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. VI. Temperaturabhängigkeit mit dem Transportmittel Chlor

New Observations on the Chemical Transport of GeO₂. VI. Temperature Dependence with the Transport Agent Chlorine In a temperature gradient T₂ ? T₁ = 100 K the chemical transport of tetragonal GeO₂ is kinetically controlled at an average transport temperature T ≤ 1080 K. The same is true for the metastable hexagonal modification at T ≤ 1150 K. An Arrhenius equation describes the rate of deposition in a satisfactory way. The activation energy amounts to 21 kcal/Mol. At higher temperatures diffusion determines the rate of transport whereby GeCl₄, Cl₂, and O₂ are looked upon as prominent gaseous molecules and the formation of a solid solution of hexagonal GeO₂ with SiO₂ is taken into account. For a transport temperature T < 1100 K GeO₂ is deposited at T₁ only if seeds of the specific modification are present. The deposition of GeO₂(hex.) ceases at temperatures lower than T ≈ 1000 K. The formation of GeO₂(tetr.) requires not only seeds but also NaCl as mineralizer and the temperature should not be lower than T ≈ 900 K.     

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.     

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.     

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.     

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).     

Die Thermodynamik des Chemischen Transports von SnS2 mit I2

Experimente zum chemischen Transport von SnS₂ mit I₂ im Temperaturgefälle zeigten, daß der Transport unterhalb 900 K zur heißeren Zone hin erfolgt (Prozeß A), und daß er bei höheren Temperaturen zur Abscheidung von SnS₂ in der weniger heißen Zone führt (Prozeß B). Die eingehende thermodynamische Rechnung ergab, daß Prozeß A durch die exotherme Reaktion beherrscht wird, und daß im Prozeß B die endotherme Reaktion vorherrschend ist. Thermodynamics of the Chemical Transport of SnS₂ by I₂ The chemical transport of SnS₂ by I₂ in a temperature gradient has been investigated. Below 900 KSnS₂ is transported into the high temperature region (A) and at higher temperatures into the low temperature region (B). The thermodynamic discussion shows that A is governed by the exothermic reaction and B by the endothermic reaction .     

Zum chemischen Transport von Wolfram mit HgBr2 – Experimente und Modellrechnungen

On the Chemical Transport of Tungsten using HgBr₂ – Experiments and Thermochemical Calculations Using HgBr₂ as transport agent tungsten migrates in a temperature gradient from the region of higher temperature to the lower temperature (e.g. 1 000 → 900°C). The transport rates were measured for various transport agent concentrations (0.64 ? C(HgBr₂) ? 11.74 mg/cm³; T? = 950°C) and for various mean transport temperatures (800 ? T? ? 1 040°C). Under these conditions tungsten crystals were observed in the sink region. To observe the influence of tungsten dioxide (contamination of the tungsten powder) on the transport behaviour of tungsten, experiments with W/WO₂ as starting materials were performed. According to model calculations the following endothermic reactions are important for the migration of tungsten: In the presence of H₂O or WO₂ other equilibria play a role, too. Using a special “transport balance” we observed a delay of deposition of tungsten (e.g. T? = 800°C; 15 h delay of deposition) with W and W/WO₂ as starting materials. The heterogeneous and homogeneous equilibria will be discussed and an explanation for the non equilibrium transport behaviour of tungsten will be given.     

Zum chemischen Transport der Wolframoxide WO2 und W18O49 mit HgI2. Experimente und Modellrechnungen

On the Chemical Transport of Tungsten Oxides WO₂ and W₁₈O₄₉ with Hgl₂. Experiments and Calculations Transport experiments with WO₂ or W + WO₂ or WO₂ + W₁₈O₄₉ show that HgI₂ is a transport agent as suitable as I₂. We observed transport rates up to 47 mg/h. We investigated the dependence of the transport rate on the concentration of the transport agent n°(HgI₂) as well as on the temperature. We also investigated the time dependence of the transport rates during transport experiments on a “transport balance”. Starting with WO₂ + W₁₈O₄₉, WO₂ is transported before W₁₈O₄₉. Thermodynamic calculations show that transport of W₁₈O₄₉ is understandable if the presence of small amounts of H₂O from the quartz glass wall are taken into consideration, while transport of WO₂ is possible with HgI₂ in the presence of H₂O as well as in absence of H₂O. is the most important reaction for the transport of WO₂.     

Zum chemischen Transport von CrOCl und Cr2O3 — Experimente und Modellrechnungen zur Beteiligung von CrOCl2,g

On the Chemical Transport of CrOCl and Cr₂O₃ - Experiments and Model Calculations for Participation of CrOCl_2,g . The migration of CrOCl in a temperature gradient (600°C→500°C) in the presence of chlorine is a result from an endothermic reaction . Above T₂ = 900°C several reactions are super imposed and Cr₂O₃, the product of the decomposition of CrOCl, migrates following the endothermic reaction . By continously monitoring the mass changes during the complete duration of the experiment the consecutive stationary deposition reactions could be registered separately and nonstationary changes in the gasphase could be recognized. The observed decomposition of solid CrOCl into Cr₂O_3,s as well as CrCl_3,g under equilibrium conditions is in accordance with thermochemical calculations assuming the heat of formation of CrOCl to be Δ_BH = - 135.3 ± 2 [kcal/mol]. Using this value the chemical transport of CrOCl with Cl₂, HCl, and HgCl₂ can be described.     

Beiträge zum chemischen Transport oxidischer Metallverbindungen. III. Der Transport von NiO und MgO über ihre Metallchloride

Transport effect of HCl on NiO and MgO according to between T₂ = 1000°C and T₁ = 800°C was calculated by the model of diffusion in dependence of total pressure; for comparison, the classical transport of α-Fe₂O₃ was analogously treated. By experimental determination of the transport rates at total pressures from 0.009 to 6 atm hitherto not considered influences of the amount and surface of the starting material, and of the transport time were found. These effects are explained by a (not in detail defined) term of ?sorption”? of the transport gas onto the powder of the starting material. For an explanation of the transport rates estimations of the diffusion coefficients of the gas pairs FeCl₃–HCl and NiCl₂–HCl were performed and the vapour pressure diagrams of NiCl₂ and MgCl₂ evaluated.     

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.     

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.     

Beiträge zum thermischen Verhalten von Oxoniobaten der Übergangsmetalle. II [1, 2]. Zum Chemischen Transport von MnNb2O6 mit Cl2 und NH4Cl. Experimente und Modellrechnungen

Contributions on the Thermal Behaviour of Oxoniobates of the Transition Metals. II. The Chemical Vapour Transport of MnNb₂O₆ with Cl₂ or NH₄Cl. Experiments and Calculations Crystals of MnNb₂O₆ were obtained by chemical transport reactions in a temperature gradient (1020°C → 960 °C) using Cl₂ (added as PtCl₂) or NH₄Cl as transport agent. As a result of thermodynamic calculations the evaporation and deposition of MnNb₂O₆ in the presence of Cl₂ can be expressed by the endothermic equilibrium (1). The endothermic reaction (2) is responsible for the migration of MnNb₂O₆ if NH₄Cl is used as transport agent. Assuming ΔH°₂₉₈(MnNb₂O_{6, s}) = ?567.6 kcal/mol a satisfying agreement between thermodynamic calculations and experimental results can be reached.     

Zum chemischen Transport von SiAs mit Iod — Experimente und Modellrechnungen

On the Chemical Transport of SiAs using Iodine — Experiments and Thermochemical Calculations Using iodine as transport agent siliconarsenide migrates in a temperature gradient. The direction of the migration depends on the chosen temperature and the concentration of the transport agent. The transport rates were measured for various transport agent concentrations (0.0002 ? C(I₂) ≥ 0,02 mmol/cm³) and for various mean transport temperatures (650 ? T? ? 1 000°C). For low temperatures (e.g. T₁ = 750°C→T₂ = 850°C), low iodine concentrations (e.g. C(I₂) = 0.001 mmol/cm³) and in the presence of H₂O (from wall of silica ampoule) the following exothermic reaction is responsible for the deposition of SiAs-crystals in the sink region:

• SiAs_s + 4HI_g = SiI_4,g + 2H_2,g + 1/4As_4,g

In case of higher temperatures (e.g. T₂ = 1 050°C→T₁ = 950°C) and higher iodine concentrations (e.g. C(I₂) = 0.02 mmol/cm³) SiI_4,g is the transport agent. According to model calculations the following endothermic reaction is responsible for the migration of SiAs to the region of the lower temperature:

• SiAs_s + SiI_4,g = 2SiI_2,g + 1/4As_4,g

The heterogeneous and homogenous equilibria will be discussed and an explanation of the non equilibrium transport behaviour of SiAs is given. Thermochemical data of SiAs are characterized by the quartzmembrane zero manometer technique and further verified by model calculations.     

Chemischer Transport fester Lösungen. 10 [1] Zum Chemischen Transport von quaternären Cobalt(II)‐Zink‐ und Mangan(II)‐Zinkgermanaten

Chemical Vapor Transport of Solid Solutions 10 [1] The Chemical Vapor Transport of quarternary Cobalt(II)‐Zinc and Manganese(II)‐Zinc Germanates By means of chemical vapor transport methods using HCl or Cl₂ as transport agent the crystalline solid solutions (Zn_xCo_1—x)₂GeO₄ and (Mn_xZn_1—x)₂GeO₄ have been prepared (1050 → 900 °C, 850 → 700 °C, respectively). ZnGeO₃ — although unknown as a pure solid — can be stabilized as a mixed crystal (Mn_xZn_1—x)GeO₃ (x > 0, 5).     

Zum chemischen Transport im System Mn?O unter Berücksichtigng des Sauerstoffkoexistenzdruckes (I)

Chemical Transport in the System Mn? O in Consideration of the Oxygen Coexistence Pressure (I) The chemical transport of the coexistent phases Mn₂O₃? Mn₃O₄ and Mn₃O₄? MnO with Cl₂, Br₂, I₂, HCl, HBr, and HI was analysed thermodynamically and experimentally. The mentioned transport agents are able to transport the following phases:

1 Index (o) bedeutet obere, (u) untere Phasengrenze (index (o) – upper phase boundary, (u) – lower phase boundary).

.     

Chemischer Transport und Sublimationsverhalten von CrBr3 — Experimente und Modellrechnungen

On the Chemical Transport and Sublimation of CrBr₃ — Experiments and Model Calculations The migration of CrBr₃ in the presence of high concentrations of bromine (for example D(Br₂) = 0,05 mmol/ml; closed silica ampoules) in the investigated temperature range (T? = 625°C to 875°C; T? = 50°C) is a result from the endothermic reaction The chemical transport of CrBr₃ is superimposed with the sublimation. With low concentrations of D(Br₂) and high temperatures T? is the sublimation decisive participated. This is a result of the homogenous equilibrium between CrBr_3,g and CrBr_4,g (2a) The reaction (2a) in comparison with the chemical transport of CrCl₃ with Cl₂ (Gl. (2b)) is more shifted to CrBr_3,g.     

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.