A process model for active brazing of ceramics

In the present investigation process modelling techniques have been applied to describe reaction layer growth during active brazing of ceramics. As a starting point, the classical solution for parabolic growth of transformation products is considered. Specific computational features are then explicitly built into the model to allow for transient effects during heating and cooling as well as changes in the growth kinetics due to depletion of the active element during brazing. This approach gives considerable scope for optimization of both process and joint properties through adjustment of the filler metal composition and the temperature-time programme under which brazing takes place. The aptness of the process model is illustrated in an accompanying paper (Part II).Nomenclature A, C B reaction products in diffusion couple - C A ceramic component - C reactive element in braze alloy - C _B concentration of element B at a given position within the reaction layer, C B (mole m^–3) - C _B ^b bulk concentration of element B in ceramic, A B (mole m^–3) - C _B ⁱ (1), C _B ⁱ (2) concentration of element B in reaction layer at C B /A B and braze metal/C B interface, respectively (mole m^–3) - C _C concentration of element C at a given position within the reaction layer, C B (mole m^–3) - C _C ⁰ initial concentration of element C in braze metal (mole m^–3) - C _C ^b bulk concentration of element C in braze metal (mole m^–3) - C _C ⁱ (1), C _C ⁱ (2) concentration of element C in reaction layer at braze metal/C B and C B /A interface, respectively (mole m^–3) - D ₀, D ₀ ^* constants in expression for diffusion coefficient (m² s^–1) - D _B intrinsic diffusivity of B in C B (m² s^–1) - D _C intrinsic diffusivity of C in C B (m² s^–1) - J _B molar flux of element B (mole m^–2 s^–1) - J _C molar flux of element C (mole m^–2 s^–1) - k ₀ constant in expression for k_p (m² s^–1) - k ₀ ^* rate constant referring to infinite diffusion couple analogue (m² s^–1) - k _p parabolic growth rate constant (m² s^–1) - L half width of braze metal zone (m) - m proportionality constant (equal to the ratio between C _C ⁱ (1) and C _C ^b ) - Q _app apparent activation energy for diffusion of C in C B (J mole^–1) - Q _app ^* apparent activation energy for diffusion of B in C B (J mole^–1) - R universal gas constant (8.314 J mole^–1 K^–1) - t time (s) - t ₀ incubation time (s) - t ₁, t ₂ limits of integration (s) - t _i isothermal hold time (s) - t _i time increment used in the numerical integration procedure (s) - T absolute temperature (K or °C) - T _c chosen reference temperature (K or °C) - T _i isothermal hold temperature (K or °C) - X thickness of reaction layer (m) - X _c contribution of the cooling leg of the brazing cycle to the total reaction layer thickness (m) - X _h contribution of the heating leg of the brazing cycle to the total reaction layer thickness (m) - X _i contribution of the isothermal hold period to the total reaction layer thickness (m) - X _lim limiting thickness of reaction layer, C B (m) - X _i increase in reaction layer thickness due to a small time increment t _i (m) - y ₁, y ₂, y ₃ molar partitioning factors - , , , , , ) molar stoichiometric factors - molar volume of reaction product, C B (m³ mole^–1)     

New process for brazing ceramics utilizing squeeze casting

A new joining process for ceramics to ceramics and ceramics to metals, SQ brazing, has been developed. This process utilizes squeeze casting; a brazing material is squeezed into the interface channel to be brazed and is solidified under a high pressure. This new process has several advantages, low cost, mass producibility, high interface strength and high reliability, no severe reaction, etc. Alumina to alumina and silicon nitride to silicon nitride brazing with pure aluminium are shown as examples. Alumina containing silica as a sintering additive brazed by a conventional method severely reacted with aluminium braze so that the joint strength was low. After SQ brazing, reaction was moderate and the strength almost reached that of the parent alumina. Silicon nitride could be brazed by SQ brazing. Although the simple SQ brazing could not make a strong interface, pre-oxidization treatment of silicon nitride increased the joint strength beyond 400 M Pa.     

钛基钎料的活性及理化性能对比研究

对两种钛基钎料非晶态箔带的氧化行为、钎焊接头性能及非晶箔带成型性能等方面进行对比研究 .研究结果对钛基钎料的选择与成分设计有一定帮助 .     

Perovskite-type BaTiO3 ceramics containing particulate SiC: Part II Microstructure and mechanical properties

Microstructure and mechanical properties of BaTiO3 and BaTiO3-based composites containing nanosized SiC particulates were investigated. Fracture behaviour and fractography were also discussed from the fracture surface observation by scanning electron microscopy and by the controlled surface flaw method. The added SiC particulates were uniformly distributed within the matrix BaTiO3 grains, with some larger particulates located at the BaTiO3 grain boundaries. The microstructure of BaTiO3 was modified by incorporation of the SiC particulate. Mechanical properties, particularly the fracture strength, were remarkably improved by adding the SiC particulate, owing to grain-size reduction and/or crystal structure change by incorporating the SiC. From the fracture surface observations, it was confirmed that the subcritical crack growth phenomenon of BaTiO3 was also improved by the nanosized SiC dispersions.     

塑料超声波焊接过程及质量研究 Ⅱ焊接接头熔化膜厚度计算模型

利用流变学基本理论 ,对焊接过程平衡阶段熔化膜的流动形态进行了理论分析 ,分别导出了工艺参数对熔化膜厚度和熔体剪切速率影响的计算公式 ,测量结果表明 ,理论计算结果和实际结果一致性较好 .     

Liquid phase sintering of Re2O3 YSZ ceramics: Part II Grain boundary electrical properties

Grain boundary electrical properties of Y₂O₃ stabilised zirconia with small additions of Er₂O₃ and Pr₂O₃ sintered via silicate liquid phase were studied by the impedance spectroscopy technique. Grain boundary specific conductivity of the praseodymium doped samples was found to be independent of sintering time, while the erbium doped sample showed high anomalous conductivity for the 1.0 h sintered samples. The electrical behaviour is explained considering the grain boundary to be a series association of the glass film and the space charge region. Specific conductivity and Debye length of the space charge region of erbium doped samples were found to be 6.7 × 10^–8 S/cm and 0.25 nm, respectively.     

Porous yttria-stabilized zirconia ceramics with ultra-low thermal conductivity. Part II: temperature dependence of thermophysical properties

This study describes the experimental results of thermal diffusivity, specific heat at constant pressure, and thermal conductivity of porous 8 mol% yttria-stabilized zirconia (YSZ) ceramics in a temperature range from room temperature to 1,400 °C. It is a follow-up study of the earlier report titled by “Porous YSZ ceramics with ultra-low thermal conductivity”, which focused on the room-temperature thermal conductivity. The thermal diffusivity of porous YSZ ceramics decreased with the increase of the measurement temperature up to 600–1,000 °C, followed by an increasing trend with increasing temperature. The specific heat did not exhibit any significant dependence on sintering temperature and agreed with literature data. The thermal conductivity of the porous YSZ ceramics showed an insensitive tendency of change with measurement temperature. The thermal conductivity fell in groups by the sintering temperature level. This investigation also discussed an appropriate sintering temperature of porous YSZ ceramics, which had both low thermal conductivity and high strength required by the practical service.     

Three-phase transformer modelling for unbalanced conditions Part II: general formulation

Part I of this two-part paper proposed a model for two-winding three-phase transformers under steadystate unbalanced conditions. Although the model is very simple (it only uses two parameters), it is valid for different core designs (three-legged, five-legged and triplex). The nodal equations for four examples are developed step by step in Part I. In Part II, phase and sequence nodal equations are systematically obtained for all possible phase shifts and all winding connections, including the isolated wyes and the impedance grounded wyes on one or both transformer sides. The proposed model encompasses the usual model in the literature. The formulation has been tested with the IEEE 4 node test feeder. Three numerical application examples illustrate that the model provides accurate results (compared with the results of a non-linear model) if the transformer is not highly saturated.     

A micromechanical compaction model for woven fabric preforms. Part II: Multilayer

With nesting between adjacent layers and inter-layer packing, the microstructure and the compaction behaviour of a multilayer woven fabric preform are much more complicated than those of a single layer fabric preform. A micromechanical model, based on the hierarchical structure characteristics of woven fabric preforms, was developed to investigate the elastic compaction behaviour of multilayer plain weave fabric preforms. The compaction mechanisms of fabrics at different hierarchical levels including deformation and compaction of yarn cross-section, flattening of yarn waveform, nesting between adjacent layers and inter-layer packing, are considered in an integrated approach in this predictive model. Effects of structural elements at different hierarchical levels on compaction behaviour of multilayer plain weave fabric preforms are investigated in detail. Both the number of layers and shifting are shown to have significant effects on compaction behavior, while the effect of nesting increases as the number of layers increases. The predictions by this model are correlated well with the experimental data.     

Mechanical properties of young concrete: Part II: Determination of model parameters and test program proposals

The paper concerns testing and modelling of the mechanical properties required as input to calculation programs made for crack risk estimation of hardening concrete structures. The results from several test series on mechanical properties of young concrete as described in Part I of this paper, are further evaluated. Model parameters for the modified CEB 1990 Model Code-equations, are determined for six concrete mixes, all having a w/b-ratio on 0.40, for compressive strength, tensile strength and E-modulus. To make the models applicable for young concrete, at ₀-parameter is introduced to fix the time at which significant mechanical properties are present. A test program to determine the model parameters is proposed, based on the experience that compressive strength tests have the smallest statistical scatter and that they are simplest to carry out.     

Synthesis and characterization of new precursors to nearly stoichiometric SiC ceramics: Part II A homopolymer route

An original route to SiC ceramics possessing low free carbon(3.8 at%) and oxygen (1 at%) contents is reported. It consists of the polycondensation of 2,4-dichloro-2,4-disilapentane, prepared by Grignard reaction of dichloromethane on methyldichlorosilane. The undesired remaining chlorine was eliminated via a subsequent reduction using lithium aluminum hydride. The synthesis of the ceramic precursor was achieved by thermolysis under argon at atmospheric pressure, with the evolution of hydrogen and methane. The pyrolysis of the precursor provided SiC ceramics in an excellent yield (79%).     

Disintegration process of yttria-stabilized zirconia ceramics using hydrothermal conditions

Capability of the recycling of high strength and high fracture toughness yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) sintered body utilizing “low-temperature annealing degradation” phenomenon was investigated. Hydrothermal treatment was employed to induce the phase transformation from tetragonal to monoclinic zirconia and to disintegrate the Y-TZP sintered body. 3 mol% Y₂O₃–ZrO₂ specimens sintered at 1,550 °C and more were disintegrated without leaving the original appearances when the treatment temperature was between 200 °C and 400 °C. The size of the disintegrated fragments of Y-TZP sintered body was much affected by hydrothermal treatment conditions. Only with hydrothermal treatment and simple ball milling, the sintered body was pulverized into the primary particle level. This technique is expected to apply to a sustainable recycling system for the zirconia ceramics, which restrains an energy consumption compared to crushing zirconia using mechanical procedures.     

纳米Cr颗粒对Sn-Zn-Bi-In/Cu钎焊焊点性能的影响

通过向Sn-Zn-Bi-In钎料中添加不同含量的纳米Cr颗粒制成新型复合钎料Sn-5Zn-10Bi-10In-xCr(x=0％,0.1％,0.3％,0.5％,质量分数),探讨纳米C r颗粒对时效前后钎焊焊点的组织形貌、元素分布、物相组成和力学性能的影响.结果表明:纳米Cr颗粒的添加能够抑制焊点金属间化合物(IMCs)的生长,随着纳米Cr颗粒含量的增加,IMCs扩散层厚度逐渐降低;界面处IMCs扩散层靠近母材Cu一侧为Cu5 Zn8相,靠近钎料区一侧为Cu6 Sn5相;随时效时间的增加,钎料侧部分Cu5 Zn8化合物长大分解,Cu3 Sn相形成;纳米Cr颗粒抑制了时效过程中IMCs扩散层的进一步长大;随着纳米Cr颗粒含量的增加,焊接焊点的剪切强度和显微硬度均先增加后下降,Sn-5Zn-10Bi-10In-0.3Cr/Cu焊点的剪切强度和硬度最高;时效后焊件的剪切强度比时效前均有所下降,但纳米Cr颗粒的添加使焊点保持了良好的剪切强度,时效后焊点钎料区显微硬度比时效前有所上升,但也始终保持在30HV0.1以下.     

Crack growth in non-homogeneous transformable ceramics. Part II: Crack deflection

Crack growth in transformation toughened ceramics is studied using a micromechanics based continuum model which accounts for both dilatant and shear transformation strain components. In the computations, the transformable phase is taken to be distributed non-homogeneously in order to model Zirconia Toughened Aluminas that have not been optimally mixed, or Duplex Ceramics in which large zirconia inclusion are dispersed in an untransformable matrix. The small scale transformation problem is solved using a finite element approach. The influence of the transformation strains around the propagating crack on the stress intensity at the crack tip is computed using the transformation domain integral. The crack is modelled as a missing row of mesh elements and crack growth is simulated by nullifying the stiffness of a crack tip element. In contrast to Part I of this paper [1], this part is concerned with cases where the transformable phase is not distributed symmetrically with respect to the x ₁-axis, which causes the crack to deflect from its original crack path due to a local shear stress intensity factor at the crack tip. A computational method is developed which is capable of simulating this, assuming that the deflections from the original crack path are small. A parametric study is carried out of the effect of crack deflection and crack meandering on the overall crack growth resistance.     

A geometrical finite element model of the sintering process of advanced ceramics

This paper describes algorithm and finite element procedure to model the warpage and shrinkage of “green” powder compacts due to density and density variations during the sintering stage. The numerical procedure has been tested against control experiments of sintering of an advanced ceramics material and found to have good agreement with actual distortions.     

Feature-based metal stamping part and process design. Part II: stamping process planning

In metal stamping development, stamping process planning plays quite an important role for the related die design. As mentioned in Part-I [Feature-based metal stamping part and process design. Part 1: stampability evaluation, 2007, 45, 2673–2695], the feature can encapsulate comprehensive engineering information; this paper proposes to realize stamping process planning based on feature mapping. A feature mapping system, between a stamping design feature space and a stamping process feature space, has been presented. The inherent mechanism of such feature mapping has been investigated with formal representations. The substance of feature mapping is the processing of related knowledge and information. Mapping rules are established to generate the stamping operation and stamping direction. Four mapping manners (direct mapping, conjugate mapping, syncretic mapping, and sequential mapping) are presented for generating the form of process feature, through geometrical information transformation from design features. Meanwhile, considering a stamping process plan as an ordered tree, this paper has introduced how to build a stamping process plan through organizing the stamping operations mapped from design features, and an illustrative example is demonstrated.     

钇掺杂ZrO_2陶瓷的制备及性能研究

采用溶胶-凝胶法结合放电等离子体烧结(SPS)技术成功制备出立方相结构的掺杂8%(原子分数)Y2O3的ZrO2(8YSZ)陶瓷块体。X射线衍射(XRD)结果表明样品烧结前后均为单一的立方相结构,但是烧结后的样品的晶粒尺寸增大,结晶性变好。Raman光谱结果表明烧结前样品为单一的立方相结构,SPS烧结后样品为单斜相和立方相的混合物,其中单斜相的含量较少。认为XRD与Raman光谱检测结果不一致是因为ZrO2的单斜相和立方相的晶格常数接近以及XRD检测精度不够高造成的。采用SPS方法制备出的样品的致密度为92.7%,显微维氏硬度为1551 MPa。