Mixture of fuels approach for the solution combustion synthesis of LaAlO3 nanopowders

A modified solution combustion approach was used in the preparation of nanosize LaAlO₃ (～23.6 nm) using mixture of citric acid and oxalic acid as fuels with corresponding metal nitrates. The synthesized and calcined powders were characterized by Fourier transform infra red spectrometry (FTIR), Differential thermal analysis-Thermogravimetry analysis (DTA–TGA), X-ray diffractometry (XRD) and Transmission electron microscopy (TEM). The FTIR spectra show the lower frequency bands at 656 and 442 cm^?1corresponds to metal–oxygen bonds (possible La–O and Al–O stretching frequencies) vibrations for the perovskite structure compound. DTA confirms the formation temperature of LaAlO₃ varies between 830–835 °C. XRD results show that mixture of fuels ratio is influential on the crystallite size of the resultant powders. The average particle size of LaAlO₃-1 as determined from TEM was about 41 nm, whereas for LaAlO₃-2 and LaAlO₃-3 samples, particles are seriously aggregated.     

Mixture of fuels approach for the solution combustion synthesis of Al2O3-ZrO2 nanocomposite

A modified solution combustion approach was used for the first time in the preparation of nanosize zirconia toughened alumina (ZTA) composite. ZTA-1 with an average particle size of ∼37 nm was prepared using corresponding metal nitrates and urea. ZTA-2 with an average particle size of <10 nm was prepared by using mixture of fuels such as ammonium acetate, urea and glycine. The products formed were characterised by powder X-ray diffractometry, Transmission electron microscopy and BET surface area analysis. By using mixture of fuels, the energetics of the combustion reaction and eventually the properties of the combustion product have been changed. A series of combustion reactions were carried out to optimise the fuel ratio combinations required to obtain <10 nm ZTA particles. The microstructure of ZTA consisted of crystallites of Al₂O₃ and ZrO₂ both of which were nanocrystalline as evident from TEM.     

Glycerol-assisted solution combustion synthesis of improved LiMn2O4

Spinel LiMn₂O₄ has been synthesized by a glycerol-assisted combustion synthesis method. The phase composition and morphologies of the compound were ascertained by X-ray diffraction (XRD) and scanning electron microscope (SEM). The electrochemical characterization was performed by using CR2032 coin-type cell. XRD analysis indicates that single phase spinel LiMn₂O₄ with good crystallinity has been obtained as a result of 5 h treatment at 600 °C. SEM investigation indicates that the average particle size of the sample is 200 nm. The initial discharge specific capacity of the LiMn₂O₄ is 123 mAh/g at a current density of 30 mA/g. When the current density increased to 300 mA/g, the LiMn₂O₄ offered a discharge specific capacity of 86 mAh/g. Compared with the LiMn₂O₄ prepared by a conventional solution combustion synthesis method at the same temperature, the prepared LiMn₂O₄ possesses higher purity, better crystallinity and more uniformly dispersed particles. Moreover, the initial discharge specific capacity, rate capability and cycling performance of the prepared LiMn₂O₄ are significantly improved.     

Solution combustion synthesis of LiMn2O4 fine powders for lithium ion batteries

In this work, fine powders of spinel-type LiMn₂O₄ as cathode materials for lithium ion batteries (LIBs) were produced by a facile solution combustion synthesis using glycine as fuel and metal nitrates as oxidizers. Single phase of LiMn₂O₄ products were successfully prepared by SCS with a subsequent calcination treatment at 600–1000 °C. The structure and morphology of the powders were studied in detail by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The electrochemical properties were characterized by galvanostatic charge–discharge cycling and cyclic voltammetry. The crystallinity, morphology, and size of the products were greatly influenced by the calcination temperature. The sample calcined at 900 °C had good crystallinity and particle sizes between 500 and 1000 nm. It showed the best performance with an initial discharge capacity of 115.6 mAh g⁻¹ and a capacity retention of 93% after 50 cycles at a 1 C rate. In comparison, the LiMn₂O₄ sample prepared by the solid-state reaction showed a lower capacity of around 80 mAh g⁻¹.     

燃烧合成ZnFe2O4的相转变与反应机制研究

采用"气窒"法实现燃烧波的"淬熄",获得了燃烧合成Zn铁氧体粉体不同阶段的燃烧产物.采用XRD、SEM和mossbauer谱研究燃烧产物的相组成和微观结构的变化.提出了燃烧合成Zn铁氧体的溶解-析出机制,即铁粉在氧气中燃烧,被氧化成Fe2O3.在高温下,Fe和Fe2O3部分熔化成液相.高熔点的ZnO颗粒悬浮于液相中并逐渐溶解,最后Zn铁氧体逐渐从液相中析出.此外,结合上述分析,给出了燃烧合成Zn-Fe2O4的反应机制模型.     

Submicronic MgAl2O4 powder synthesis in supercritical ethanol

Submicronic MgAl₂O₄ powders have been synthesized by decomposition of the double alkoxide Mg[Al(O-secBu)₄]₂ in supercritical ethanol. The influence was studied of various experimental parameters (reaction time and temperature, reactional medium concentration and density) on the main characteristics of the obtained materials. Particularly, sample crystallinity and chemical composition, particle size distribution and mean diameter were investigated by X-ray diffraction, IR spectroscopy, laser scattering and electronic microscopy.     

煅烧工艺对溶液燃烧合成超细ZrO2粉末的影响

以硝酸锆-尿素为原料,采用溶液燃烧合成(SCS)法制备了超细ZrO2粉末,着重研究了后续煅烧处理对合成的ZrO2粉末结构、尺寸和团聚程度的影响.结果表明,利用SCS法可以合成超细ZrO2粉末,对之进一步进行煅烧处理,可明显降低合成产物中残留有机物的含量,并改善合成粉末的分散性;在700℃煅烧1.0h的条件下,可以获得分散性好,粒径小(＜200nm)的ZrO2粉末.     

Facile synthesis and crystal growth dynamics study of MgAl2O4 nanocrystals

Well-dispersed MgAl₂O₄ nanocrystalline was prepared by a convenient salt-assistant combustion process using low-toxic glycine as fuel, low-cost Mg(NO₃)₂·6H₂O and Al(NO₃)₃·9H₂O as raw materials. By changing the reaction condition such as glycine dosage, the amount of inert-salt, inert-salt types and calcinations temperature, the optimal conditions to synthesize MgAl₂O₄ nanocrystalline were explored. The obtained results showed that parameters inert-salt and fuel dosage in the process have an important effect on the structure properties of samples. The obtained products were characterized by XRD and TEM. TEM images revealed that the products were composed of well-dispersed cube-like nanocrystals with the size of 60–80 nm. The synthesis mechanism of MgAl₂O₄ nanocrystals was discussed. The crystal growth dynamics of MgAl₂O₄ nanocrystals was studied and the activation energy of MgAl₂O₄ nanocrystals was 39.1 kJ/mol.     

镁铝尖晶石超微粉的制备方法

主要介绍了国外近年来制备MgAl_2O_4超微粉的各种方法、特点,并比较了它们的优缺点,根据合成原理将制备方法分为二大类:液相法和固相法。     

Fracture of single crystal MgAl2O4

The fracture of single crystal, stoichiometric MgAl₂O₄ spinel, was investigated for the (1 0 0), (1 1 0), and (1 1 1) from room temperature to 1500° C. Two regions of fracture behaviour were observed; a low temperature elastic region where K _lc decreased with increasing temperature, and an elevated temperature region where K _lc increased rapidly with increasing temperature. The elastic region is explained primarily by the decrease of elastic modulus with increasing temperature, whereas the rapid increase of K _lc at elevated temperature is attributed to plastic flow in the vicinity of the crack tip     

Novel synthesis of high surface area MgAl2O4 spinel as catalyst support

A modified sol–gel route, by combining gelation and coprecipitation processes, was developed for the synthesis of high surface area MgAl₂O₄ spinel precursors. The obtained precursors were then calcined in flowing air at temperatures ranging from 500 to 900 °C. The formation of new phases upon calcinations was investigated using X-ray diffraction, thermal gravimetric analysis, and Fourier transform infrared spectroscopy (FTIR). Single-phase spinel powder with uniform pore size distribution was formed at temperatures as low as 600 °C. It was found that the thermal stability of the as-synthesized spinels is higher than that reported by other preparation methods. After calcinations at 800 and 950 °C for 8 h, the specific surface area reaches a level of 182 and 136 m²·g⁻¹, respectively. And the degree of crystallinity is higher than other preparation methods as illustrated by samples calcined at 800 °C. The amount of PVA added significantly affects the surface area of the samples. With increasing the ratio of M/PVA, the surface area of the resulting spinels increased accordingly.     

MgAl2O4尖晶石涂层的显微结构

解XRD红外吸收光谱、ＴＥＭ和ＥＤＳ等方法研究了等离子喷涂尖晶石涂层的显微结构，结果表明，用等离子喷涂能获得结晶良好、阳离子分布比较有序的尖晶石涂层，涂层中尖晶石晶粒细小、均匀涂层热稳定性非常好，可在高温下长时间使用杂质可能导致涂层局部形成玻璃相涂层中尖晶石晶粒内存在大量位错，可能是在喷涂沉积过程中产生的变形所致     

Influence of fuels and combustion aids on solution combustion synthesis of bi-phasic calcium phosphates (BCP)

The possibility to obtain biphasic HA/TCP composite by solution combustion synthesis method using urea and glycine as fuels was investigated. Calcium nitrate was taken as a source of calcium, and diammonium hydrogen phosphate served as a source of phosphate ions. Nitric acid and nitrate ions were used as oxidizers. The effect of the nature of fuel (urea and glycine) and fuel to oxidizer ratio on the combustion behavior, as well as, chemical composition and morphology of as-formed powders was investigated. It was found that only monoclinic-TCP was formed in the combustion end-product when glycine was used. In contrast to this, the use of certain amount of urea led to the formation of rhombohedral-TCP. A series of combustion reactions were carried out to study the influence of fuel to oxidizer ratio on HA to TCP ratio in synthesized product.     

焙烧温度对醋酸盐体系液相燃烧合成尖晶石型LiMn2O4的影响

以醋酸锂、醋酸锰为原料，尿素为燃料，用液相燃烧合成方法制备尖晶石型LiMn2O4物质，考察了焙烧温度（300-800℃焙烧5h）对产物的组成结构、晶粒大小及电化学性能的影响。实验结果表明，未焙烧产物中主晶相为LiMn2O4及少量Mn2O3，但在300-800℃焙烧5h后都可得到单相的LiMn2O4粉体材料，焙烧温度为900℃时，LiMn2O4部分分解为Mn3O4；产物颗粒随焙烧温度升高而长大，≤600℃时，产物颗粒〈100nm，≥700℃时产物颗粒〉100nm，可观察到LiMn2O4的特征八面体结构；在焙烧温度800℃以下，产物的电化学性能随焙烧温度的升高而增加。当电流密度为C/3时，焙烧温度为800℃的首次放电容量为105mAh／g，但循环性能较差，30次循环后仅剩83％。     

The effect of MgAl2O4 on the formation kinetics of Al2TiO5 from Al2O3 and TiO2 fine powders

The formation of Al_2(1–x)Mg _x Ti_(1+x)O₅ solid solutions from Al₂O₃-TiO₂-MgAl₂O₄ powder mixtures of 1 m particle size and moderate purity has been studied at 1300°C for different final composition values: x=0 (pure Al₂TiO₅), 10^–3, 10^–2 and 10^–1. Analysis of the kinetic data and microstructural observation indicates that MgAl₂O₄ affects the mechanism of Al₂TiO₅ formation by providing active nuclei for the growth of the new phase. These nuclei are probably constituted by Mg_0.5AlTi_1.5O₅, i.e. the equimolar Al₂TiO₅-MgTi₂O₅ solid solution, and are formed by reaction between MgAl₂O₄ and TiO₂ at temperatures above 1150 °C. As the value of x increases, the number of titanate particles per unit volume accordingly increases and the conversion of the original oxides is faster. At values of x10^–2, the prevailing mechanism is the nucleation and growth of Al₂TiO₅ nodules for fractional conversion up to 0.8. Further conversion of the residual Al₂O₃ and TiO₂ particles dispersed into the titanate nodules is slower and controlled by solid-state diffusion through Al₂TiO₅. At x=0.1, a large number of nucleation sites is present, and solid-state diffusion through Al₂TiO₅ becomes important even in the initial stage of reaction, as the diffusion distances are strongly reduced. The study of Al₂TiO₅ formation under non-isothermal conditions in the temperature range 1250–1550°C shows that reaction proceeds between 1300 and 1350 °C for x=0.01 and between 1250 and 1300 °C for x=0.1. Densification of the titanate becomes important at temperatures above 1300°C for x=0.1, but only above 1450 °C for x=0.01.     

Glycerol-assisted gel combustion synthesis of nano-crystalline LiNiVO4 powders for secondary lithium batteries

Glycerol assisted gel combustion route (GGCR) was used for the synthesis of nano-crystalline inverse spinel LiNiVO₄ powders, with metal nitrates, citric acid and glycerol for three different total metal ions to glycerol ratios (M/G) as 1:1, 1:2 and 1:3. In this study, the effect of total metal ion to glycerol ratio on the formation of the nano-crystalline LiNiVO₄ phase was investigated and discussed. The thermal behavior and structural coordination of the as prepared as well as the calcined polymeric intermediates at different temperatures were investigated, respectively, using the DSC and FTIR measurements. The microstructure of the polymeric intermediates was identified by taking the scanning electron microscopy (SEM) pictures. The formation of the inverse spinel LiNiVO₄ phase, obtained from the calcined intermediates, was confirmed by comparing the observed X-ray diffraction spectra with the respective JCPDS standard. The crystallite size was calculated using the Scherrer's formula and the smallest crystallite size of 39 nm was observed for the LiNiVO₄ powders obtained from the polymeric intermediate, calcined at 450 °C for 12 h, synthesized using the GGCR with M/G=1:1.     

Direct combustion synthesis of SiC powders

In this study, we examine the possibility to direct combustion synthesize SiC powders from elements using four reactant combinations (Si and graphite; Si and carbon black; ground Si and carbon black; both Si and carbon black ground) under two reacting environments (heated by an oxy-acetylene torch in air, and heated by tungsten coils in vacuum). The experimental results showed that high-purity SiC product with no oxide present was obtained if pellet, made of Si and carbon black, was burned by an oxy-acetylene torch in air. Also, ground reactant particles yielding deteriorated effect on product conversion is also discussed.