The effect of silicon on the structure and properties of Fe-Cr-Co permanent magnet alloys

The influence of silicon in amounts of up to 4.33 wt% was studied in terms of its effect on micro- and macrostructure, on non-metallic inclusions, on mechanical hardening and on magnetic properties of Fe-28Cr-24Co alloy of equiaxial grain structure and of partially ordered structure-the so-called semicolumnar structure. Optimum magnetic properties in the examined alloy are attained with silicon content Si = 1.04 wt%. Magnetic properties of an alloy of semicolumnar structure are markedly superior to those of the alloy devoid of that structure. With up to 1.04 wt% of silicon, the macrostructure of the alloy remains practically unchanged, while with higher contents of silicon a refinement of the structure takes place. The stereometric parameters of non-metallic inclusions (Al₂O₃, SiO₂, Al₂O₃·SiO₂) increase with silicon contents Si > 1.04 wt%. The hardness of the alloy after homogenizing is practically constant up to silicon content Si = 1.65 wt%, but it increases slightly with Si > 1.65 wt%. The examined alloy with silicon up to 1.65 wt% is ductile in the quenched state and could also be cold-worked.     

Pressure study of monoclinic ReO2 up to 1.2 GPa using X‐ray absorption spectroscopy and X‐ray diffraction

The crystal and local atomic structure of monoclinic ReO₂ (α‐ReO₂) under hydrostatic pressure up to 1.2 GPa was investigated for the first time using both X‐ray absorption spectroscopy and high‐resolution synchrotron X‐ray powder diffraction and a home‐built B₄C anvil pressure cell developed for this purpose. Extended X‐ray absorption fine‐structure (EXAFS) data analysis at pressures from ambient up to 1.2 GPa indicates that there are two distinct Re—Re distances and a distorted ReO₆ octahedron in the α‐ReO₂ structure. X‐ray diffraction analysis at ambient pressure revealed an unambiguous solution for the crystal structure of the α‐phase, demonstrating a modulation of the Re—Re distances. The relatively small portion of the diffraction pattern accessed in the pressure‐dependent measurements does not allow for a detailed study of the crystal structure of α‐ReO₂ under pressure. Nonetheless, a shift and reduction in the (011) Bragg peak intensity between 0.4 and 1.2 GPa is observed, with correlation to a decrease in Re—Re distance modulation, as confirmed by EXAFS analysis in the same pressure range. This behavior reveals that α‐ReO₂ is a possible inner pressure gauge for future experiments up to 1.2 GPa.     

Surface segregation of InGaAs films by the evolution of reflection high-energy electron diffraction patterns

Surface segregation is studied via the evolution of reflection high-energy electron diffraction (RHEED) patterns under different values of As 4 BEP for InGaAs films. When the As 4 BEP is set to be zero, the RHEED pattern keeps a 4×3/(n×3) structure with increasing temperature, and surface segregation takes place until 470 C. The RHEED pattern develops into a metal-rich (4×2) structure as temperature increases to 495 C. The reason for this is that surface segregation makes the In inside the InGaAs film climb to its surface. With the temperature increasing up to 515 C, the RHEED pattern turns into a GaAs(2×4) structure due to In desorption. While the As 4 BEP comes up to a specific value (1.33×10 4 Pa–1.33×10 3 Pa), the surface temperature can delay the segregation and desorption. We find that As 4 BEP has a big influence on surface desorption, while surface segregation is more strongly dependent on temperature than surface desorption.     

Magnetic properties of Mn3B4 under high magnetic field

The magnetic properties of antiferromagnetic compound Mn₃B₄ were examined using pulsed magnetic field up to 470 kOe. It was found that Mn₃B₄ shows a clear metamagnetic transition under 360 kOe at 4.2K. The present results are not explained by the magnetic structure proposed by Neov and a modified magnetic structure is proposed to understand the present magnetization curve. The magnetic phase diagram in the H-T plane is shown.     

The study of the vibrational spectra of NH4Cl and NH4Br at high pressures

The vibrational spectra of NH₄Cl at pressures of up to 2.6 GPa and of NH₄Br at pressures of up to 7 GPa are investigated by the method of inelastic incoherent scattering of neutrons. It is found that a linear baric dependence of a librational mode changes its slope above the pressure of transition from a disordered cubic phase into an ordered cubic phase with a CsCl-type structure. The slope of the baric dependence of the transverse optical translational mode remains invariant. Estimates for the Grüneisen parameters are presented and the shape of the potential function is calculated in the one-dimensional approximation for librational vibrations in disordered and ordered cubic phases with a CsCl-type structure. It is shown that the phenomena observed are attributed to the high anharmonicity in the disordered phase.     

Formation of nanotubes TiO2 from layered titanate particles by a soft chemical process

Nanotubes TiO₂ were synthesized from layered titanate Na₂Ti₃O₇ particles by a soft chemical process. These nanotubes have a hollow structure with an opening end. The wall of nanotubes is very well crystallized multi-wall structure. The inner diameters of nanotubes are ca. 30-40 nm and the length up to several hundreds nanometers. XRD results show that TiO₂ phase was formed during the procedure of exfoliating Na₂Ti₃O₇ particles into nanosheets. Based on our experimental results, an exfoliating-rolling model was proposed for formation of nanotube structure.     

Plastic deformation of polycrystals of Co3(Al,W) with the L12 structure

The plastic behaviour of Co₃(Al,W) polycrystals with the L1₂ structure has been investigated in compression from 77 to 1273?K. The yield stress exhibits a rapid decrease at low temperatures (up to room temperature) followed by a plateau (up to 950?K), then it increases anomalously with temperature in a narrow temperature range between 950 and 1100?K, followed again by a rapid decrease at high temperatures. Slip is observed to occur exclusively on {111} planes at all temperatures investigated. The rapid decrease in yield stress observed at low temperatures is ascribed to a thermal component of solid-solution hardening that occurs during the motion of APB-coupled dislocations whose core adopts a planar, glissile structure. The anomalous increase in yield stress is consistent with the thermally activated cross-slip of APB-coupled dislocations from (111) to (010), as for many other L1₂ compounds. Similarities and differences in the deformation behaviour and operating mechanisms among Co₃(Al,W) and other L1₂ compounds, such as Ni₃Al and Co₃Ti, are discussed.     

Magnetic properties of the pseudo-binary Fe2(Nb1-xMnx)

The magnetic behavior of the pseudo-binary system Fe₂(Nb_1-xMn_x) is investigated by means of the experimental techniques of X-ray diffraction (XRD), Mössbauer Spectroscopy (MS) and magnetization studies. The XRD results indicate that, up to x=0.3, all samples are single phase with hcp structure. This corresponds to the solubility limit of manganese in this phase. Above x=0.3, all prepared samples present the coexistence of three phases, two with hcp structure and one fcc. The magnetization measurements at low temperatures indicate that the transition temperature increases with the addition of Mn atoms in the Fe₂Nb host (T_N=10 K) up to 58 K for x=0.1. The Mössbauer spectra were fitted with a quadrupole splitting distribution, which indicates that the average quadrupolar splitting increases slightly with the increase of the manganese concentration.     

Temperature and pressure dependence of Raman active lattice mode frequencies of cubic C8H6(Br)2, evidence of a phase transition

The temperature dependence of the Raman active frequencies of the lattice modes of dibromocubane is measured below room temperature. The frequencies are linearly dependent on the temperature decreasing with increased temperature with a marked change in slope occurring at 200?K indicative of a structural phase transition. The pressure dependence of the frequencies measured at room temperature up to 24?KBar shows no evidence of a phase change. Density functional calculations of the structure and Raman frequencies of the internal modes of an isolated molecule of C₈H₆(Br)₂ indicate the molecule is slightly distorted from a cubic carbon structure.     

Raman and photoluminescence spectroscopy study of benzoic acid at high pressures

Benzoic acid (C₆H₅COOH, BA) has been studied by high pressure Raman and fluorescence spectroscopy up to about 13.40 GPa using a diamond anvil cell at room temperature. The changes of lattice modes are interpreted as the crystal structure transformation. Three possible phase transitions, with the pressure increasing up to about 0.55, 3.67 and 11.10 GPa, are, respectively, elucidated as crystalline-to-crystalline, crystalline-to-amorphous transitions. A new material formed when the pressure is up to above 11.10 GPa remains stable after the pressure is released.     

High-pressure crystal structure of thorium disulfide and diselenide and uranium disulfide

Abstract

The crystal structure of ThS₂, ThSe₂ and US₂ has been investigated for pressure up to 60GPa using x-ray powder diffraction. The bulk moduli are 175(10), 155(10) and 155(20) GPa, respectively. A pressure-induced phase transformation occurs at about 40 GPa for ThS₂, 30 GPa for ThSe₂ and 15 GPa for US₂. The results for ThSe₂ indicate that its high-pressure phase has a monoclinic structure. The same structure is compatible with the observed high-pressure spectra of ThS₂ and US₂. However, the crystal system assignment is less certain for these compounds.     

Neutron diffraction investigation of the structural transition in HgSe1−x Sx ternary mercury chalcogenide systems at high pressures

The structure of HgSe_1?x S_x ternary mercury chalcogenides at high pressures up to 35 kbar is investigated by neutron diffraction. It is found under pressure, that the HgSe_1?x S_x compounds undergo, a phase transition from the cubic sphalerite-type to the hexagonal cinnabar-type structure, which is accompanied by a jump-wise change in the unit cell volume and interatomic distances. The unit cell parameters and the positional parameters of Hg and Se (S) atoms in the high-pressure hexagonal phase are determined. A two-phase state is revealed in the phase transition region.     

Pressure-induced phase transition in defect Chalcopyrites HgAl2Se4 and CdAl2S4

Results of angle dispersive X-ray diffraction (ADXRD) measurements on the defect chalcopyrites (DCP), HgAl₂Se₄ and CdAl₂S₄ up to 22.2 and 34 GPa, respectively, are reported. The ambient tetragonal phase is retained in HgAl₂Se₄ and CdAl₂S₄ up to 13 and 9 GPa respectively. The values of the bulk modulus estimated from the Equation of State is 66(1.5) and 44.6(1) GPa for HgAl₂Se₄ and CdAl₂S₄ in the chalcopyrite phase. At higher pressure a disordered rock-salt structure and on pressure release a disordered zinc blende structure with broad X-ray diffraction lines are observed as is the case for several defect chalcopyrites.     

Effect of Si in reactively sputtered Ti-Si-N films on structure and diffusion barrier performance

34 Si₂₃N₄₃ (b3) and Ti₃₅Si₁₃N₅₂ (c3), are synthesized by reactively sputtering a Ti₅Si₃ or a Ti₃Si target, respectively. The silicon-lean film (c3) has a columnar structure closely resembling that of TiN. As a diffusion barrier between a shallow Si n⁺p junction diode and a Cu overlayer, this material is effective up to 700 °C for 30 min annealing in vacuum, a performance similar to that for TiN. The silicon-rich (b3) film contains nanocrystals of TiN, randomly oriented and embedded in an amorphous matrix. A film of (b3) maintains the stability of the same diode structure up to 850 °C for 30 min in vacuum. This film (b3) is clearly superior to TiN or to (c3). Similar experiments performed with Al instead of Cu overlayers highlight the importance of the thermodynamic stability of a barrier layer and demonstrate convincingly that for stable barriers the microstructure is a parameter that directly determines the barrier performance. Received: 18 November 1996/Accepted: 22 January 1997     

The superconductivity of Sc5T4Si10 (T = Co,Rh, Ir) and isomorphous compounds

Superconductivity is reported for a new class of ternary compounds of general formula R₅T₄X₁₀ where R is Sc, Y or a rare earth, T a transition metal of the platinum or iron groups and X either Si or Ge. Critical temperatures up to 8.6 K are observed. The relationship with non-superconducting compounds of similar composition and structure is discussed.     

First-principles calculations for elastic and electronic properties of ZnSnO3 under pressure

The LiNbO₃ (LN)-type structure and the ilmenite (IL)-type structure of ZnSnO₃ are investigated with the ultrasoft pseudopotential scheme in the frame of the local density approximation (LDA). The calculated lattice parameters of ZnSnO₃ under zero pressure and zero temperature are in very good agreement with the existing experimental data. The pressure dependences of the elastic constants, Debye temperatures, Poisson's ratio, sound velocity, mechanical stability and mechanical anisotropy of the LN-type structure of ZnSnO₃ have also been investigated. We find that the LN-type structure of ZnSnO₃ is a mechanically stable phase under pressures up to 21 GPa; however, the mechanical anisotropy weakens with the increasing pressures. In addition, the calculated band structure indicates that LN-ZnSnO₃ has a direct band gap of 1.669 eV, and the total and partial densities of states, under diverse pressures of the LN-type structure ZnSnO₃ have also been obtained.     

Magnetic properties of incommensurate antiferromagnet β-KMnCl3

Magnetic properties of orthorhombic incommensurate antiferromagnet β-KMnCl₃ have been studied as a function of temperature down to 0,5K and in magnetic fields up to 70 k0e. The exchange parameters were estimated with molecular-field theory. Magnetic field behaviour of β-KMnCl₃ is in accordance with change of magnetic structure from helical into fan structure and then into induced ferromagnetic state. The Neel temperature decreases with increase magnetic field overage 0,05K on 1 k0e.     

The structure of FeCl2 at high pressures

A full three-dimensional structure determination of FeCl₂ has been performed at hydrostatic pressures up to 6.4 kbar at room temperature. These studies demonstrate the feasibility of four-circle neutron diffraction studies under pressure, and show that above 5.8 kbar FeCl₂ has a structure isomorphous to FeBr₂. The pressure dependence of chlorine position parameter and lattice parameters has been studied over the entire pressure range.     

Structural changes in gas hydrates and existence of a filled ice structure of methane hydrate above 40 GPa

Recent studies have contributed a great deal to our understanding of the structural changes in gas hydrates under pressure. A feature of structural changes in terms of pressure and guest size is summarised in the former part of this paper. The gas hydrate with the guest size from argon to methane finally take a common filled ice I_h structure, although they have different initial and intermediate structures. In the latter part, retention of the filled ice I_h structure of methane hydrate up to 42 GPa is described. In situ X-ray diffractometry and optical observation revealed the existence of the filled ice structure with the volume change of 40% and large anisotropic compressibility. The distances between oxygen atoms estimated from the unit cell parameters suggested that symmetrization of hydrogen bonds occurred in the structure.