Y2O3添加量对Al2O3/ZrO2共晶陶瓷显微组织及力学性能的影响

为探索第三组元Y₂O₃添加对Al₂O₃/ZrO₂共晶陶瓷显微组织与机械性能的影响,本文利用低温度梯度的高温熔凝法制备了直径为20 mm的Al₂O₃/ZrO₂(Y₂O₃)共晶陶瓷块体,采用SEM、EDS及XRD技术对共晶陶瓷进行微结构分析,并利用维氏压痕法对其硬度和断裂韧性进行测试。SEM结果表明,凝固组织由群集的共晶团结构组成,随着Y₂O₃添加量的增加,共晶团形态由胞状转变为枝晶状,内部相间距在1~2 μm范围内变化。力学测试表明,Y₂O₃摩尔分数小于1.1%时,由于组织内部存在低硬度m-ZrO₂及微裂纹缺陷,故陶瓷硬度较低,约为(9.53±0.22 )GPa;当Y₂O₃摩尔分数为1.1%时,陶瓷硬度最大,约为(18.05±0.27)GPa;当Y₂O₃的摩尔分数大于1.1%时,由于共晶团边界区内气孔缺陷及粗大组织增多,引起陶瓷硬度值略有下降。低Y₂O₃摩尔分数添加时,陶瓷断裂韧性相对较高,约为(6.30±0.16)MPa·m^1/2,这与其内部存在大量微裂纹缺陷有关;随着Y₂O₃添加量的增加,陶瓷的微裂纹数量减少、边界区内缺陷增多,断裂韧性降低。     

First principles investigation on the structural, mechanical and electronic properties of OsC2

The structural, mechanical and electronic properties of OsC₂ were investigated by use of the density functional theory. Seven structures were considered, i.e., orthorhombic Cmca (No. 12, OsSi₂), Pmmn (No. 59, OsB₂) and Pnnm (No. 58, OsN₂); tetragonal P4₂/mnm (No. 136, OsO₂) and I4/mmm (No. 139, CaC₂); cubic Fm–3m (No. 225, CaF₂) and Pa-3 (No. 205, PtN₂). The results indicate that Cmca in OsSi₂ type structure is energetically the most stable phase among the considered structures. It is also stable mechanically. OsC₂ in Pmmn phase has the largest bulk modulus 319 GPa and shear modulus 194 GPa. The elastic anisotropy is discussed.     

Microstructure and mechanical properties of ZrB2-SiC composites

A ZrB₂-based composite containing 20 vol.% nanosized SiC particles (ZSN) was fabricated at 1900 °C for 30 min under a uniaxed load of 30 MPa by hot-pressing. The microstructure and mechanical properties of the composite were investigated. It was shown that the grain growth of ZrB₂ matrix was effectively suppressed by submicrosized SiC particles located along the grain boundaries. In addition, the mechanical properties of ZSN composite were strongly improved by incorporating the nanosized SiC particles into a ZrB₂ matrix, especially for flexural strength (925 ± 28 MPa) and fracture toughness (6.4 ± 0.3 MPa•m^1/2), which was much higher than that of monolithic ZrB₂ and ZrB₂-based composite with microsized SiC particles, respectively. The formation of intragranular nanostructures plays an important role in the strengthening and toughening of ZrB₂ ceramic.     

Synthesis and mechanical properties of high-purity Cr2AlC ceramic

High-purity and dense Cr₂AlC has been successfully fabricated by hot-pressing, using Cr, Al and graphite as raw materials. Delamination, kink bands, monolamellar kink, transgranular crack and transgranular fracture of bulk Cr₂AlC are found during the room-temperature test. The density, Vickers hardness, flexural strength, Young's modulus, compressive strength and fracture toughness of the Cr₂AlC are 5.17 g/cm³, 4.9 GPa, 469 ± 27 MPa, 282 GPa, 949 ± 22 MPa and 6.22 ± 0.26 MPa m^1/2, respectively. The strength of Cr₂AlC could be greatly improved by second phase of Cr₇C₃. And the slipping of basal planes and slip system cold be hindered by Cr₇C₃, thus resulting in a lower toughness.     

Synthesis, structure and exchange bias in Cr2O3/CrO2/Cr2O5 particles

We report on the synthesis, structure and magnetic properties of a novel exchange bias system with Cr₂O₃/CrO₂/Cr₂O₅ interfaces. Chromium oxide particles with mixed chromium valences were prepared by sintering CrO₃ in air. X-ray diffraction patterns show that CrO₃ lost its oxygen gradually with increasing temperature and time through Cr₃O₈, Cr₂O₅, CrO₂, and finally Cr₂O₃ at temperatures above 760 K. X-ray photoelectron spectra indicate a low CrO₂ content and a binding energy of 579.3 eV for Cr 2p_3/2 photoelectrons in Cr₂O₅. Chromium dioxide was found to stably coexist with Cr₂O₃ and Cr₂O₅ in the particles. Magnetic measurements show hysteresis loop shifts in the sample, indicating an exchange bias induced by antiferromagnetic Cr₂O₃/Cr₂O₅ in ferromagnetic CrO₂. An exchange bias of 9 mT at 5 K and a coercivity of 26.3 mT were observed in the chromium oxide particles containing CrO₂.     

Mechanical properties and microstructure of Al2O3/TiAl in situ composites doped with Cr2O3

Al₂O₃/TiAl composites were successfully fabricated from powder mixtures of Ti, Al, TiO₂ and Cr₂O₃ by a hot-press-assisted exothermic dispersion method. The effect of the Cr₂O₃ addition on the microstructures and mechanical properties of Al₂O₃/TiAl composites was characterized, and the results showed that the Rockwell hardness, flexural strength and fracture toughness of the composites increased as the Cr₂O₃ content increased. When the Cr₂O₃ content was 2.5 wt%, the flexural strength and the fracture toughness attained peak values of 925 MPa and 8.55 MPa m^1/2, respectively. This improvement of mechanical properties was due to the more homogeneous and finer microstructure developed from the addition of Cr₂O₃ and an increase in the ratio of α₂-Ti₃Al to γ-TiAl matrix phases.     

Effect of Nb2O5 on the microstructure and mechanical properties of TiAl based composites produced by hot pressing

In situ composites of TiAl reinforced with Al₂O₃ particles are successfully synthesized from an elemental powder mixture of Ti, Al and Nb₂O₅ by the hot-press-assisted reaction synthesis (HPRS) method. The as-prepared composites are mainly composed of TiAl, Al₂O₃, NbAl₃, as well as small amounts of the Ti₃Al phase. The in situ formed fine Al₂O₃ particles tend to disperse on the matrix grain boundaries of TiAl resulting in an excellent combination of matrix grain refinement and uniform Al₂O₃ distribution in the composites. The Rockwell hardness and densities of TiAl based composites increase gradually with increasing Nb₂O₅ content, and the flexural strength and fracture toughness of the composites have the maximum values of 634 MPa and 9.78 MPa m^1/2, respectively, when the Nb₂O₅ content reaches 6.62 wt.%. The strengthening mechanism was also discussed.     

Synthesis of Mo5SiB2 based nanocomposites by mechanical alloying and subsequent heat treatment

In this study, systematic investigations were conducted on the synthesis of Mo₅SiB₂-based alloy by mechanical alloying and subsequent heat treatment. In this regard, Mo-12.5 mol% Si-25 mol% B powder mixture was milled for different times. Then, the mechanically alloyed powders were heat treated at 1373 K for 1 h. The phase transitions and microstructural evolutions of powder particles during mechanical alloying and heat treatment were studied by X-ray diffractometry and scanning electron microscopy. The results showed that the phase evolutions during mechanical alloying and subsequent heat treatment are strongly dependent on milling time. After 10 h of milling, a Mo solid solution was formed, but, no intermetallic phases were detected at this stage. However, an α-Mo-Mo₅SiB₂ nanocomposite was formed after 20 h of milling. After heat treatment of 5 h mechanically alloyed powders, small amounts of MoB and Mo₂B were detected and α-Mo-MoB-Mo₂B composite was produced. On the other hand, heat treatment of 10 h and 20 h mechanically alloyed powders led to the formation of an α-Mo-Mo₅SiB₂-MoSi₂-Mo₃Si composite. At this point, there is a critical milling time (10 h) for the formation of Mo₅SiB₂ phase after heat treatment wherein below that time, boride phase and after that time, Mo₅SiB₂ phase are formed. In the case of 20 h mechanically alloyed powders, by increasing heat treatment time, not only the quantity of α-Mo was reduced and the quantity of Mo₅SiB₂ was increased, but also new boride phases were formed. Finally, after 5 h heat treatment, the Mo phase completely disappeared and a Mo₅SiB₂-based composite was completely formed.     

Effect of the post-deposition annealing on electrical characteristics of MIS structures with HfO2/SiO2 gate dielectric stacks

In this work, we report on effects of post-deposition annealing on electrical characteristics of metal–insulator–semiconductor (MIS) structures with HfO₂/SiO₂ double gate dielectric stacks. Obtained results have shown the deterioration of electro-physical properties of MIS structures, e.g. higher interface traps density in the middle of silicon forbidden band (D_itmb), as well as non-uniform distribution and decrease of breakdown voltage (U_br) values, after annealing above 400 °C. Two potential hypothesis of such behavior were proposed: the formation of interfacial layer between hafnia and silicon dioxide and the increase of crystallinity of HfO₂ due to the high temperature treatment. Furthermore, the analysis of conduction mechanisms in investigated stacks revealed Poole–Frenkel (P–F) tunneling at broad range of electric field intensity.     

Synthesis and characterization of nanocrystalline manganese pyrophosphate Mn2P2O7

Mn₂P₂O₇ polyhedral particles were synthesized by simple and cost-effective method using manganese nitrate hydrate and phosphoric acid in the presence of nitric acid with further calcinations at the temperature of 800 °C. The crystallite size obtained from X-ray line broadening is 31 ± 13 nm for the Mn₂P₂O₇. The X-ray diffraction and SEM results indicated that the synthesized nanoparticles have only the structure without the presence of any other phase impurities. The FT-IR and FT-Raman spectra show characteristic bands of the P₂O₇⁴⁻ anion. The UV–Vis–NIR spectrum confirms the octahedral coordination of Mn²⁺ ion.     

Effect of annealing treatment on mechanical properties of a ZrB2-SiC-graphite ceramic

The ZrB₂-SiC-graphite (ZSG) ceramic was annealed at 1600, 1700 and 1800 °C for different times, respectively. It was revealed that the annealing treatment is favorable to increase the interface bonding and relative density, and to decrease the thermal residual stress, to result in the crimp of the graphite flake. It was the optimal annealing treatment process conditions of 1700 °C and 90 min according to the best combination of the mechanical properties. To compare the mechanical properties of the specimen before the annealing treatment, the hardness, strength and toughness of the specimen annealed at 1700 °C for 90 min were enhanced by 20.6% and 20.2%, decreased by 8.2%, respectively.     

Growth studies and characterization of silicon nitride thin films deposited by alternating exposures to Si2Cl6 and NH3

We deposited silicon nitride films by alternating exposures to Si₂Cl₆ and NH₃ in a cold-wall reactor, and the growth rate and characteristics were studied with varying process temperature and reactant exposures. The physical and electrical properties of the films were also investigated in comparison with other silicon nitride films. The deposition reaction was self-limiting at process temperature of 515 and 557 °C, and the growth rates were 0.24 and 0.28 nm/cycle with Si₂Cl₆ exposure over 2 × 10⁸ L. These growth rates with Si₂Cl₆ are higher than that with SiH₂Cl₂, and are obtained with reactant exposures lower than those of the SiH₂Cl₂ case. At process temperature of 573 °C where the wafer temperature during Si₂Cl₆ pulse is 513 °C, the growth rate increased with Si₂Cl₆ exposure owing to thermal deposition of Si₂Cl₆. The deposited films are nonstoichiometric SiN, and were easily oxidized by air exposure to contain 7-8 at.% of oxygen in the bulk film. The deposition by using Si₂Cl₆ exhibited a higher deposition rate with lower reactant exposures as compared with the deposition by using SiH₂Cl₂, and exhibited good physical and electrical properties that were equivalent or superior to those of the film deposited by using SiH₂Cl₂.     

First-principles study of structural and mechanical properties of AgB2 and AuB2 compounds under pressure

In this work, density functional theory calculations on the structural, mechanical, and lattice dynamical properties of AgB₂ and AuB₂ compounds in AlB₂, OsB₂, and ReB₂ structures are reported. Generalized gradient approximation has been used for modeling exchange-correlation effects. The detailed information is given for the energetically most stable structure for AgB₂ and AuB₂ compounds. Specifically, the lattice parameters, bulk modulus, cohesive energies, elastic constants, shear modulus, Young’s modulus, Poison’s ratio, Debye temperature, sound velocities, and anisotropic factors are studied. The elastic properties are also studied under pressure. The phonon dispersion curves and corresponding phonon density of states are calculated and discussed. Our structural and some other results are in agreement with the available experimental and theoretical data.     

Synthesis of SrAl2O4 from a mixed-metal citrate precursor

A mixed-metal citrate precursor method was used to synthesize SrAl₂O₄. The effects of the pH of the starting solutions and the molar ratio of citric acid to total metal cations concentration (CA/M) on the formation of SrAl₂O₄ were studied. DTA, TG, FT-IR, XRD and field emission scanning electron microscopy (FESEM) were used to characterize the precursors and the derived oxide powders. XRD analysis showed that single-phase SrAl₂O₄ was synthesized from CA/M = 2 precursors at a temperature of 900 °C for 2 h, without the formation of any intermediate phase.     

Study of the mechanical properties of CeO2 layers with the nanoindentation technique

The mechanical properties of CeO₂ layers that are undoped or doped with other elements (e.g. Zr and Ta) are a topic of special interest specially in the manufacturing of superconductor buffer layers by pulsed electron deposition. Nowadays, the trend is to produce small devices (i.e. coated conductors), and the correct mechanical characterization is critical. In this sense, nanoindentation is a powerful technique widely employed to determine the mechanical properties of small volumes. In this study, the nanoindentation technique allow us determine the hardness (H) and Young's modulus (E) by sharp indentation of different buffer layers to explore the deposition process of CeO₂ that is undoped or doped with Zr and Ta, and deposited on Ni–5%W at room temperature. This study was carried out on various samples at different ranges of applied loads (from 0.5 to 500 mN). Scanning electron microscopy images show no cracking for CeO₂ doped with Zr, as the doping agent increases the toughness fracture of the CeO₂ layer. This system, presents better mechanical stability than the other studied systems. Thus, the H for Zr–CeO₂ is around 2.75 · 10⁶ Pa, and the elastic modulus calculated using the Bec et al. and Rar et al. models equals 249 · 10⁶ Pa and 235 · 10⁶ Pa respectively.     

Comparative properties of ternary oxides of ZrO2-TiO2-Y2O3 obtained by laser ablation, magnetron sputtering and sol-gel techniques

Thin films of ternary oxides of zirconium (Zr), yttrium (Y) and titanium (Ti) were obtained by the sol-gel, magnetron sputtering and laser ablation techniques. Zirconium propoxide, titanium isopropoxide and yttrium nitrate hexahydrate reagents were used as precursors in the sol-gel process. The Zr/Ti and Zr/Y molar ratios have been controlled by varying the precursors and surfactant concentration. The obtained gels were supported by dip coating on α-alumina supports, dried at 373 K and calcinated at 873 K. The powders obtained after the removal of carbon residuals were subsequently pressed and calcinated for using as targets for the magnetron sputtering and pulsed laser deposition techniques. The chemical composition was determined by Auger Electron Spectroscopy. The surface topography was investigated by Atomic Force Microscopy, while the crystallinity was evaluated from X-ray diffraction. The electrical response of the deposits to nitrogen oxides (NO_x) toxic gas is discussed according to the experimental conditions.     

Thermo-optic properties of TiO2, Ta2O5 and Al2O3 thin films for integrated optics on silicon

The direct measurement of the thermo-optic coefficients of aluminium oxide, tantalum pentoxide and titanium dioxide thin films is presented. Using ellipsometry on monolithically integrated permutations of the layers of silicon, silicon dioxide and the material under test, allows the direct measurement of the overall thermo-optic coefficient accounting for thermally induced changes in the dielectric permittivity and density of the materials as well as the elasto-optic effect due to the non-matching thermal expansion coefficients of the different materials.     

Synthesis of new binary cobalt iron pyrophosphate CoFeP2O7

A new binary cobalt iron pyrophosphate, CoFeP₂O₇ was synthesized through solid phase reaction using cobalt carbonate, iron metal and phosphoric acid in the presence of water-methanol system with further calcinations at the temperature of 500 °C. The particle size obtained from X-ray line broadening is 36 ± 7 nm for the CoFeP₂O₇. FTIR spectrum of CoFeP₂O₇ is assigned based on the P₂O₇⁴⁻ ion in the structure. The XRD, UV-Vis near IR and FTIR results of the synthesized CoFeP₂O₇ appear to be very similar to that of M₂P₂O₇ (M = Fe and Co), which indicates the monoclinic phase with space group C2/m.     

Influence of N2 partial pressure on mechanical properties of (Ti,Al)N films deposited by reactive magnetron sputtering

The influence of the nitrogen partial pressure on the mechanical properties of (Ti,Al)N films deposited by DC reactive magnetron sputtering using a Ti-Al mosaic target at a substrate bias of −100 V is investigated. Nanoindentation tests reveal that with increasing N₂ partial pressure, the film hardness and elastic modulus increase initially and then decrease afterwards. The maximum hardness and elastic modulus are 43.4 GPa and 430.8 GPa, respectively. The trend is believed to stem from the variations in the grain size and preferential orientation of the crystals in the (Ti,Al)N films fabricated at varying N₂ partial pressure. The phenomenon is confirmed by results acquired using glancing angle X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS).     

The cryogenic thermal expansion and mechanical properties of plasma modified ZrW2O8 reinforced epoxy

In this article, epoxy resin reinforced by negative thermal expansion material, ZrW₂O₈, was fabricated. The surface modification of ZrW₂O₈ particles was performed via plasma enhanced chemical vapor deposition (PECVD) process. As a result, a thin film was uniformly deposited on the surfaces of the ZrW₂O₈ particles, leading to an improvement of compatibility and dispersion of ZrW₂O₈ fillers inside epoxy matrix. Moreover, the coefficients of thermal expansion (CTEs) of the composite material containing 0-40 vol.% fillers were studied under cryogenic temperatures. The results showed a significant reduction in thermal expansion with increasing ZrW₂O₈ content. The cryogenic mechanical properties of ZrW₂O₈/epoxy composites were also investigated, showing the properties were improved by adding ZrW₂O₈ to certain content. In addition, the mechanical strength and modulus of the composite were observed significantly higher at cryogenic temperature than that at room temperature because of the thermal shrink effect and the frozen epoxy matrix.