Oxidation process of lanthanum hexaboride ceramics

Oxidation process of lanthanum hexaboride (LAB6) ceramic powder was investigated. The LaB6 powder sampies were heated continually fTom room temperature to 1 473 K at a heating rate of 10 K/min by differential scanning calorimetry. The oxidation tests were conducted at different exposure temperatures. The phases and morphologies of the samples before and after exposure were analyzed by XRD and SEM. It was pointed out that before 1 273 K, LaB6 has high oxidation resistant ability, which was due to that the oxide layer hinders the oxygen diffusion fTom outer to the surface of LaB6 grains. The oxide layer was composed of the transition phases, which were composed of La203 and B2O3 formed fi‘om the initial oxidation; when the oxidation temperature exceeded 1 273 K, protective layer was destroyed due to the vaporization of liquid B2O3. Based on the results of X-ray diffraction analysis, oxidation process of LaB6 ceramicpowder can be described as follows: Before 1 273 K, lanthanum borate, La(BO2)3 was formed on the surface of samples,then lanthanum oxide (La2O3) and boron oxide (B2O3) were present on the surface of samples oxidized when the tempera-ture reached to 1 473 K.     

多元镧基六硼化物固溶体材料研究进展

多元镧基六硼化物固溶体是异质金属元素进入六硼化镧(LaB₆)晶格中取代部分La原子而形成含3种及以上元素的化合物。相比于单一LaB₆，多元镧基六硼化物固溶体具有更低的电子逸出功和可调节的光吸收性能，在电子和光学领域应用前景广阔，受到了众多研究者的关注。由于异质金属元素种类众多，且组合方式多样，导致多元镧基六硼化物固溶体材料体系种类庞大且复杂多变。为此，本文系统梳理了近年来多元镧基六硼化物固溶体材料在晶体结构、制备工艺及光电性能3个方面的研究进展情况，分析了当前多元镧基六硼化物固溶体材料所面临的难题，并从理论、技术及应用3个方面对多元镧基六硼化物固溶体材料的未来发展方向进行了展望。     

Mechanism of chlorinating lanthanum oxide and cerium oxide with ammonium chloride

1　INTRODUCTIONThelightrareearthoressuchasbastnasiteandBeiyuneboitearetheoneswithlanthanumandceriumelementasmaincomponent (thecontentoflan thanumandcerium >85 % (massfraction) ) .General ly ,thelightrareearthoresaredecomposedbyacidoralkali,thentherareearthproductscouldbefurtherrecovered[1,2 ] .Someshortagesexistintheconven tionalprocessofrecoveringrareearth ,suchaslongflow sheet ,multi transferringformoftherareearthandsoon .Furthermore ,thewasteacidoralkaliandgasemissionsuchasHF ,SO2 pr…     

La3+离子在EMIMBF4离子液体中的电化学行为研究

采用三电极体系对La3+离子在0.01mol/LLaCl3-0.1mol/LLiCl-EMIMBF4离子液体电化学还原制备金属镧的电极过程进行了研究。循环伏安法结合恒电位电解法研究结果表明:La3+离子的电化学还原是不可逆过程,还原步骤为一次完成La3++3e-→La。计时电流法结合恒电位电解法研究表明:La3+离子的还原过程受到扩散控制,传递系数和扩散系数分别为0.0492和(1.07～1.19)×10-6cm2/s。     

LaB6改性C/C-ZrC-SiC复合材料的制备及其抗烧蚀性能

采用料浆浸渍结合树脂浸渍裂解法制备了含9.73 wt.% LaB6的LaB6-C/C预制体,再利用反应熔体浸渍法（RMI）制备了LaB6改性C/C-ZrC-SiC复合材料,考察了材料的微观结构和烧蚀行为,探究LaB6对材料抗烧蚀性能的作用机理。结果表明：在热流密度为2380 kw/m2的氧乙炔焰烧蚀120 s后,LaB6改性C/C-ZrC-SiC复合材料的质量烧蚀率和线烧蚀率分别为1.05×10-3 g/s和2.17×10-3 mm/s,较未改性C/C-ZrC-SiC复合材料分别降低了74.8 %和61.9 %。烧蚀过程中,LaB6发生氧化反应生成La2O3和B2O3,La2O3与ZrO2之间的固溶作用以及化学反应,再加之液态B2O3具有促进固相反应传质的作用,使得材料表面形成大面积连续稳定的ZrO2-La2Zr2O7-La0.1Zr0.9O1.95熔融态保护层,这是材料优异抗烧蚀性能的主要原因。     

Investigation of high-temperature oxidation of lanthanum hexaboride in oxygen and the effect of internal oxidation on the protective properties of the scale

The vacuum microbalance method, as well as petrographic and x-ray phase analysis were employed to investigate the oxidation of lanthanum hexaboride as a function of temperature (700–1200°C), oxygen pressure (0.1–740 Torr) and porosity of the sample. It has been established that LaB₆ is highly resistant to oxidation in oxygen up to a temperature of 1200°C. The protective properties of the scale are determined by the peculiarities of the process of internal oxidation, during which a lower lanthanum boride, LaB₄, was observed to be enveloped by the vitreous oxide phase, B₂O₃.     

掺铝铬酸镧烧结过程的相变特征

用溶胶一凝胶法制备出铬酸镧超细粉体，然后掺入铝粉制成试样。用DSC和XRD等手段对掺铝铬酸镧烧结过程的相变进行研究。结果表明，掺铝La0.9Ca0.1CrO3烧结时铝首先熔化，然后与氧发生反应生成Al2O3，后者发生晶型转变后再与La0.9Ca0.1CrO3反应生成六铝酸镧。掺铝烧结体密度较素坯有所减小，电阻率随铝掺入量的增加而显著增大。     

火法气态传输分离混合稀土氯化物

本文以氯化铝作为络合物配体与稀土氯化物形成气态络合物。研究了相邻稀土元素氯化物之间火法分离的可能性，结果表明，二元系分离系数Ｌａ：Ｃｅ＝１２．３０，Ｐｒ：Ｃｅ＝５．７９，Ｐｒ：Ｎｄ＝１．３２；四元系中Ｐｒ－Ｎｄ的分离系数高达Ｐｒ：Ｎｄ＝１．９８．     

在含有饱和LaCl_3的AlCl_3-EMIC中电沉积铝

将氰化镧溶解到AlCl3-EMIC室温离子液体(N=0.667)中,制备氯化镧饱和的AlCl3-EMIC离子液体;采用循环伏安法、计时电流法研究了在饱和LaCl3-AlCl3-EMIC中钨电极上的阴极过程和成核机理;并在不同的电流密度下制备了镀层,利用扫描电镜对镀层表面形貌进行了分析.结果表明:氯化镧的加入使得铝的沉积电位负移,由拟合曲线可知其成核过程为三维瞬时形核,在12 mA/cm2的电流密度下可以获得光滑致密的镀层,氯化镧的加入可以细化晶粒.     

镧化合物对煤间接液化柴油碳烟颗粒摩擦学特性的影响

目的 研究镧化合物对含煤间接液化柴油碳烟润滑油摩擦磨损性能的影响.方法 利用四球长时摩擦磨损试验机,分别考察不同浓度氟化镧(LaF3)和氧化镧(La2O3)对含煤间接液化柴油碳烟(Fischer-Tropsch synthesis diesel soot,F-T DS)与0#柴油碳烟(Diesel soot,DS)白油的润滑性能的影响,利用多功能万能工具显微镜、扫描电镜与能谱、X射线光电子能谱等仪器对磨斑直径、结构、形貌、组成及元素价态进行表征.结果 在含F-T DS白油体系中,添加0.4％(质量分数)镧化合物LaF3或La2O3时,减摩抗磨效果最优,平均摩擦系数(AFC)分别为0.070、0.058,较未添加时(0.094)分别下降25.5％和38.3％,平均磨斑直径(AWSD)分别为0.4318、0.4207 mm,较未添加时减少了33.6％和35.2％.在含DS白油体系中,添加0.6％ 镧化合物LaF3或La2O3时,减摩效果最佳,AFC分别下降11％和16％;添加0.8％ 镧化合物时,抗磨效果最优,AWSD降幅分别为24.8％和33.7％,La2O3抗磨效果优于LaF3.结论 添加少量镧化合物可以改善含F-T DS与DS白油的减摩抗磨效果.其减摩抗磨机理可能是,通过形成转移膜,或在摩擦进程中添加剂吸附在接触表面形成镧系边界润滑保护膜,改善了润滑状态,起到了减摩抗磨作用.     

反应时间对溶剂热法制备Bi_2Se_3纳米片的影响

以BiCl3和Se粉为原料,二甘醇(DEG)作为溶剂,Na2SO3作为还原剂,碱性条件,在160℃的反应温度下,利用溶剂热法成功地合成Bi2Se3纳米片,并研究不同反应时间(20,22,24h)作用下所得产物结构形貌的变化。利用XRD和SEM对所合成产物的晶相结构及表面形貌进行表征。结果显示:在不同反应时间下所合成的粉体均为纯物相(均为六方晶相)的纳米片状Bi2Se3,且反应22h条件下所得产物形貌为大小均一、分散性良好的六边形片状结构,其厚度为10～50nm。并分析片状结构Bi2Se3的形成机制。     

Combustion synthesis of Ni3Al by SHS with boron additions

The preparation of Ni₃Al intermetallic compound from elemental powder compacts with the addition of boron was conducted by self-propagating high-temperature synthesis (SHS) in this study. Effects of the boron content, preheating temperature, and particle size of the reactants on the combustion characteristics, as well as on the composition and morphology of final products were studied. Experimental observation indicates that the addition of boron results in a significant increase in the flame-front propagation velocity, leading to a two-stage combustion process preceded by the fast propagation of the flame front and consecutively followed by prolonged bulk combustion. However, the combustion temperature was slightly decreased by adding boron in the sample. The boron addition also contributed to a decrease in the activation energy associated with the combustion synthesis of Ni₃Al from 97.8 to 86.4 kJ/mol. In contrast to the presence of an intermediate phase (NiAl) and the unreacted Ni in the final burned product of the boron-free samples, complete reaction yielding a single-phase product of Ni₃Al was achieved for the boron-added samples. Moreover, the addition of boron also increased the density of the final products up to above 80% relative to the density of the Ni₃Al compound.     

化学共沉淀法制备正铌酸镧过程中杂相的形成与控制

从多酸化学的角度 ,讨论了在化学共沉淀法制备正铌酸镧 (LaNbO4 )过程中 ,不同 pH条件对产物相组成的影响。结果表明 :在酸性条件下 ,由于同多酸盐的析出 ,产物中有部分偏铌酸镧相K2 LaNb5O15存在 ;在碱性条件下 ,由于铌酸盐的水解受到抑制 ,可以制备得到相组成相对单一的正铌酸镧相 ;对酸性条件下得到的无定形前驱体粉末进行碱化处理可以降低烧成正铌酸镧相中的杂相含量。     

Neodymium Production Processes

This overview describes the major processes for rare earth metals production, including chloride electrolysis for producing cerium, lanthanum and misch metal; oxide electrolysis for producing neodymium metal and its alloys; calciothermic reduction for producing neodymium commercially; a reduction-diffusion process for making powders of SmCO₅; and a co-reduction process for producing powders of rare earth metal alloys. A new molten salt extraction process, “NE0CHEM,” has been developed by which neodymium metal or its alloys can be produced by the reduction of Nd₂0₃, with sodium in the presence of CaCl₂ by the reaction Nd₂O₃ + 3 CaCl₂ + 6 Na → 2 Nd + 3 CaO + 6 NaCl. In comparison to existing processes, the proposed process holds great promise toward lowering the cost of neodymium.     

镧助剂对Pd/Al_2O_3催化剂性能的影响

一氧化碳(CO)-亚硝酸甲酯(MN)气相氧化偶联合成草酸二甲酯(DMO)的反应是煤制乙二醇的关键转化步骤,现用的Pd/Al_2O_3催化剂中钯负载量高达1%。在催化剂制备过程中添加不同助剂,用催化燃烧热阻型检测系统考察其对合成反应的催化性能影响。表征分析表明,镧助剂可改善钯在载体上的分散;当La_2O_3/Al_2O_3=14 mg/g时,La_2O_3在载体表面以单层分散,制成的催化剂中钯分散性最好。性能评价结果显示,载钯量0.2%的催化剂具有较好的稳定性,在优化反应条件下,其性能甚至优于工业催化剂。     

Investigation of pretreatment effects on the formation of lanthanum based conversion coating on AA2024-T3

The effects of a pretreatment process on the formation and properties of lanthanum based conversion coatings on AA2024-T3 was investigated using optical and scanning electron microscopy (SEM), X-ray diffraction (XRD) and open circuit potential (OCP) measurements. The results indicate that the conventional alkaline activation pretreatment with acidic desmutting could not create a suitable condition to produce lanthanum based conversion coating on AA2024-T3. Therefore, in this study, a new approach to produce lanthanum based conversion coating on AA2024-T3 by acidic pretreatment is proposed. The pretreatment of the alloy in acidic and chloride environments (90 s at 30 vol % HCl) creates more cathodic sites on AA2024-T3 compared to alkaline activation with acidic desmutting. Additionally, it was found that the formation stages of lanthanum based conversion coating is similar to those of cerium based conversion coating on AA2024-T3 except lanthanum ions require more localized pH increase to deposit on the alloy.     

Application of controlled and electrical discharge assisted mechanical alloying for the synthesis of nanocrystalline MgB2 superconducting compound

In this paper we present the results of our efforts to synthesize the nanocrystalline MgB₂ superconducting compound from elemental Mg and B powders by combination of controlled mechanical pre-alloying in a magneto-mill Uni-Ball-Mill 5 under shearing mode followed by electrical discharge (ED) assisted mechanical alloying (MA). There is no conclusive evidence of MgB₂ formation in the Mg-2B mixture using crystalline boron after controlled mechanical alloying (CMA) under protective argon or helium atmosphere as well as subsequent ED assisted alloying. There seems to be some XRD evidence of the strongest (1 0 1) MgB₂ peak presence in the Mg-2B mixture processed using both crystalline and amorphous boron after CMA under hydrogen as well as subsequent ED assisted alloying but this evidence is rather ambiguous. We postulate here that it is highly likely that a certain critical Mg nanograin size must be achieved before a successful reaction to form nanocrystalline MgB₂ is going to be completed. Following recent report by Gümbel et al. [Appl. Phys. Lett. 80 (2002) 2725] this critical value can be roughly estimated at 15 nm or less. Calculations of the Mg nanograin size in the present work show that only three Mg-2B powders ball milled under hydrogen meet this critical nanograin size criterion for the Mg phase. However, a massive formation of the β-MgH₂ hydride in these powders consumes the available Mg in the reaction with hydrogen which may leave inadequate concentration of Mg to form MgB₂ even though the nanograin size of Mg is sufficiently refined, say below 15 nm.     

掺Li量和烧结温度对铬酸镧烧结行为的影响

研究了用Pechini法制备掺Li铬酸镧超细粉体在烧结过程中掺Li量和烧结温度对粉体烧结行为及烧结体组织的影响.结果表明,在相同烧结温度下,LaCr1-yLiyO3烧结体收缩率随掺Li量的增加而升高;当掺Li量y为0.2时收缩率达到最大;此后随着掺Li量增加而降低.就相同掺Li量的烧结体而言,烧结温度越高组织越致密.致密烧结体的断口特征为穿晶和沿晶混合断裂,这表明晶界结合强度接近晶内.烧结过程中晶粒不断合并、长大,其长大机制为台阶方式.     

水合草酸镧的热分解动力学(英文)

以PEG20000为表面活性剂在撞击流反应器中制备La2O3超细粉体的前驱体十水草酸镧(La2(C2O4)3.10H2O)。在室温至900°C下研究La2(C2O4)3.10H2O的热分解过程,通过FTIR和DSC-TG对其反应中间物及最终固体产物进行分析。结果表明,该热分解过程由5个连续的反应阶段组成。采用Flynn-Wall-Ozawa(FWO)和Kissinger-Akahira-Sunose(KAS)法对活化能E进行求取,结果显示E值随着α的变化而变化,说明草酸镧的分解为复杂的热分解过程。采用多元非线性回归分析法对动力学方程和相关动力学参数进行拟合,得到动力学模型为G(α)=[1-(1+α)1/3]2。采用该动力学模型求得的活化能平均值与采用FWO法和KAS法计算而得的活化能平均值十分接近,其拟合曲线与样品的热重分析曲线吻合。     

LaB_6和La_2O_3添加剂对ZrB_2-SiC超高温陶瓷结构与性能的影响

采用SPS工艺制备添加La_2O_3或LaB_6的ZrB_2-SiC陶瓷,测量试样的密度和力学性能,利用扫描电镜和透射电镜观察试样的微观形貌,研究添加镧的不同化合物对ZrB_2-SiC陶瓷显微结构和力学性能的影响,分析添加量对材料力学性能的影响.同时对ZrB_2-SiC-La_2O_3和ZrB_2-SiC-LaB_6陶瓷进行热处理,考察热处理对其力学性能的影响.结果表明,加入2.5%或5%(质量分数, 下同)的La_2O_3或LaB_6添加剂后,材料的室温强度、高温强度、断裂韧性都比无添加剂时要高;当含量相同时,加入LaB_6比La_2O_3更有利于提高陶瓷材料的室温强度;当添加剂的含量为2.5%时,材料的室温强度比较好,当添加剂的含量为5%时,材料的高温强度和断裂韧性比较高.热处理可以提高ZrB_2-SiC-La_2O_3和ZrB_2-SiC-LaB_6陶瓷材料的高温强度.