Emissionselektronenmikroskopische Untersuchungen am Zweistoffsystem Niob-Eisen

Results from emission electron microscope investigations of thermal and mechanical behaviour of the Laves-phase NbFe₂ and of two-phase areas in the binary system Nb Fe are presented. The melting process of NbFe₂ samples with included μ-phase NbFe was observed in the emission microscope and supported the existence of a eutectic between both intermetallic compounds. The Laves-phase NbFe₂ was deformed plastically by a microhardness tester and also homogenously by a compressive device during electron microscope observations. Both yield stress and microhardness of NbFe₂ were measured as a function of temperature up to 1550 K. The relation between a critical temperature T₀ of the hardness-temperature curve and the melting temperature T_m was T₀/T_m = 0,64, in good agreement with results from other Laves-phases. During the heating of NbFe₂ samples a niobium oxide decoration structure appeared at residual air pressures from 1 · 10⁻⁶ torr to some 10⁻⁴ torr. The observed density of decoration points amounted to (1…5) · 10⁸ cm⁻². A discussion shows the possibility of a connection between decoration points and dislocations emerging from the crystal.     

The thermodynamic equilibrium of the formation process of High-Tc superconducting phase YBa2Cu3O7−x

Analysing the equilibrium state of the formation reaction of the high-T_c superconducting phase 1:2:3 (YBa₂Cu₃O_7−x) on the base of the pressure-temperature-oxygen content (P-T−x) correlation research and using the Van't-Hoff equation, the Gibbs free energy (ΔG_m) of the reaction has been calculated for the first time. It is seen that the absolute value of ΔG_m is not large and of the order of 2.5 kcal · mole⁻¹. This leads to an instability of the 1:2:3 phase. Also, from the calculated values of ΔG_m relative stability regions of the 1:2:3 phase are determined for different values of x. Combinatively analysing the relation lg P = A−B/T (where A – the x-dependent constant, and value of B was determined to be equal to 10518 atm. K) and the value of ΔG_m, we come to the conclusion that the synthesis of YBa₂Cu₃O_{7 − x} when x → 0 must be carried out under high oxygen pressure and low temperature for a lasting time, because the reaction rate is limited by slow diffusion in the solid phase.     

A study of the Growth-Temperature-dependent Surface Morphology in InSb Liquid Phase Epitaxy

InSb LPE layers were grown on (111) InSb substrates, their phase diagram and growth rates studied. The InSb activity coefficient α(T), determined by fitting the experimental data, had a value of 2556–11.11 · T cal · mole⁻¹. The terraces on the surface were reduced by increasing the temperature to 350 °C. Hall measurements of InSb/CdTe heterostructure failed due to the high Te segregation. The carrier concentration of these InSb epilayers, however, was determined by C-V measurements with values between 7.5 · 10¹⁴ cm⁻³ and 5 · 10¹⁵ cm⁻³.     

Scandium thin films-structure and electrical resistance (I). A study on films obtained in a vacuum of 10−5 up to 10−6 Torr

A structure as well as a specific electrical resistance of scandium films having a various thickness (300 up to 2800 Å) obtained at a condensation rate: w⋍ 1 up to 155 Å/s have been studied in the present work. Depending on condensation conditions in films a hydride ScH_x, h.c.p. — Sc, or ScH_x + Sc alternatively, is formed, a phase containing a distorted hexagonal lattice evolving into the ScH_x over a period of a few hours. The ScH_x and h.c.p-Sc change to Sc₂O₃ when annealed in a vacuum of 5 · 10⁻⁵ Torr at a T ≈ 800°C, while in a vacuum of 3 · 10⁻⁸ Torr a change to h.c.p.-Sc with negligible inclusions of oxidic Sc₂O₃ particles takes place. Formation of oxide is observed also when heating the films with an electron beam directly in a microscope under vacuum of ∼ 8 · 10⁻⁵ Torr. The specific electrical resistance ϱ of the film just after a condensation has taken place is approximatly by an order higher as against the ScH_x- and h.c.p.-Sc solid specimens. An irreversible gain in said resistance is observed while holding at a room temperature or annealing in a vacuum of 5 · 10⁻⁵ Torr, this being associated with absorption of H₂, O₂ and, possibly, of water steam, and then with formation of Sc₂O₃. Annealing in a vacuum of 5 · 10⁻⁸ up to 10⁻⁷ Torr involves a temperature relationship for ϱ of an intricated nature, this being accounted for by a structure transition as follows: A diode-effect diminishing with a temperature rise is observed in annealed films.     

Plastic Deformation of Cesium Iodide Single Crystals at High Temperatures

The plastic deformation of CsI crystals of three crystallographic orientations 〈100〉. 〈110〉. and 〈111〉 at temperatures from 423 to 773 K (0.5 to 0.86T_m) and strain rates from 2 × 10⁻³ to 10⁻⁵ s⁻¹ is studied. Four-stage stres-strain curves were found, three stages being more and more distinct with an increase in temperature up to 623 K above which stage III prevails. For all the temperatures, strain rates and oreintations studied the superplasticity features (jerky flow, deformation localization, active dynamical recovery etc.) were observed. The thermal activatoin analysis has shown that the rate of high temperature deformation of CsI is controlled by dislocation climb limited in its turn by mobility of cation vacancies (quasiviscous creep).     

Growth of bulk zinc chalcogenide single crystals from the vapour phase and their use for laser screens of projection colour television

Results on ZnSe, ZnSe_xS_1−x and ZnS crystal growing from the vapour phase up to 7.5 cm³ in volume are described. Crystals were grown on sapphire, ZnS, ZnSe_xS_1−x and quartz glass substrates without a contact of the growing crystal with a growth ampoule and using the molten tin as a heating medium. Large high-purity crystals with a density of etch pits of 10³ cm⁻² were obtained They exhibited an effective exciton luminescence and rather high radiation efficiency (30 ± 10% for ZnSe at T = 77 K). This made it possible to use these crystals for fabricating laser screens for a cathode ray tube. The main laser parameters obtained on a ZnSe screen at T = 80 and 300 K using a 75 keV electron beam excitation are presented. The light power output reached 0.8 W at T = 80 K; this allowed to obtain a 10 cd · m⁻² TV image of 1.5 × 2 m² in area.     

Structure and electrical resistance of thin scandium films (III). Study on electrical properties

The specific resistance-thickness of ScH_x (vacuum 10⁻⁵ to 5 · 10⁻⁶ Torr) and Sc films (vacuum 1 · 10⁻⁸ to 3 · 10⁻⁸ Torr) relationships have been obtained by keeping a record of a film resistance directly in the course of condensation. The results are discussed according to the theory FUCHS -SONDHEIMER (ScH_x, Sc) and MAYADAS -SHATZKES (Sc) assuming a diffuse scattering of the carriers (P = 0) on the external surface of the films. A mean free path of the current carriers is 188 to 355 Å (T ⋍ 300 K) for ScH_x and 400 Å with grain-boundary scattering parameter from 0.1 to 0.3 for Sc. The temperature dependence of specific resistivity of Sc films was obtained in the interval from 300 to 650 K during the annealing in vacuum 5 · 10⁻⁹ Torr.     

Preparation and some properties of thallium orthothioarsenate single crystals

Growing of large (Ø 25–30 mm, L = 50–60 mm) optically homogenous single crystals of Tl₃AsS₄ using the Bridgman-Stockbarger method is described. Tl₃AsS₄ is orthorhombic (Pnma), a = 8.85 ± 0.03, b = 10.86 ± 0.03, c = 9.18 ± 0.03 Å; z = 4; ϱ_x = 6.15 g · cm⁻³; ϱ_p = 6.15 ± 0,03 g · cm⁻³, T_m = 424 ± 3°C, Moh's hardness = 3, microhardness and ultrasonics velocity along x, y, z are 65–85 kg · mm⁻² and 2.16 – 2.37 · 10⁵ cm · sec⁻¹, respectively. The crystals possess perfect cleavage plane (010). Their transparency range is 0.6–12 microns. – Unsuccessful attempts to obtain Tl₃AsS₄ and its alloys in the vitreous state were taken. The possibilities of glass formation and boundaries of the vitreous region in the system Tl–As–S based on the characteristic features of the melt forming structural units are analyzed.     

Plastische Verformung von Siliziumeinkristallen unterschiedlicher Ausgangsversetzungsdichte im Streckgrenzenbereich

Floating-zoned Silicon crystals orientated for single slip both dislocation-free and with grown-in dislocations (EPD₀ = 6,5 · 10⁴ cm⁻²) were dynamically deformed in compression in the temperature range 1120 … 1300 K up to the lower yield point. The dislocation structure of plastically deformed crystals (T = 1138 K; a₀ = 1,45 · 10⁻⁴ s⁻¹) was examined by etching and high-voltage electron microscopy. The crystals exhibit essential differences in deformation behaviour and in the resulting dislocation structure. Typical for the dislocation-free basic material are (i) an inhomogeneous deformation in the range of the upper yield point (ii) a greater portion of sessile lomer dislocations at the lower yield point in relation to the crystal with grown-in dislocations. The measured stresses were related to those values calculated from structure data by employing plasticity theory of diamond-like semiconductors.     

EPR and Optical Absorption Spectra of Cr3+ Ions in Lithium Sulphate Single Crystals

Optical absorption and EPR spectra of Li₂SO₄ · H₂O crystals doped with Cr³⁺ are studied at liquid nitrogen temperature. The bands are found in absorption spectra with maxima about 17000, 23 800 and 37 200 cm⁻¹, assigned to the ⁴A₂ → ⁴T₂, ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₁ (⁴P) transitions, respectively. The crystal field theory parameters were determined and appeared to be as follows: Dq = 1700 cm⁻¹, B = 667 cm⁻¹, C = 3002 cm⁻¹. The lines resulting from Cr³⁺ ions are found in EPR spectra. All lines are doublets, which is indicative of presence of two magnetically unequivalent centre positions, and have the hyperfine structure resulting from interaction of the unpaired electron spin with Cr⁵³ isotope nucleus. Centres are oriented in such a way, that z-axes, corresponding to two centre positions, are situated at both sides of a-axis at an angle of about 3°. Spin Hamiltonian parameters were found as follows: g_x = 1.985, g_y = 1.984, g_z = 1.988, D = 0.130 cm⁻¹, E = 0.016 cm⁻¹, |A| = 17.8 · 10⁻⁴ cm⁻¹.     

Formation Entropy of Schottky Defects and Free Formation Enthalpy of Vacancy Pairs in Alkali Halide Crystals

The melting of alkali halides occurs probably at a critical mole fraction x_c = 3.2 · 10⁻⁴ of thermally created vacancies (Schottky defects). Using this value the formation entropy of Schottky defects is found to be s_S = (962.1 K mol/kJ L/T_m – 16.1) k (L, T_m heat and temperature of fusion, respectively). As to the free association enthalpy g_A = h_A – T_sA of (impurity-vacancy) complexes it is s_A = (10.8 h_A/eV – 2.56) k. Using this relation the concentration vacancy pairs is found to be smaller than that of isolated vacancies up to the melting point.     

Convective Temperature Fluctuations in Liquid Gallium in Dependence on Static and Rotating Magnetic Fields

The influence of static axial, static transversal, and rotating magnetic fields on convective temperature fluctuations in liquid gallium (temperature range around 800 °C, Prandtl number 2.5 · 10⁻³) has been investigated. For a Rayleigh number of 6.3 · 10⁻⁵ (ΔT = 10°C, h = 50 mm), convective temperature fluctuations with peak to peak values of 3 °C have been measured. Depending on the strength, the frequency, and the configuration of the field, they could be eliminated to ΔT < 0.01 °C (static axial field of 182 mT), damped to a nearly periodic state with 0.1 °C amplitudes (static transversal field of 45 mT), or reduced to 0.03 °C oscillations by applying a rotating field.     

Thermodynamische Berechung des Systems Ge/H2O/H2 und experimentelle Untersuchung mit der Nahabstandsanordnung

In the temperature range from 950 to 1200 K K_p-values result from 5 · 10⁻⁵ to 6,9 · 10⁻³ for the heterogeneous reversible reaction Ge_(s) + H₂O_(g) ⇋ GeO_(g) + H₂O_(g), the average reaction enthalpy being 46,5 ± 05 kcal/mol. Etching rates calculated with these equilibrium constants for closed systems are 20-25% larger than the experimental etching rates for (110)-Ge in the sandwich device. The ratio of the etching rates is for (111)-, (100)- and (110)-Ge 1:1,4:1,8; the average reaction enthalpy calculated experimentally is 46 ± 1 kcal/mol for the temperature range 1000-1200 K. The dependence of transport rate on distance between source and substrate characterizes the sandwich device as a quasi-closed system with the diffusion as the rate controlling step of the material transport.     

Determination of Nitrogen Concentration in GaP Epitaxial Layers by Two Independent Methods

The nitrogen concentration in GaP is determined by optical absorption in the A-line at T = 77 K. The concentration of the isolated nitrogen atoms is given for T = 77 K by the modified LIGHTOWLERS' relation [N_A] cm⁻³ = 8.2 · 10¹⁴ phα_max, where α is measured in cm⁻³, h in meV. p is a dimensionless line shape factor. It is shown that at higher N concentrations considering only the A-line absorption the impurity density is underestimated because the nitrogen atoms included in NN_i pairs give no contribution to the absorption. The measurements have been made in the range from [N] = 5 · 10¹⁶…︁ 10¹⁹ cm⁻³ in layers grown by vapour phase epitaxy. The results are compared with nitrogen concentrations obtained by precision lattice parameter measurements. The change of the lattice parameter is calculated using VEGARD's law. The good agreement between the nitrogen densities obtained by two different independent methods reveals (i) that the LIGHTOWLERS' calibration factor is valid also at higher N-concentrations and (ii) that the nitrogen atoms are predominantly incorporated into P-lattice sites.     

Effect of the oxygen pressure during annealing on Tc and lattice parameter c of Bi-2212 single crystals

The influence of oxygen partial pressure (p_o2 = 10⁻⁴–1.5 · 10² atm) during annealing on the T_c and lattice parameter c of Bi-2212 single crystals has been investigated. Antibate correlation between non-monotonous alteration of the c parameter and T_c with the increase in oxygen content has been revealed.     

Temperature dependence of electrical conductivity and hall effect of Ga2Se3 single crystal

Electrical conductivity (σ) and Hall coefficient (R_H) of single crystal grown from the melt have been investigated over the temperature range from 398 K to 673 K. Our investigation showed that our samples are p-type conducting. The dependence of Hall mobility an charge carrier concentration on temperature were presented graphically. The forbidden energy gap was calculated and found to be 1.79 eV. The ionization energy of impurity level equals 0.32 eV approximately. At 398 K the mobility equals to 8670 cm² V⁻¹ s⁻¹ and could described by the law μ = aTⁿ (n = 1.6) in the low temperature range. In the high temperature range, adopting the law μ = bT^–m (as m = 1.67), the mobility decreases. This result indicates that in the low temperature range the dominant effect is scattering by ionized impurity atoms, whereas in the high temperature range the major role is played by electron scattering on lattice vibrations (phonons). At 398 K the concentration of free carriers showed a value of about 1.98 × 10⁷ cm⁻³.     

Optimized growth conditions of GaN epitaxial layers

Optimized growth conditions of the epitaxial GaN layers on the (0001) oriented sapphire substrates in the Ga/HCl/NH₃/H₂ system have been proposed. The corresponding growth rate varied about the value 0.5 μm · min⁻¹. The study of surface morphology and X-ray diffraction have confirmed the single crystalline character of the layers even though the surface shows a faceted structure. The c/a ratio calculated from our measured data of the lattice parameters was found 1.624 which is relatively close to the ideal close-packed wurtzite structure value. The cathodoluminescent spectra of the layers with a sufficient thickness were characterized by a peak at 3.35 eV having a halfwidth of about 0.2 eV.     

Structures of the Triclinic and the Monoclinic Modification of Divascan®

Trielinic modification: C₁₂N₄O₃H₁₆. Mr = 264.26, triclinic P 1 , a = 7.702(3), b = 9.420(11),c = 10.893(8) A, α = 73.37(8), β = 82.22(5), γ = 66.30(6)°, V = 638(1) A³, Z = 2, D_m = 1.360 Mg m⁻³, D_x = 1.362 Mg m⁻³, λ(MoK_α) = 0,71062 A, μ = 0.1084 mm⁻¹, F(000) = 280, T = 296 K, final R = 0.0742, wR = 0.0758 for 1842 reflections. Monoclinic modification: C₁₂N₄O₃H₁₆·, 3/2H₂O, M_r = 291.28, monoclinic P 2₁,/c, α = 14.075(5), b = 12.631(6), c = 16.234(5) A, β = 99.25(3)°, V = 2849(1) A³,Z = 8, D_m = 1.338 Mg m⁻³, D_m = 1.359 Mg m⁻³, λ(MoK_α) = 0.71062 A, μ = 0.1134 mm⁻¹, F(000) = 1240, T = 296 K, final R = 0.0557, wR = 0.0691 for 1266 reflections. Comparison of the two independent Divascan® molecules of the monoclinic modification with the one of the triclinic modification demonstrates good agreement of related bond distances and angles in the molecules. Disorder of the hydroxyl groups of one Divascan® molecule of the monoclinic modification is announced by high vibration parameters of the related oxygen atom and a very short C O distance, which is not adequate to a real C O bond length.     

Crystal structure determination of 2-acetylamino-4-benzoyl-5,5-dimethyl-4,5-dihydro-1,3,4-thiadiazolin,C13H15N3O2S

Thiosemicarbazone is receiving increasing interest because of the biological activity of some of its derivatives. Mr = 277.35, space group P2₁2₁2₁, Z = 4, a = 9.290(3), b = = 11.512(2), c = 13.286(2) Å, V = 1421 (1) ų, D_m = 1.29(1) Mg m⁻³, D_x = 1.296 Mg m⁻³, λ(MoK_α) = 0.71062 Å, μ = 0.230 mm⁻¹, F(000) = 584, T = 296 K, final R = 0.028 for 1255 reflections. All hydrogen atoms were located. The structure determed by X-ray analysis confirms the chemical results. The structure analysis of the title compound is described and its molecular and crystal structure discussed.     

Debye temperature and melting point of II‐VI and III‐V semiconductors

In this paper, simple relations are proposed for the calculation of Debye temperature θ_D and melting point T_mof II‐VI and III‐V zincblende semiconductors. Six relations are proposed to calculate the value of θ_D. Out of these six relations, two are based on plasmon energy data and the others on molecular weight, melting point, ionicity and energy gap. Three simple relations are proposed to calculate the value of T_m. They are based on plasmon energy, molecular weight and ionicity of the semiconductors. The average percentage deviation of all nine equations was calculated. In all cases, except one, it was estimated between 3.34 to 17.42 % for θ_D and between 2.37 to 10.45 % for T_m. However, in earlier correlations, it was reported between 10.59 to 33.38% for θ_D and 6.96 to 14.95% for T_m. The lower percentage deviation shows a significant improvement over the empirical relations proposed by earlier workers. The calculated values of θ_D and T_m from all equations are in good agreement with the available experimental values and the values reported by different workers. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)