o′-β′ sialon ceramics

Powders of Si₃N₄, Al₂O₃ and SiO₂ were mixed with Y₂O₃ as sintering aid and hot-pressed to form o- sialon. During sintering, the o phase preferentially precipitated in the temperature region of 1550 to 1600 °C, and the phase precipitated at temperatures of 1700 °C and above. The resultant microstructure consisted of dual phases of o and . The mechanical properties of o- sialon were improved with increasing amount of the phase. When SiO₂ required for the formation of the o phase was all included in the grain boundary phase, o- sialon exhibited a three-point bending strength as much as 1400 MPa. The oxidation resistance of o- sialon is expected to improve due to the presence of the o phase, but actually was not so good due to the presence of the grain-boundary glassy phase and impurities.     

Charged defects-controlled conductivity in Ge-ln-Se glasses

The variation of the d.c. electrical conductivity, , with composition and temperature was investigated for glasses of the Ge-In-Se system. The results indicate a decrease in the activation energy for electrical conductivity, E, and an increase in on introduction of indium into Ge-Se glasses. The changes in E and with composition (selenium content in the glasses) are identical for the Ge_x In₅ Se_95–x and Ge_x In₈Se_92–x families. The results have been traced to the conduction controlled by charged defects in these chalcogenide glasses. The changes in E and have been explained by a shift in the Fermi level, being brought by the introduction of indium.     

The formation of α-Al2O3 from θ-Al2O3: The relevance of a “critical size” and: Diffusional nucleation or “synchro-shear”?

The coarsening of -Al₂O₃ crystals to a 'critical size' is often interpreted as the first step in the shear nucleation of -Al₂O₃. The existence of this so-called critical size has also been used to explain the observation that -Al₂O₃ nuclei are generally twice as large as the crystals in the -Al₂O₃ matrix. This paper discusses the important issues in the nucleation of -Al₂O₃ from -Al₂O₃. A few key experiments are also presented to clarify the nucleation process. It is concluded that a critical -Al₂O₃ crystal size is not a prerequisite for -Al₂O₃ nucleation, but is primarily a result of the incubation time required to produce -Al₂O₃ nuclei by diffusional nucleation. It is proposed that the large observed -Al₂O₃ crystal size also does not result from a shear nucleation event in a 'critical size' -Al₂O₃ crystal, but is due to the intrinsically low -Al₂O₃ nucleation density, together with rapid growth of -Al₂O₃ after nucleation.w     

Some studies of the optical properties of molybdenum-phosphate glasses

A series of binary MoO₃-P₂O₅ and ternary MoO₃-In₂O₃-P₂O₅ glasses was prepared and their optical properties were investigated. The optical absorption edge of the glasses was measured for specimens in the form of thin blown films. It was found that the fundamental absorption edge of these glasses usually occurs in the ultraviolet region. The linear variation of ()^1/2 with where is the absorption coefficient and is the incident photon energy, is taken as evidence of non-direct interband transitions. The linear dependence of optical gap with molybdenum content indicates that the molybdenum content controls the absorption phenomena in this glassy system. The infrared spectra of all the glasses appeared to be almost the same, indicating that the infrared bands arise primarily from the vibrations of the phosphate and molybdate groups. The structure of molybdenum phosphate glasses is discussed in terms of the formation of mixed phosphate and MoO₄ tetrahedra.     

The critical curve in an antiferromagnetic superconductor with nonmagnetic impurities

The critical curve of a transition of the second kind in an antiferromagnetic superconductor (AFS) with nonmagnetic impurities has been studied. The AFS is described by using the mean-field model given by Nass, Levin, and Grest and assuming a one-dimensional electron band. We find that the points on the critical curve satisfy the thermodynamic stability condition for 0₁/₀5.04 and 0.49H_Q/₀1.64.Here ₁ is the inverse lifetime of a conduction electron for nonmagnetic impurity scattering,H _Q is the antiferromagnetic molecular field, ₀ is the zero-temperature order parameter of a superconductor in the absence ofH _Q and impurities. Further, ₁ and H_Q denote the values of these quantities for points on the critical curve. For ₁/₀>5.04 and H_Q/₀>1.64, the phase transition from the superconducting to the normal state is always of the second kind. Some thermal properties of the system near the critical curve have also been investigated and we find that these depends dramatically on the impurity concentration.     

Functional derivative ofT c with α2(ω)F(ω) for weak coupling anisotropic superconductors

Approximate analytic calculations of the functional derivative ofT _c with respect to ²()F() for anisotropic superconductors are presented, with the primary purpose of identifying the determining material parameters. The square-well model for the phonon-mediated electron-electron interaction, the weak coupling limit ( _c /2T _c 1), and separable anisotropy are used. The general behavior of T _c /²()F() is the same as that found in numerical calculations for the case of small anisotropy, a ²/(–*)1; the regime of * is also discussed.     

Phase decomposition in extruded Zn-Al based alloy

Ageing characteristics of an extruded eutectoid Zn-Al based alloy were investigated using X-ray diffraction and scanning electron microscopy techniques. The extruded alloy consisted of Al rich phase and Zn rich _E and phases. The original cast eutectoid Zn-Al alloy was extruded at 250 °C. Both supersaturated _s and _s phase decomposed during extrusion and appeared as fine and coarse lamellar structures. The _E and phases particles formed in the original interdendritic region. It was found that two Zn rich phases _E and decomposed sequentially during ageing at 170, 140 °C. The decomposition of the _E phase occurred as a discontinuous precipitation in the early stage of ageing and the decomposition of the phase took place in a four phase transformation: + T + in the prolonged ageing. Two typical morphologies of the decomposition of the Zn rich phases _E and were distinctive in back-scattered scanning electron microscopy.     

Effect of Gamma Irradiation on the Piezoelectric Properties of K2Sr4Nb10O30–K6Li4Nb10O30 Solid Solutions

The effect of gamma irradiation on the dielectric ( ₃₃/₀, tan) and piezoelectric (d ₃₁, g ₃₁, K _p, Q _m) properties of ₂Sr₄Nb₁₀O₃₀–K₆Li₄Nb₁₀O₃₀ solid solutions (tetragonal tungsten bronze structure) was studied as a function of K₆Li₄Nb₁₀O₃₀ content. The results demonstrate that increasing the gamma dose to 9 × 10⁵ Gy reduces ₃₃/₀ and tan. With increasing Li⁺ content (filling of triangular channels), d ₃₁, g ₃₁, K _p, and Q _m increase. The results are interpreted in terms of the generation of stable defects and effective redistribution of the energy of gamma radiation over the ceramic sample.     

Formation and characterization of oxynitride glasses in the Si-Ca-Al-O-N and Si-Ca-Al,B-O-N systems

Oxynitride compositions in the Si-Ca-Al-O-N and Si-Ca-Al, B-O-N systems were melted and furnace-cooled in BN crucibles at temperatures from 1650 to 1850° C under dry nitrogen atmosphereS. Glass formation, phase stability and crystallization were studied by characterizing the cooled melts by X-ray diffraction, DTA, and electron microscopy. Oxynitride batches with nitrogen content up to 11 at % formed glasses in the Si-Ca-Al-O-N system. Glasses in the Si-Ca-Al, B-O-N system could be formed only when the B₂O₃ content of the batch was less than 3 wt %. Oxynitride glasses in these boron-containing systems were characteristically inhomogeneous, difficult to process, and prone to crystallization. In both the systems, glasses exhibited glass transitions beginning at 1000° C and crystallization at 1300 to 1500° C. Nitrogen-containing crystalline phases were identified in devitrified glasses via microstructural and micro-mechanical analyses.     

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.     

Joining Ti-47Al-2Cr-2Nb with a Ti/(Cu,Ni)/Ti clad-laminated braze alloy

The joining of Ti-47Al-2Cr-2Nb using Ti-15Cu-15Ni (wt%) as braze alloy was investigated. Experiments were conducted at 980 and 1000°C for 10 min. The microstructure and the chemical composition of the interfaces were studied by scanning electron microscopy (SEM) and by energy dispersive X-ray spectroscopy (EDS), respectively. For both processing conditions the reaction between the -TiAl alloy and the braze alloy produced layered interfaces, which are essentially composed of ₂-Ti₃Al and of Ti-Ni-Cu-Al and Ti-Ni-Cu intermetallic compounds. Microhardness tests showed that all reaction layers are harder than either the or the (₂ + ) lamellar grains of the intermetallic alloy.     

Notch tip stress distribution in strain hardening materials

Using the results of elastic-plastic stress analyses for notched bars, it is shown that a modified form of slip-line field solution can satisfactorily explain the variation of longitudinal stress ahead of notch tips in strain hardening materials.

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Résumé En utilisant les résultats d'analyses de contrainte élastoplastique dans le cas de barres entaillées, on montre qu'il est possible d'utiliser une forme simplifiée de solution du champ des lignes de glissement pour expliquer de façon satisfaisante la variation des contraintes longitudinales en avant d'extrémités d'entaille dans des matériaux susceptibles d'un écrouissage.

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Nomenclature _yy longitudinal tensile stress in the notch tip plastic zone - _xx transverse stress in the x-direction - _zz transverse stress in the z-direction - k yield stress in shear - ₀ yield stress in tension - ₀ ^* strain hardened yield stress (flow stress) - _{0/* c} flow stress at notch tip - _total total strain _pl plastic strain _l principal strain - _{1 c} maximum principal strain at notch tip - _1pl plastic strain in they-direction - _{1 cp1} E_{1 pl} at notch tip - _eff effective plastic strain - _{c eff eff} at notch tip - ₀ yield strainC Stress decay constant in the notch tip region - /e_pl linear strain hardening rate - n strain hardening exponent in power hardening law - 2 flank angle of notch - distance from notch tip - p notch tip radius - k _I applied stress intensity for Mode I loading - E Young's modulus - V _c crack tip opening displacement     

In-Plane Fourfold Symmetry of the Upper Critical Field Observed in La(Sr)214 Single Crystals

In order to establish the existence of in-plane anisotropy of the upper critical field H _c2 (), the out-of-plane resistivity _c measurements were performed on La(Sr)214 single crystals with rectangular and cylindrical shape under rotating magnetic field applied within the ab-plane. Although observed _c shows non-sinusoidal twofold symmetry, clear fourfold symmetry was obtained after subtracting twofold sinusoidal component in _c which is due to the unavoidable misalignment of the magnetic field with respect to ab-plane. H _c2 () is estimated from the extracted fourfold component of _c with the flux flow theory. Angular dependence of H _c2 () was well fitted by cos(4). Since the fourfold component of H _c2 () was largest at = n/2, which corresponds to the a-axis direction, the present results strongly suggest the type symmetry in La(Sr)214. In addition, the difference in fourfold component of H _c2 at = n/2 and n/2+/4 was found to increase with decreasing temperature.     

Theory of the hall effect in strongly correlated Fermi systems

The anomalous properties of the Hall constant in the normal state of high-T_c superconductors are investigated within the Hubbard model. In Mori theory, the frequency dependent Hall constant is given as the sum of its infinite frequency limit and a memory function contribution. The first term (R _H ) was already considered by Shastry et al.¹ In perturbation theory and in the limit of infinite spatial dimensions, the memory function contribution causes the Hall constant to change sign as a function of doping () and to decrease as a function of temperature, if we allow U to be 2W (U: Coulomb repulsion; W: bandwidth). In the limit U , the memory function is calculated via its moments. For frequencies W U, this leads to a decrease of R _H by a factor of (1 + )/2.     

Investigations of the copper isotope effect in oxygen-deficient YBa2Cu3O7−δ

We present data on the copper isotope effect (⁶³Cu-⁶⁵Cu), C_u =-nT_c/nm_Cu, for two isotopic pairs of oxygen-deficient YBa₂Cu₃O_7–, where varies between 0.06 and 0.52. _Cu is below 0.01 at =0.06 (fully oxygenated), it takes values between –0.14 and –0.34 in the 60 K plateau. Larger negative values of _Cu are observed away from the plateau. The dependence of _Cu is similar to that of the pressure effect dnT_c/dP.     

Structure of unidimensional temperature stresses

Using the structural approach, the temperature stresses are examined in a semiinfinite rod, insulated on the lateral faces and rigidly fixed at the end. A comparative analysis is made for three heat-transfer models.Notation k(t) heat flux relaxation function - (t) internal energy relaxation function - T rod temperature - ambient temperature - t time - x coordinate along the rod - _xx(x, t) stress - u(x, t) displacement - (x, t) deformation - c₀=(E/)^1/2 speed of sound in the rod under isothermal conditions - E elasticity modulus - density of the material - _t coefficient of thermal expansion - thermal-conductivity coefficient - a thermal-diffusivity coefficient - b thermal-activity coefficient - c_q=(a/_r)^1/2 velocity of heat propagation - _r heat flux relaxation time - (t) unique Heaviside function Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 5, pp. 912–921, November, 1977.     

Preparation and transport properties of aqueous sol-gel synthesized NaCo2O4−δ

We have successfully prepared thermoelectric materials NaCo₂O_4– at lower temperature (700°C) using aqueous sol-gel methods. We find that the electrical resistivity and thermopower of NaCo₂O_4– depend strongly on the sintering temperature and partial pressure of oxygen. The oxygen content of NaCo₂O_4– is found to decrease with increasing sintering temperature. The sample prepared at the combination of 700°C and pure flowing oxygen exhibits the highest value of power factor.     

Ferroelectric Lead Zirconate Titanate Films Prepared by Spray Pyrolysis of Carboxylate Solutions

Ferroelectric PbTi_0.6Zr_0.4O₃films 0.5–1.5 m in thickness were produced on platinum substrates by spray pyrolysis of carboxylate solutions. The optimized compositions of the precursor solutions, containing methacrylic acid and ethylene glycol, are stable under normal conditions, allow the annealing temperature to be reduced, and lead to higher quality film surfaces and large grains. The film exhibit the following electrical properties: T _C= 360–460°C, _max= 1750 at T _C, tan = 0.02–0.1 at 1 kHz and room temperature, P _{s max} = 18 C/cm², P _{r max} = 15 C/cm²at 50 Hz, and E _c= 42–120 kV/cm.     

Subsolidus phase relations in the ZnS-In2S3 system: 600 to 1080° C

Eleven ternary compounds were synthesised, of which nine are new. The seven most ZnS-rich compounds form a series. Each has a primitive hexagonal lattice witha=3.85±0.01 Å. Thec-dimension varies from 37.47 Å (39 ZnS8 In₂S₃) to 18.63 Å (18 ZnS8 In₂S₃) with a periodicity of 3.14 Å which can be related to a decrease in the ZnS content. One of the four remaining compounds (12 ZnS8 In₂S₃) is triclinic, but at least two polytypes are present. The more common variety has the parameters:a=3.86 Å,b=15.48 Å,c=3.54 Å,=90° 16,=120° 02 and=89° 47. The structure of the other polytype and of compounds 17 ZnS:8 In₂S₃ and 10 ZnS8 In₂S₃ have not been determined. Stoichiometric ZnIn₂S₄ has a rhombic hexagonal lattice (a=3.86 Å,c=36.95 Å). This ternary compound which has been synthesised previously is the only one to show appreciable solid solution. Approximately 2 mole % ZnS is soluble in ZnIn₂S₄ at 600° C; this increases to 8 mole % at 1080° C.     

Formation and characterization of the solid solutions (CrxFe1−x)2O3, 0⩽x⩽1

The solid solutions (Cr_xFe_1–x)₂O₃, 0 x 1, were prepared by traditional ceramic procedures. The samples were characterized using X-ray diffraction, Mössbauer, Fourier transform infra-red (FT-IR) and optical spectroscopic measurements. In the whole concentration range two phases exist phase F, -(Cr_xFe_1–x)₂O₃, which is isostructural with -Fe₂O₃ and phase C, which is closely related to Cr₂O₃. Phase F exists in samples heated up to 900°C, for 0 x 0.95. Phase C exists from x0.27 to x=1 for samples heated up to 900°C and from x0.65 to x=1 for samples heated up to 1200 °C. For samples heated up to 900 °C, the solubility limits were 27.5 ± 0.5 mol% of Cr₂O₃ in -Fe₂O₃ and 4.0 ± 0.5 mol % of -Fe₂O₃ in Cr₂O₃. For the samples heated at 1200 °C the diffraction peaks for the F and C phases in the two phase region were severely overlapped and thus the solubility limits could not be determined accurately as for previous samples. ⁵⁷Fe Mössbauer spectra of the samples heated up to 1200 °C showed significant broadening of spectral lines and a gradual decrease of the hyperfine magnetic field with increase of x up to 0.50. For x0.7, a paramagnetic doublet with collapsing sextet was observed. The spectra were interpreted in terms of an electronic relaxation effect; however, an agglomeration of iron ions which would contribute to the superparamagnetic effect could not be excluded. The FT-IR spectra showed transition effects in accordance with the X-ray diffraction results. The most intense absorption bands, observed for the samples heated up to 1200 °C, were located at 460 and 370 nm (22 000 and 27 000cm^–1) for x 0.5, 500 and 360 nm for x < 0.3, and might be correlated with the strong enhancement of the pair transitions through antiferromagnetic interactions. The intensification of the ⁶A₁ 4T₁ Fe³⁺ ions in all spectra and the development of the absorption at 13000 cm^–1 due to a metal-metal charge transfer (Cr³⁺ Fe³⁺) transition, might be explained by exchange coupling which has been observed in some spinel compounds.