Characterization of hydrothermally prepared BaTi1−xZrxO3

BaTiO₃ (BTO) and BaTi_0.8Zr_0.2O₃ (BZT) powders were prepared using the hydrothermal method, starting from BaO, TiO₂ and Zr(NO₃)₂, 7H₂O. X-ray diffraction analysis showed that the cubic phase is stable at room-temperature and the pure perovskite phase is obtained after heating the powders for 2 h at 1280 °C. The temperature dependence of the dielectric constant points to ferroelectric behavior. This ferroelectric behavior can likely be due to the presence of a possible quadraticity gradient in the grains since the cubic phase may not be ferroelectric. The diffuse character of the transition is attributed to this quadraticity gradient, to grain size distribution and (for BZT) to spatial fluctuations in the concentrations of the substituted ion (Zr) leading to the coexistence of regions of different Curie temperatures.     

Novel Crystallization Behaviors of Zr-Based Metallic Glass Under Thermo-Mechanical Coupled Fatigue Loading Condition

Novel crystallization behaviors of Zr_(55)Cu_(30)Al_(10)Ni_5 bulk metallic glass are investigated.On the one hand,mixed oxides,including CuO,CuAlO_2,CuAl_2O_4 and ZrO_2,show sequential oxidation process determined by coupling effects of specific cyclic load and temperature.On the other hand,at a temperature(100 ℃)by far lower than Tg of 412 ℃,under cyclic loading condition,non-oxidized binary alloy CuZr_2 is precipitated;the thermo-mechanical coupled effects of temperature below Tg and fatigue accumulation on the non-oxidized crystallization behaviors are revealed.Meanwhile,at a constant temperature of 400℃, by comparing among the XRD patterns,respectively,obtained from tensile,creep and fatigue fractures,the dominating effect of cyclic load on the generation of non-oxidized CuZr_2 is verified.Furthermore,the crystallization behavior of amorphous phases under cyclic loading condition is observed through TEM micrograph and diffraction pattern at 100 ℃.     

TC4合金表面扩散渗硅涂层的高温氧化行为和失效机制

为提高钛合金的高温抗氧化性能,采用固体粉末扩散渗方法在其表面制备了扩散渗硅涂层,研究了涂层的组织结构、高温氧化行为和失效机制。结果表明,所制备涂层具有致密的多层梯度组织结构,主要由TiSi₂外层,TiSi中间层和Ti₅Si₄+Ti₅Si₃内层组成。高温氧化实验结果表明,涂层在850℃空气中氧化时表面形成了由SiO₂和TiO₂混合组成的保护膜,高温抗氧化性能优良;氧化过程中,涂层与基体合金中Si和Ti的互扩散引起氧化膜内TiO₂含量增加及Si源不足,导致氧化膜保护性变差;氧化产物与涂层之间较高的P-B比、氧化膜与涂层组织间热膨胀系数不匹配导致了氧化膜开裂和剥落。     

Effects on direct synthesis of large scale mono-disperse Ni0.5Zn0.5Fe2O4 nanosized particles

Mono-disperse Ni_0.5Zn_0.5Fe₂O₄ spinel ferrite particles have been synthesized directly via the hydrothermal method using sodium dodecyl sulfate (SDS) as surfactant. Particle size could be varied from 6 to 19 nm by changing the experiment parameters. X-ray diffraction, high resolution TEM images confirmed the high crystallinity of ferrite nanocrystals. The effects of precursor suspension pH value, reaction temperature and surfactant (SDS) concentration on phase purity, particle size and dispersed property were discussed. The results indicated that mono-disperse Ni_0.5Zn_0.5Fe₂O₄ spinel ferrite nanoparticle had been obtained at pH value range (8–9), reaction temperature (90 °C) and moderate SDS concentration (>0.2 mM). The magnetic measurement shows that as prepared Ni_0.5Zn_0.5Fe₂O₄ nanoparticle possesses good super-paramagnetic behavior. We also put forward a primary experimental model to shed light on the controllability of the monodispersity of the nanosized particles.     

Multiferroic Consequence of Porous (BiFeO_3)_x–(BiCrO_3)_(1-x) Composite Thin Films by Novel Sol–Gel Method

In this work, we have presented a spin-coating method to produce thin films started with pure BiCrO_3(BCO) and ended up with BiFeO_3(BFO) by increasing x values in the(BiFeO_3)_x–(BiCrO_3)_(1-x)composites. All the produced thin films have been crystallized at the annealing temperatures of 400 °C for 0.5 h. The XRD and EDAX spectrums give insight that the two crystal phases related to BCO and BFO stayed together within the thin film matrices. SEM analysis showed that the prepared composite had macroporous morphology with interconnected pores and its width(size) decreased with increasing x values. The strong correlations are observed among the microstructure, dielectric, ferroelectric, ferromagnetic properties and Fe concentration. Among all composites, the composition of 0.75 shows an attractive magnetization, polarization, switching and improved dielectric behaviors at room temperature. Significant increase in the multiferroic characteristics of 0.75 composition is due to arise of lower leakage current by causing reduction in oxygen vacancy density, and enhancement of super-exchange magnetic interaction between Fe~(3+) and Cr~(3+) at BFO/BCO interface layers. Our result shows that the thin layer on Pt(111)/Ti/SiO_2/Si substrate possesses simultaneously improved ferroelectric and ferromagnetic properties which make an inaccessible potential application for nonvolatile ferroelectric memories.     

Gold nanoparticles stabilized by a flake-like Al2O3 support

A flake-like alumina with rough surface and small mesopores has been prepared by a hydrothermal method. Remarkably, such alumina was able to stabilize Au nanoparticles, predominantly ∼2.2 nm in size, even up to an annealing temperature of 700°C. The catalytic activity was tested using the CO oxidation model reaction where a complete conversion of 1% CO in air at 30°C was obtained.     

Microstructure and High-Temperature Oxidation Behavior of Dy-Doped Nb–Si-Based Alloys

Microstructures and oxidation behaviors of four Dy-doped Nb–Si-based alloys at 1250℃ were investigated. The nominal compositions of the four alloys are Nb–15Si–24Ti–4Cr–2Al–2Hf–xDy(at.%), where x = 0, 0.05, 0.10 and 0.15,respectively. Results showed that the four alloys all consisted of Nbss, αNb_5Si_3 and γNb_5Si_3, and the addition of Dy produced no obvious effect on the phase constitution and the microstructures of Nb–Si-based alloys. After oxidation at 1250℃ for 58 h, it was found that the addition of Dy accelerated the oxidation rate of Nb–Si-based alloys and caused a larger weight gain, accompanied by the formation of a more porous and less protective oxide scale. The oxides of Nb_2O_5,Ti_2Nb_(10)O_(29), TiNb_2O_7, Ti_(0.4)Cr_(0.3)Nb_(0.3)O_2 and glassy SiO_2 were formed on Dy-doped Nb–Si-based alloys. The hightemperature oxidation mechanism of Dy-doped Nb–Si-based alloys was discussed.     

309SMOD奥氏体耐热不锈钢的高温氧化行为

采用增重法分析了309SMOD奥氏体不锈钢板材在不同温度下的氧化行为,获得了该钢高温氧化的抛物线动力学曲线,利用SEM、EDS及XRD对氧化物的形貌和物相进行了分析。结果表明,800 ℃氧化物形貌为板状和块状,900 ℃、1000 ℃的氧化物主要为尖晶石颗粒。309SMOD奥氏体不锈钢表面由于高温氧化生成具有3层结构的混合氧化物膜,最外层结构为MnCr₂O₄和FeCr₂O₄,次外层结构的氧化物为Cr₂O₃,最内层结构的氧化物为SiO₂,这种结构的氧化膜使得309SMOD奥氏体不锈钢具有良好的抗高温氧化性能。     

Oxidation induced ductility enhancement of Zr based metallic glass ribbons in vicinity of glass transition temperature

Zr-based metallic glasses(MGs)possess a wide supercooled liquid region,which gives a wide processing window for superplastic forming to make microdevices with demanding size accuracy and surface finishing.The existence of oxygen may have an influence on the thermoplastic deformation process.Therefore,the effect of oxidation on the mechanical behavior of the MGs in the vicinity of glass transition temperature is of great significance for practical forming of MG components.In the present study,the effect of oxidation on tensile properties of Zr50Cu40Al10 metallic glass was investigated.The tested samples were characterized by XRD and SEM analysis.For the samples tested in air,the strength decreases 187 MPa,61 MPa and 59 MPa and the ductility increases 0.31,0.36,and 0.77 at 420℃,430℃,and 440℃,respectively,compared with those tested in flowing argon.ZrO₂ preferentially formed during the tensile testing at 420℃,and both ZrO₂ and Al₂O₃ oxides formed at 430℃.The dilution of Zr elements in the remaining amorphous matrix caused by preferential oxidation on the surface layer attributes to the decrease in strength and enhancement in ductility of the Zr₅₀Cu₄₀Al₁₀ metallic glasses.     

气体渗氮对F92钢在700 ℃空气中氧化行为的影响

利用扫描电镜(SEM)、能谱分析(EDS)、X射线衍射分析(XRD)研究了F92钢供货态和渗氮后在700 ℃静态空气中的高温氧化行为,阐述了气体渗氮对F92钢氧化行为的影响。研究表明,渗氮后F92钢表面形成了氮的过饱和膨胀铁素体相和CrN相。供货态F92钢表面生成了薄且致密的(Cr, Fe, Mn)₂O₃和MnCr₂O₄氧化层,具有良好的保护性。此外,供货态F92钢表面有两种瘤状氧化物生成,一种由连续的Fe₂O₃构成,另一种独立的由外层Fe₂O₃和内层富Cr的(Cr, Fe, Mn)₃O₄组成。渗氮加剧了F92钢的氧化,基体内部观察到了内氧化区。渗氮试样表面氧化膜呈双层结构,其中,外层为Fe₂O₃,内层为富Cr的Fe-Cr氧化物。F92钢渗氮过程中形成的膨胀氮过饱和铁素体相和CrN相以及氧化过程中析出的CrN沉淀相降低了铬的活性,阻碍了有保护性的富铬氧化物的生成,从而导致抗氧化性能的下降。     

Critical properties of symmetric nanoscale metal–ferroelectric–metal capacitors

The size, surface and interface effects on the magnitude and stability of spontaneous polarization in a symmetric nanoscale ferroelectric capacitor were studied by analyzing its evolutionary trajectory based on a thermodynamic model. Analytic expressions of the Curie temperature, spontaneous polarization, critical thickness and the Curie–Weiss relation were derived, taking into account the effects of the depolarization field, built-in electric field, interfaces and surfaces. Our results show that the critical properties are not only functions of the ambient temperature, misfit strain and electromechanical boundary conditions, but also depend on the characteristics of electrodes, surfaces and interfaces, through the incomplete charge compensation, near-surface variation of polarization and work function steps of ferroelectric–electrode interfaces, which are adjustable.     

Structure, hyperfine interactions and magnetization studies of mechanically alloyed Fe50Ge50 and Fe62Ge38

X-ray diffraction, Mössbauer spectroscopy and magnetization measurements were used to study the structure and some magnetic properties of Fe₅₀Ge₅₀ and Fe₆₂Ge₃₈ prepared by mechanical alloying from the elemental powders. In both cases in the early stages of milling the intermediate paramagnetic FeGe₂ phase was formed. The mechanical alloying process of Fe₅₀Ge₅₀ resulted in the formation of the paramagnetic FeGe (B20) phase with an average crystallite size of about 15 nm. In the case of the Fe₆₂Ge₃₈, the ferromagnetic Fe₅Ge₃ (β) phase with a Curie temperature of about 430 K was obtained. The average crystallite size was about 9 nm. The average hyperfine magnetic field of about 16 T allowed it to determine that more than four germanium atoms exist in the nearest environment of the ⁵⁷Fe isotopes in the Fe₅Ge₃ phase.     

Preparation of Flaky Hexagonal Nanoporous Au-Cu and Au Particles via Dealloying

Nanoporous metals have attracted significant attention owing to their excellent physical, chemical, and biological properties. However, preparing ultrafine nanoporous metal particles (1-5 μm) with specific geometries remains challenging. Herein, we report a simple strategy to prepare ultrafine flaky hexagonal nanoporous Au-Cu and Au particles via dealloying. Mg-based alloy ribbons with ultrafine flaky hexagonal Mg-Au(Cu)-Gd particles dispersed in a Mg-Cu(Au)-Gd metallic glassy matrix were prepared. The size and morphology of the precipitated flaky hexagonal Mg-Au(Cu)-Gd particles were controlled by the solidification process of a Mg₆₁Cu₂₁Au₇Gd₁₁ alloy melt. Ultrafine flaky hexagonal nanoporous Au-Cu particles (diagonal diameter 2.58 ± 0.44 μm, ligament size ~ 28 nm), Au_-1 particles (diagonal diameter 2.38 ± 0.35 μm, ligament size ~ 83 nm) and Au_-2 particles (diagonal diameter 2.39 ± 0.44 μm, ligament size ~ 66 nm) were prepared via ultrasonic-assisted dealloying of Mg₆₁Cu₂₁Au₇Gd₁₁ alloy ribbons in 0.25 M HCl/ethanol, 1 M HCl/ethanol and 0.25 M HNO₃/ethanol solutions, respectively. The ultrafine flaky hexagonal nanoporous Au-Cu and Au particles with a large specific surface area have a uniform particle size and shape, implying that they possess adequate powder fluidity and excellent catalytic properties. Moreover, the formation mechanism of the MgAu(Cu)Gd phase in solidified Mg-Cu-Au-Gd alloys was discussed. This study provides a novel approach for synthesizing nanoporous metal particles with a specific geometry.     

In Situ Transmission Electron Microscopy Observations of the Growth Process of Fe_3O_4–Ag Nanoparticles

The properties of nanocrystals are highly dependent on their morphology, composition and structure. To obtain full control over their properties, the behavior of nanocrystals under external stimuli, such as heat treatment, needs to be understood. Herein, to in situ observe their microstructure and morphology changes, Fe_3O_4–Ag heterodimers were selected as a model system. Their structural changes after heat treatment were investigated by in situ transmission electron microscopy. A combination of real-time imaging with elemental analysis enabled observation of the transformation of Fe_3O_4–Ag heterodimers having a loose interface configuration to those with a Janus structure at the atomic scale after heating from room temperature to 600 °C. After incubation at 600 °C for 32 min, two kinds of Janus structures could be seen, including a clear linear interface in the Fe_3O_4–Ag heterodimers and a semi-crescent-shaped interface between the Ag and Fe_3O_4 nanoparticles(NPs). These dynamic observations provide unique insights into NP growth mechanisms, which are essential for understanding and controlling the structure and morphology of nanoparticles.     

Investigation of the influence of thermal treatment on the morphologies, dielectric and ferroelectric properties of PZT-based ceramics

Pb[(Zr_1/2Ti_1/2)_0.9(Zn_1/3Nb_2/3)_0.1]O₃ (PZT–10PZN) powder was prepared using the columbite precursor method. The phase development of calcined powder precursors was analyzed by X-ray diffraction. Dielectric and ferroelectric properties of the as-sintered and annealed samples were measured and correlated with the microstructure. The morphological evolution was determined by scanning electron microscopy (SEM). The as-sintered ceramic exhibited weak normal-ferroelectric behavior, with a relatively low dielectric constant maximum measured at 1 kHz (_rmax at 1 kHz) of 13,000. Annealing resulted in a transition to relaxor-ferroelectric-like behavior, a shift in the dielectric maximum temperature from 360 to 350 °C, and a dramatic increase in _rmax at 1 kHz to a maximum value of 35,000 for the longer anneal. Furthermore, after thermal annealing at 900 °C for 1 week the composition shifted close to the MPB with a great reduction in the transition temperature and a broadening of the dielectric constant maximum. A strong enhancement of the remanent polarization (P_r) was also observed.     

Double SHS of ZrB2 powder

Production of ZrB₂ powder through self-propagating high-temperature synthesis (SHS) from ZrO₂, Mg and H₃BO₃ mixture often leads to incomplete conversion. A new technique, called DSHS (double SHS) has been developed, wherein the reaction product of the first SHS is mixed with calculated amounts of Mg and H₃BO₃ powder and subjected to a second SHS. The ZrB₂ powder produced by DSHS technique yields increased conversion. The NaCl is used as a diluent during SHS to control the particle size of the product. The average particle size of SHS ZrB₂ powder found to be 75–125 nm in range, which decrease to 25–40 nm after DSHS.     

Crystallisation of oxygen-stabilised amorphous phase in a Zr50Cu50 alloy

The crystallisation of the oxygen-stabilised amorphous phase in a Zr₅₀Cu₅₀ alloy has been investigated by means of neutron diffraction and electron microscopy. The crystallisation microstructure consists of ZrO₂, Zr₂Cu and Zr₇Cu₁₀. A two-stage crystallisation mechanism is suggested: (i) primary crystallisation of Zr₂Cu and (ii) formation of nanocrystals ZrO₂ and Zr₇Cu₁₀. In (i), it is proposed, Zr₂Cu crystallises from the oxygen-stabilised amorphous phase, leaving an oxygen- and copper-enriched matrix ; Zr₂Cu rapidly grows and eventually attains a grain size of about 100 nm. In (ii), it is suggested, the residual amorphous matrix crystallises into nanocrystals ZrO₂ and Zr₇Cu₁₀ due to the sluggish growth of ZrO₂ and to the already formed ZrO₂ which acts as a growth barrier to Zr₇Cu₁₀. In this case there is no particular orientation relationship between Zr₂Cu and Zr₇Cu₁₀.     

Synthesis and catalytic property of Cu-Mn-Ce／γ-Al2O3 complex oxide

A new type of catalytic material for purification of automobile exhaust, Cu-Mn-Ce-O/γ-Al2O3, has been studied. The factors affecting its catalytic activity, such as calcination temperature and the period of calcinations and so on have been investigated. Its catalytic activity after SO2-poisoning was determined in a hxed-bed reactor by exposing the sample to the atmosphere of 160 mL/min SO2/air. The study reveals that the catalyst has shown high catalytic activities for the conversion of NH3 oxidation by NO after sulfate. The conversion of NO reduction over the sulfated catalyst is somewhat higher than that over the fresh catalyst except that the optimum temperature has increased about 100℃. Also at the optimum process for the experiment, the selective catalytic oxidation of CO by NO is Over 76 % and the conversion of NO reduction is over 80 % by NH3.     

Fe3O4特性对单晶SiC固相芬顿反应研磨丸片性能的影响

为提高单晶SiC研磨加工质量和加工效率,制备固相芬顿反应研磨丸片,研究固相催化剂Fe₃O₄特性(粒径和质量分数)对研磨丸片的硬度、抗弯强度、气孔率、催化性能及其对单晶SiC研磨加工性能的影响。结果表明：随着Fe₃O₄粒径的增大,丸片的硬度和抗弯强度减小、气孔率增大、催化性能减弱,材料去除率MMRR从43.12 nm/min降到36.82 nm/min,表面粗糙度Ra从1.06 nm增大到3.72 nm。随着Fe₃O₄质量分数的增大,丸片的硬度和抗弯强度减小、气孔率增大、催化性能增强,MMRR从40.14 nm/min降到33.51 nm/min,表面粗糙度Ra先减小后增大,分别为3.25 nm、1.75 nm和1.88 nm。     

Enhancing High-Temperature Strength and Thermal Stability of Al_2O_3/Al Composites by High-Temperature Pre-treatment of Ultrafine Al Powders

Amorphous Al_2 O_3-reinforced Al composite(am-Al_2 O_3/Al) compacted from ultrafine Al powders for high-temperature usages confronts with drawbacks because crystallization of am-Al_2 O_3 at high temperatures will result in serious strength loss.Aiming at this unsolved problem,in this study,high-temperature Al materials with enhanced thermal stability were developed through introducing more thermally stable nano-sized particles via high-temperature pre-treatment of ultrafine A1 powders.It was found that the pre-treatment at ≤550℃ could introduce a few Al_2 O_3 in the Al matrix and increase the strength of the composites,but the strength was still below that of am-Al_2 O_3/Al because without being pinned firmly,grain boundaries(GBs) were softened at high temperature and intergranular fracture happened.When the pre-treatment was carried out at 600℃,nitridation and oxidation processes happened simultaneously,producing large numbers of intergranular(AlN+γ-Al_2 O_3) particles.GB sliding and intergranular fracture were suppressed;therefore,higher strength than that of am-Al_2 O_3/Al was realized.Furthermore,the(AIN+γ-Al_2 O_3)/Al exhibited more superior thermal stability compared to amAl_2 O_3/Al for annealing treatment at 580℃ for 8 h.Therefore,an effective way to fabricate high-temperature Al composite with enhanced thermal stability was developed in this study.