Abstract—

Ln₂O₃ + HfO₂ (Ln = Nd, Dy) powders and ceramics have been studied in an oxidizing (O₂) and a mild reducing (He) atmosphere using differential scanning calorimetry (DSC), thermogravimetry, mass spectrometric analysis of released gases, X-ray diffraction, IR spectroscopy, and Raman spectroscopy. The results demonstrate that both a mechanically activated oxide mixture of appropriate composition and the powders and ceramics prepared by heat-treating the mixture contain carbon-containing compounds (basic rare-earth carbonates and hydroxycarbonates) and/or at least 0.2–0.5 wt % carbon (X-ray amorphous or crystalline). As a result, during heating in an oxidizing atmosphere all of the samples release CO₂ in the same temperature ranges (250–600 and 750–1200°C), which is accompanied by exothermic peaks in their DSC curves. The CO₂ release in the range 250–600°C is due to the onset of decomposition of the basic rare-earth carbonates and hydroxycarbonates, which are present in small amounts in the starting mixture, powders, and ceramics. The CO₂ release in the range 750–1200°C is due to the burnout of strongly bonded carbon and thermally stable carbon-containing compounds (rare-earth dioxymonocarbonates, Ln₂O₂CO₃). The exothermic peaks in the DSC curve are due to fluorite LnHfO_{4 – δ} (Ln = Nd, Dy) crystallization processes. We believe that synthesis in air, involving the formation of X-ray amorphous (fine-particle and nanocrystalline) precursors containing rare-earth oxides, which tend to form basic rare-earth carbonates and hydroxycarbonates in air, will always yield high-temperature ceramics containing carbon compounds and at least 0.5 wt % X-ray amorphous carbon and/or graphite. The amount of carbon and carbon-containing compounds in the dysprosium-containing ceramics is markedly smaller (~0.2%) than that in the neodymium-containing ceramics. The crystallization of the rare-earth hafnates is a rather slow process that can begin at temperatures as low as 550°C. The formation of Nd₂Hf₂O₇ with the pyrochlore structure involves fluorite NdHfO_{4 – δ} formation as an intermediate step, and a single-phase product can only be obtained by high-temperature firing at ~1600°C. Phase-pure DyHfO_{4 – δ} with the fluorite structure can be obtained by firing at 1200°C.

中文翻译：

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机械活化的Ln 2 O 3 + 2HfO 2（Ln = Nd，Dy）混合物制备的陶瓷中含碳杂质的相关系和行为

摘要—

Ln ₂ O ₃ + HfO ₂（Ln = Nd，Dy）粉末和陶瓷已在氧化（O ₂）和轻度还原（He）气氛中使用差示扫描量热法（DSC），热重分析，释放的质谱分析进行了研究。气体，X射线衍射，IR光谱和拉曼光谱。结果表明，适当成分的机械活化氧化物混合物以及通过热处理该混合物而制得的粉末和陶瓷均包含含碳化合物（碱性稀土碳酸盐和羟基碳酸盐）和/或至少0.2-0.5 wt％的碳（X射线无定形或晶体）。结果，在氧化气氛中加热期间，所有样品均释放CO _2。在相同温度范围（250–600和750–1200°C）下，其DSC曲线伴随着放热峰。在250-600°C范围内释放CO _2的原因是碱性稀土碳酸盐和羟基碳酸盐开始分解，而这些分解物中少量存在于起始混合物，粉末和陶瓷中。在750–1200°C范围内释放CO ₂是由于强烈键合的碳和热稳定的含碳化合物（稀土二氧一碳酸酯，Ln ₂ O ₂ CO ₃）的烧尽。DSC曲线中的放热峰是由于萤石LnHfO _{4 –δ}（Ln = Nd，Dy）结晶过程。我们认为，在空气中进行合成，包括形成含有稀土氧化物的X射线无定形（微粒和纳米晶体）前体，往往会在空气中形成碱性稀土碳酸盐和羟基碳酸盐，将始终产生高温陶瓷。包含碳化合物和至少0.5 wt％的X射线无定形碳和/或石墨。含的陶瓷中的碳和含碳化合物的量明显小于含钕的陶瓷中的碳和含碳化合物的量（〜0.2％）。稀土盐酸盐的结晶过程相当缓慢，可以从低至550°C的温度开始。Nd ₂ Hf ₂ O ₇的形成具有烧绿石结构的萤石是作为中间步骤的萤石NdHfO _4-δ的形成，仅在约1600°C的高温下燃烧才能获得单相产物。具有萤石结构的纯相DyHfO _4-δ可以通过在1200°C焙烧获得。