Study of calcium phosphate precipitation at 37 °C

The nature of the calcium phosphate formed when equal volumes of 20 mM CaCl₂ and 15 mM or 12 mM K₂HPO₄ with pH 7.4 are rapidly mixed at 37 °C has been investigated by following the pH of the suspension as a function of time, by calorimetric and light scattering methods, chemical analysis and TEM. It is found that in the early stages of this process, there appears a spherular amorphous form of calcium phosphate, called ACP1, which transforms into a floccular amorphous form, called ACP2. We suggest that this transformation is solution-mediated. ACP2 has not previously been clearly described as a separate phase and has not before been shown to be amorphous.     

Mechanical Behaviour of LiF Single Crystals in Compression and Tension at Temperatures from −196°C to 800°C

The work hardening characteristics of LiF single crystals in compression and tension has been investigated as a function of temperature. It was shown that in temperature range from 200 °C to 600 °C four stages of hardening were observed and the main part of plastic flow proceeds in stage IV. The parabolic hardening was not observed. The flow stresses and hardening rates in all stages in compression are higher than those in tension. As a temperature increase this difference decreases. The hardening rates, stresses which correspond to the beginning of the linear hardening stages and the duration of those stages decrease as a temperature increase. At temperatures above 600 °C the yield drop occured. Taking into account a good agreement between the temperature dependences of the critical resolved shear stress and τIII the assumption was made that τIII corresponds to resolved shear stress in a secondary glide system.     

Growth mechanisms in GaAs-VPE at low deposition temperature

The VPE growth of GaAs in the system GaAs AsCl₃ with either H₂ or He as the Carrier gas was studied in the range of low deposition temperatures. Down to about 670 °C the growth in the H₂-system is limited by the kinetics of chlorine desorption by molecular hydrogen. The increase in growth rate at lower temperatures results from the onset of GaCl₃-desorption (disproportionation mechanism). The addition of NH₃ to the vapour phase enhances this effect. Below about 700 °C the growth rate in the GaAs AsCl₃ H₂ NH₃ system becomes comparable to that in the GaAs AsCl₃ He-system, where the GaCl₃ desorption mechanism is the only possible growth mechanism.     

On the reversion behaviour of an Al-15 at.% Zn alloy between 155°C and 175°C

The structural changes going on in an Al-15 at.% Zn alloy postaged in the vicinity of the kink of the C-curve (161°C) in the T.T.T.-diagram after pre-ageing at R. T. or 90°C (till start radii r_S between 1.0 and 4.0 nm were obtained) were followed by means of SAXS-investigations. The main results are the following:

• 1 No indication could be found that the reversion behaviour is remarkably changed by crossing the temperature of the kink of the C-curve in the T.T.T.-diagram, i.e. the reversion as well as the growth mechanism of the zones does not differ considerably below and above 161°C.
• 2 The decrease of the integral intensity Q₀ during the reversion process is mainly due to both the loss of zinc content of the zones and its rise in the matrix and not essentially by the diminution of the volume fraction of the precipitates.
• 3 Zones of a start radius of about 4 nm are much more stable against the decrease of the zine content during the reversion treatment that those of 2 nm.

     

Structural Changes during Post-Ageing of an Al-Zn (15 at.%) Alloy between 125 °C and 215 °C

The structural changes going on in an Al Zn (15 at.%) alloy postaged between 125 °C and 215 °C after pre-ageing between 20 °C and 90 °C (till start r_s = (20…︁ 55) Å are obtained) were followed by means of XSAS-investigations. The main results are the following:

• 1 An increase of r_s does not favour the growth of the precipitates during the reversion treatment.
• 2 At later reversion times the happening is obviously independent of the pre-history of the samples.
• 3 The integral intensity is decreasing with increase of T_rev, but the mean distance (d)-size (r_G) relation between the precipitates is not changed remarkably up to T_rev ≈ 200 °C in the early reversion stage.

Conclusions from this behaviour are drawn.     

Isothermal Ageing in the T Range from 0°C to 180°C of a Technical Al-2.0 at.% Zn-1.3 at.% Mg Alloy after Direct Quench

The variations of the structure and the mechanical properties had been investigated by means of TEM investigations and determination of the microhardness number in the T_a range between 0 °C and 180 °C. The main results are the following: – At the starting period of the isothermal ageing an incubation time, t_inc, occurs. t_inc plotted versus T_a yields two intersecting C curves at about 100 °C. – At T_a = 70 °C the only decomposition product are G.P. zones. – At 70 °C <T_a <100 °C the G.P. zones formed in the first period of decomposition can be transformed into η-phase or spherical hexagonal (S.H.) zones. – Above about 110 °C the precipitates present in the matrix are quasi-homogeneously formed at nucleation sites delivered by clusters of impurity atoms.     

Microampere laser threshold at 80°C with InGaAs/GaAs/InGaP buried heterostructre strained quantum well lasers

An extremely low CW threshold current of 670 μA and a high slope efficiency of 0.14 W/A at a high junction temperature of 80°C were obtained with a 200 μm long Al-free InGaAs/GaAs/InGaP buried heterostructure (BH) quantum well laser grown by three-step metal organic vapor phase epitaxy (MOVPE). The maximum energy conversion efficiency of a 500 μm long laser was as high as 50% at a output power level of 1 mW. Regrowth conditions of InGaP layers were found to be crucial for planarizing the grown surface to realize the high performances.     

Kinetics of crystallization of potassium chloride from aqueous solutions at 30 °C

The aim of this work is to use a diffusion layer model for the determination of individual rate constants of reaction and diffusion step of KCl at 30 °C. Crystal growth rate was measured by travelling microscope technique. From the measured values of the mean linear growth rate the resulting individual rate constants and the effectiveness factors (GARSIDE) are evaluated as a function of the driving force and of the flow velocity of solution. The significance of diffusional and surface reaction resistances against mass transfer is discussed in relation to the experimental conditions used.     

A facile route to prepare boron nitride hollow particles at 450 °C

Hexagonal boron nitride (h-BN) particles including hollow spheres (with a proportion of ~30–40%) and nanotubes (10%) have been synthesized by using sodium fluoroborate and sodium azide at 450 °C for 20 h. X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) studies show that the as-obtained BN hollow particles are crystalline. The total specific surface area of the product calculated from Brunauer–Emmentt–Teller (BET) absorption measurement is 89.79 m²/g, indicating that it may be utilized as a promising candidate for hydrogen storage container or catalyst. Thermal gravimetric analysis (TGA) result reveals its excellent thermal stability below 800 °C. Its possible growth mechanism and the effects of reaction parameters were also briefly discussed.     

Mechanical behaviour of magnesium oxide single crystals at temperatures from 196 to 2400 °C

The stress — strain characteristics of MgO single crystals in compression are investigated in the temperature range from –196 °C to 2400 °C on samples of the same shape and size. It is shown that the stress — strain curves may be subdivided into three basic types, each of them corresponding to the definite temperature range. The work hardening rate at stage I is temperature independent and at stage II lineary decreases with temperature increase in overall temperature range where work hardening is observed. The high temperature maximum in the temperature dependence on the proportional limit is found which is not due to the effects of impurities. The effect of geometrical form of specimens on stress-strain parameters and the local true strain distribution in compressed specimens have been studied.     

Die Diffusion von Wasserstoff in Nickel-Ein- und -Polykristallen bei 60°C

Diffusion of hydrogen in nickel mono- and polycrystals is investigated to find the influence of grain boundaries. The result is that the small differences in the diffusion coefficients cannot unambiguously related to the differences between the polycrystalline material and the monocrystals. The grain boundaries do not influence essentially the mechanism of the diffusion.     

Growth and investigation of substituted gadolinium gallium garnet laser crystals

Gadolinium gallium garnets substituted with (Ca,Zr), (Mg,Zr) and (Ca,Mg,Zr) are considered to be potential new laser materials. Such substituted GGG boules double-doped with (Nd,Cr) and single-doped with Cr have been grown by the Czochralski method. Color center investigation and lattice constant determination have been carried out. Laser output slope efficiency for a GGG(Ca,Mg,Zr) : (Nd,Cr) rod of dimensions 5 mm diameter by 40 mm long is greater than 0.7%.     

The properties of hydrogen forms of zeolite T obtained by the treatment with hydrochloric acid at 22°C

Hydrogen forms of zeolite T obtained by the treatment of synthetic zeolite of erionite-offretite type with hydrochloric acid at 22°C were tested with the methods of chemical analysis, sorption and X-ray diffraction. The influence of the variations in the number of treatments with acid, the concentration of acid and the calcination at 500°C on the properties of the samples was observed. It was found that the modification runs in two steps. The existence of the threshold of stability of the zeolite T structure under the treatment with 0.1 N HCl was indicated at the degree of decationization of 45–48% and the degree of dealumination of 25–28%. It was also found that about 30–33% of alkali cations in zeolite T are at the more accessible positions.     

Nature of luminescence and strain in gallium nitride nanowires

Photo- and cathodo-luminescence measurements of a variable-diameter ensemble of GaN nanowires revealed a diameter-dependent, spectral emission distribution between 350 nm and 850 nm. Spectral analysis indicated that wires with a diameter less than 400 nm were dominated by a yellow luminescence with a weaker near UV/violet emission also present. Examination of this ensemble showed that there was a general trend in the ratio of near-UV-to-yellow emission intensities with increasing nanowire diameter. Additionally, a broad green emission appears in the nanowires with a diameter above approximately 200 nm. A calculation based on the nanoheteroepitaxy model indicates that this diameter represents a transitional thickness where strain is relieved by defect formation mechanisms with a characteristic green emission.     

Phase Equilibrium in the System HgSeO4 – H2SeO4 – H2O at 100°C

The solubility of the system HgSeO₄ – H₂SeO₄ – H₂O was studied at 100 °C. The fields of crystallization of HgSeO₄ H₂O and HgSeO₄ were established. The compounds obtamed were identified by means of chemical, X-ray and crystal-optical analysis. The mechanism of the thermal dissociation of the compounds obtained was studied.     

The influence of sodium chloride on the driving force of the crystallization of potassium chloride from aqueous solutions at 25 °C

Admixtures at higher concentrations affect the growth rate in two kinds by changing the kinetic phenomena and also the driving force. Based on measurements of the osmotic coefficient as a function of the total ionic strength of the solution at 25 °C different kinds of driving force are calculated. Usefull for the discussion of the admixture problem is the mean molal free enthalpy of crystallization. NaCl increases the driving force of the crystallization of KCl. It shows that the reason for the decreasing effect of NaCl in growth rate is a kinetic one.     

The 30° rotation domains in wurtzite ZnO films

The 30° rotation domains in ZnO films were studied by transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The cross-section and plane-view observations reveal that the 30° rotation domains have elliptical cylindrical shape, with the longitude axis along one of the 1 1 2¯ 0 directions and the short axis along one of the 1¯ 1 0 0 orientations, respectively. The volume fraction of the 30° rotation domains is about 1%. Due to the elliptical shape of the domain boundaries along the [0 0 0 1]_ZnO direction, partially disordered superlattice-like structures are formed. As shown by the HREM images and Zn elemental mapping, these super-lattices are most likely caused by periodical segregation and depletion of Zn along the domain boundary for compensating the mismatched lattice strain.     

The Ln–Ba–Cu–O (Ln = Lanthanoid) systems: Compounds and compatibilities in air at 900–980°C

Comparison of phases observed in air at 900–980°C in the Ln–Ba–Cu–O (Ln = Lanthanoid) systems is reported. On the basis of the occurrence of compounds found in these Ln₂O₃–BaCO₃–CuO systems they must be divided into at least three subgroups: the first is characteristic of La, Pr and Nd, the second specific for Sm and Eu, and the third common to the smaller lanthanoids, the Y-type elements, with some variation within the groups.