Electrical properties of ferromagnetic Gd3−xS4 near its Curie temperature

Nonstoichiometric $\text{Gd}{}_{\mathrm{<mtext>3?</mtext><mtext>x</mtext>}}\text{S}{}_4\text{(0 <}\text{x}\text{<}\text{1}\text{3}\text{)}$, which exhibits a metallic behavior, was obtained by heating an insulating Gd₂S₃ at various temperatures under a vacuum. Electrical and magnetic properties of the samples obtained have been investigated from 4.2 K to 300 K. A maximum in resistivity was observed in the curve of resistivity vs. temperature. The temperature T_P, at which the maximum emerged, was very close to the Curie temperature T_C for the sample. An increase in resistivity at T_P, Δ_?, is proportional to ?_P exp ($\text{E}\text{k}{}_{\mathrm{B}}\text{T}{}_{\mathrm{P}}$), where ?_P is the resistivity obtained by the extrapolation from the linear portion to T_P in the ? vs. T curve. Using the relationship obtained, a model based on the formation of magnetic polaron was proposed for this system.     

Properties of bone at low temperatures and its potential as cryostat support material

The ultimate compressive strength, $\text{σ}{}_{\mathrm{<mtext>f</mtext>}}$, Young's modulus, E, and the integrated thermal conductivity k_T1–T2, of bone have been measured between 20 and 80 K. The two figures of merit indicating the quality for transmission of forces to low temperature apparatus are determined as $\text{σ}{}_{\mathrm{<mtext>f</mtext>}}\text{/}\text{k}{}_{\mathrm{4–77}}\text{= 42.5 ± 4 Ms m}{}^{\mathrm{?2}}$, and E/k_4–77 = 3.5 ± 0.2 Gs m^?2. According to these figures bone is comparable or superior to the best glass-fibre composites. Some observations on creep strength are added.     

Syntheses and magnetic properties of Eu3B2O6 and Sr3B2O6

Europium orthoborate and strontium orthoborate crystallize in the rhombohedral system with two formula units in a cell of dimensions $\text{a}{}_{\mathrm{R}}\text{=6.697}\text{A}\text{?}$, α_R=85.17° for Eu₃B₂O₆, and $\text{a}{}_{\mathrm{R}}\text{=6.695}\text{A}\text{?}$, α_R=85.00° for Sr₃B₂O₆. The equivalent hexagonal lattice parameters are $\text{a}{}_{\mathrm{H}}\text{=9.069}\text{A}\text{?}$, $\text{c}{}_{\mathrm{H}}\text{=12.542}\text{A}\text{?}$, and $\text{a}{}_{\mathrm{H}}\text{=9.046}\text{A}\text{?}$, $\text{c}{}_{\mathrm{H}}\text{=12.566}\text{A}\text{?}$ respectively. Eu₃B₂O₆ appears to be ferromagnetic below 7.5K.     

Control of the growth-induced magnetic anisotropy in ferrimagnetic garnet films grown by liquid-phase epitaxy

The influence of melt composition and growth temperature T_g on the growth-induced magnetic anisotropy constant K^g_u in Bi substituted LPE ferrimagnetic garnet films has been investigated. In films grown from Bi₂O₃ based and from $\text{PbO}\text{B}{}_2\text{O}{}_3$ based melts the value of K^g_u increased upon decreasing T_g, associated with an increasing incorporation of B1. However, at given Bi content the films grown from Bi₂O₃ based fluxes generally exhibited smaller values of K^g_u than those from $\text{PbO}\text{B}{}_2\text{O}{}_3$ based melts. The addition of small amounts of Ca²⁺ to the Bi₂O₃ based melts resulted in (Y,Bi)-garnet films changing from n- to p-type electrical conductivity at a minimum value of K^g_u. Additions of Si⁴⁺ to a $\text{PbO}\text{B}{}_2\text{O}{}_3$ based melt reduced the uniaxial magnetic anisotropy of La-YIG-Ga films accompanied by a change from p- to n-type conductivity. Similarly, (Gd,Bi)-garnet films from $\text{PbO}\text{B}{}_2\text{O}{}_3$ based melts changed from n- to p-type conductivity upon decreasing T_g where K^g_u changes sign from negative to positive. From the temperature dependence of the electrical conductivity the electron and hole concentrations in some of these films were estimated suggesting that the sign change of K^g_u in the investigated (Gd,Bi)-garnet films and the minimum of K^g_u in the investigated (Y,Bi)-garnet films and (La,Y)-garnet films occur at donor-acceptor compensation. This experimental evidence led us to conclude that the growth induced magnetic anisotropy may be correlated with the presence of donor and acceptor centers in the garnet structure.     

Distribution coefficients in (YSmLuCa)3(FeGe)5O12 film growth

The composition of bubble garnet films has been analyzed by Inductively Coupled Plasma Emission Spectroscopy (ICP) to determine the distribution coefficients for different growth conditions. Under typical growth conditions, the distribution coefficients, k, of each element are as follows: k^Y = 2.15, k^Sm = 1.56, k^Lu = 1.32, k^Ca = 0.45, k^Fe = 0.98, k^Ge = 1.10. As the supercooling temperature (growth rate) increases, k^Ca, which is the smallest and deviates most from 1.0, changes in the direction approaching 1.0. For charge compensation, k^Ge also increases, consequently, k^Fe must decrease. Also k^Y, which is the largest, $\text{k}{}^{\mathrm{Sm}}\text{k}{}^{\mathrm{Y}}$ and $\text{k}{}^{\mathrm{Lu}}\text{k}{}^{\mathrm{Y}}$ change in the direction approaching 1.0. On the other hand, as the melt parameter R₁ (≡ $\text{Fe}{}_2\text{O}{}_3\text{Σ}\text{Ln}{}_2\text{O}{}_3$) increases, k^Fe decreases, and k^Y, k^Sm and k^Lu increase, whereas $\text{k}{}^{\mathrm{Sm}}\text{k}{}^{\mathrm{Y}}$ and $\text{k}{}^{\mathrm{Lu}}\text{k}{}^{\mathrm{Y}}$ remain constant at 0.73 and 0.61, respectively.     

Corresponding states treatment of refrigerants and cryogenic fluidsLe traitement des états correspondants des frigorigènes et des cryofluides

An analysis of temperature dependence of saturated liquid density along the liquid vapour coexistence curve shows that a plot of reduced properties T_r(=T/T_c) against T_r?_r (=T?/T_c?c), goes through a maximum at a particular T_r for all the following liquids: Ne, Ar, Kr, Xe, N₂, O₂, CH₄, C₂H₆, C₂H₄, CO₂, R11, 12, 12B1, 13, 13B1, 14, 21, 22, 23, 32, 40, 113, 114, 115, H₂O and NH₃. For all these liquids, except NH₃, T_r is ≈ 0.82, whether they are polar, non-polar, monoatomic or polyatomic. This characteristic temperature, $\text{T}{}^1\text{(=T}{}_{\mathrm{<mtext>c</mtext>}}\text{T}{}_{\mathrm{<mtext>r</mtext>}}\text{)}$, has been used to evaluate the energy parameter and the corresponding molar volume has been used to evaluate the length parameters for calculation of reduced critical properties. $\text{T}{}^1$ is also found to be a point of inflexion on the saturated liquid viscosity versus temperature plot.     

A two material constant,nonlinear elastic stress constitutive equation including the effect of compressibility

A new constitutive equation for compressible, nonlinear elasticity is derived. The stress-deformation relation takes the final form $\text{σ}{}_{\mathrm{ij}}\text{=}\text{3k}\text{2J}\text{(J}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{- 1) δ}{}_{\mathrm{ij}}\text{+}\text{μ}\text{J}\text{(B}{}_{\mathrm{ij}}\text{- j}{}^{\mathrm{<mtext>2</mtext><mtext>3</mtext>}}\text{δ}{}_{\mathrm{ij}}\text{)}$ where B_ij is the left Cauchy-Green deformation tensor. J is the Jacobean of the deformation, and k and μ are the two governing mechanical properties having the same interpretation as in linear, isotropic elasticity theory. In this nonlinear theory, k and μ are separately determinable from large deformation conditions of pure dilatation and simple shear, respectively. Under isochoric conditions, the distortional part of the theory takes the form of the kinetic theory of rubber elasticity. Under conditions of small volume change, the theory coincides with the molecular theory of Flory derived for elastomers, but given explicit from only in the one dimensional case.     

A modified thickness criterion for fracture toughness testing

A modified criterion is developed on an empirical basis for the minimum thickness $\text{B}{}_{\min }$ of a plane strain fracture toughness test specimen: $\text{B}{}_{\mathrm{<mtext>min</mtext>}}\text{= 400}\text{K}{}_{\mathrm{Ic}}{}^2\text{Eδ}{}_{\mathrm{Y}}$ where $\text{K}{}_{\mathrm{Ic}}$ is the plane strain fracture toughness, $\text{E}$ is the Young's modulus and δ_Y is the yield stress of the material. The modified criterion is tested alongside the ASTM thickness criterion against published data on the variation of $\text{K}{}_{\mathrm{c}}$ with thickness, and shows significantly the better agreement with observed values of $\text{B}{}_{\mathrm{<mtext>min</mtext>}}$ for a wide range of materials.An attempt has been made to rationalise this criterion. The expression is considered to take into account two major factors which determine $\text{B}{}_{\mathrm{<mtext>min</mtext>}}$, the attainment of plane strain in the specimen interior ahead of the crack tip, and the role of microstructure in determining how far the quasi-plane strain fracture (square fracture) extends beyond the region of true plane strain.     

Crystal data and thermal expansion of tricalciumborate

Single crystals of tricalciumborate were grown by the flux method. Ca₃B₂O₆ crystallizes in the rhombohedral system with the unit cell parameters $\text{a}{}_{\mathrm{R}}\text{= 6.3577(7)}\text{A}\text{?}$, α_R = 85.68(8)°, Z = 2. The equivalent hexagonal lattice parameters are a_H = 8.640(1), $\text{C}{}_{\mathrm{H}}\text{= 11.854(1)}\text{A}\text{?}$. The anisotropic thermal expansion parameters α_a = (1.00 ± 0.14) × 10^?5, α_c = (3.60 ± 0.47) × 10^?5 were determined by X-ray methods in the temperature interval 25 – 1000°C.     

Study of the barrier against flux exit from type II superconductors

The expulsion of flux from flat samples (ribbons and discs) of Nb, NbTa, V and PbIn cooling, from T_c to 4.2 K, in static magnetic fields, $\text{H}{}_{\mathrm{o}}$, has been investigated. A dramatic increase in expulsion of the component of the magnetic flux threading the specimen parallel to its flat faces is observed when $\text{H}{}_{\mathrm{o}}$ is tilted to pierce these broad surfaces. We attribute this phenomenon to the extinction of a surface barrier against flux exit by H_⊥, the transverse component of $\text{H}{}_{\mathrm{o}}$. The effect of B_⊥ = μ_oH_⊥ on the flux profiles is also examined. The concept of equilibrium diamagnetism is applied to this novel situation. A simple picture, incorporating prevailing views, is developed. With this picture, although crude approximations are used in the computations, we reproduce several families of data curves quite well. The barrier against flux exit is seen to be an appreciable fraction of H_c1 and information on its variation with the tilt angle of $\text{H}{}_{\mathrm{o}}$ is obtained.     

Dependence of the crystallization rate of vacuum-deposited amorphous antimony films on the film thickness

The crystallization rate of vacuum-deposited amorphous antimony (a-Sb) films was investigated as a function of the film thickness d. Experimental plots of the observed growth rate v against d^-1 for systems such as Sb/glass and Sb/Ge show that the relationship between v and d^-1 are convex towards the origin. Such a feature is well interpreted by a model in which

$\text{v=}\text{1}\text{d}\text{∫}\text{d}\text{0}\text{u}\text{d}\text{z}$

where u is the actual growth rate of the crystallite in the a-Sb film and z the distance from the film surface adjacent to the substrate. The quantity u is assumed to vary as follows: u = αzⁿ + u_s when 0 ? z ? d_s0, u = u_i when d_s0 ? z ? d ? d_v0 and u = β(d ? z)ⁿ + u_v, when d ? d_v0 ? z ? d where $\text{α =}\text{(u}{}_{\mathrm{i}}\text{? u}{}_{\mathrm{<mtext>s</mtext>}}\text{)}\text{d}{}_{\mathrm{<mtext>s</mtext><mtext>0</mtext>}}{}^{\mathrm{n}}$, $\text{β =}\text{(u}{}_{\mathrm{i}}\text{? u}{}_{\mathrm{<mtext>v</mtext>}}\text{)}\text{d}{}_{\mathrm{<mtext>v</mtext><mtext>0</mtext>}}{}^{\mathrm{n}}$, d_s0 and d_v0 are thickness of surface regions near the substrate and the vacuum respectively, u_i is the growth rate inside the film, u_s and u_v are the rates at surfaces adjacent to the substrate and the vacuum respectively and n an adjustable numerical parameter. As a typical example, for the Sb/Ge system at 30°C, u_i, u_s and u_v are estimated to be 139 μm s^-1, 25.4 μm s^-1 and 0.2 μm s^-1 respectively and d_s0 and d_v0 to be 133 Å and 143 Å respectively with n = 3.     

Sr2Mn2O5, an oxygen-defect perovskite with Mn(III) in square pyramidal coordination

An oxygen-defect perovskite Sr₂Mn₂O₅ was isolated by reduction of SrMnO_3?x perovskites in the presence of zirconium. Its structure, similar to that of Ca₂Mn₂O₅, has been determined by X-ray powder diffraction and HREM. The orthorhombic cell has the parameters : $\text{a}\text{= 5.523(1)}\text{A}\text{?}$, $\text{b}\text{= 10.761(5)}\text{A}\text{?}$, $\text{c}\text{= 3.811(1)}\text{A}\text{?}$. The possible space groups are Pbam and Pba2. The framework is built up from corner-sharing MnO₄ pyramids forming pseudo-hexagonal tunne$\text{l}$s running along 〈001〉 and perovskite tunnels running along 〈110〉 and 〈110〉. This oxide is antiferromagnetic with $\text{T}{}_{\mathrm{N}}\text{? 380 ± 10}\text{K}$ and $\text{θ}{}_{\mathrm{<mtext>P</mtext>}}\text{? 300 ± 10}\text{K}$.     

Sur l'existence des oxydes rhomboe driques A(III) B(II) B′(III)O4

The symmetry group of the rhombohedral structure of the mixed oxides A(III)B(II)B′(III)O₄ is R3&#x0304;m: A is in an octahedral coordination, B and B′ occupy the same crystallographic sites and are in a pyramidal coordination with a triangular basis.The existence of this structural type with Ni²⁺ and Cr³⁺ ions is incompatible with this coordination close to tetrahedral.The geometric conditions necessary for adoption of the rhombohedral structure have been clearly defined by comparing the cation-oxygen and oxygen-oxygen distances determined on 4 monocrystals (InCuAlO₄, InFe₁, ₇₅Si_O, ₂₅O₄, YbFe₂O₄ and Yb_O,₅Eu_O,₅Fe₂O₄).The shortest cation-oxygen distance in the pyramidal site is designated by d′: it can be calculated through the relation: by using the ionic radii of ions in tetrahedral coordination, given by R.D. Shannon.The oxygen - A³⁺ cation distance of the octahedron, d, is close to the sum of the ionic radii of the ions in coordinence 6.Mixed oxides ABB′O₄ can crystallize in the rhombohedral system if the ratio d′/d is between 0.842 and 0.888 and these two limiting values can be refined for each couple B, B′.     

Copper doped zinc oxide Y2BaZnO5: ERS and optical investigation

In copper doped Y₂BaZnO₅ oxides, copper exhibits a distorted square pyramidal coordination which is consistant with the values of g and A tensors obtained from O band ERS spectrum for a sample containing about 1 % Cu. Three values for g and A are observed, g₁ = 2.049₅, g₂ = 2.051₅, g₃ = 2.275, $\text{¦}\text{A}{}_1\text{¦ = 13 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$, $\text{¦}\text{A}{}_2\text{¦ = 10 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$ and $\text{¦}\text{A}{}_3\text{¦ = 147.5 10}{}^{\mathrm{?4}}\text{cm}{}^{\mathrm{?1}}$. Since $\text{g}{}_1\text{?}\text{g}{}_2$ an approximate C_4v point symmetry can be assumed for copper. The electronic spectrum shows three bands at 11700, 14500 and 20500 cm^?1 which can be assigned to the transitions A₁ → B₁, B₂ → B₁ and E → B₁ respectively. The orbital reduction parameters are calculated and the bonding covalency is discussed.     

Nature de la fluorescence de l'europium divalent dans les fluorures

The levels of the ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ (4f⁷) and 4f⁶5d¹ states of europium (+II) have been determined from fluorescence spectra in a series of fluorides M_xB_yF_z (M = alkaline-earth element, B = Li, Be, Mg, Y, Si). As in BaY₂F₈, SrSiF₆ and BaSiF₆, the ${}^6\text{P}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ level lies even at room temperature far below the 4f⁶5d¹ band, only the f → f emission is observed. The influence of the choice of the alkaline-earth and the B cations and of the coordination of europium on the relative positions of the energy levels is discussed.     

Optimization of growth conditions for ZnTe films

An optimum substrate temperature T₀ of 573 K was found for the preparation of ZnTe thin films by vacuum deposition. Films prepared at this temperature are stoichiometric and exhibit a maximum electrical resistivity. The observations are explained on the basis of the Vincett-Barlow-Roberts theory which predicts a value of nearly 0.33 for $\text{T}{}_0\text{T}{}_{\mathrm{<mtext>b</mtext>}}$, where T_b is the boiling point.     

Models for the effect of chemisorbed gases on the electrical resistance of metal films

Three hypotheses are considered for the proportionate change of resistance $\text{ΔR}\text{R}{}_1$ when a continuous metal film of resistance R₁ adsorbs gas: (a) a loss of metallic properties at the surface; (b) a change in the electron scattering parameter at the surface; and (c) a change in the electron concentration throughout the film. Two models of the film structure are used to compare the predictions of these hypothesis with our experimental data for the systems $\text{O}{}_2\text{Er}$, $\text{CO}\text{Er}$, $\text{CO}\text{Ti}$ and $\text{O}{}_2\text{Ti}$. The flat plate model accounts for neither R₁ nor $\text{ΔR}\text{R}{}_1$. A simple version of the columnar model accounts for $\text{ΔR}\text{R}{}_1$ by either (a) or (b) for the first three systems and by (c) for the fourth, but it does not explain all the properties of the clean films. The linear approximation given by Mola and Heras for the columnar model of Mayadas and Shatzkes accounts satisfactorily for R₁ and $\text{ΔR}\text{R}{}_1$ for $\text{O}{}_2\text{Er}$, $\text{CO}\text{Er}$ and $\text{CO}\text{Ti}$ by increases in the electron scattering coefficient at the grain boundaries. It does not account for $\text{ΔR}\text{R}{}_1$ with $\text{O}{}_2\text{Ti}$ because that system incorporates gas into the metal lattice.     

Viscous dissipation effects on mixed convection about a rotating sphere

The effect of viscous dissipation on mixed convection flow about a rotating sphere is investigated analytically. A method based on Merk's type of series expansions is used to obtain the heat transfer rate and the skin-friction coefficients. Numerical computations were carried out for Eckert number Ec ranging from 0 to 0.1, rotation parameter B = 0, 1, 4 and buoyancy parameter ranging from 0 to 1 at various angular positions. As viscous dissipation increases heat transfer rate decreases. It is found that for Ec > 0, a critical value of B (say B_cri) exist such that $\text{Nu Re}{}_{\mathrm{R}}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$ increases as B increases up to B = B_cri and then decreases. B_cri decreases with Ec when the angle is fixed or decreases with angle when Ec is fixed. When Ec > 0, possibly there exist a λ, say λ_cri, such that $\text{Nu Re}{}_{\mathrm{R}}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}$ increases with λ up to λ = λ_cri and then decreases. Also the effect of viscous dissipation on skin friction is investigated.     

Refinement of the structure of K2Cr2O7

The parameters for the structure of K₂Cr₂O₇ have been refined from 7511 observed reflections; $\text{a}{}_{\mathrm{o}}\text{= 7.4200(6)}\text{A}\text{?}$, $\text{b}{}_{\mathrm{o}}\text{= 13.399(3)}\text{A}\text{?}$, $\text{c}{}_{\mathrm{o}}\text{= 7.3845(9)}\text{A}\text{?}$, cosα = ?0.1396(2) (α = 98°2'), cosβ = ?0.0154(2) (β = 90°53'), cosγ = ?0.1078(2) (γ = 96°11'). The discrepancy factor R(F_o²) = 0.0702 with a type 2 extinction correction. The average CrO distances are 1.609Å (unshared) and 1.783Å (shared).