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 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 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.     

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 thermischen Verhalten von Oxoniobaten der Übergangsmetalle. IV. Zum Chemischen Transport von CoNb2O6 mit Cl2, NH4 und HgCl2. Experimente und Modellrechnungen

Contributions on the Thermal Behaviour of Oxoniobates of the Transition Metals. IV The Chemical Vapour Transport of CoNb₂O₆ with Cl₂, NH₄Cl, or HgCl₂. Experiments and Calculations Well shaped crystals of CoNb₂O₆ were obtained by CVT using Cl₂ (added as PtCl₂), NH₄Cl or HgCl₂ as transport agents (1020°C → 960°C). As a result of thermodynamic calculations the evaporation and deposition of CoNb₂O₆ in the presence of Cl₂ can be expressed by the heterogenous endothermic equilibrium (1). The endothermic reaction (2) is responsible for the CVT of CoNb₂O₆ if NH₄Cl is used as transport agent: The unfavourable site of the equilibrium (3) causes the small transport effect using HgCl₂ as transport agent. Assuming Δ_BH°₂₉₈(CoNb₂O_6,s) = ?524.7 kcal/mol a satisfying agreement between thermodynamical calculation and experimental results can be reached.     

Experimente und Modellrechnungen zum chemischen Transport von WO2 mit HgCl2 oder HgBr2

Experiments and Calculations on the Chemical Transport of WO₂ with HgCl₂ or HgBr₂ Transport experiments with WO₂ or WO₂ + W₁₈O₄₉ or W + WO₂ as starting phases show that HgCl₂ or HgBr₂ are suitable transport agents. When using HgBr₂ we observed (in customary silica ampoules) unusual high transport rates n′ > 1000 mg/h. Experimental and calculated results agree to a large extent if the presence of small amounts of H₂O from the quartz glass wall and the resulting gaseous particles (for example HCl or HBr) formed under equilibrium conditions as well as an influence of convection are taken into consideration.     

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 Chrom- und Manganmonophosphid mit Iod. Experimente und Modellrechnungen

On the Chemical Vapour Transport of Chromium and Manganese Monophosphide. Experimental Results and Thermochemical Calculations Using iodine as transport agent well shaped crystals of a volume up to V ≈︁ 50 mm³ (CrP) or an edgelength of approximately 1 ≈︁ 10 mm (MnP) can be grown. CrP has been deposited at the lower temperature of a temperature gradient (1050 → 950°C). At a density of the transport agent higher than D = 26 · 10⁻⁶ [mol I₂/cm³] CrP and CrI_2,1 coexist in the deposition region at the lower temperature. The determined composition of the condensed phases under equilibrium conditions are in accordance with thermochemical calculations assuming the heat of formation of CrP to be Δ_FH= −25.5 ± 2 [kcal/mol]. Furthermore these calculations show that the solution of CrP in the gas phase leads to CrI_2,g, Cr₂I_4,g, P_2,g and P_4,g, while I_2,g, HI_g, PI_3,g and P₂I_4,g have to be considered as transport agents. The migration of MnP (1000 → 1100°C) results from an exothermic reaction. MnP_s exists besides MnI_2,1 in the source region. Thermochemical calculations are in good agreement with the experimental results and suggest the following heterogenous equilibrium to be responsible for the observed behaviour: .     

Chemischer Transport von Cr2O3 mit Brom und mit CrBr3/Br2 — Experimente und Modellrechnungen zur Beteiligung von CrOBr2,g und CrO2Br2,g

On the Chemical Transport of Cr₂O₃ with Br₂ and CrBr₃/Br₂ — Experiments and Model Calculations for Participation of CrOBr_2,g and CrO₂Br_2,g Gaseous chromium oxybromides that were unknown up to now cause the migration of the starting material Cr₂O₃ in the temperature gradient from T₂ = 1000°C to T₁ = 900°C when Br₂ or Br₂/CrBr₃ respectively is added. Model calculations show that under the influence of H₂O (from the wall of the silica ampoule) or O₂ (from a homogenous equilibrium between H₂O/Br₂) the transport takes place via the oxybromide CrO₂Br₂ of the hexavalent chromium (eq. (1) and (2)). For thermodynamical reasons eq. (2) seems to be more favourable. At higher temperature the less oxygen containing gas species CrOBr_2,g has also to be taken into account if H₂O is excluded. An addition of CrBr₃ lowers the partial pressure of oxygen (and of H₂O as well) in the system Cr₂O₃/Br₂. Under this conditions CrOBr_2,g becomes an important species for the transport of the solid phase (eq. (4)) and CrBr_4,g has to be considered as transport agent. Estimated values of the enthalpies of formation were fixed more precisely by thermodynamic model calculation. For CrOBr_2,g (system Cr₂O₃/CrBr₃/Br₂) Δ_fH°₂₉₈ = ?70 kcal/mol and for CrO₂Br₂ (Cr₂O₃/Br₂) Δ_fH°₂₉₈ = ?107,4 kcal/mol was found. The estimated limits of error for the enthalpies of formation given for both oxybromides are smaller than ±5 kcal/mol.     

Zum Chemischen Transport von Cr2O3 mit Cl2 und mit HgCl2 ? Experimente und Modellrechungen

On the Chemical Transport of Cr₂O₃ with Cl₂ and with HgCl₂ — Experiments and Model Calculations The migration of Cr₂O₃ in a temperature gradient (1 000°C → 900°C) in the presence of low concentrations of chlorine and water (from the wall of silica ampoules) is a result from the endothermic reactions (1) Cr₂O_3,s + H₂O_g + 3 Cl_2,g = 2 CrO₂Cl_2,g + 2 HCl_g (2) Cr₂O_3,s + 1/2 O_2,g + 2 Cl_2,g = 2 CrO₂Cl_2,g With higher concentrations of chlorine, the transport reaction is (3) Cr₂O_3,s + 5/2 Cl_2,g = 3/2 CrO₂Cl_2,g + 1/2 CrCl_4,g The gas phase of the transport system Cr₂O₃/Cl₂ can be reduced step by step by adding small amounts of chromium, so that CrCl₃ and finally also CrCl₂ become more important. Further, at a lower ratio n°(Cl)/n°(Cr) three transport reactions have to be taken into consideration; with the participation of CrOCl_2,g (5). (4) Cr₂O_3,s + 9/2 CrCl_4,g = 3/2 CrO₂Cl_2,g + 5 CrCl_3,g (5) Cr₂O_3,s + 3 CrCl_4,g = 3 CrOCl_2,g + 2 CrCl_3,g (6) Cr₂O_3,s + H_2,g + 4 HCl_g = 2 CrCl_2,g + 3 H₂O_g The reactions (1), (2) and (6) become possible through the cooperation of two transport agents at a time. The migration of Cr₂O₃ with HgCl₂ can also be described with reactions (1) – (3). The decomposition of HgCl₂ Produces the small chlorine pressure for the transport reaction. The oxidation potential of the transport agent HgCl₂ is too low for the oxidation of Cr^III to Cr^VI.     

Beiträge zum thermischen Verhalten von Oxoniobaten der Übergangsmetalle. V. Zum Chemischen Transport von NiNb2O6 mit Cl2 und NH4Cl. Experimente und Modellrechnungen

Contributions on the Thermal Behaviour of Oxoniobates of the Transition Metals. V. Chemical Vapour Transport of NiNb₂O₆ with Cl₂ or NH₄Cl. Experiments and Calculations Well shaped crystals of NiNb₂O₆ were obtained by CVT using Cl₂ (added as PtCl₂) or NH₄Cl as transport agents (1020°C → 960°C). As a result of thermodynamic calculations the migration of NiNb₂O₆ in the temperature gradient in the presence of Cl₂ can be expressed by the heterogenous endothermic equilibrium (1). Assuming Δ_BH(NiNb₂O_{6, s}) = ?524.4 kcal/mol a satisfying agreement between thermodynamical calculation and experimental results can be reached. NH₄Cl is less suitable as transport agent, because Ni²⁺ is partly reduced to the metal by NH₃. The additionally H₂O produced by this reduction leads to a less favourable equilibrium position of (2) and to low deposition rates. .     

Chemischer Transport von Cr2O3 mit CrI3/I2 – Experimente und Modellrechnungen zur Beteiligung von CrOI2,g

On the Chemical Transport of Cr₂O₃ with CrI₃/I₂ – Experiments and Model-Calculations for Participation of CrOI_2,g Gaseous chromium oxyiodides that were unknown up to now cause the migration of Cr₂O₃ in the temperature gradient 1 000°C→900°C when iodine (e. g. 0.1 mmol/ml) and CrI₃ is added (eq. (1)). Transport agent for Cr₂O₃ is gaseous CrI₄. With a smaller concentration of iodine (D(I₂) ? 0.016 mmol/ml) and lower temperatures (e.g. T? = 850°C) the influence of H₂O (from the wall of the silica ampoule) becomes more important. Under these conditions the transport of Cr₂O₃ is a result from the endothermic reactions (2), (3) and (4). H_2,g has on the basis of the decomposition of HI_g a positive difference of the solubility and H_2,g should not to be considered as a transport agent. Because of the range of equilibrium-values the reaction 4 has to be taken into consideration. Estimated value of the enthalpie for CrOI_2,g is fixed more precisely by thermodynamic model calculation to Δ_fH°₂₉₈(CrOI_2,g) = ?51.4 kcal/mol. The estimated limit of error for the enthalpie of formation is smaller than ± 5 kcal/mol. Without an addition of CrI₃ is in the system Cr₂O₃/I₂ a migration of Cr₂O₃ not observable.     

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.     

Beiträge zum thermischen Verhalten von Sulfaten. XIII. Zum chemischen Transport von Cr2(SO4)3 mit Cl2 und mit HCl. Experimente und Modellrechnungen

Contributions on the Thermal Behaviour of Sulfates. XIII. The Chemical Vapour Transport of Cr₂(SO₄)₃ with Cl₂ and with HCl. Experiments and Calculations Well shaped crystals (cubical or rectangular parallel-epiped respectively, edge length up to 1 mm) of rhombohedral Cr₂(SO₄)₃ can be grown in the less hot zone of a closed silica ampoule by means of CVT using Cl₂ or HCl as transport agents in endothermal transport reactions. The influence of the mean transport temperature as well as the concentration of the transport agents on the deposition rates was investigated. On the basis of thermochemical calculations an explanation of the transport mechanism is given in the present paper.     

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.     

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.     

Beiträge zum thermischen Verhalten von Sulfaten. XII. Zum chemischen Transport von In2(SO4)3 mit Cl2 und mit HCl. Experimente und Modellrechnungen

Contributions on the Thermal Behaviour of Sulfates XII. The Chemical Vapour Transport of In₂(SO₄)₃ with Cl₂ and HCl. Experimental Results and Calculations By means of CVT (T₁ between 500°C and 825°C; ΔT = 50°C), well shaped crystals of anhydrous In₂(SO₄)₃ can be grown in the less hot region of a closed silica ampoule. We investigated the dependence of the deposition rate on the variation of the concentration of the transport agent (system In₂(SO₄)₃/Cl₂) and on the variation of the transport temperature (In₂(SO₄)₃/Cl₂ as well as In₂(SO₄)₃/HCl). A comparison of the experimental results with thermodynamical calculations shows a satisfying agreement. The influence of the variation of some additional parameters (H₂O from the wall of the ampoule; Δ_BH(In₂(SO₄)₃)) on the deposition rate is discussed.     

Experimente und Modellrechnungen zum chemischen Transport von V2O5 mit NH4Cl

Experiments and Calculations for the Chemical Transport of V₂O₅ with NH₄Cl For the chemical transport of V₂O₅ in a temperature gradient (T₂ – T₁ = ΔT = 100 K) the influence of temperature (T₂ = 770 K to 970 K) on the transport rate n′(V₂O₅) using an admixture of 2 to 8 mg NH₄Cl (2,3 to 9,2 × 10^?3 mmol/cm³) has been investigated. Also the dependence of n′ on the admixture of the transport agent has been examined from 2 to 52 mg NH₄Cl (T₂ = 850 K, T₁ = 750 K). We observed that n′ increases with increasing temperature and increasing admixture of NH₄Cl. The model calculations show the opposite tendency of the dependence on temperature; for all experiments the value of n′ was lower by a factor of 10 to 320 than the calculated one. These deviations indicate, that our knowledge on the gas phase of this system is incomplete.