Effect of surface oxidation on the hydriding and dehydriding of Mg<Subscript>2</Subscript>Ni alloy produced by hydriding combustion synthesis

Hydriding combustion synthesis (HCS) has been regarded as an innovative process for the preparation of high active magnesium-based hydrogen storage alloys. For the purpose of understanding the interrelation of the unique hydrogen storage properties and the surface characteristics of the HCS product, the samples of Mg₂Ni alloy/hydride with and without exposure to air were prepared from the HCS product of Mg₂NiH₄. The hydriding and dehydriding properties were compared and the surface compositions were analyzed by means of X-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES). It was shown that the air exposure considerably decreases the hydriding activity of Mg₂Ni. Absorbing of 3.0 wt.% of hydrogen under the conditions of 603 K and 3.0 MPa after the air exposure takes 1500 s, which is six times longer than for the unexposed alloy. The hydrogen desorption of the hydride are also impeded by the air-exposure, which results in the increase of dehydriding temperature from 450 K to 540 K. XPS and AES analyses indicated that Mg segregates and exists in the form of hydroxide on the surface of the air-exposed sample, which is responsible for the degradation of the hydriding and dehydriding properties. It was confirmed that the fresh surfaces generated during the dehydriding process of the as-synthesized hydride product contributes to the high activity of the HCS product in the first cycle of the hydriding determination.     

不同制样方式对氢化燃烧合成Mg2NiH4的影响

采用XRD、SEM、TG DSC研究了不同的制样方式对氢化燃烧合成Mg2NiH4的影响,结果表明:相同条件下,镁粉、镍粉直接氢化燃烧合成比镁粉、镍粉压片和镀镍镁片更容易合成Mg2NiH4。生成的Mg2NiH4更纯,放氢温度也相对集中。     

Combustion synthesis and characterization of spherical PZT powder

A simple and effective method for the synthesis of PZT powder is reported. Fine spherical powders of PZT [Zr]/[Ti]=52/48 in mol ratio were prepared by solid-state combustion synthesis method based on the reaction of Pb₃O₄, ZrO₂, and TiO₂, activated by KClO₃+1.5C mixture. The powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and LPSA methods. The PZT powder consists of nearly homogenous and spherical grains with an average diameter of 1 μm. An approximate explanation of spherical shape formation is proposed.     

Hydriding of Mg<Subscript>2</Subscript>Ni in ammonia

The chemical interaction between the intermetallic compound Mg₂Ni and ammonia in the presence of NH₄Cl as an activator is investigated at temperatures from 100 to 450 °C, and the reaction scheme is presented. The results demonstrate that the use of ammonia for hydriding/nitriding the intermetallic compound makes it possible to prepare various magnesium compounds (Mg₂NiH₄, Mg₃N₂, and Mg (NH₂)₂) in a highly dispersed state.     

氢化燃烧合成Mg2NiH4产物高活性特征及分析

氢化燃烧合成法(HCS法)是镁基储氢合金制备的新方法,具有省能、省时、产物高活性等显著优点,近年来引起了国内外广泛关注.本文通过XRD、SEM和PCT等手段研究了氢化燃烧合成Mg2NiH4产物高活性的特征,并从物相组成、颗粒特性和氢化反应等方面揭示了HCS产物高活性的机理.结果表明,氢化燃烧合成Mg2NiH4产物在没有任何活化处理的前提下,第一次吸氢就能达到饱和吸氢量,且在不同温度下均具有比传统熔炼法产物Mg2Ni高的氢化活性;纯的物相组成、多孔的颗粒特性、大量微裂纹的存在和小的晶粒尺寸是其高活性的内在原因.本研究对控制HCS产物的微结构并进一步改善其储氢性能具有重要意义.     

Mg95Ni5-30wt%La0.7Mg0.3Ni2.8Co0.5复合物的吸放氢性能研究

先采用氢化燃烧合成法制备Mg95Ni5,然后将氢化燃烧合成产物与30%(质量分数)La0.7Mg0.3Ni2.8Co0.5合金进行机械球磨复合,球磨时间分别为5、10、15和20h;将Mg95Ni5的氢化燃烧合成产物直接球磨10h用于对比研究.采用X射线衍射仪、扫描电镜、能谱仪及气体反应控制器研究了材料的相组成、微观形貌、颗粒化学成分以及吸放氢性能.研究表明,球磨10h的Mg95Ni5/La0.7Mg0.3Ni2.8Co0.5复合物具有最佳的吸放氢性能,在373K,50s内基本达到饱和吸氢量3.78%(质量分数);在523K,1800s内放氢量为3.83%(质量分数);其起始放氢温度为425K,与Mg95Ni5相比降低了35K,吸放氢性能的改善与复合物的组织结构密切相关.此外,La0.7Mg0.36Ni2.8Co0.5的加入改善了复合物的放氢动力学性能.     

Mechanically activated combustion synthesis of Ti3AlC2/Al2O3 nanocomposite from TiO2/Al/C powder mixtures

Ti₃AlC₂/Al₂O₃ nanocomposite powder was synthesized by mechanical-activation-assisted combustion synthesis of TiO₂, Al and C powder mixtures. The effect of mechanical activation time of 3TiO₂-5Al-2C powder mixtures, via high energy planetary milling (up to 20?h), on the phase transformation after combustion synthesis was experimentally investigated. X-ray diffraction (XRD) was used to characterize as-milled and thermally treated powder mixtures. The morphology and microstructure of as-fabricated products were also studied by scanning electron microscopy (SEM) and field-emission gun electron microscopy (FESEM). The experimental results showed that mechanical activation via ball-milling increased the initial extra energy of TiO₂-Al-C powder mixtures, which is needed to enhance the reactivity of powder mixture and make it possible to ignite and sustain the combustion reaction to form Ti₃AlC₂/Al₂O₃ nanocomposite. TiC, AlTi and Al₂O₃ intermediate phases were formed when the initial 10?h milled powder mixtures were thermally treated. The desired Ti₃AlC₂/Al₂O₃ nanocomposite was synthesized after thermal treatment of 20?h milled powder and consequent combustion synthesis and FESEM result confirmed that produced powder had nanocrystalline structure.     

Low temperature solid-phase synthesis of tetragonal BaTiO3 powders and its characterization

We report an alternative synthesis route for tetragonal and submicron barium titanate particles through a low temperature isothermal heat treatment of a precursor powder obtained from the BaO₂-TiO₂-C mixture utilizing combustion synthesis technique. Effort has been concentrated on the investigation and optimization of the synthesis process including combustion temperature and velocity, heat treatment procedure, and the characteristics of the powder obtained. It was shown that precursor powder prepared by the combustion method easily transformed to the tetragonal BaTiO₃ starting from 700 °C. Characterization of the products was performed by powder X-ray diffraction, scanning electron microscopy (SEM) and BET. From the results, it has been found that the formation of tetragonal BaTiO₃ powder at low temperature is conditioned by high chemical activity and specific characteristics of combustion products. The BaTiO₃ specimens were found to possess high dielectric constant and low dissipation factor at room temperature.     

The study of combustion synthesis of fine-particle γ-lithium aluminate

Fine-particle -lithium aluminate has been prepared by the combustion of mixtures of the metal nitrates, M(NO₃) _x (M = Li, Al) as oxidizers and urea and citric acid as fuels, at low temperature and short reaction time. The combustion products were identified from X-ray powder diffraction (XRD) patterns, scanning electron microscopy (SEM), and pore size distribution measurements. As the total composition of oxidizing and reducing elements of the reactants leave related to the thermochemistry of the system, a simple method of calculating the elemental stoichiometric coefficient was introduced. The physical significance of such a correlation based on thermochemical reasoning was investigated. It was found that a significant improvement was achieved in the reactant mixture with a molar ratio of urea:citric acid = 9:1. An improvement in the combustion synthesis of -lithium aluminate is due to the formation of an activated complex of the fuel with the corresponding oxidizer.     

Hydrogen storage properties and phase structures of RMg2Ni (R = La,Ce, Pr,Nd) alloys

RMg₂Ni alloys were prepared by inductive melting where R is rare earth (R = La, Ce, Pr, Nd). X-ray diffraction (XRD) patterns revealed a single-phase composition of RMg₂Ni phase when R was one of the three elements (La, Pr, Nd), and a double-phase composition of CeMg₂Ni and CeMg₃ phases when R was Ce. In the hydriding process, RMg₂Ni phases transformed to rare earth hydrides (R-H) and Mg₂NiH₄ phase, and for CeMg₃ phase, it is decomposed to CeH_2.74 and MgH₂ phases. The enthalpy change of Mg₂Ni phase in RMg₂Ni alloys during the hydriding/dehydriding process was smaller compared with that of pristine Mg₂Ni alloy, which could be attributed to the existence of R-H. The hydrogen storage properties of RMg₂Ni alloys changed with different R compositions in R-H. At 573 K, the NdMg₂Ni alloy had the highest hydrogen storage capacity and dehydriding plateau, and the descending order of hysteresis was PrMg₂Ni < NdMg₂Ni < CeMg₂Ni < LaMg₂Ni, which suggested that the PrMg₂Ni alloy exhibited a better cycling stability and reversibility than the other three alloys. At 523 K, the uptake time of RMg₂Ni alloys to reach 90% of the maximum hydrogen storage capacity was 75 s, 34 s, 65 s and 52 s, respectively, compared with 110 s of pristine Mg₂Ni alloy. Therefore, we believed the R-H in the alloys not only improved their thermodynamic properties but also accelerated their hydriding kinetics.     

The structure of the low temperature phase Mg2NiH4(LT)

The crystal structure of a low temperature modification of Mg₂NiH₄(LT), stable below 235°C, has been determined from Guinier-Hägg powder diffraction data. The unit cell dimensions are $\text{a}\text{= 6.497(2)}\text{A}\text{?}$, $\text{b}\text{= 6.414(1)}\text{A}\text{?}$, $\text{c}\text{= 6.601(2)}\text{A}\text{?}$ and β = 93.23(2)°. The structure has been refined by profile analysis from a sample also containing MgH₂ and small amounts of two other phases, viz. the high temperature modification Mg₂NiH_3.9(HT) and Mg₂NiH_x(LT). It is indicated that the phase transformation of Mg₂NiH_3.9 (HT) at 235°C is eutectoid, giving mainly Mg₂NiH₄(LT) but also small amounts of a less hydrogen containing phase Mg₂NiH_x(LT) (x≈2).     

On the mechanism of mechanochemical synthesis of Ti2SC from Ti/FeS2/C mixture

In this study, the synthesis of Ti₂SC MAX phase by high energy ball milling, and the effects of heat treatment on ball milled powder was investigated. To this aim, a mixture of Ti, FeS₂ (as sulfur source), and C according to Ti₂SC stoichiometry, were ball milled by a planetary ball mill for different milling periods up to 10 h. The structural evolution, and the morphology of the products was studied by x-ray diffraction (XRD), and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), respectively. The results showed that after 10 h of ball milling, the raw materials reacted together and resulted in the formation of Ti₂SiC and TiC phases. The ball milled powder was then compacted and heat treated at 1000 and 1200 °C. Heat treatment caused the progressing of synthesis reactions, and led to increasing the purity of Ti₂SC phase. The heat-treated powder was leached in 1 M HCl for 2 h to remove iron from the product. The XRD results confirmed successful iron removal by leaching. SEM micrographs of the final product revealed the specific lamellar structure of MAX phases. Elemental mapping confirmed the homogeneous distribution of Ti, S and C elements.     

Nanoparticles of silver powder obtained by mechano-chemical process

The silver (Ag) powder was synthesised in a mechano-chemical (MC) process by inducing a solid-state displacement reaction between silver chloride (AgCl) and copper (Cu). The AgCl and Cu were ground in atmosphere conditions using a planetary ball mill. The reaction caused the mixture of AgCl and Cu to change the composition of the mixture, such as Ag and copper chloride (CuCl). CuCl was separated from MC product by leaching with ammonium hydroxide and we obtained Ag powder as the final product. Moreover, ascorbic acid (C₆H₈O₆) was used as the additive to improve dispersion of Ag powder during MC process. The ground powders, formed in the presence of additive, were characterised by X-ray diffraction (XRD) and scanning electron microscope (SEM). The XRD determined that the reaction between AgCl and Cu was complete in almost all the experiments carried out. SEM examinations revealed that the size of the particles in the synthesised metallic Ag powder was in the range of 30–300 nm.     

Mechanochemical Synthesis of Intermetallic Hydrides at Elevated Hydrogen Pressures

An apparatus is described for producing gas pressures of up to 10 MPa in the grinding vessels of a planetary mill. Mechanochemical reactions are carried out for the first time at gas pressures above 1 MPa. A novel approach to the synthesis of intermetallic hydrides is proposed which involves mechanical activation of a metal mixture at a high hydrogen pressure. Using this approach, two new hydrides are synthesized: Mg₂NiH₆ and a magnesium copper hydride with the approximate composition MgCuH₂.     

Effect of HCl concentration on TiB2 separation from a self-propagating high-temperature synthesis (SHS) product

This paper presents a synthesis of TiB₂ powder via self-propagating high-temperature synthesis (SHS) method using a mixture of TiO₂, B₂O₃ and Mg followed by acid leaching. In the acid leaching step, the MgO content in the SHS product was leached in different HCl concentrations. X-ray diffraction (XRD) analysis showed that when 9.3 M HCl was used, the leached SHS product was found similar to that of the commercial TiB₂ powder. However, scanning electron microscopy (SEM) and BET surface area analysis revealed that the leached product was agglomerated and exhibited very high surface area.     

Mechanochemical assisted synthesis of B4C nanoparticles

The present work reports on the preparation of boron carbide nanoparticles by the reduction of boron oxide with magnesium in the presence of carbon using the mechanochemical processing. The phase transformation and microstructure of powders during ball milling were investigated by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results showed that during ball milling the B₂O₃–Mg–C reacted with a self-propagating combustion mode producing MgO and B₄C compounds. To separate B₄C from the milled powder mixture, an appropriate leaching process was used. After leaching, the purified powder mixture was characterized using XRD and transmission electron microscope (TEM). XRD studies indicated that the prepared particles were single phase crystalline B₄C. Moreover, TEM studies showed the size of B₄C particles were ranging from 10 to 80 nm.     

Verification of the rate-controlling steps in the hydriding reaction of Mg2Ni

The rate-controlling steps in the hydriding reaction of Mg₂Ni are verified by comparing the incubation periods in the hydriding reaction of the Mg₂Ni alloy and a mechanically-alloyed mixture of 2Mg and Ni, by using gas mixtures of hydrogen and argon, and by varying the sample weight in the hydriding reaction.     

Evaluation and characterization of nanostructure hydroxyapatite powder prepared by simple sol-gel method

Many attempts have been focused on preparing of synthetic hydroxyapatite (HA), which closely resembles bone apatite and exhibits excellent osteoconductivity. Low temperature formation and fusion of the apatite crystals have been the main contributions of the sol-gel process in comparison with conventional methods for HA powder synthesis. This paper describes the synthesis of nano-HA particles via a sol-gel method. Nanocrystalline powder of hydroxyapatite (HA) was prepared using Ca(NO₃)₂·4H₂O and P₂O₅ by a simple sol-gel approach. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for characterization and evaluation of the phase composition, morphology and particle size of products. The presence of amorphous and crystalline phases in the as-dried gel precursor was confirmed by the evaluating technique. Single phase of HA was also identified in the heat treated powder by XRD patterns. SEM and TEM evaluations showed that the obtained powder after heat treatment at 600 °C was agglomerated and composed of nanocrystalline (25-28 nm) HA particles. Increasing the sintering temperature and time could cause decomposition of HA into β-tricalcium phosphate and calcium oxide. The prepared nanocrystalline HA is able to improve the contact reaction and the stability at the artificial/natural bone interface for medical applications.     

镁基储氢合金氢化物Mg2NiH4的制备及性能研究

以不经压制的Mg、Ni混合粉末为原料,利用氢化燃烧合成法在合成温度850 K和1.8 MPa初始合成氢压下制备了镁基储氢合金氢化物Mg2NiH4,并利用XRD及PCT仪分析了其物相组成和储氢性能.研究表明,产物由单一物相Mg2NiH4组成,无未反应的Ni和不完全氢化的Mg2NiH0.3;相对于传统熔炼法制备的Mg2Ni,氢化燃烧合成产物具有更高的氢化活性,在没有任何活化处理的前提下,第一次吸氢就能以很快的速度达到饱和吸氢量,同时在任何吸氢温度下均具有较好的吸氢动力学性能,且随温度的降低,最大吸氢量降低幅度较小,平台压和吸放氢温度的关系为:lgP(0.1 MPa)=-3 187.6/ T 6.362 4(吸氢),lgP(0.1 MPa)=-3 468.4/T 6.694 3(放氢).     

Synthesis and regulation of α-LiZnPO4·H2O via a solid-state reaction at low-heating temperatures

A simple and novel route for the synthesis of a lithium zinc phosphate hydrate, α-LiZnPO₄·H₂O, was studied, and the target product was obtained with LiH₂PO₄·H₂O and ZnCO₃ as raw materials and polyethylene glycol-400 (PEG-400) as a surfactant via a one step solid-state reaction at room temperature (25 °C). The product was characterized with X-ray powder diffraction (XRD), thermogravimetric analysis and the 1st derivativative of thermogravimetric analysis (TG/DTG) and Fourier transform infrared spectroscopy (FTIR). The comparison experimental results suggested that aging temperature controlled the products of the synthesis, that is, the α-LiZnPO₄·H₂O was formed when the reaction mixture was aged at room temperature, and the α-LiZnPO₄ was obtained when the reaction mixture was aged at 80 °C.