Effect of water on the α-β phase transition in a surface quartz layer

The temperature dependence of the α-phase concentration in surface layers and in the bulk of quartz plates cut out at a distance of ～2 mm from the natural growth surface of druses extracted at the Dodo deposit in the Polar Urals has been studied using infrared and Raman spectroscopy. It has been found that, in the bulk of the sample, the temperature dependence behaves as expected for a first-order phase transition; more specifically, below 800 K, it remains unchanged and, at high temperatures, approaches zero. In surface layers with thicknesses of ～0.15 and ～0.8 μm, the α-phase concentration decreases monotonically by approximately 10% with an increase in the temperature to 780 K. The temperature dependence of the α-phase concentration in the layer at a depth of ～6 μm passes through two minima, namely, at ～370 and ～570 K, at which the concentration of this phase decreases by about one half. This is accompanied by an increase in the concentration of the β-phase. The revealed behavior of the α-phase concentration with an increase in the temperature has been assigned to the influence of water on crystal lattice distortions near growth dislocations. At 370 K, free water evaporates from grain boundaries, and at 570 K, the water bound by hydrogen bonds to the SiOH groups. The evaporation of water affects stresses at grain boundaries, and it is this factor that brings about a change of the α-phase concentration. It has been demonstrated that tensile stresses generated with increasing temperature in a near-surface quartz layer to ～0.8 μm thick can reach ～170 MPa. The stresses create microcracks, which culminate in destruction of the sample. The generation of the tensile stresses is explained by an increase in the volume of the microcrystal layer located at a depth from ～1 to ～8 μm from its surface as a result of the increase in the β-phase concentration in it.     

Temperature-induced phase transition in quartz nanocrystals dispersed in pseudotachylite

The size and concentration of α-quartz nanocrystals dispersed in samples of pseudotachylite and the internal stresses in these nanocrystals have been determined using infrared spectroscopy in the temperature range 300–800 K. Pseudotachylite is a product of intense crushing of granite that undergoes in the Earth’s crust faults. It has been found that the size of the nanocrystals is ～20 nm and does not depend on temperature. As the temperature increases, their concentration decreases monotonically and tends to zero at ～650 K. This process is paralleled by a growth of the concentration of β-quartz nanocrystals. The α-quartz nanocrystal concentration regains its initial level with decreasing temperature. Thus, the α → β phase transition in quartz nanocrystals in pseudotachylite starts at temperatures lower by ～500 K than that in the bulk of the macrocrystal (846 K), and is stretched by ～350 K. At room temperature, the unit cell of nanocrystals is compressed by surface tension forces. These forces retard the α → β phase transition. The thermal expansion coefficient of nanocrystals is larger than that of macrocrystals, which entails a decrease of compression and a monotonic decrease of the concentration of α-quartz nanocrystals with increasing temperature.     

Magnetic state of a system of barium hexaferrite nanocrystals in the vicinity of the curie temperature

In order to study the magnetic state of a system of nanocrystals of highly anisotropic hexagonal barium ferrite in the vicinity of the Curie temperature, a macrocrystal surface layer comparable in thickness to the particles of the system is chosen as a model. As revealed by simultaneous application of gamma-, x-ray, and electron Mössbauer spectroscopy, the transition of a ～200-nm-thick surface layer to the paramagnetic state starts 55 K and is complete 3 K below the Curie temperature of the bulk of the crystal. This temperature range overlaps the range where the particles transfer from the magnetically stable to the superparamagnetic state. The data obtained made it possible to refine the high-temperature part of the H-T diagram for particles with a close-to-critical volume. The regions of the diagram where mixed magnetic states originating simultaneously from the size and surface factors coexist are identified.     

Depth profiling of D implanted into Ti at different temperatures

Depth proliles of 6.6 keV D⁺ implanted into titanium in the temperature range between 140 K and 500 K have been studied using the D(³He, α) H nuclear reaction.

At 140 K the trapped amount is close to 100% at low doses and reaches saturation at about 2 × 10¹⁸D/cm², whereas at room temperature no saturation could be reached up to 2 × 10¹⁹ D/cm². At higher temperatures the amount decreases until no deuterium could be detected in the surface layer above 500 K.

The depth profiles are strongly dependent on temperature. At 140 K the deuterium is found in a surface layer of about 2000 Å with a maximum ratio of deuterium to metal atoms of 2.5. At room temperature a hydride layer of TiD^1.8, forms. The thickness of the hydride layer depends on deuterium dose and extends to 1.5 μm at 2 × 10¹⁹ D/cm². At higher temperatures the atom concentrations are lower and the deuterium seems to diffuse deeply into the bulk.

These results are discussed in terms of diffusion of deuterium i n Ti and titanium hydride.     

Synthesis,phase composition,and magnetic properties of iron nanowires prepared in the pores of polymer track-etched membranes

Arrays of iron nanowires prepared by the method of galvanic filling of polymer track-etched membrane pores (matrix synthesis) under different electrolysis modes and electrolyte temperatures have been studied. The conditions of the synthesis have been analyzed. The optimal composition and electrolyte temperature have been found. The phase composition and magnetic properties of nanowires have been studied using the methods of electron microscopy, X-ray diffraction, elemental energy-dispersive microanalysis, and Mössbauer spectroscopy. The average nanowire diameter is 100–200 nm. The length varies from 6 to 10 μm. The surface density is ～10⁸ cm^?2 at the average distance to each other of about 1 μm. It has been established that the basis of nanowires is formed by the metal iron nanocomposite that manifests the magnetic properties of bulk α-Fe. It has been found that the preferred orientation of the magnetization inside the iron nanowires arises for an array prepared at a potential of ?750 mV.     

Chemisorption of CO on tungsten (100): Combined flash desorption and electron stimulated desorption study. II

The chemisorption of CO on W(100) at ~ 100K has been studied using a combination of flash desorption and electron stimulated desorption (ESD) techniques. This is an extension of a similar study made for CO adsorption on W(100) at temperatures in the range 200–300K. As in the 200–300 K CO layer, both α₁-CO and α₂-CO are formed in addition to more strongly bound CO species upon adsorption at ~ 100K; the α-CO states yield CO⁺ and O⁺ respectively upon ESD. At low CO coverages, the α₁ and α₂-CO states are postulated to convert to β-CO or other strongly bound CO species upon heating. At higher CO coverages, α₁-CO converts to α₂-CO upon thermal desorption or electron stimulated desorption. There is evidence for the presence of other weakly-bound states in the low temperature CO layer having low surface concentration at saturation. The ESD behavior of the CO layer coadsorbed with hydrogen on W(100) is reported, and it is found that H(ads) suppresses either the concentration or the ionic cross section for α₁ and α₂-CO states.     

The crystal structures and phase transition of methyl bromide

The detailed crystal structures of deuterated methyl bromide in both its α-and β-phases have been determined at 175 K and 146 K respectively by neutron powder profile measurements. The α-phase has space group Cmc2₁ while the β-phase has space group Pnma and both have a quasi-two-dimensional character. The major characteristic of the first order transition is a large change in the relative orientation of the two molecules lying in the mirror planes. The transition is successfully interpreted in terms of changes in the topology of the lattice energy surface calculated as a function of temperature from pair-wise interatomic potentials.     

67Zn Mössbauer study of Zn metal and Cu−Zn alloys at high pressure

A high pressure Mössbauer spectrometer for the 93.3keV resonance in⁶⁷Zn has been used to investigate Zn metal and two Cu?Zn alloys with Zn concentrations of 41.9at% and 48.0at% at 4.2K and pressures up to ～58kbar. For Zn metal the c/a-ratio plays an important role to describe the change of the Mössbauer parameters with pressure. Most striking is the increase of the recoil-free fraction by a factor of ～3. In the brass alloys a new phase is observed at 4.2K and high pressures which we tentatively identify with a martensitic α₁-phase.     

Type-II superlattice detectors for free space optics applications and higher operating temperature conditions

The utmost limit performance of interband cascade detectors optimized for the longwave range of infrared radiation is investigated in this work. Currently, materials from the III–V group are characterized by short carrier lifetimes limited by Shockley-Read-Hall generation and recombination processes. The maximum carrier lifetime values reported at 77 K for the type-II superlattices InAs/GaSb and InAs/InAsSb in a longwave range correspond to ～200 and ～400 ns. We estimated theoretical detectivity of interband cascade detectors assuming above carrier lifetimes and a value of ～1–50 μs reported for a well-known HgCdTe material. It has been shown that for room temperature the limit value of detctivity is of ～3–4×10¹⁰ cmHz^1/2/W for the optimized detector operating at the wavelength range ～10 μm could be reached.     

Critical point wetting in NbH0.31

The local α-α' phase distribution in a thin Nb single crystal loaded with the critical H-concentration was investigated with X-rays. The sample used in the experiment showed a macroscopic hydrogen density mode, leading to a curvature of the whole crystal plate with the α-phase on the concave side and the α'-phase on the convex side. In addition an α-phase wetting layer was found on top of the α'-phase separated from the bulk α'-phase by a mixed α-α' phase region. In a simple model their thickness were determined to about $\text{\$}\text{?}$1.0 and 7?0μm, resp. This thin wetting layer leads to a new interpretation of the phase transition of H and Nb, involving the familiar long-range elastic interaction as well as a short-range surface free energy contribution.     

Ammonium dynamics in the disorder a-phase of K1- x (NH4) x Y (Y=Cl,Br, I). a neutron scattering study

Temperature and concentration effects on the lattice parameters and the amplitude weighted phonon density of states (AWPDS) in the mixed salts of ammonium-potassium halides were investigated by neutron powder diffraction (NPD) and incoherent inelastic scattering (IINS). In the disorder α-phase (NaCl type), ammonium ions perform a fast stochastic reorientation at phonon frequencies rate, down to ca. 80 K. The IINS spectra at 10 K displayed the four distinct ammonium excitations, two (resonant) modes below and two (localised) above the cut-off frequency of the AWPDS of potassium halides. The high frequency localised modes correspond to the translational and librational vibrations of NH₄ ions. These modes are typical for ordered phases of ammonium halides. Ammonium concentration effects on the localised and resonant modes were studied for the K_{1? x} (NH₄) _x I mixed salts and the harmonic excitations of ammonium in the hypothetical low temperature α-phase of NH₄I were approximated to ca. 30, 95, 155 and 250 cm^?1. In the real low temperature ordered γ-phase of NH₄I, translational ammonium vibrations were observed at ca. 140-160 cm^?1 and librational at ca. 300 cm^?1.     

Accumulation of deuterium in radiation-damaged tungsten

Experimental studies have been performed to determine the effect of high-level radiation damage on the accumulation of deuterium and erosion of tungsten samples exposed to deuterium plasma. Tungsten samples were exposed first to fast helium ions having an energy of 3–4 MeV (providing from one to ten displacements per atom) and then to deuterium plasma up to a dose of 10²⁵ ion/m². The effects of deformation and modification of the surface microstructure have been observed. The concentrations of helium and deuterium have been measured by the methods of elastic nuclear proton backscattering and nuclear recoil detection of helium ions. A high concentration of deuterium in the damaged layer of a tungsten sample has been measured, and helium has been detected in a layer ～5 μm thick. The proposed method shows promise for determining the lifetime of materials used in fusion reactors and measuring the concentration of tritium accumulated in these materials.     

Ferrite stabilization induced by molybdenum enrichment in the surface of unalloyed iron sintered in an abnormal glow discharge

Unalloyed iron was sintered using an abnormal glow discharge with confined anode-cathode configuration. This electrode arrangement allows heating the component as well as enriching the surface with atoms from the cathode. In order to sinter unalloyed iron with molybdenum enrichment, the samples were placed on a holder acting as the discharge anode, inside a cylindrical cathode of Mo. The cathode was heated by the bombardment of ions and fast neutrals accelerated in the cathode sheath and as a consequence of the confined geometry the sample was efficiently heated by thermal radiation. On heating, the ion and neutral species bombardment produces sputtering of atoms of the cathode, which by diffusion in the gas phase deposit on the sample surface. During sintering the deposited atoms diffuse into the sample, resulting in a layer enriched with elements sputtered out from the cathode. Mo enrichment was observed on the sample surface (more than 4 wt%), which resulted in stabilization of the Fe α-phase at the treatment temperature. Under these conditions, as the diffusion coefficient in the Fe α-phase is about 10² times higher than in the γ-phase, the sintering of this enriched phase is activated.     

Theoretical modelling of XBn T2SLs InAs/InAsSb/B-AlAsSb mid-wave detector operating below thermoelectrical cooling

The paper reports on the barrier mid-wave infrared InAs/InAsSb (x_Sb = 0.4) type-II superlattice detector operating below thermoelectrical cooling. AlAsSb with Sb composition, x_Sb = 0.97; barrier doping, N_D < 2×10¹⁶ cm^?3 leading to valence band offset below 100 meV in relation to the active layer doping, N_D = 5×10¹⁵ cm^?3 was proved to be proper material not introducing extra barrier in valence band in the analyzed temperature range in XBn architectures. The detectivity of the simulated structure was assessed at the level of ～ 10¹¹ Jones at T ～ 100 K assuming absorber thickness, d = 3 μm. The detector’s architecture for high frequency response operation, τ_s = 420 ps (T ～ 77 K) was presented with a reduced active layer of d = 1 μm.     

PAC study of nuclear relaxation in solids:181Ta in hafnium hydrides

The electric quadrupole interaction (QI) at the site of¹⁸¹Ta in different hafnium hydrides HfH_x has been investigated as a function of temperature by TDPAC measurements. Below 300 K the QI is static within a 300 nsec time window. Above 300 K a dynamic QI related to the diffusion of hydrogen atoms leads to relaxation effects in the TDPAC spectra. The average residence time between hydrogen jumps, evaluated on the basis of the theory of stochastic perturbations, ranges from 100 nsec at 400 K in the ɛ-phase to 1 nsec at 700 K in the α-phase of the hafnium-hydrogen system. The activation energy for hydrogen jumps was found to be independent of the hydrogen concentration: E_a=0.53(12) eV.     

Kalorimetrische Bestimmung der Übergangstemperatur zur Supraleitung von verdünnten Titan-Rhodium-Legierungen

The specific heats of Ti-Rh alloys have been measured between 0.4 and 4 °K for Rh concentrations between 0 and 3 atomic percent. The measurements have been carried out in a simple He³ cryostat. The transition temperature and the electronic specific heat are increasing functions of the Rh concentration. In quenched alloys the α′-phase can be stabilized up to an average Rh-concentration of 2.5 At-% for the whole sample.     

Influence of surface on the effective magnetic fields in α-Fe2O3 and FeBO3

First measurements of the effective magnetic fields as a function of the depth at which the iron ions are in the surface layer of α-Fe₂O₃ and FeBO₃ single crystals are reported. The method used is the depth-selective conversion-electron Mössbauer spectroscopy. An analysis of experimental spectra revealed that the magnetic properties of the crystal surface vary smoothly from the bulk to surface characteristics within a layer ～100 nm thick. The layers lying below ～100 nm from the surface are similar in properties to the bulk of the crystal, and their spectra consist of narrow lines. The spectral linewidths increase smoothly as one approaches the crystal surface. The spectra obtained from a ～10-nm thick surface layer consist of broad lines indicating a broad distribution of effective magnetic fields. Calculations show that the field distribution width in this layer is δ=2.1(3) T, for an average value H _eff=32.2(4) T. It has been experimentally established that, at room temperature (291 K), the effective magnetic fields smoothly decrease as one approaches the crystal surface. The effective fields in a 2.4(9)-nm surface layer of α-Fe₂O₃ crystals are lower by 0.7(2)% than the fields at the ion nuclei in the bulk of the sample. In the case of FeBO₃, the effective fields decrease by 1.2(3)% in a surface layer 4.9(9) nm thick.     

稀土元素Ce热引入Si单晶中及Ce在Si中的扩散系数

利用真空淀积和真空热处理(1050℃,20h)向Si单晶中引入了稀土元素Ce,热处理过程中Ce首先与Si形成合金,然后向Si中扩散,于是在Si中形成Ce的扩散层。用二次离子质谱(SIMS)技术测定了Ce的纵向相对浓度分布,并据此分析了Ce在Si中的扩散系数。并在77—450K范围内测量了扩散层的平均电导率。 关键词：     

Subnanosecond multiphonon relaxation of divalent copper impurities in LiTaO3

The temperature dependence of the lifetime of the excited ²T₂ state of the Cu²⁺ ion in a LiTaO₃ crystalline host has been measured with picosecond optical pulses. Its value ranges from 450 psec at 22°K to 10 psec at 423°K. Consistent with this result is the observation of weak fluorescence at 1.75 μm having a low temperature quantum efficiency of ～ 10^?6. The experimental results are in reasonably good agreement with recent theoretical models based on strong coupling of the Cu²⁺ impurities to the host lattice.     

Time decay of optically injected free carriers in highly excited GaAs

The time decay of large densities of optically injected free carriers in epitaxial GaAs has been obtained from time-resolved mm wave reflectivity at 300°K. The decay time increases from 5 to 200 nsec as the epitaxial layer thickness increases from 1 to 72 μm, but is independent of injection level (Δn ～ 10¹⁶to 10¹⁸cm_?3) and doping level (norp ～ 10¹⁴to 10¹⁶cm^?3).