Thermal analysis and crystal growth of doped Nb2O5

Abstract

The systems Nb 2 O 5 -Ta 2 O 5 and Nb 2 O 5 -V 2 O 5 were investigated using thermal analysis, X-ray powder diffraction and thermodynamic simulations. Solid solution formation is possible for both systems; furthermore, both contain one intermediate compound, VNb 9 O 25 or Ta 2 Nb 4 O 15 , respectively. Phase relationships for pure niobium(V)-oxide and tantalum(V)-oxide were studied under ambient pressure. It was found that both compounds can occur in two stable solid modifications. For niobium(V)-oxide this are the monoclinic high-temperature modification (H-Nb 2 O 5 ) and an orthorhombic low-temperature modification (T-Nb 2 O 5 ) and for tantalum(V)-oxide a tetragonal high-temperature form (α-Ta 2 O 5 ) and an orthorhombic low-temperature form (β-Ta 2 O 5 ). Based on these results, crystal growth experiments with various compositions from both systems were carried out using the optical floating zone (OZF) technique.

Phase equilibria in the ZrO2–Ta2O5–Nb2O5 system: Experimental studies and thermodynamic modeling

“…Besides, the Nb 4 Ta 2 O 15 compound is calculated to form at 1840 K and decompose at 1703 K, which is consistent with the available experimental results as shown in Figure 8. A peritectic reaction of Liq + Nb 4 Ta 2 O 15 → Nb 2 O 5 _ss was calculated to be existent at 1803 K, corresponding to the DTA results of Hidde et al 28 The calculated invariant temperatures and compositions of this system are summarized in Table 4.…”

“…The peritectic reaction of Liquid+Nb 4 Ta 2 O 15 →Nb 2 O 5 _ss and peritectoid reaction of αTa 2 O 5 _ss +Nb 4 Ta 2 O 15 →βTa 2 O 5 _ss were proposed in their phase diagram. Recently, Hidde et al 28 determined the phase transformation of the Nb 2 O 5 –Ta 2 O 5 system by thermal analysis, and a calculated phase diagram was presented. In general, their experimental data at the high temperature are consistent with the results released by Holtzberg and Reisman, 27 but exhibited some contradictions at the low temperature.…”

“…The solidus from 0 to 20 mol% Ta 2 O 5 was determined using thermal analysis in their work. Additionally, a compound with the formula of 2Nb 28 determined the phase transformation of the Nb 2 O 5 -Ta 2 O 5 system by thermal analysis, and a calculated phase diagram was presented. In general, their experimental data at the high temperature are consistent with the results released by Holtzberg and Reisman, 27 but exhibited some contradictions at the low temperature.…”

Consolidation and high‐temperature properties of ceramics in the TaC–NbC system

“…Furthermore, one of the main features of the TaC–NbC system is that the majority of oxides should melt below 2000°C 51–53 . Also, the Ta 2 O 5 ‐rich section of the pseudo‐binary Ta 2 O 5 –Nb 2 O 5 would be a liquid with Ta 2 O 5 above 1600°C.…”

“…Similar steps may be implemented if the secondary phases are undesirable for creep performance. 12 Furthermore, one of the main features of the TaC-NbC system is that the majority of oxides should melt below 2000 • C. [51][52][53] Also, the Ta 2 O 5 -rich section of the pseudo-binary Ta 2 O 5 -Nb 2 O 5 would be a liquid with Ta 2 O 5 above 1600 • C. This should facilitate the reaction between the free-carbon with an oxide at elevated temperatures. As a rule, the majority of oxides would have up to 2-3 wt% of free carbon.…”