Effects of particle size of reactant on characteristics of combustion synthesis of TiC-Fe cermet

Four Ti-C-Fe powder mixtures, with a same molar ratio but a different particle size, were used for a combustion synthesis of TiC-Fe cermet to investigate effects of the particle size on the characteristics of the combustion synthesis. The results showed that the mixture with the finer Ti powder gave out a higher combustion temperature, a higher reaction velocity, a higher product density, a layer-shaped pore, and a greater size of TiC particles whether the Fe powder was finer or coarser. While in the case of the coarser Ti powder used, a small amount of residual phase remained in the product, and what is more interesting that the mixture with the finer Fe powder gave out a lower reaction velocity than that by the mixture with the coarser Fe powder. These effects were successfully explained with the previously proposed mechanisms of the combustion synthesis of TiC-Fe, and the mechanisms were proved to be valid thereby.     

Ti-C-xFe体系自蔓延高温合成及机理

采用自蔓延高温合成新技术合成ＴｉＣ／Ｆｅ复合材料,研究了原料组份、粒度对合成过程及产物特征的影响。探讨了燃烧反应及结构形成机理。结果表明,随Ｆｅ含量的增大,燃烧合成温度降低,合成ＴｉＣ粒度变细,燃烧波速度在Ｆｅ含量为１０％（质量分数）时出现极大值。与碳黑作碳源对比,细粒石墨在反应合成过程中表现出更大的活性。用铜楔块燃烧波淬息法研究了合成过程。整个合成过程经历了金属的熔融、碳向熔融体中的溶解扩散、大团聚的形成、小的ＴｉＣｘ颗粒生长及ＴｉＣ长大等阶段。     

Al-TiO2-C-Ti-Fe体系的燃烧模式研究

采用与计算机相连的快速图像采集系统对Al-TiO2-C-Ti-Fe体系的燃烧模式进行研究。结果表明，燃烧波蔓延模式随着Fe含量的增中即绝热温度的降低，由稳定的平面燃烧逐渐转化为非稳定的多点波动燃烧，然而随着Fe含量进一步增加，燃烧模式又转化为稳定的平面燃烧。     

高压均质技术对低熔点玻璃粉的细化及烧结性能的影响

利用羧甲基纤维素(CMC)作为低熔点玻璃粉水分散液的防沉剂, 用高压均质机对玻璃粉进行细化处理, 使其粒度减小. 探讨了不同高压均质时间对玻璃粉的粒度大小和烧结性能的影响; 结果表明: 在相同的压强下(50 MPa), 高压均质时间越长则玻璃粉粒度越小、粒度分布范围越窄, 其软化温度和烧结温度也越低. 因此可以利用高压均质技术来降低玻璃粉的粒度, 从而降低玻璃粉的软化温度和烧结温度.     

原料组份、粒度对Ti-C-Fe体系自蔓延高温合成的影响

本文探讨了Fe含量、碳源及Ti、C颗粒大小对Ti－C－Fe体系自蔓延高温合成过程及产物结构特征的影响．结果表明：Fe含量增高,燃烧温度降低,产物颗粒变细,而燃烧波速度在10wt％Fe时出现极大值,反映了Fe液相的作用．石墨作碳源燃烧合成的TiC更接近于化学计量的TiC,且TiC颗粒较粗,燃烧温度、燃烧波速度均较高,反映了碳源结构差异对燃烧合成的影响．Ti、C颗粒越细,越有利燃烧反应合成．随着Fe含量增高,Ti－C－Fe体系燃烧方式由稳态变为振荡式及螺旋式燃烧．Fe含量＞60wt％;反应则不能自持．     

热等静压温度和粉末粒度对Ti2AlNb合金组织与性能的影响

采用无坩埚感应熔炼超声气体雾化法制备了成分为Ti-22Al-24Nb-0.5Mo(原子分数, %)的预合金粉末,通过预合金粉末热等静压工艺制备了Ti₂AlNb粉末冶金合金。研究结果表明,热等静压温度显著影响Ti₂AlNb粉末冶金合金的显微组织,需严格控制。为了对比研究,选取了平均粒度分别为70 μm和200 μm的两种Ti₂AlNb预合金粉末,制备坯料并测试性能,探讨了粉末粒度的选取原则,分析了粉末粒度对Ti₂AlNb粉末冶金合金显微组织和力学性能的影响。研究结果表明,粉末粒度对合金室温拉伸强度无显著影响,但会对高温拉伸强度和高温持久寿命产生显著影响,由粗粉(平均粒度200 μm)制成的合金高温持久寿命较细粉(平均粒度70 μm)的降低大约40%。     

Reaction synthesis of TiC–TiB<Subscript>2</Subscript>/Al composites from an Al–Ti–B<Subscript>4</Subscript>C system

The TiC–TiB₂/Al composites were fabricated by self-propagating high-temperature synthesis (SHS) from Al–Ti–B₄C compacts. The addition of Al to the Ti–B₄C reactants facilitates the ignition occurrence, lowers the reaction exothermicity, and modifies the resultant microstructure. The maximum combustion temperature and combustion wave velocity decrease with the increase in the Al amount. The B₄C particle size exerts a significant effect on the combustion wave velocity and the extent of the reaction, while that of Ti has only a limited influence. The reaction products are primarily dependent on the B₄C particle size and the Al content in the reactants. Desired products consisting of only the TiC, TiB₂, and Al phases could be obtained by a cooperative control of the B₄C particle size and the Al content.     

工艺参数对熔盐法制备钛酸铋粉体影响的研究

研究了工艺参数对熔盐法制备钛酸铋粉体晶粒形貌和尺寸的影响.发现温度、盐的种类和盐的用量能够较显著地影响晶粒的形貌和尺寸.在氯盐中,晶粒尺寸随着温度的升高而逐渐增大,晶粒最终发育为较规则的圆片状,晶粒生长按照Ostwald液相生长机理来进行.在硫酸盐中,晶粒尺寸明显大于相同条件下在氯盐中的尺寸,且晶粒形貌呈不规则片状.当原料与盐的比例<1时,随着盐用量的增加,晶粒尺寸减小,形貌更加不规则.通过调整工艺参数能够控制颗粒的尺寸和形貌,制备具有各向异性生长的钛酸铋片状粉体.     

燃烧合成MgB2超导材料

采用自蔓延及热爆两种燃烧模式合成了较纯的MgB2金属间化合物。用XRD和SEM对Mg-B体系燃烧合成产物进行了研究,结果表明,压坯压力越大合成产物越致密,孔隙明显减少,产物更加均匀,但过大的压坯压力,降低了体系的燃烧温度,影响Mg、B间的反应程度;而对于细小的Mg粒,Mg-B间充分反应产生较高的燃烧温度,导致MgB2分解出现MgB4,影响合成产物纯度。     

高活性纳米金属铝的研究进展

综述了高活性金属粉,特别是高活性纳米铝粉的研究进展。详述了纳米铝粉的粒径对其熔点、表面氧化层厚度、氧化起始温度以及纳米铝粉的点火燃烧特性的影响,分析了高活性纳米金属铝目前存在的问题,指出了今后高活性纳米金属铝的研究方向。     

Formation mechanism of silicide nanoparticles by induction thermal plasmas

The condensation mechanism of metal mixture in thermal plasmas was investigated experimentally and numerically to prepare nanoparticles of silicon base intermetallic compounds. Silicon powder premixed with metal powder (Mo, Ti, Co, Fe, Cr, or Mn) was introduced into the plasma. The nanoparticles were prepared on condition that metal vapor was quickly quenched by the water-cooled copper coil. The nucleation rate expression was used for the estimation of critical saturation ratio. The nucleation temperature of the metal almost corresponds to the melting temperature, while silicon has wide liquid range between the nucleation and melting temperature, resulting in better preparation of silicide. For Mo–Si system, nucleation position of Mo is different from that of Si. Therefore, quenching position has strong effect on the particle composition of molybdenum silicide nanoparticles.     

Formation mechanism of silicide nanoparticles by induction thermal plasmas

The condensation mechanism of metal mixture in thermal plasmas was investigated experimentally and numerically to prepare nanoparticles of silicon base intermetallic compounds. Silicon powder premixed with metal powder (Mo, Ti, Co, Fe, Cr, or Mn) was introduced into the plasma. The nanoparticles were prepared on condition that metal vapor was quickly quenched by the water-cooled copper coil. The nucleation rate expression was used for the estimation of critical saturation ratio. The nucleation temperature of the metal almost corresponds to the melting temperature, while silicon has wide liquid range between the nucleation and melting temperature, resulting in better preparation of silicide. For Mo–Si system, nucleation position of Mo is different from that of Si. Therefore, quenching position has strong effect on the particle composition of molybdenum silicide nanoparticles.     

Effect of the TiO2 nanoparticle size on the decomposition behaviors in aluminum matrix composites

The decomposition behaviors and the effect of particle size on the kinetic rate are studied for Al–3 vol.% titanium dioxide (TiO₂) composites by using three different types of TiO₂ particles (15, 50, and 300 nm). Thermal analysis shows that the reaction is stepwise with the first reaction starting before the melting temperature of Al. Since the high chemical potential of nanoparticles enhances reactivity, the TiO, Al₃Ti, and α-Al₂O₃ phases are found to be formed during the first reaction regardless of particle size. Based on observations of microstructure, the formation mechanism of Al₃Ti and α-Al₂O₃ is understood to be solution precipitation. Non-isothermal kinetic analysis reveals that the reaction mechanism is closely related to the three-dimensional continuous nucleation and the growth limited by diffusion. Particle size is found to be having considerable effect on the kinetic rate. As the particle size decreases, the rate constant increases, while the pre-exponential factor and the activation energy decreases. A non-linear relationship between the rate constant and the reciprocal of the size is found and evaluated.     

固相反应法合成碳化硼纳米粉体

以六方氮化硼和炭黑(或石墨)为原料, 采用固相反应法合成了碳化硼粉体. 碳源、反应气氛和温度对粉体合成产生重要影响. 以炭黑为碳源, 在1900℃真空下保温5 h, 得到了平均粒径约为100 nm的碳化硼纳米粉体. 与商业粉体相比, 合成的粉体具有较好的烧结活性. 在2000℃/30 MPa/1 h条件下烧结, 样品的相对密度达到97.9%(商业粉体样品为93.1%), 这可归结于合成的粉体具有细小的粒径、低的氧含量和一定程度的孪晶结构.     

Assessment of Microwave Heating for Sintering of Al/SiC and for in‐situ Synthesis of TiC

This work describes sintering of SiC‐reinforced Al‐matrix composites and in‐situ synthesis of TiC in a powder mixture of Ti and C. In the first case, microwave energy is absorbed by SiC grains, heating the metal matrix to sintering and even melting temperature. The composite is processed at <1 kW microwave power. Microwave absorption and the heating rate increase with decreasing SiC particle size. Composites with high SiC content (70 vol.‐%) are processed at 650 °C/1 h in the microwave furnace, whereas conventional resistive heating at the same temperature did not allow sintering of the sample. In the second case, radiative energy allowed the heating of Ti/C samples up to 950 °C, and microwave assistance enhanced the reaction sintering of Ti/C powder mixtures forming TiC at the border of the Ti particles. The results are compared with conventional processing. Optical images and XRD patterns confirmed the formation of TiC for both techniques.     

Reaction kinetics of fly ash geopolymerization: Role of particle size controlled by using ball mill

Fly ash is milled for 0, 30 and 90?min and used to study the role of particle size on the kinetics of geopolymer formation. The increase in particle fineness is very prominent in the initial milling stage, and then slows down due to agglomeration effect of finer fraction. The fly ash geopolymerization kinetics and its mechanism is determined using heat of reaction data measured by isothermal conduction calorimeter. The improvement in reaction rate with milling is correlated with the median particle size of the fly ash. The apparent activation energy decreases with size reduction because finer fractions are more prone to alkali activation. Although the kinetics changes with particle fineness, but no alternation is detected in the reaction mechanism, governed by nucleation and growth. The apparent activation energy evaluated by rate method is showing three major steps of geopolymerization such as dissolution, gel formation and restructuring.     

高活性铝钆合金的制备及热氧化特性研究

为降低纯铝粉燃料的点火温度,提升体积燃烧热,改善热氧化特性,采用真空感应熔炼法制备了铝钆合金Al-10Gd和Al-2Gd,并破碎成粉制得样品。SEM、EDS图像显示:样品呈片状,元素分布均匀。结合微机全自动量热仪与真密度测试仪对样品的燃烧热进行测量与计算。结果表明:铝钆合金的质量燃烧热与纯铝粉相当,体积燃烧热要高于纯铝粉,在装药体积有限的情况下可以释放更多的能量。采用同步热分析仪研究了样品的热氧化特性。结果表明:铝钆合金的熔点和初始氧化温度均低于纯铝粉,反应放热至少持续到1 550.0 ℃以上,增重约60.42%。金属钆会促进铝粉的热氧化,从而降低燃料的着火点,使燃料迅速升温放热。     

空气中氮气或氩气浓度对锆粉尘层着火温度的影响

为了探索抑制锆包壳剪切过程中锆粉着火的方法,采用粉尘层最低着火温度测定仪、红外热成像仪、真空手套箱等测定了不同粒径的锆粉尘层在空气和含不同浓度氮气、氩气的空气中的最低着火温度和火焰温度。结果得出:锆粉的中位粒径从2.4 μm升至71.7 μm,粉尘层最低着火温度从200 ℃升至390 ℃,表明粒径越小的锆粉着火敏感性越高;4种粒径的锆粉燃烧火焰最高温度都在1 776～1 913 ℃范围内,锆粉粒径较大时,燃烧的剧烈程度较低;氮气或氩气体积分数从60%～65%升至70% 85%时,锆粉尘层最低着火温度升至400 ℃,表明空气中高浓度的氮气或氩气对锆粉燃烧有抑制作用,且浓度越高,抑制作用越强,锆粉粒径越小,抑制效果越好。氩气的抑制效果强于氮气。     

Nanocrystalline ceria powders through citrate-nitrate combustion

Nanocrystalline ceria powders have been synthesized by combustion technique using citric acid as a fuel and nitrate as an oxidizer. The auto-ignition of the gels containing cerium nitrate and citric acid resulted in ceria powders. A theory based on adiabatic flame temperature for different citric acid-to-cerium nitrate molar ratios has been proposed to explain the nature of combustion reaction and its correlation with the powder characteristics. Specific surface area and primary particle size of the ceria powder obtained through fuel-deficient precursor was found to be approximately = 127 m2/g and 2.5-10 nm, respectively. The combustion synthesized ceria powder when cold pressed and sintered in air at 1250 degrees C for 1 hour resulted in approximately = 96% of its theoretical density with sub-micron grains.     

TiB2-TiC复合粉的自蔓延高温还原合成

热分析结果表明,对于B₂O₃-TiO₂-Mg-C体系,可利用SHS还原技术合成出TiB₂-TiC陶瓷复合粉。其化学反应机理为:Mg先还原B₂O₃和TiO₂,新生的Ti与B和C反应生成TiB₂和TiC; TiO₂的还原经历了TiO₂→TiO→Ti的逐步过程。采用一定的酸洗工艺得到了纯净的TiB₂-TiC陶瓷复合粉。复合粉中包含六方片状TiB₂和圆球状TiC;复合粉中1μm以下颗粒质量百分数超过45%,87%以上的颗粒大小在3μm以下。在TiB₂-TiC中,TiC_<em>y以一种贫碳结构存在,物料中Ti被B或C结合形成TiB₂和TiC_<em>y,y的值为0.7483。