液压集成块内弯曲流道流场数值计算与分析

本文对液压技术中广泛应用的液压集成块中常见的“S”型流道的流场用有限元方法进行数值计算,并将计算所获得的流场的结构以可视化的图像形式给出,为分析流道中的能量损失和优化设计流道提供了理论依据。     

Effects of alumina additions on sintering behavior of Ce0.8Sm0.2O1.9 ceramics synthesized by Pechini method

The effects of alumina additions on the densification behavior and grain growth of Ce_0.8Sm_0.2O_1.9 ceramics were investigated by the use of powders synthesized by Pechini method. Both the sintered density and grain size were found to increase rapidly up to 1 mol% Al₂O₃, and then to decrease with further additions. A pinning effect on grain growth was found at a higher Al₂O₃ content above 2 mol%. In the dopant content range of 0–5 mol%, 1 mol% Al₂O₃ was the optimum doping level in promoting densification and grain growth of Ce_0.8Sm_0.2O_1.9 ceramics.     

Low temperature sintering of ZrC-SiC composite

High-energy ball milling and spark plasma sintering were adopted to prepare ZrC-SiC composite. Zirconium carbide, silicon, and graphite powders were used as raw materials. ZrC-30 vol.%SiC was sintered to a relative density of >96.1% at 1800 °C. The composite showed a fine microstructure. The fracture strength reached up to 523.4 MPa, Vickers’ hardness 18.8 GPa, fracture toughness 4.0 MPa m^1/2, and elastic modulus 390.5 GPa.     

起重机轨道接头处断裂分析及其强化方法探讨

现场轨道断裂部位主要集中于轨道接头伸缩缝处。轨道承接部分的主要作用是保证轨道与轨道之间的平稳运行,减小车轮在伸缩缝处受到的冲击。吊车运行时,轨道伸缩缝一方面受到冲击,另一方面轨道接头伸缩缝安装错位,截面突变处受到的力矩最大,容易导致断裂。另外,伸缩缝位置处轨道挡板连接不佳也弱化了此处的强度。为了解决上述问题,利用有限元仿真软件ANSYS建立模型,分析伸缩缝附近轨道接头截面变化处的应力,发现采用传统挡板连接时,轨道端头应力过大,是造成其断裂的主要原因。根据所得结果在对应位置处替换优化后的双翼挡板,通过计算,此挡板能有效解决轨道端头处应力集中现象,增大轨道接头处的强度。     

模具的热电属性对电流烧结温度场的影响

建立了电流烧结温度场的有限元模型,通过有限元模型对电流烧结过程中的粉末体、模冲及阴模的温度分布进行了数值模拟。结果表明,模具的热电属性对电流烧结温度场的影响较大。在相同的烧结工艺条件下,与采用传统的石墨材料相比,采用高电阻、低热导率的陶瓷材料可显著提高粉末体的升温速率和最终烧结温度,但同时也带来了较大的温度梯度。进行了纯钛粉电流烧结试验,试验结果与有限元分析一致,验证了采用有限元分析的可行性。     

液压锥阀的数值模拟

液压锥阀是液压驱动系统的最基本控制部件,其性能的优劣直接影响系统的品质。本文用有限元方法建立了锥阀式结构的数学模型,给出了研究区域的边界条件,得到了锥阀工作过程中的有限元数值解,得到了锥阀工作过程中流体在阀内部的流函数曲线分布,并为研究锥阀的其它特性参数奠定了基础。     

New dielectric material system of La(Mg1/2Ti1/2)O3–Ca0.6La0.8/3TiO3 for microwave applications

The microstructure and microwave dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics system with ZnO additions (0.5 wt.%) investigated by the conventional solid-state route have been studied. Doping with ZnO (0.5 wt.%) can effectively promote the densification and the dielectric properties of xLa(Mg_1/2Ti_1/2)O₃–(1 − x)Ca_0.6La_0.8/3TiO₃ ceramics. 0.6La(Mg_1/2Ti_1/2)O₃–0.4Ca_0.6La_0.8/3TiO₃ ceramics with 0.5 wt.% ZnO addition possess a dielectric constant (_r) of 43.6, a Q × f value of 48,000 (at 8 GHz) and a temperature coefficient of resonant frequency (τ_f) of −1 ppm/°C sintering at 1475 °C. As the content of La(Mg_1/2Ti_1/2)O₃ increases, the highest Q × f value of 62,900 (GHz) for x = 0.8 is achieved at the sintering temperature 1475 °C. A parallel-coupled line band-pass filter is designed and simulated using the proposed dielectric to study its performance.     

Study of (1 − x)(Mg0.6Zn0.4)0.95Co0.05TiO3–xCa0.61Nd0.26TiO3 microwave dielectrics

The microwave dielectric properties and the microstructures of the (1 − x)(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–xCa_0.61Nd_0.26TiO₃ ceramic system were investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. Ca_0.61Nd_0.26TiO₃ has a large positive temperature coefficient of resonant frequency. (Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃ possesses a negative temperature coefficient of resonant frequency. By appropriately adjusting the x value in the (1 − x)(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–xCa_0.61Nd_0.26TiO₃ ceramic system, a near-zero τ_f value can be obtained. A new microwave dielectric material of 0.8(Mg_0.6Zn_0.4)_0.95Co_0.05TiO₃–0.2Ca_0.61Nd_0.26TiO₃ possesses the excellent dielectric properties of a dielectric constant of 28.6, a Q × f value of 80,600 GHz and a temperature coefficient of resonant frequency of 4.1 ppm/°C and has a lower sintering temperature of 1250 °C.     

Liquid-phase Infiltration of Thermal Sintered Skeletons by Low-temperature Gold Eutectic Alloys

The flexibility of powder-based Rapid Prototyping (or Solid Free Form Fabrication) techniques and the densification method described in this paper would open a whole new range of applications where thermal constraints exist. This work examines the infiltration of sintered skeletons made of pure gold spherical powder ranging between 5 and 40 μm in size, by binary gold eutectics based on silicon, germanium and tin, with a melting point as low as 278° C. Experiments have shown that high densification rate, good surface finish and improved mechanical properties can be achieved with this process. The material properties influencing the process performance are pointed out.     

Fracture toughness measurements of plasma-sprayed thermal barrier coatings using a modified four-point bending method

In this study, the adhesion strength of thermal barrier coatings 8YSZ (ZrO₂ + 8 wt.% Y₂O₃) deposited on NiCrAlY bond coats by atmospheric plasma spraying is investigated experimentally. A modified four-point bending specimen that can generate a single interface crack to facilitate the control of crack growth was adopted for testing. The fracture surfaces were examined using a scanning electron microscope. Images show that cracks are initiated along YSZ/NiCrAlY interfaces, then kink and grow uniformly within the YSZ layer. The load-displacement curves obtained indicate three distinct stages in crack initiation and stable crack growth. Based on a microstructural model, finite element analyses were performed to extract the bonding strength of the thermal barrier coatings. The fracture toughness of the plasma-sprayed 8YSZ coatings, in terms of critical strain energy release rate G_c, can be reliably obtained from an analytical solution or from a numerical simulation of the cracking process using compliance methods.     

大型整体壁板用扁挤压筒的温度场及热应力分析

采用弹性有限元法对生产大型整体壁板用扁挤压筒的温度场和热应力场进行了数值模拟 ,获得了扁挤压筒在不同加热条件和装配条件下的温度分布及热应力分布情况。结果表明 ,对内孔尺寸为 850 mm× 2 50 mm的三层和四层扁挤压筒 ,有中间层加热圈时温度场的分布比无加热圈时的情形较为均匀 ;内孔尺寸为 850 mm× 2 50 mm的四层扁挤压筒 ,由于扁挤压筒内的温度不均匀分布所引起的最大热应力高达 50 0 MPa以上 ;有中间层加热圈时其最大等效热应力水平有所下降     

Sintering-modified mixed Ni-Co-Cu oxymanganospinels for NTC electroceramics

Mixed Ni-Co-Cu oxymanganospinels of Cu_0.1Ni_0.8Co_0.2Mn_1.9O₄ composition with improved functional reliability are first developed for possible application as high-precise NTC thermistors. It is established the amount of additional rock-salt NiO phase in these ceramics, which was not externally introduced at the initial stages of ceramics processing, but extracted during sintering route occurs a decisive role to inhibit the parasitic degradation caused by thermal storage at the elevated temperatures. This effect is well revealed only in ceramics having a character fine-grain microstructure obtained due to injection of small amount of thermally transferred energy, while structural monolithization caused by great value of thermally transferred energy into ceramics bulk reveals an opposite influence. The fact, the ceramics with fine-grain microstructure and large content of rock-salt NiO extractions demonstrate the best suitability for stretched-exponential relaxation kinetics (the most appropriate one for describing degradation kinetics in structurally dispersive solids like ceramics) serves as additional confirmation to the above conclusion.     

液压锥阀流场的有限元法解析

本文用近年来发展起来的有限元法,对流体传动与控制系统中广泛使用的液压阀的内流场进行数值模拟,并给出了锥阀流场的速度矢量和流线图谱。这对于进一步分析阀口处的噪声和能量损失、优化设计阀内流道等具有重要的实际意义。     

Numerical and experimental study on the gravitation and surface tension effects on flow injection in center-gated disks

Flow injection in center-gated disks is numerically and experimentally studied in this paper for possible applications in the manufacturing of lightweight and high performance composite materials in space. A comprehensive three-dimensional model, which combines the Galerkin finite element method with a predictor/corrector scheme, is employed to determine the transient flow field. The effects of gravitation and surface tension on the development of flow front shape, velocity field pressure distribution, and advancement of the flow front are examined for a wide range of the governing parameters (namely, the capillary and Bonds numbers). It has been found that surface tension tends to hold the flow front in symmetric shape between disks while gravitation is to distort it. The balance of these two forces has significant effects on the front shape, front tip traveling speed and required injection pressure. Good agreement is found between the prediction and experimental measurements concerning the development of flow front shape and advancement of the flow front. The present results provide useful information in the design of resin transfer molding process in space.     

Optimization of crystal microstructure in piezoelectric ceramics by multiscale finite element analysis

A multiscale finite element method based on homogenization theory is employed to optimize the microstructure of polycrystalline piezoelectric ceramics so as to maximize the homogenized macrostructural piezoelectric response. This analysis reveals that two specific microstructures, layered or alternating [1 1 1]-oriented structures, result in a piezoelectric response that exceeds that of the single-crystal. The layered structure consists of ordered 120°-rotated layers maximizing d₃₃₃, while the alternating structure consists of adjacent grains rotated by 180° in three dimensions optimizing d₃₁₁. These heterogeneous structures maximize the internal strain in polycrystalline aggregates by optimizing electrical and mechanical effects.     

Effect of addition of Ba(W0.5Cu0.5)O3 in Pb(Mg1/3Nb2/3)O3-Pb(Zn1/3Nb2/3)O3-Pb(Zr0.52Ti0.48)O3 ceramics on the sintering temperature, electrical properties and phase transition

In this work, we report on the Pb(Mg_1/3Nb_2/3)O₃-Pb(Zn_1/3Nb_2/3)O₃-Pb(Zr_0.52Ti_0.48)O₃ (PMN-PZN-PZT) ceramics with Ba(W_0.5Cu_0.5)O₃ as the sintering aid that was manufactured in order to develop the low-temperature sintering materials for piezoelectric device applications. The phase transition, microstructure, dielectric, piezoelectric properties, and the temperature stability of the ceramics were investigated. The results showed that the addition of Ba(W_0.5Cu_0.5)O₃ significantly improved the sintering temperature of PMN-PZN-PZT ceramics and could lower the sintering temperature from 1005 to 920 °C. Besides, the obtained Ba(W_0.5Cu_0.5)O₃-doped ceramics sintered at 920 °C have optimized electrical properties, which are listed as follows: (K_p = 0.63, Q_m = 1415 and d₃₃ = 351 pC/N), and high depolarization temperature above 320 °C. These results indicated that this material was a promising candidate for high-power multilayer piezoelectric device applications.     

Sintering effect on electrical properties of ZnO-V2O5-MnO2-Nb2O5 ceramics

The microstructure and electrical properties of quaternary ZnO-V₂O₅-MnO₂-Nb₂O₅ ceramics were investigated at different sintering temperature (875-950 °C). The average grain size increased from 4.4 μm to 9.6 μm with increasing sintering temperature. The breakdown field decreased from 6991 V/cm to 943 V/cm with increasing sintering temperature. Proper sintering for quaternary ZnO-V₂O₅-MnO₂-Nb₂O₅ ceramics led to surprisingly high nonlinear coefficient (50). The donor concentration increased from 3.33 × 10¹⁷ cm⁻³ to 7.64 × 10¹⁷ cm⁻³ with increasing sintering temperature and the barrier height exhibited the maximum value (1.07 eV) at 900 °C.