Improved quantum efficiency of Sr(Al,Si)5(O,N)7:Ce3+ by selection of Ce raw material

The luminescence properties of yellow-emitting Ce³⁺-doped Sr-containing sialon phosphor Sr(Al,Si)₅(O,N)₇:Ce³⁺ were notably improved by the Ce raw material selection. By changing the Ce raw material from oxides to nitrides or chlorides, the emission wavelength shifted to above 560 nm, which is beneficial for higher color rendering index white light-emitting diodes. This result from an increase in the covalency of the host crystal being associated with a decrease in the oxygen content. When Ce chloride was used, both the absorption and internal quantum efficiency increased, resulting in an increase in the external quantum efficiency up to 65%–72%. Inductively coupled plasma mass spectrometry, X-ray diffraction, and electron spin resonance measurements showed that the reason for the absorption increase is an increase in Ce³⁺ content and suppression of the generation of the second phase, and the reason for the increase in the internal quantum efficiency is a decrease in the host crystal absorption via suppression of anion vacancy generation. It was found that Ce chloride not only suppresses oxygen impurities but also acts as a flux that results in improved crystallinity.     

Preparation and Luminescence Spectra of Calcium- and Rare-Earth (R = Eu, Tb, and Pr)-Codoped α-SiAlON Ceramics

Rare-earth-doped oxynitride or nitride compounds have been reported to be luminescent and may then serve as new phosphors with good thermal and chemical stabilities. In this work, we report the photoluminescence (PL) spectra of europium-, terbium-, and praseodymium-doped Ca-α-SiAlON ceramics. The highly dense ceramics were prepared by hot pressing at 1750°C for 1 h under 20 MPa in a nitrogen atmosphere. Europium-doped Ca-α-SiAlON displayed a single broad emission band peaking at λ= 550–590 nm depending on the europium concentration. The emission bands in the spectra of europium-doped Ca-α-SiAlONs were assigned to the allowed transition of Eu²⁺ from the lowest crystal field component of 4 f ⁶5 d to ⁸S_7/2 (4 f ⁷) ground-state level. The emission spectra of terbium- and praseodymium-doped Ca-α-SiAlON ceramics both consisted of several sharp lines, which were attributed to the ⁵D₄→⁷F _j ( j = 3, 4, 5, 6) transitions of Tb³⁺ and ³P₀→³H _j ( j = 3, 4, 5) transitions of Pr³⁺, respectively. In particular, the terbium-doped α-SiAlON ceramics showed a strong green emission among these phosphors.     

SiCf/O′-SiAlON composite: properties and oxidation retained properties

A composite of O′SiAlON (Si_2-xAl_xN_2-xO_1+x, with x 0.14) reinforced with 20 vol.% SiC monofilaments was fabricated by hot-pressing, at 1600°C, for 2 h under 34 MPa pressure. The mechanical and interfacial properties of the composites, as-fabricated as well as post-oxidized, were, investigated. The composite exhibited a significant improvement in ultimate flexure strength (640 MPa) and work of fracture (42 kJ m⁻²) compared with that (350 MPa and 1.8 kJ m⁻², respectively) of the monolithic material. These mechanical properties were slightly increased after the composite was heat treated for 24 h in air at 1200 and 1300°C. However, the composite exhibited a significant degradation in ultimate strength, while the work of fracture (WOF) remained unchanged after exposure in air at temperatures beyond 1400°C. The as-fabricated composite revealed a low interfacial shear strength (6.2 MPa) and a frictional sliding stress (3.2 MPa). After the composite was oxidized at elevated temperatures, the interfacial bonding and sliding stresses were reduced to noticeable extents, resulting from the degradation of the carbon coating layer of the SiC monofilaments.     

Grain-Boundary Wetting-Dewetting in z= 1 SiAlON Ceramic

The grain-boundary structure of a model SiAlON polycrystal with nominal composition Si₅AlON₇ was characterized by transmission electron microscopy (TEM) both in an equilibrium (as-processed) state at room temperature and after quenching from elevated temperature. In addition, low-frequency (1–13 Hz) internal friction data were recorded as a function of temperature, showing a pronounced grain-boundary sliding peak positioned at 1030°C. High-resolution transmission electron microscopy (HRTEM) of the equilibrated low-temperature microstructure revealed residual glass only at multigrain junctions, but no amorphous intergranular films were observed. The detection of clean interfaces in the as-processed sample contradicts the internal friction data, which instead suggests the presence of a low-viscosity grain boundary phase, sliding at elevated temperatures. Therefore, a thin section of the as-sintered material was heated to 1380°C and rapidly quenched. HRTEM analysis of this sample showed, apart from residual glass pockets, wetted grain boundaries, which is in line with the internal friction experiment. This wetting-dewetting phenomenon observed in z = 1 SiAlON is expected to have a strong impact not only on high-temperature engineering ceramics but also on geological, temperature-activated processes such as volcanic eruptions.     

Highly Transparent Lu-α-SiAlON

Novel Lu-α-SiAlON ceramics were produced by hot pressing mixtures of Si₃N₄, Lu₂O₃, AlN, and Al₂O₃ at 1950°C for 2 h in a nitrogen atmosphere. The resultant SiAlON was fully dense and possessed a uniform, equiaxed microstructure with a grain size of ∼1 μm, which resulted in a high hardness of >19 GPa. In addition to high hardness, the sample showed very high optical transparency in the visible light region, with >70% transmission at higher wavelengths. This high transparency was attributed to the uniform, dense microstructure and lack of residual grain-boundary phase.     

天然原料碳热还原氮化合成β′-SiAlON的研究进展

本文介绍β′-SiAlON的结构、基本性能及应用领域,阐述了合成β′-SiAlON的原材料和各种合成方法,重点介绍了利用天然原料合成β′-SiAlON的还原剂外加法和还原剂内加法即有机插层法。详细阐述了各工艺条件(反应原料的组成、反应温度、反应时间、反应气氛、流速及反应物颗粒尺寸、添加剂等)对合成β′-SiAlON反应结果的影响。同时本文指出目前该研究可能存在的问题及解决的途径。     

碳还原剂加入方式对粉煤灰氮化反应的影响

通过碳还原氮化反应,从工业废料粉煤灰制备氮化陶瓷类材料.采用氮/氧分析仪测定产物氮含量,评价氮化反应程度;以BET氮气吸附法测定比表面积,分析粉体活性;以扫描电镜分析样品表面形貌;以热重分析仪测定碳含量,X射线能谱仪估计元素组成.比较机械混合法、超声法和高能球磨法3种碳加入方式.结果表明,粉煤灰自身含碳的还原活性处于机械混合法和高能球磨混合法之间,高能球磨法对提高反应混合物活性最有效,所得产物结晶氮含量最高,质量分数可由0.3 %提高到23.1%.     

Structures and mechanical properties of Fe- and Cr-incorporated β-Si5AlON7: First-principles study

The incorporation of Fe and Cr atoms into β-Si₅AlON₇ and the effects on the mechanical properties of β-Si₅AlON₇ were theoretically studied at the GGA-PBE/USP level of theory. The incorporation of Fe and Cr atoms shows remarkable site preferences in β-Si₅AlON₇. The binding energies between the incorporated Fe/Cr atoms and the parent β-Si₅AlON₇ are ~5.0 eV, indicating both Fe@β-Si₅AlON₇ and Cr@β-Si₅AlON₇ are thermodynamically stable. Fe incorporation at the A, B and G sites induces remarkable increase in the shear modulus and Young's modulus; all other Fe/Cr incorporated β-Si₅AlON₇ structures exhibit lowered shear modulus and Young's modulus than the parent β-Si₅AlON₇. Except Cr^G@β-Si₅AlON₇, the Poisson's ratio of β-Si₅AlON₇ decreases in all cases of Fe and Cr incorporation.     

用广西高岭土制备β''''-SiAlON材料研究

以广西高岭土、炭黑为原料,白云石、CaO、TiO2为烧结助剂,采用碳热还原氮化法(CRN法)制备了β'-SiAlON材料.研究了烧成温度、保温时间、不同烧结助剂、成型压力等因素,对β'-SiAlON材料制备的影响.     

Incorporation of the transition metals (Cr and Fe) into β-SiAlON crystal structure

Outstanding properties for SiAlON ceramics can be obtained by tailoring the microstructure through α-SiAlON and β-SiAlON phase content as well as a type of secondary phases. It is so far well known that while some of the elements could be accommodated in α-SiAlON structure, β-SiAlON does not easily accommodate different elements in its structure. In this work, SiAlON ceramics were produced by using β-Si₃N₄ starting powder containing small amount of iron and chromium and the possible incorporation of iron and chromium into β-SiAlON structure was investigated by using analytical transmission electron microscopy (TEM) techniques. As a result of analytical TEM analysis, it is found that transition metals (Cr and Fe) can enter into the β-SiAlON crystal structure.