsystems obeying the same power laws but with a Gaussian distribution ofT _N's having a width of 1.1×10^–3 K. ForT>T _N, we findC _P ^–0.12, essentially as predicted for three-dimensional Ising models in the critical region. ForT<T _N and 10^–3 10^–1,C _P ln , which approximates Ising model behavior in this interval but is not expected to be valid for 10^–4. ForT>T _N and 2.5×10^–1, C_P agrees well with predictions for the classical Heisenberg model. This crossover at 10^–1 is consistent with the known anisotropy of the salt and with present theory. The data forT>T _N in the interval 10^–4 10^–3, while not in the range of obvious rounding, appear to be strongly influenced by the mechanism responsible for that rounding.Work supported by the National Science Foundation and the Office of Naval Research. Based on a thesis submitted by L.W.K. to Carnegie-Mellon University in partial fulfillment of the requirements for the Ph.D. degree. A preliminary account of this work was presented at the Atlantic City meeting of the American Physical Society, March 1972 [Bull. Am. Phys. Soc. 17, 299 (1972)].     

Crystal growth of thin-plate β-alumina by liquid transport

Single crystals of -alumina (Na₂0.11Al₂O₃) were grown by liquid transportat 1030–1150 °C and for durations of 4–140 h, using an Na₃AlF₆ solvent and - or -alumina as the nutrient. The maximum size of -alumina crystal grown using a small platinum crucible was 1.1 cm in length and 0.6 mm in thickness. On the other hand, a single crystal 1.8 m long and 0.7 mm thick was grown using a large crucible. These grown single crystals were hexagonal, thin platelike, and very transparent. Screw dislocation-like patterns were observed on the surfaces of grown -alumina single crystals; each pattern showed a right-handed rotation. Small crystals on a matrix crystal grew spirally about the c-axis and formed a new, flat surface on the matrix crystal, evidently an intrusive twin. The angle between the two inclined crystals was 2.9–2.8 °. The twin face seemed to be w(14, 14, 3). The growth rates of -alumina single crystals treated for less than 12 h using large and small crucibles were 3.3 × 10^–1 and 4.0 × 10^–1 mm h^–1, respectively. For treatment ranging from 12 to 40 h growth rates were 1.1 × 10^–1 and 1.5 × 10^–1 mm h^–1 for the large and small crucible, respectively; rates for 40–100 h treatment were 0.3 × 10^–1 mm h ^–1 for both crucibles. The grown -alumina single crystals had fairly good crystallinity. The growth mechanism of the -alumina single crystal can be estimated from transport through liquid film in the crucible, i.e. a creeping phenomenon.     

Phase transformation behavior of nanocrystalline χ-alumina powder obtained by thermal decomposition of AIP in inert organic solvent

Thermal decomposition of aluminum isopropoxide (AIP) in inert organic solvents (toluene and mineral oil) resulted in the formation of -alumina. Phase evolution by calcination at various temperatures for this alumina was studied via X-ray diffraction. The results suggest a direct transformation from -alumina to -alumina at approximately 1180°C, without the formation of -alumina phase, while still maintaining the small particle size (<100 nm). The transformation behavior was observed by TEM and the crytallite size was calculated by the Scherrer equation. The results indicate one -alumina crystal transforms into one -alumina crystal at its critical size in a nucleation step. This crystal exhibits a rapid grain growth following the transformation.     

Magnetic field dependence of the density of states of YBa2Cu3O6.95: Implications for the vortex structure

The total specific heat of YBa₂Cu₃O_6.95 single crystals includes contributions from phonons and spin-1/2 particles, as well as electronic contributions. The electronic specific heat is described by a quadratic term T² in zero field and a linear term [(0)+(H)]T which is increased when a magnetic field H is applied perpendicular to the CuO₂ planes. In agreement with d-wave superconductivity, we find that _n/T_c and (H)_n(H/H_c2)^1/2, where _n is the coefficient of the normal-state linear term. The H^1/2 dependence of the density of states at the Fermi level was predicted by G. Volovik for lines of nodes in the gap: the quasiparticles which contribute to this density of states are close to the nodes in momentum space and are located outside the vortex core.     

Thermodynamic properties of iron doped beta″-alumina by e.m.f. measurements with beta-alumina electrolytes

Thermodynamic studies of the non-stoichiometric iron doped beta-alumina (ID) phase were carried out by electrochemical measurements coupled with coulometric titration using the cell Na_liq/Li-alumina/ID. Hot pressing and glass sealing techniques were developed and employed to obtain a suitable and stable Li-alumina/ID interface. The equilibrium e.m.f. of the cell was determined as a function of sodium concentration over the temperature range 444 to 523 K. The range of sodium concentrations over which the ID phase is stable was also determined. The relative partial molar thermodynamic quantities of sodium, , , and in ID alumina as a function of sodium concentration were obtained from cell e.m.f. data.     

Mössbauer study on the magnetic state of iron particles in Fe-N and Fe-Zr-N soft magnetic thin films

The magnetic state of -Fe particles and the behaviour of nitrogen and zirconium during annealing in Fe₉₆N₄ and Fe_85.6Zr_7.6N_6.8 magnetic thin films have been studied by conversion electron Mössbauer spectroscopy for ⁵⁷Fe. The crystalline phases present in the Fe-N annealed films were -Fe and -Fe₄N, and those in the Fe-Zr-N annealed films were -Fe and ZrN. In the Fe-N films annealed below 300°C, about 60% nitrogen is incorporated interstitially into -Fe and the rest is used for the formation of -Fe₄N. In the Fe-N film annealed at 500°C, almost all nitrogen participates in the formation of -Fe₄N, leading to the grain growth of -Fe particles and an increase in coercive force. The values (291–325 kOe) of internal magnetic field of iron sites in -Fe in the Fe-Zr-N films are much smaller than that (333 kOe) of the iron site in pure -Fe. Even if the Fe-Zr-N films were annealed at 500–700°C, some zirconium and nitrogen is still incorporated substitutionally and interstitially into -Fe, respectively. In particular, the substitutional zirconium depresses the grain growth of -Fe particles, perhaps due to a chemical interaction between zirconium and iron.     

Bifurcation of crack pattern in arrays of two-dimensional cracks

Theoretical calculations based on simple arrays of two-dimensional cracks demonstrate that bifurcation of crack growth patterns may exist. The approximation used involves the dipole asymptotic or pseudo-traction method to estimate the local stress intensity factor. This leads to a crack interaction parametrized by the crack length/spacing ratio =a/h. For parallel and edge crack arrays under far field tension, uniform crack growth patterns (all cracks having same size) yield to nonuniform crack growth patterns (bifurcation) if is larger than a critical value _cr. However, no such bifurcation is found for a collinear crack array under tension. For parallel and edge crack arrays, respectively, the value of _cr decreases monotonically from (2/9)^1/2 and (2/15.096)^1/2 for arrays of 2 cracks, to (2/3)^1/2/ and (2/5.032)^1/2/ for infinite arrays of cracks. The critical parameter _cr is calculated numerically for arrays of up to 100 cracks, whilst discrete Fourier transform is used to obtain _cr for infinite crack arrays. For infinite parallel crack arrays under uniaxial compression, a simple shear-induced tensile crack model is formulated and compared to the modified Griffith theory. Based upon the model, _cr can be evaluated numerically depending on (the frictional coefficient) and c ₀/a (c ₀ and a are the sizes of the shear crack and tensile crack, respectively). As an iterative method is used, no closed form solution is presented. However, the numerical calculations do indicate that _cr decreases with the increase of both and c ₀/a.     

Ionic conductivity of Na,K and Ag β″-alumina-ZrO2 composites

Composite samples of Na -alumina with 15 vol % Zr0₂ stabilized with 4.5 wt % Y₂O₃ were fabricated using two sintering schedules. K and Ag -alumina composites were prepared from the Na -alumina composite by ion exchange. The ionic conductivities of the Na, K, and Ag composites at 300 °C were 1.3×10^–1, 5.9×10^–2 and 6.8×10^–3 S cm^–1, respectively. K _1c for the K composite was as high as 3.7 MPam^1/2.     

Limits on Superconductivity in Nanoparticles and Nanowires

Recent experimental results are consistent with the prediction that superconductivity is suppressed in aluminum nanoparticles which are so small (10 nm diameter) that the electronic energy level spacing exceeds the superconducting energy gap in bulk material. Very recent experiments on nanowires (diameter 5–10 nm, length 150 nm) of MoGe indicate that a dissipative phase transition between superconducting and normal behavior occurs when the normal resistance of the wire equals the superconducting quantum resistance h/4e² 6.5 k.     

Design of the Target Assembly for Accumulation of 199Tl by Irradiation on a U-120 Accelerator

A target assembly for accumulation of ^{1 9 9}Tl by irradiation of 30-40-m gold foils was developed and designed. As alternative to traditional design, sandwich cyclotron targets consisting of materials with different coefficients of linear thermal expansion are proposed. The efficient mechanical contact between the gold foil and spherical support (prepared from D-16 alloy) can be ensured by an outer clamp, e.g., 0.2-mm vanadium gauze transparent for -particles to 85-90%.     

Investigation of Phase Evolution During the Thermochemical Synthesis of Tricalcium Phosphate

The formation of tricalcium phosphate (TCP) from dicalcium phosphate and calcium carbonate was investigated using differential thermal analysis (DTA), thermogravimetric analysis (TGA), and powder X-ray diffraction (XRD). DTA showed three distinct thermal events attributed to dehydration of dicalcium phosphate dihydrate (brushite, DCPD) to dicalcium phosphate anhydrous (monetite, DCPA), the formation of beta-calcium pyrophosphate (-Ca₂P₂O₇), and the calcination of calcium carbonate. TGA showed three weight losses corresponding to the three thermal events, respectively. XRD analysis showed that -TCP formed, beginning at about 900°C, by the reaction of -Ca₂P₂O₇ with CaO and -TCP changed to -TCP above 1200°C. Further examination of the formation of TCP from calcium hydroxide (Ca(OH)₂) and DCPA showed that -TCP would form only after the decomposition of Ca(OH)₂ to CaO and from the reaction of CaO with -Ca₂P₂O₇ at a fairly low temperature of 800°C. In addition, by naturally cooling -TCP, formed at 1300°C within the heating furnace, to room temperature, it was difficult to obtain a pure phase of -TCP. The proposed mechanism of the reaction to form TCP may include the dehydration of brushite to monetite, dehydration of monetite to beta-calcium pyrophosphate, decomposition of calcium carbonate, the formation of -TCP, and phase transition.     

The martensitic transformation in cold-worked Fe-Mn alloys studied by Mössbauer spectroscopy

The effect of cold-working on 16, 23 and 30 at% Mn iron-manganese alloys (C<0.05 at%) has been examined using Mössbauer spectroscopy and X-ray diffractometry. The induced martensitic transformation , depends on the composition and on the initial structure. It is found that the transformation in presence of the phase occurs only at high deformations.     

The dielectic response of K x Al x Ti8−x O16 and K x Mg x/2 Ti8−x/2 O16

Materials of the hollandite structure with the general formulae K_x Al_x Ti_8–x O₁₆ and K_x Mg_x/2 Ti_8–x/2 O₁₆ have been synthesized in the composition range 1.6x2.0 and their dielectric properties have been measured in the temperature range 77 to 800 K and the frequency range 10^–3 to 10⁶ Hz. The observed response shows a whole range of features characteristic for both charge carrier and dipolar polarization processes and these are seen as being associated with the one-dimensional transport in channels in the hollandite structure. At low temperatures the dominant response is the universal dielectric relation in which the loss follows the law x() ^n–1, with the exponent n<1 and equal specifically to approximately 0.7. This is followed at 120 to 180 K by a distinct loss peak superimposed on the above law, and finally at higher temperatures by a region of strong dispersion which is associated with strongly interacting many-body processes between charged carriers restricted by defects to move in limited regions of the channels.