The oxidation kinetics of magnesium at low temperatures and low oxygen partial pressures

The initial oxidation of magnesium at oxygen partial pressures between 1.3 × 10^− 8 Pa and 1.3 × 10^− 5 Pa and at temperatures just above room temperature has been investigated in situ with X-ray photoelectron spectroscopy (XPS) and ellipsometry. Quantitative analysis of the XPS spectra showed that the initially formed oxide has a higher Mg/O ratio (> 1.3) than bulk MgO. Ellipsometry measurements indicated that the band gap values of the oxide layers are considerably smaller than the value expected for bulk MgO ( 2.5 eV vs. 7.8 eV). From the XPS and ellipsometry data recorded as a function of oxidation time the oxidation kinetics have been determined. The kinetics has been described quantitatively with a coupled currents model, involving simultaneous transport of electrons and ions through the oxide layer.     

Microstructure and properties of Cu–Mg alloy treated by internal oxidation

A Cu–0.24Mg alloy bar was treated by internal oxidation using Cu₂O as an oxidiser at 1123–1273?K for 10?h. The thermodynamic diagram for oxidation of a Cu–Mg alloy was confirmed by the critical oxygen pressure. The results show that the thickness of the MgO/Cu layer and the electrical conductivity of the Cu–0.24Mg alloy increase with increasing internal oxidation temperature, whereas the average hardness of the MgO/Cu layer initially increases and subsequently decreases. Examination of the microstructure of the MgO/Cu layer revealed that Mg precipitation in the form of MgO particles and their uniform dispersion in the Cu matrix were the primary reasons for increases in the comprehensive properties of the Cu–Mg alloy treated by internal oxidation.     

镁铝合金直接燃烧法合成AlN晶体

以球形镁铝合金(Al₁₂Mg₁₇)颗粒为原料, 在空气中直接燃烧合成氮化铝(AlN)晶体。实验样品堆积在直径为 1 cm的区域内, 使用乙烷火焰点燃。使用高速摄像仪记录燃烧合成过程。借助XRD和SEM对原料和产物的组成及结构进行分析, 并使用TG-DSC分析合金的热力学性质。结果表明: 镁铝合金中的铝可以全部转化为AlN晶体。合金的点火温度约为494.4 ℃, 一旦点燃, 不需要外界热源的持续加热, 样品可持续燃烧。燃烧开始后, 合金颗粒中镁快速汽化, 与空气中氧发生优先反应, 并耗掉颗粒周围的氧气, 使氮气进入液态铝表层, 生成氮化铝。燃烧产物有明显分层, 检测结果表明上层产物为白色氧化镁, 下层产物为黑色氮化铝晶体。合成过程中, 镁对氮化铝的形成起着积极的促进作用。     

Microwave dielectric properties of W-doped Ba0.6Sr0.4TiO3 thin films grown on (001) MgO by pulsed laser deposition with a variable oxygen deposition pressure

The microwave dielectric properties of Ba_0.6Sr_0.4TiO₃ 1 mol% W-doped thin films deposited using pulsed laser deposition, are improved by a novel oxygen deposition profile. The thin films were deposited onto (001) MgO substrates at a temperature of 720 °C. A comparison is made between three different oxygen ambient growth conditions. These include growth at a single oxygen pressure (6.7 Pa) and growth at two oxygen pressures, one low (6.7 Pa) and one high (46.7 Pa). Films were deposited in a sequence that includes both a low to high and a high to low transition in the oxygen deposition pressure. Following deposition, all films were post-annealed in 1 atm of oxygen at 1000 °C for 6 h. The dielectric Q (defined as 1 / tanδ) and the dielectric constant, ε_r, were measured at room temperature, at 2 GHz, using gap capacitors fabricated on top of the dielectric films. The percent dielectric tuning (defined as (ε_r(0 V) − ε_r(40 V)) / ε_r(0 V) × 100) and figure of merit (FOM) (defined as percent dielectric tuning × Q(0 V)) were calculated. The film deposited using the two-stage growth conditions, 6.7 / 46.7 Pa oxygen, showed a maximum Q(0 V) value with high percent dielectric tuning and gave rise to a microwave FOM twice as large as the single stage growth condition. The improved dielectric properties are due to initial formation of a film with reduced interfacial strain, due to the formation of defects at the film/ substrate interface resulting in a high Q(0 V) value, followed by the reduction of oxygen vacancies which increases the dielectric constant and tuning.     

Microstructural development in the directed melt-oxidized (DIMOX) Al-Mg-Si alloys

Metal-ceramic composites produced via directed melt oxidation (DIMOX) of aluminium alloys are of recent interest. Thein situ composite forming method is based on the reaction of a molten alloy with a gaseous oxidant. In the present study, Al-Mg-Si alloys were subjected to directed melt oxidation and the progressive microstructural evolution was examined by interrupted growth experiments. In the early stages, liquid alloy oxidizes to form a duplex oxide layer (MgO+MgAl₂O₄) on the surface. The openings in these oxide layers allow the liquid alloy to wick through to form small nodules on the surface. When further wicking occurs through these nodules, a cauliflower type of colonies is formed. During the early part of the second stage, spinel growth dominates to form a multi-layered structure. In the final stage, as the magnesium reaches low levels, Al₂O₃ formation dominates the growth, and alumina crystals grow continuously for several tens of micrometres. The oxygen required for alumina formation is expected to come from two sources: (i) from the ingress of oxygen through microcracked oxide layers, and (ii) demixing of magnesium-containing oxides in the underneath layers.     

Effect of partial oxygen pressure on the initial stages of high-temperature oxidation of γ-NiCrAl alloys

Abstract

The effect of the partial oxygen pressure (pO₂) on the initial stages of oxide-layer growth of a γ-Ni-27Cr-9Al (at.%) alloy at 1373 K was studied using X-ray photoelectron spectroscopy, X-ray diffractrometry, scanning electron microscopy and electron probe X-ray microanalysis. The alloy was oxidised at a pO₂ of 0.1 and 0.2 × 105 Pa in an UHV processing chamber and a furnace, respectively. For both pressures, a double-layered oxide structure evolved upon oxidation, which consisted of a NiCr₂O₄/Cr₂O₃ layer on top of an α-Al₂O₃ layer. Two different oxidation stages were identified: an initial, very fast oxidation stage followed by a second stage of slow, parabolic growth. The initial oxidation stage was associated with the preferential oxidation of Al, followed by the internal oxidation of Al and simultaneous formation of Cr₂O₃ (and some NiO). The onset of the second stage ran parallel with the coalescence of internal α-Al₂O₃ crystallites into a continuous closed layer. Then, the internal oxidation of Al and growth of Cr₂O₃ ceased. A lower pO₂ reduced the activity of oxygen at the oxide/alloy interface during the initial, fast oxidation stage, thereby enhancing the rate of formation of a continuous closed α-Al₂O₃ layer and suppressing the formation of Cr₂O₃ (and NiO).     

The influence of reactive element additions to β-NiAlCr alloys on the morphology of thermally grown oxides

Abstract

The effect of the reactive elements (REs), Y and Zr, on oxidation of β-NiCrAl alloy at 1373 K in a gas mixture of argon with 20 vol.% oxygen at atmospheric pressure was evaluated using X-ray diffractometry, scanning electron microscopy, electron probe X-ray microanalysis and secondary ion mass spectroscopy. The oxide surfaces and interface morphologies, compositions and growth kinetics were studied for alloys with 0.32 at.% Zr and 0.24 at.% Y additions and for an undoped alloy. The oxide layer produced on the three different alloys contains mainly α-alumina and some intermediate alumina modification, Cr₂O₃ and RE-oxides. A needle-like morphology was seen on top of the oxide layer for the undoped and Zr alloy. Needle formation on the Y alloy was suppressed by the formation of a thin Y₂O₃ layer during the initial stage of oxidation. Needles were maintained to long oxidation times for the undoped alloy, but disappeared on the doped alloys indicating that some cation diffusion is possible when REs are present. Fewer intermediate alumina modifications are seen for the oxide layers on the RE alloys showing that the REs promote the formation of the α-alumina phase. Oxide layer growth occurs in two stages for all alloys. Initially, oxide growth is rapid with outward diffusion of aluminium. The second stage of oxidation is slow and is initiated by the formation of a closed α-alumina layer limiting further oxidation to inward oxygen diffusion. This stage is characterised by parabolic growth kinetics associated with a constant aluminium interface concentration. The oxide layer is thinnest for the Y alloy due the fine Y₂O₃ layer acting as a diffusion barrier. The oxide/alloy interface for the undoped alloy is flat and shows many voids, whereas voids are not seen for the RE alloys. This is due to the promotion of a closed α-alumina layer giving predominantly inward growth early in the oxidation process. Oxide pegs of the RE are also seen growing into the alloys. The lack of voids and the oxide pegs are advantageous for oxide layer adhesion to the doped alloys.     

Effects of p(O2) and p(CO2) on epitaxial growth of BaTiO3 thin films on MgO(100) substrates by using metal organic acid salts

BaTiO₃ thin films were prepared by using metal organic acid salts on MgO(100) substrates, which have large lattice-misfit with BaTiO₃. Amorphous films prefired at 470°C were crystallized to BaTiO₃ phase by heat treatment at higher temperature. Crystallinity and in-plane alignment of the prepared films were found to depend on the heat-treatment conditions. BaTiO₃ films with high crystallinity but poor (100)-orientation were obtained in air at higher than 1200°C. Whereas, (100)-oriented epitaxial BaTiO₃ film was fabricated by annealing at 900°C under low oxygen partial pressure (p(O₂)). Low carbon dioxide partial pressure (p(CO₂)) is also found to be essential for preparation of epitaxial BaTiO₃ films on MgO substrates by using metal organic acid salts.     

Kinetics of α-FeO(OH)Formation in Weak Acidic Medium

The kinetics of α-FeO(OH) formation in weakacidic medium was studied.The effects of the initialconcentration,temperature,pH value,partial pres-sure of oxygen,air flow rate and agitating intensityon the oxidation rate of Fe(OH)_2 suspensions havebeen investigated.It is confirmed that the reactionorder for Fe(Ⅱ) is zero at the stage of α-FeO(OH)seeds formation,and about 0.5 at the stage of theseeds growth,and in the whole process partial pres-sure of oxygen appears in first order.The activationenergy is determined.The dissolution-oxidationequilibrium which exists in the process of seedsformation has been suggested,by which the zeroorder reaction for Fe(Ⅱ) and the plateau in thepH-t curve were explained.Physical process is therate-determining step of the heterogeneousreaction.The study on the kinetics of α-FeO(OH)formation may provide some fundamental informa-tion for the reactor design,the engineering amplifi-cation and the optimization control in the industrialprocess.     

20wt% SiO2/Al-Mg复合材料的界面反应及其微结构

采用粉末冶金法制备了20wt% SiO₂/Al-Mg复合材料。研究了SiO₂和基体元素Al,Mg反应机制,研究表明:在原SiO₂颗粒内,形成MgAl₂O₄,MgO,Mg₂Si和少量Al和Si。MgAl₂O₄呈不规则形状,而且MgAl₂O₄往往和Al相邻;MgO和Mg₂Si形成片层状共析体;经620℃烧结30min,SiO₂被完全反应掉。反应生成物Si多数被排到Al基体中;原Al-Mg基体中主要物相为:Al,Mg₂Si和Si,Mg₂Si颗粒的尺寸小于0.2μm。原Al-Mg基体中,单质Mg已不存在,Mg反应形成Mg₂Si。     

Oxidation kinetics of low-oxygen silicon carbide fiber

The effect of partial pressure and temperature on the oxidation rate of low-oxygen silicon carbide fiber (Hi-Nicalon) has been investigated. The initial oxidation rate was described by a two-dimensional disc contracting formula for reaction control, and the activation energy was 155 kJ/mol. The rate at the later stage of oxidation obeyed the equation for diffusion control, and the activation energy was 109 kJ/mol. Both the rate constants were proportional to oxygen partial pressure. The diffusion species through the SiO₂ film are considered to be oxygen molecules.     

合金元素复合化对Ti_(2)AlNb合金高温抗氧化性能影响EI北大核心CSCD

Ti_(2)AlNb合金具有良好的工艺性能、综合力学性能和较低的密度等性能优势,是新型航空发动机的重要选材之一。为拓宽Ti_(2)AlNb合金的应用范围,需对传统Ti_(2)AlNb合金进行合金成分优化和工艺组织调控以进一步增强其高温抗氧化性能。本研究在传统Ti-Al-Nb三元合金体系基础上,综合设计Mo,Zr,W等合金复合化的方法提高Ti_(2)AlNb合金的抗氧化能力,通过对新型Ti_(2)AlNb合金在750℃和850℃的氧化增重行为分析、氧化层特征结构分析、表面氧化物种类和合金成分过渡分布分析等,发现Mo合金元素引起Ti_(2)AlNb合金在750℃上升至850℃时抗氧化性能的明显下降,Zr合金元素则始终保持着Ti_(2)AlNb合金良好的高温抗氧化能力;更为深入的截面试样SEM表征可将氧化层结构细分为氧化物层、富氧扩散层和组织演变层,Zr和W合金元素对850℃高温氧化过程中不同氧化层结构具有协同抑制作用,因此提出通过Zr和W合金元素复合的方法作为新型Ti_(2)AlNb合金抗氧化合金成分优化方向。     

SiO2对铝合金熔体直接氧化的影响

本文研究了在高温空气氛中涂覆在Al-Mg-Si合金表面的SiO₂对铝合金直接氧化的影响规律。实验揭示了SiO₂对Al-Mg-Si合金熔体直接氧化生长表面的形态的影响规律,发现SiO₂有助于Al₂O₃/Al复合材料以光滑的方式进行氧化生长,提高了材料的致密度。实验还发现,SiO₂可消减Al-Mg-Si合金熔体直接氧化所需的孕育期,缩短Al₂O₃/Al复合材料的生长时间。     

Cracking behavior of oxide scale formed on Ti3SiC2-based ceramic

The cyclic oxidation and acoustic emission (AE) tests were carried out for studying cracking behavior of oxide scales formed on Ti₃SiC₂-based ceramic at 1100 °C. A duplex oxide scale with an outer layer of pure TiO₂ and an inner layer of a mixture of TiO₂ and SiO₂ was formed. The oxide scale did not spall from substrate during the cyclic oxidation at 1100 °C for 360 times. However, a great number of micro-cracks penetrating whole inner oxide layer were detected. AE test showed that the oxide scale did not crack during the isothermal oxidation at 1100 °C for 1 h, however, the scale cracked during the cooling stage. Comparing the growth rate and thickness between the oxide layers formed during the isothermal oxidation and cyclic oxidation, respectively, indicated that cracks in the inner oxide layer served as paths mainly for outward diffusion of titanium and for inward diffusion of oxygen, resulting in increased growth rate of the outer oxide layer. Because of entire and compact TiO₂ consisted of outer oxide layer, and low thermal stress resulting from small mismatch of thermal expansion coefficients between the oxides and the substrate, Ti₃SiC₂ exhibited excellent cyclic oxidation resistance at 1100 °C for 360 cycles.     

Influence of MgO nano-crystals on the thermodynamics, hydrogen uptake and kinetics in Mg films

Hydrogen uptake in thin sputtered magnesium films covered with a Palladium layer was studied by resistance measurements. During growth, oxygen was introduced into the chamber in small amounts and the effect on the resistance, measured in situ, while growing, was monitored. This resulted in the formation of a mixture of MgO nanocrystals and layers in the Mg films. The measurements were made in situ in the sputtering chamber where the samples were grown. The aim of the study was to study the effect of oxygen contamination on hydrogen uptake in Mg films, as well as studying the uptake kinetics. Previous work on clean Mg films has shown that hydride formation at the surface reduces greatly the rate of hydrogen uptake further inside the film. These measurements show that the presence of oxygen contamination initially increases the rate of uptake greatly but decreases it when the Mg film is contaminated further with more oxygen.     

补加合金成分对铝合金熔体直接氧化生长的影响

为了控制DIMOX工艺中复合材料的生长速度和体积,采用压差法补加合金,使剩余铝合金熔体的成分得到调整并与正在氧化生长的Al₂O₃/Al复合材料层保持连续接触。试验研究了补加合金成分对Al-3Mg-10Si合金氧化生长的影响作用。结果表明:补加纯Al或低Mg、Si含量的铝合金,能够降低合金熔体的含镁量,缩短材料生长前沿合金熔体成分到达Al₂O₃-(Al,Mg)两相区的时间,促进传质过程,加速材料生长,有利于获取较大体积的复合材料。     

Al含量对漂珠/镁合金可溶复合材料组织与性能的影响

采用搅拌铸造法制备了不同Al含量下的漂珠（FAC）/镁合金可溶复合材料。采用金相显微镜、SEM及XRD观察分析了FAC/镁合金可溶复合材料的微观组织、溶解表面形貌及溶解产物的物相组成,采用力学性能试验机研究了复合材料的压缩性能,采用电化学工作站对复合材料进行电化学性能测试,在常温及水浴锅内进行复合材料在不同温度下的KCl溶液中的溶解试验。结果表明：该FAC/镁合金可溶复合材料主要由α-Mg基体相、β-Mg₁₇Al₁₂相、Mg₂Si相和MgO相组成。随着Al含量的增加,FAC/镁合金可溶复合材料的溶解速率先变快后减慢,在80℃的3wt% KCl溶液中,含15wt% Al的FAC/镁合金复合材料溶解速率最快,为56 mg/（h·cm²）。Al-FAC/镁合金可溶复合材料的抗压强度随Al含量的增加先提高后下降,四种合金的抗压强度均大于300 MPa,最高强度达到372 MPa。     

Formation of ZnO/metal composites by rapid oxidation of Zn melts

A novel ZnO-based composite material is fabricated by oxidation of Zn alloy melts, in which outward growth of the reaction products and rapid reaction kinetics are observed. Additions of Na and Bi to the Zn melts are found to be essential to increase the matrix growth rate to a practical level. Optimum reaction rates are observed for Zn-3Na-5Bi alloys oxidized at temperatures between 450 and 550°C in pure oxygen. Processing parameters, such as alloy composition and oxygen activity, also have a profound impact on the resulting microstructure and reaction kinetics. The evidences supported that the continuous outward growth of the matrix is achieved via modification of the ZnO defect structure and partial wetting of the growing oxide by the melts.     

Ni-Cr-Ta-Al-Co-Mo合金在900℃和1000℃的恒温氧化行为

用热重分析法测定了氧化动力学曲线,并采用XRD和SEM（EDAX）等手段,研究了Ni12.59Cr6.74Ta4.04A15.16Co1.35Mo6.29W合金在900℃和1000℃的高温氧化行为。结果表明,在氧化初期合金的氧化质量增加较快,随着氧化时间的延长质量增加的速率逐渐降低,其氧化动力学曲线符合抛物线规律。在9...     

Growth kinetic and characterization of Mg(x)Zn1-xO nanoneedles synthesized by thermal oxidation

Mg(x)Zn1-xO nanoneedles were synthesized on alumina substrate by using thermal oxidation technique under normal atmosphere. Zn powder and MgO powder were mixed and heated to form Mg(x)Zn1-xO with x content of 0-0.3 by mol at heating temperature and time of 400-1000 degrees C and 24 h. The morphology of Mg(x)Zn1-xO nanoneedle was characterized by filed emission scanning electron microscope (FE-SEM). It was found that the needle-like nanostructures with the sharp ends were observed outward from microparticles at 400-800 degrees C. From EDS, XRD, and TEM analysis, it was suggested that Mg(x)Zn1-xO alloy was formed with no segregation of MgO in Mg(x)Zn1-xO alloy after thermal oxidation process. Also, from reflectance spectra, the Mg(x)Zn1-xO nanoneedle exhibited higher energy gap than that of ZnO films for entire Mg content indicating widening band gap energy due to alloying effect. Moreover, we have proposed the growth mechanism of Mg(x)Zn1-xO nanoneedles based on growth kinetic of nucleation formation. This growth model can be explored to explain nanostructure of other metal-oxide alloy prepared by thermal oxidation.