Oxygen ion concentration in solvents for the growth of garnets

A concentration cell for the determination of the oxygen ion concentration by means of e. m. f. measurements is described which is suitable for oxidic melts with low viscosity and high attack on ceramic materials. Applying this concentration cell binary mixtures of oxides and fluorides used as solvents for the garnet preparation were investigated. The oxygen ion concentration is given in dependence on the composition of the mixtures. By comparison with simple models conclusions on the structure of the melt are made.     

New solvents for the growth of substituted BaFe12O19 single crystals from high-temperature solutions

The solubility of a series of hexaferrite derivatives of BaFe₁₂O₁₉ in solvents of the system Na₂O-B₂O₃ with oxide ratios of 7:3 and 3:2 has been investigated. The temperature dependences of the saturation concentration in these solvents are determined for ferrites with the nominal compositions Ba_0.8Pb_0.2Fe₁₂O₁₉, BaFe₁₀Ga₂O₁₉, BaFe₁₀Al₂O₁₉ and BaFe₈Mn₂ Ti₂O₁₉. Single crystals of BaFe₁₂O₁₉, in which part of the metal ions are replaced by various amounts of Pb²⁺, Ga³⁺, Al³⁺ and Mn²⁺ + Ti⁴⁺ ions, are g rown from the solutions by the slow cooling technique. The distribution coefficients of the substituting ions and the compositions of the crystals obtained are established by microprobe analysis (EPMA). Information on the position of substitution is obtained from the Mössbauer spectra.     

Defect model for the mixed mobile ion effect revisited: An importance of deformation rates

The progress in understanding the behavior of glassy mixed ionic conductors within the concept of the defect model for the mixed mobile ion effect [V. Belostotsky, J. Non-Cryst. Solids 353 (2007) 1078] is reported. It is shown that in a mixed ionic conductor (e.g., mixed alkali glass) containing two or more types of dissimilar mobile ions of unequal size sufficient local strain arising from the size mismatch of a mobile ion entering a foreign site cannot be, in principle, absorbed by the surrounding network-forming matrix without its damage. Primary site rearrangement occurs immediately, on the time scale close to that of the ion migration process, through the formation of intrinsic defects in the nearest glass network. Neither anelastic relaxation below glass transition temperature, T_g, nor viscoelastic or viscous behavior at or above T_g can be expected being observed in this case because the character of the stress relaxation in a wide temperature range is dictated above all by the deformation rates employed locally to the adjacent network-forming matrix. Since the ion migration occurs on the picosecond time scale, the primary rearrangement of the glass network adjacent to an ionic site occurs at rates orders of magnitude higher than those of the critical minimum values, so the matrix demonstrates brittle-elastic response to the arising strain even at temperatures well above T_g, which explains, among other things, why mixed alkali effect is observable in glass melts.     

Hydrogen and oxygen concentration analysis of porous silicon

Elastic recoil detection and Rutherford backscattering spectrometry combined with the nuclear reaction analysis method have been applied for the determination of oxygen, hydrogen and carbon concentration depth profiles of aged p⁺-type porous Si layers of different low and medium porosities. The plan view and cross-section scanning electron microscopy images have provided with information about both the diameter and silicon skeleton structure of the pores. The concentration depth profiles reveal the existence of a non-homogeneous subsurface porous film several hundred nanometers thick for all the studied samples. Differences in the atomic composition among low and medium porosity layers and the possible origin of various impurities are discussed. The maximum H content in PSi has been observed at the depth of 200–600 nm, while the highest oxygen concentration is typical of 200 nm thick subsurface layers. The highest obtained ratio of H/Si atomic concentrations reaches the value of 2 for the PSi samples with porosity P of 66%, comparing to N_H/N_Si = 0.27 in the case of P = 25% PSi.     

Study on the oxygen concentration reduction in heavily Sb-doped silicon

It was determined that oxygen concentration in heavily Sb-doped silicon was about 40% lower than that in the lightly doped Czochralski grown silicon and decreased with increasing content of Sb by means of coincident elastic recoil detection analysis. Through thermodynamic calculation, the oxygen loss by evaporation from the free surface of melt is only due to the formation of SiO, and Sb₂O₃ evaporation can be neglected. The basic reason for oxygen concentration reduction in heavily Sb-doped CZSi was that oxygen solubility decreased when element Sb with larger radius doped degenerately into silicon crystal.     

The effect of fluoride ion concentration on apatite formation and on its crystal habit

The effect of fluoride ions on the rate of apatite [ca_10(PO4)6(F,OH)₂] formation and on its crystal habit was investigated. It was found that both at high and at low fluoride concentrations the rate formations is retarded. At two optimal concentrations, namely at 3.4 and 5.5 fluoride: apatite excess of the stoichiometric ratio, the rate is the highest. The crystal habit of the apatite formed at high fluoride concentrations is hexagonal plate-like, while the apatite formed at low fluoride concentrations is dendritic or needle-like. The concentrations of fluoride in the crystals were determined by specific fluoride electrode technique.     

State of the chromium ion in soda silicate glasses under various oxygen pressures

Optical absorption and ESR spectra of chromium in soda silicate glasses were measured to characterize the electronic environment of the chromium ion in these glasses. Glasses produced in very strongly reducing conditions showed a broad optical absorption in the wavenumber range of 10 000–25 000 cm^?1 without any ESR absorption, which suggested the formation of Cr²⁺ ions. Glasses produced in air or moderately reducing conditions showed ESR signals suggesting that there are three different states of Cr³⁺ ions, the strongly coupled Cr³⁺ ion pairs, the weakly coupled Cr³⁺ ion pairs and the isolated Cr³⁺ ions in elongated tetragonal sites or sites with lower symmetry. The presence of Cr⁵⁺ ions in glasses produced in air was also suggested. It was indicated that the critical partial oxygen pressure of the formation of Cr²⁺ ions is in the vicinity of P_O2 = 10^?8 atm and that Cr²⁺ ions do not coexist with Cr³⁺ ions homogeneously in soda silicate glasses.     

Influences of Yb3+ ion concentration on the spectroscopic properties of silica glass

We investigated optical spectroscopic properties of silica glass doped with ytterbium (Yb³⁺) ions to assess their lasing performance in diode-laser pumped systems. The Yb-doped silica glass preforms were fabricated by solution doping technique using the MCVD process. The stimulated emission cross-section and laser performance parameters were determined from the measured absorption spectra using the method of reciprocity. Fluorescence decay characteristics were observed to be deviated from exponential behavior as the Yb³⁺ ions doping level increases and there exists a lifetime quenching behavior related to the cluster effect of Yb³⁺ ions into silica glass. Nevertheless, lasing parameters indicated that clustering of Yb³⁺ ions does not significantly affect the spectral properties relevant to the predicted lasing performance when concentration is low, but becomes predominant at higher concentration.     

Enhancement effect of nitrogen co-doping on oxygen precipitation in heavily phosphorus-doped Czochralski silicon during high-temperature annealing

Oxygen precipitation in conventional and nitrogen co-doped heavily phosphorus (P)-doped Czochralski silicon (CZ-Si) crystal subjected to various high-temperature annealing in the range of 1000–1150 °C was comparatively investigated. It was revealed that oxygen precipitates hardly generated in conventional heavily P-doped CZ-Si; while they remarkably generated in the nitrogen co-doped one. Moreover, nitrogen doping could enhance oxygen precipitation during the prolonged annealing with a rapid thermal process (RTP) pre-treatment, but it has neglectable influence on oxygen precipitation for short-time annealing. It was believed that nitrogen co-doped heavily P-doped CZ-Si possesses nitrogen-related complexes that act as heterogeneous nuclei for super-saturated interstitial oxygen and then enhanced oxygen precipitation. Finally, it was found that nitrogen doping could hardly enhance oxygen precipitation in heavily P-doped CZ-Si at 1200 °C.     

Equilibrium concentration of oxygen rich and deficient defect centers in germanosilicate glasses

Equilibrium concentrations of oxygen rich and deficient defect centers are calculated as a function of temperature and glass redox condition for germanosilicate glasses. We have here used the approach of Silin and Lace [J. Non-Cryst. Solids 149 (1992) 54-61] but extended it to include the case of binary system of germanosilicate glasses. A set of 23 reactions is identified as the possible pathway for formation of different defect centers. Each of these reactions are represented by forward and backward steps with Arrhenius kinetics and their activation energies are estimated based on the relevant bond energies of the involved species. Equilibrium concentrations are determined by setting rates of each of these reactions equal to zero. Typical results are presented for glasses which are: (i) stoichiometric, (ii) glasses with excess oxygen levels of 10¹⁶ to 10²⁰ cm⁻³, and (iii) glasses which are oxygen deficient by 10¹⁶ to 10¹⁸ cm⁻³.     

Nature of large-scale oscillations in oxygen concentration along the growth axis in Czochralski-grown silicon crystals

Oxygen distribution in a Si crystal (100 mm in diameter) has been studied by the absorption method in the range of the absorption band of interstitial oxygen, λ = 5.81 μm. Large-scale fluctuations (～1 cm) of the oxygen concentration (N ₀) along the growth axis were determined. Depending on the melt height, the regions of the chaotic and quasiperiodic changes were established, as well as the region of the constant N ₀ value, and their relation to turbulent, quasiperiodic, and stationary modes of melt convection in crystallization. The values of the critical Rayleigh number for the melt transition from stationary to quasiperiodic (3 × 10³) and from quasiperiodic to turbulent (1.7 × 10⁴) convection modes are determined for growth of silicon crystals by the Czochralski method. The dominating modes of N ₀ concentration oscillations at two incommensurable frequencies, f ₁ = 1.3 × 10^?3 and f ₂ = 6 × 10^?4 Hz, are assumed to be related to the oscillatory transfer of oxygen from the walls of the quartz crucible to the crystallization front and restructurization of the convective flow pattern of the melt in the course of crystal growth.     

Relative importance of hydrogen atom flux and ion bombardment to the growth of μc-Si:H thin films

An investigation of the relative importance of H atoms and ions to the transition from amorphous to microcrystalline silicon growth was performed by applying in situ plasma diagnostics and a 2D simulator of SiH₄/H₂ discharges. The growth transition was achieved by reducing the % SiH₄ in the SiH₄/H₂ discharges while keeping all the other plasma parameters constant. The distribution of the main species in the discharge space, as well as the flux of H atoms and ions per monolayer and the energy transferred by each to the growing film surface, was estimated from the simulation results. H atoms flux was found to be much higher compared to ions but the total amount of energy transferred from both H atoms and ions was found to be much lower than the activation energy required for crystallization of stable a-Si:H films with low H-content. These results support the theory that in the present conditions the formation of microcrystals proceeds via the initial growth of an unstable a-Si:H with high H content, which reduces significantly the energy barrier for crystallization.     

Defect model for the mixed mobile ion effect

This paper presents a new defect model for the mixed mobile ion effect. The essential physical concept involved is that simultaneous migration of two unlike mobile ions in mixed ionic glass is accompanied by expansion or contraction of the guest-occupied sites with distortion of surrounding glass matrix; in many cases, an intensity of the local stresses in glass matrix surrounding ionic sites occupied by foreign ions is much greater than, or at least comparable to the glass network binding energy. Hence, when the stress exceeds the breaking threshold, relaxation occurs almost immediately via the rupture of the bonds in the nearest glass matrix with generation of pairs of intrinsic structural defects. The specificity of the mechanism of defect generation leads to the clustering of negatively charged defects, so that rearranged sites act as high energy anion traps in glass matrix. This results in the immobilization of almost all minority mobile species and part of majority mobile species, so mixed mobile ion glass behaves as single mobile ion glass of much lower concentration of charge carriers. Generation of defects leads also to the depolymerization of glass network, which in turn results in the reduction of the glass viscosity and T_g as well as in the compaction of glass structure (thermometer effect). In the spectra of mechanical losses of mixed alkali glasses it reflects as a shift of the maximum in mechanical losses corresponding to the glass transition to lower temperatures, and the dramatic increase of the maximum corresponding to the movement of non-bridging oxygens (so-called mixed alkali peak). The magnitude of the mixed mobile ion effect is defined by the size mismatch of unlike mobile ions, their total and relative concentrations, the binding energy of the glass-forming network, and temperature. Although the proposed model is based upon the exploration of alkali silicate glass-forming system, the approach developed here can be easily adopted to other mixed ionic systems such as crystalline and even liquid ionic conductors.     

The properties of application of the electron probe X-ray microanalysis in the high-temperature range particularly for the study of diffusion processes and phase formations

In this study the efficiency of a manufactored heating stage for the electron probe microanalyzer is presented and a methodology (High-Temperature Electron Probe X-Ray Micro Analysis, HT-EPXMA seldom used till now for the determination of diffusion coefficients and activation energies, for the discovery, observation and characterization of phases in the heating-up process as well as for the investigation of crystallographic parameters of phase transformations is obtained.     

The effect of solvents and crystallization conditions on crystal habit of cholesterol

The effect of various organic solvents as well as the crystallization conditions on the crystal habit of anhydrous cholesterol have been studied. Both plates and needle-like crystals can be obtained in a particular solvent depending on the degree of supersaturation of the solution. However, the crystals grown at about the same supersaturation, from different solvents, show differences in their habit indicating dependence on the solvent-solute interaction. Thus, it is advisable not to identify needle-like crystals with anhydrous-cholesterol and plates with mono-hydrate cholesterol without specifying the crystallization conditions. A prediction on the crystal habit length to width ratios can be made using those parameters.     

The effects of argon gas flow rate and furnace pressure on oxygen concentration in Czochralski silicon single crystals grown in a transverse magnetic field

The effects of the argon gas flow rate and furnace pressure on the oxygen concentration in a transverse magnetic field applied Czochralski (TMCZ) silicon single crystals were examined through experimental crystal growth. A gas controller which had been proposed by Zulehner was used for this series of experiments. In the TMCZ gas-controlled crystals, a decrease in the oxygen concentration with a decrease in furnace pressure was found. A clear relationship between the oxygen concentration and the argon gas flow rate was not obtained due to the limited experimental conditions. The relationships between the oxygen concentration and the furnace pressure and the argon gas flow rate previously observed for Czochralski (CZ) crystals by a similar gas controller were confirmed by the present gas controller. The oxygen concentration changes in the TMCZ and the CZ crystals were analyzed in terms of the calculated flow velocity of the argon gas between the gas controller and the silicon melt surface. In contrast with the CZ gas-controlled crystals, the oxygen concentration was decreased with an increase in the flow velocity of argon gas in the TMCZ gas-controlled crystals. The surface temperature model and the melt flow pattern model which had been proposed in the previous report are discussed again in light of the present experimental results.