Neue Beobachtungen zum chemischen Transport von GeO2 IV. Temperaturabhängigkeit mit dem Transportmittel Wasserstoff

Novel Observations on the Chemical Transport of GeO₂. IV. Temperature Dependence with the Transport Agent Hydrogen The chemical transport of GeO₂ with H₂ proceeds on the basis of reaction (1) In the case of a filling pressure of 1 atm H₂ a micro crystalline coating of GeO₂(hex.) is obtained at T₁, using a temperature gradient T₂ ? T₁ = 100 K. In addition acicular, colourless crystals are growing. The shape depends on the mean transport temperatures T?. Besides GeO₂ a small amount of Ge is obtained at temperatures T? < 1023 K in the colder region of the ampoules. This additional Ge-Transport is not to be expected under equilibrium conditions. Model calculations show, that it is due to a kinetic inhibition of the deposition of GeO₂. In a wide range of temperature the experimentally determined rates of transport are in accordance with the expected values.     

Neue Beobachtungen zum chemischen Transport von GeO2 II. Transportmittel Wasserstoff

Novel Observations on the Chemical Transport of GeO₂. II. Transport Agent Hydrogen The endothermic reaction (1) (2) is the basis of the deposition of GeO₂ in the temperature gradient T₂ ? T₁ = 100 K at the low temperature T₁ = 1023 K. The quartz modification of GeO₂ is obtained as micro crystalline coating and acicular, colourless crystals adherent to it. The gaseous molecules H₂, H₂O and (GeO)_n (with n = 1, 2, 3) participate in the chemical transport. The chemical transport depends on the concentration of H₂. At low H₂-concentration GeO₂ is transported, with growing H₂-pressure at the beginning of the experiments Ge and GeO₂ are deposited simultaneously. A further increase of the H₂-pressure leads again to a chemical transport of GeO₂, followed by a range of Ge-deposition. After reaching a steady state in each experiment only one phase is transported. The sequence of deposition can be explained by model calculations. A comparison of experimentally determined rates of transport with calculated values shows that under the present conditions at a total pressure of ∑P = 1 atm (298 K) no kinetic inhibition of the phase transfer reactions exists.     

Neue Beobachtungen zum chemischen Transport von GeO2. V. Zur Mineralisatorwirkung von Na-Verbindungen auf den Modifikationswechsel von GeO2

New Observations on the Chemical Transport of GeO₂. V. Minerlizer Effect of Sodium Compounds on the Transformation of GeO₂ Modifications The formation of GeO₂(tetr.) which is kinetically hindered is catalyzed by all Na-containing substances which react with GeO₂ to Na₄Ge₉O₂₀. Since Na₄Ge₉O₂₀ doesn't migrate over the gas phase in a closed system the effect of mineralization is limited to the region where the mineralisator is placed. In the presence of chlorine the formation of GeO₂(tetr.) is catalyzed by NaCl if there are also seed crystals of this modification. Cl₂ reacts with Na-Germanates and Na-Silicates to NaCl. In this case the effect of mineralization is not limited to the region where the mineralizator was initially placed. The temperature of 1306 K at which the modification of GeO₂(tetr.) changes into GeO₂(hex.) can be shifted to lower values by small amounts of SiO₂ built into the hexagonal modification. The substitution of SiO₂ for GeO₂ is catalyzed either in the presence of NaCl by a gas phase containing halogen or by a Na₂O/GeO₂/SiO₂-melt.     

Zum Verständnis der Flüchtigkeit von GeO2 in Gegenwart von WO2

On the Understanding of the Volatility of GeO₂ in the Presence of WO₂ The equilibria composition of the gaseous and the solid phase in the system GeO₂/WO₂ is calculated with an improved thermodynamical program for temperatures 1100 < T < 1400 K and constant volume. By means of the results the experimental observed migration of GeO₂ in the presence of WO₂ in a temperature gradient T₂ → T₁ (1200 → 1100 K) in sealed evacuated silica tubes is dued to a chemical transport with H₂ as the transporting agent. The H₂ is formed by H₂O which is desorbed by the quartz glass of the ampoules. The also observed volatility of WO₂ and its deposition in form of Ge_0.75 W₃O₉ at the “cold” end (T₁) of the tubes is performed by gaseous GeWO₄. The calculated and experimental transport rates are compared and discussed.     

Neue Beobachtungen zum chemischen Transport von GeO2 I. Transportmittel Chlor

New Observations in the Chemical Transport of GeO₂. I. Transporting Agent Chlorine By using Chemical transport reactions with Cl₂ as transporting agent, the rutile modification of GeO₂ is obtainable with deposition temperature below 900°C. The shape of the crystals is columnar, and they show a colour which varies from yellow to amber. A comparison of calculated and observed rates of transport shows that an inhibition of the reaction exists. This inhibition is partially reduced if alkaline chlorides are added. In the presence of Mn(II) the calculated rates of transport are observed. In the presence of NaCl and KCl the crystals of GeO₂ (rutile) are needle shaped and colourless, in presence of Mn(II) the needles are brown.     

Der chemische Transport von Platin mit Chlor. Experimentell bearbeitet mit Marita Trenkel

The Chemical Transport of Platinum with Chlorine Experiments show that the chemical transport of platinum by means of chlorine within a temperature gradient at temperatures below ≈ 1000°K goes into the hot temperature region, but at higher temperatures in the reverse direction. From the thermodynamic discussion it can be seen, that the platinum content of the gas phase at low temperatures is governed by the exothermic formation of Pt₆Cl_12,g, and at higher temperatures by the endothermic formation of PtCl_3,g and PtCl_2,g. The platinum content of the gas phase passes a minimum at ≈ 1000°K, if P(Cl₂) = 3.5 atm. This result is in agreement with the observed inversion of the transport direction.     

Beiträge zum thermischen Verhalten von Sulfaten. XVI. Zum chemischen Transport von Ga2(SO4)3 mit Cl2 und mit HCl. Experimente und Modellrechnungen

Contributions on the Thermal Behaviour of Sulphates. XVI. The Chemical Vapour Transport of Ga₂(SO₄)₃ with Cl₂ and HCl. Experimental Results and Calculations Crystals of anhydrous Ga₂(SO₄)₃ can be grown by means of CVT (e. g. 525°C → 475°C) in the less hot region of a closed silica ampoule. We investigated the dependance of the deposition rate on the concentration of the transport agent (Cl₂, HCl) and the transport temperature (475°C ≤ T ≤ 750°C; T₂ > T₁; ΔT = 50°C; T = 0.5(T₁ + T₂)). Experimental results and thermodynamic calculations on the basis of Δ_FH ${}_{\text{298}}^{\text{º}}$ (Ga₂(SO₄)₃) = ?686.5 kcal/mol show a good agreement.     

Chemischer Transport fester Lösungen. 8 [1] Zum Chemischen Transport von ternären und quaternären Cobalt(II)‐ und Nickel(II)‐germanaten

Chemical Vapor Transport of Solid Solutions. 8 The Chemical Vapor Transport of Ternary and Quarternary Cobalt and Nickel Germanates By means of chemical vapor transport methods using HCl as transport agent CoGeO₃, Co₂GeO₄, and Ni₂GeO₄ have been prepared (1000 → 900 °C and 900 → 700 °C). In this system the formation of a continuous crystalline solid solution of Co₂GeO₄ and Ni₂GeO₄ was found as well as the deposition of the compound NiGeO₃ which — although unknown as a pure solid — can be stabilized as a mixed crystal Ni_xCo_1—xGeO₃ (0 < x < 0, 6).     

Wanderung von SiAs im Temperaturgradienten ohne Zusatz eines Transportmittels – Experimente und Modellrechnungen

On the Migration of SiAs without using a Transport Agent – Experiments and Thermochemical Calculations SiAs migrates in a temperature gradient (T = 0.5 · (T₁ + T₂) = 850 to 1000°C) without adding a transport agent, into the cooler part of the silica ampoule. The migration rate depends on the temperature and the partial pressure of elemental arsenic in the silica tube. The migration rates were measured for various arsenic concentrations (0 ≤ n(As) ≤ 4 mmol/20 cm³) and for various mean transport temperatures (850 ≤ T le; 1000°C). In case of increasing the temperature the migration rate rises (e.g. T = 850°C, ?(exp.) = 0.006 mg/h; T = 1000°C, ?(exp.) = 0.044 mg/h). Adding arsenic (e.g. n(As) = 0.11 mmol, ?(exp.) = 0.067 mg/h; n(As) = 4.0 mmol, ?(exp.) = 0.82 mg/h), gives also the result of an increasing migration rate. Augmenting the pressure by adding argon as inert gas has only a small effect to the migration rate of SiAs. To explain the mechanism of the migration by using model calculations, the thermochemical data of the gaseous species SiAs_g and SiAs_{3, g} have to be estimated. According to model calculations an endothermic reaction like the following one is responsible for the migration of SiAs the region of the lower temperature: SiAs_s + 2 As_{n, g} = SiAs_{3, g} (1 ≤ n ≥ 4).     

Neue Beobachtungen zum chemischen Transport von GeO2. III. Transportraten mit dem Transportmittel Wasserstoff

Novel Observations on the Chemical Transport of GeO₂. III. Rates of Transport with Hydrogen In the course of a chemical transport in the system GeO₂/H₂ the composition of the solid phases can change until a steady state is reached. In this case an iterative calculation of the equilibrium is now possible. The method takes into acount the mutual interaction of the processes in the source and in the zone of deposition and allows a general application. The participation of H₂ implies a large difference in the coefficients of diffusion which demands the estimation of the interaction of diffusion and laminar flow. The comparision of experimentally determined rates of transport with calculated values shows now that the increasing laminar flow is antagonistic to the deposition of GeO₂.     

Die relative Unterkühlung an Quarzglasoberflächen und anderen Substraten beim chemischen Transport fester Stoffe über die Gasphase

The Relative Undercooling on Silica Glass Surfaces and other Substrates during the Chemical Transport of Solids via the Vapor-phase . The relative undercooling on etched surfaces of silica glass at the chemical transport of ZnS in a stream of gaseous iodine at 652–854°C is found to be ΔT = 37–43°. Scrapers on silica glass lead to a substantial smaller undercooling. During the deposition of ZnS (made from “pure” components) in a temperature gradient one finds a remarkable fractionation. In closed (etched) silica glass ampoules the relative undercooling is determined for the systems ZnS/I₂, ZnSe/I₂, ZnS/HCl; ZnS/H₂, CdS/I₂, CdSe/I₂, Al₂S₃/I₂ and Nb₂O₅/NbCl₅ using a special furnace, The region free of nuclei (or crystals) for instance at T₂ ≈ 800°C depending on the system (and T₂) is ΔT ≈ 13–45°. The variation of the substrates showed: for fire polished silica surfaces for ZnS/I₂ is ΔT ≈ 56°; for the different quartz-faces and ZnS/I₂ is ΔT ≈ 13–35°. Generally, on different substrates (broken pieces, fragments) one finds for ZnS/I₂ ΔT ≈ 20–28°. Using another way for the systems MoO₃/Cl₂, MoO₃/Cl₂, Ar, MoO₃/Cl₂, O₂ and MoO₃/HgCl₂ with T₂ a strongly decreasing value of ΔT is found.     

Zum chemischen Transport von Molybdän mit SbBr3 – Experimente und Modellrechnungen

On the Chemical Transport of Molybdenum using SbBr₃ – Experiments and Thermochemical Calculations Mo migrates in a temperature gradient from the region of higher temperature to the lower temperature using SbBr₃ as transport agent. For various mean transport temperatures (750 ? T ? 1000°C; T = 0,5 (T₁ + T₂); T₂ ? T₁ = 100°C) we observed small transport rates (? ? 0,6 mg/h) which rise up to 16 mg/h for higher transport agent concentrations. Small amounts of MoO₂ and Sb were detected beside Mo in the sink. The observed solid phases in the sink are in agreement with thermodynamical calculations by CVTrans which also demonstrate that the formation of MoO₂ and Sb as well as the transport effect of SbBr₃ are caused by traces of H₂O from the quartz glass wall. The sequence of deposition of Mo, MoO₂ and Sb in the examined temperature range can be calculated (CVTrans) and measured with the transport balance.     

Zum chemischen Transport von Zinkoxid und zur Bestimmung seiner Phasenbreite

Chemical Transport of Zinc Oxide and the Estimation of its Phase Width The chemical transport behaviour of zinc oxide using different transport agents and the influence of the transport conditions (T, ΔT, p) on transport rate and deposition form was studied. The range of homogeneity of ZnO is very narrow. The best agents for deposition of ZnO with the upper phase boundary are Cl₂ and Br₂, for the deposition of the lower phase boundary HBr and NH₄Br are more suitable. The deviation from stoichiometry depends on the deposition temperature and amounts to 10 till 250 ppm in the range of 1073 up to 1373 K.     

Chemischer Transport fester Lösungen. 9 [1] Zum Chemischen Transport fester Lösungen in den Systemen Eisen(II)/Cobalt(II)‐ und Mangan(II)/Cobalt(II)‐germanat

Metagerma‐Chemical Vapor Transport of Solid Solutions. 9. The Chemical Vapor Transport of Solid Solutions in the System Iron(II)/(Cobalt(II)‐and Manganese(II)/Cobalt(II) Germanate By means of chemical vapor transport methods (900 → 700 °C) using HCl as transport agent FeGeO₃, Fe₂GeO₄ and MnGeO₃ have been prepared. Co₂GeO₄ and Fe₂GeO₄ as well as CoGeO₃ and FeGeO₃ form continuous crystalline solid solutions, whereas in MnO/CoO/GeO₂ two different phases (Mn_xCo_1‐x)GeO₃ are formed. All of these systems show congruent transport behaviour. Chemical vapor transport has been proved a suitable method to prepare solid solutions.     

Darstellung,Struktur und thermisches Verhalten einer neuen hexagonalen Ge-W-Bronze

Preparation, Structure, and Thermal Behaviour of a New Ge? W Bronce A hexagonal Ge-tungsten bronce with the composition Ge_0.24 WO₃ is obtained by chemical vapour deposition of a mixture of GeO₂ and WO₂(temperature gradient: 930 to 830°C). By means of an X-ray crystal structure analysis the space group was determined to be P622 and the lattice constants a = 744.0 pm, c = 381.7 pm. In absence of oxygen Ge_0.24 WO₃ is stable up to 850°C; in contact with air it is oxydized at temperatures T > 670°C. An anomaly in the thermal lattice expansion at temperatures T > 260°C is discussed.     

Der chemische Transport von Nickel mit Indiumjodid

Chemical Transport of Nickel by Indium Iodide At higher temperatures (1273 → 1073 K) the chemical transport of nickel by means of indium iodide going into the zone with lower temperature is caused by the endothermic reaction Ni + InJ₃O,_g = NiJ₂,_g + InJ_,g At lower temperatures (873 → 973 K) this reaction is superimposed by the formation of gas complexes. These exothermic reactions cause transport in the inversed direction.     

Zum chemischen Transport von Molybdän mit HgBr2 — Experimente und Modellrechnungen

On the Chemical Transport of Molybdenum using HgBr₂ ? Experiments and Thermochemical Calculations . Mo migrates under the influence of HgBr₂ in a temperature gradient (e.g. 1 000→900°C). Besides elementary Mo we observed in some experiments the occurence of MoBr₂ and MoO₂ (from oxygen containing impurities) respectively. The transport behaviour (deposition sequence; deposition rates of various phases) has been enlightened by continous measurement of the mass change during the transport experiments using a special “transport balance”. Thus obtained deposition rates m(Mo) for molybdenum reached in the temperature region 800 ≤ T ≤ 1 040°C a maximum at T = 980°C independend from the starting material (Mo or Mo/MoO₂ mixtures). For variable densities D of transport agent at a constant temperature (T = 950°C) increasing values for m(Mo) were observed (m(Mo) = 23 mg/h, D_max = 8.61 mg HgBr₂/cm³). Thermochemical calculation give strong evidence for the migration of Mo via the endothermal reaction . The experimental deposition rates are about half as large than the calculated values. Good agreement between calculations and experiments were obtained only assuming the presense of oxygen in the starting materials.     

Laser-Assisted Chemical Vapor Deposition of Boron

Ar⁺ laser-induced chemical vapor deposition (LCVD) of B from gaseous mixtures of BCl₃ and H₂ has been investigated for temperatures between 1000 K and 2250 K and for partial pressure within the ranges 25 mbar ≤ p(BCI₃) ≤ 800 mbar and 10 mbar ≤p(H₂) ≤ 400 mbar. For the lowest temperatures, deposition is controlled by the chemical kinetics which, with p(BCI₃) = p(H₂) = 100 mbar, is characterized by an apparent chemical activation energy of about 26.5 kcal/mol. In this regime, the deposited boron is amorphous. At high temperatures, deposition becomes limited by gas-phase transport and polycrystalline boron with βrhombohedral structure is obtained.     

Untersuchungen zur Kristallisation von Rhodium(III)-Oxoverbindungen unter Beteiligung der Gasphase – Der chemische Transport von Rh2O3 mit Chlor

Investigations on the Crystallization of Rhodium(III) Oxo Compounds – Chemical Vapour Transport of Rh₂O₃ using Chlorine Rh₂O_3,s migrates in chemical transport experiments with chlorine as transport agent from the higher (T₂) to the lower (T₁) temperature of a gradient (Δ_T = 100°) due to endothermal reactions (900°C < T ≤ 1050°C; T = 0,5 · (T₂ + T₁)). Under the conditions of transport experiments RhCl_3,s is observed in most experiments as equilibrium solid besides the sesquioxide. The transport rates for Rh₂O_3,s and the sublimation rates for RhCl_3,s grow with increasing temperature T . The composition of the equilibrium solids, the rates of migration and the sequence of deposition (1. RhCl_3,s, 2. Rh₂O_3,s) is well reproduced by thermodynamic model calculations. As a result of this calculations the transport behaviour of the system Rh₂O_3,s/Cl₂ is determined by the two equilibria The influence of RhCl_2,g and RhCl_4,g on the transport behaviour of Rh₂O_3,s as well as the possible occurence of RhOCl_2,g in the equilibrium gas phase will be discussed. Predictions of the transport behaviour of ternary rhodium(III) oxo compounds will be made.