Silicon Nitride Joining

Hot-pressed Si₃N₄ was joined using an Mgo-A1₂O₃-SiO₂ glass composition chosen to approximate the oxide portion of the grain-boundary phase in the ceramic. After it has been heated at 1550° to 1650°, the interface of the joined ceramic is an interlocking mixture of Si₂N₂O, β-Si₃N₄, and a residual oxy-nitride glass. The kinetics of reactions between Si₃N₄ and the molten joining composition were studied by X-ray diffraction analysis of the phases present in Si₃N4 powder-glass mixtures quenched after varied heat treatments. Analytical transmission electron microscopy of the composition and micro-structure of the reaction zone in joined specimens, together with the X-ray diffraction results, suggests that the driving force for joining is the lowering of the Si₃N₄ interfacial energy when it is wet by the molten silicate, augmented by the negative Gibbs energy for the reaction SiO₂( l ) + Si₃N₄= 2Si₂N₂O.     

Joining carbon nanotubes

To fully exploit the exceptional electronic and mechanical properties of carbon nanotubes in real-world applications, it is desirable to create carbon nanotube networks in which separate, multiple nanotubes are joined so that as many as possible of the properties of single nanotubes are conserved. In this review we summarize the progress made towards this goal, covering techniques including electron and ion beam irradiation, Joule heating and spark plasma sintering.     

Joining nitride ceramics

Nitride ceramics are gaining importance in advanced technology applications. Examples include high temperature engine components of Si₃N₄, heat dissipating electronic substrates of AlN, and chemically inert cutting tools of cubic BN. The effective and widespread use of nitrogen ceramics in these and other applications, however, depends on the ability to bond them to metals, as well as to other ceramics.

Bonding ceramics to metals is inherently difficult because of their non-metallic characteristics, and this can be aggravated for nitride ceramics by their tendency to dissociate in vacua. The successful production of reliably strong joints by diffusion bonding and brazing with reactive filler metals is discussed. Reference will be made to the joining of AlN, BN and particularly Si₃N₄ using alloys containing reactive elements, and comments will be made on the influence of process and parameters such as temperature, environment and ceramic surface preparation on the microstructure, mechanical properties and high temperature stability of interfaces.     

Large products from refractory carbides

Conclusions A technology was developed for making large blanks of zirconium and niobium carbides. The technique includes slip preparation, preforming of the blanks by hot casting or hot extrusion, and hot die casting in vacuum at 2200°C.Using this method articles were made whose density was 96–97% of the theoretical. There were no internal defects according to ultrasonic control checks.Translated from Ogneupory, No. 6, pp. 49–52, June, 1967.     

Hot-Isostatic-Press Joining of Cemented Carbides

Hot isostatic pressing (HIPing) is investigated as a technique for joining the cermet WC-15% Co to itself. Encapsulation of the specimens prior to HIPing was carried out using steel encapsulation, glass encapsulation, and self-encapsulation. The bonds were evaluated using a four-point-bend method. It is shown that the glass and steel encapsulation methods have a number of inherent problems which make them inappropriate for near net shape processing. In contrast the novel self-encapsulation method, described for the first time in this communication, is both simple and effective, producing joined material with bulk strength. The concept of self-encapsulation is potentially widely applicable for joining composite materials.     

Capacitor-Discharge Joining of Oxide Ceramics

A capacitor-discharge joining technique has been used to fabricate alumina–metal–alumina and zirconia–metal– zirconia joints using thin foils of aluminum, titanium, or amorphous Al_x Ni_y Y_z alloys as interlayers. The technique involves passing a high-energy pulse through a conductive interlayer, so converting the interlayer into a liquid-vapor energized foil which wets and bonds the pieces of ceramic being joined. The bond strengths of the joints were mea-sured by shear testing which showed that the highest bond strengths for both alumina and zirconia substrates were obtained when an amorphous Al_x Ni_y Y_z interlayer was used. An investigation of the interfacial structure of the joints revealed that there is a distinct reaction layer between the ceramic substrate grains and the amorphous Al_x Ni_y Y_z interlayer.     

精细陶瓷的微波焊接

利用精细陶瓷吸收微波能而自身发热进行焊接是近年迅速发展的一门新技术,它具有焊接接头强度高,升温速度快,局部加热,易于控制温度和节能等优点。本文在讨论微波加热焊接的原理同时,对微波焊接Al_2O_3、Si_3N_4、SiC陶瓷的焊接特性及工艺进行详细介绍。     

Synthesis of nonstoichiometric high entropy Nb-added carbides

A novel class of nonstoichiometric high-entropy carbide (HEC_x) materials, namely, Nb/TiC/TaC, Nb/TiC/TaC/VC and Nb/TiC/TaC/VC/WC, were produced by mechanically milled and spark plasma sintering (SPS) from Nb and carbides. XRD, SEM-EDS and S/TEM-EDS were used to characterize the phase constitution, microstructure and compositional distribution of samples, respectively. HEC_x exhibits a single-phase rock-salt crystal structure with a relatively uniform elemental distribution. Among the three different HEC_x materials, Nb/TiC/TaC/VC/WC with an average grain size of 2.15 μm sintered at 1600 °C shows an enhanced fracture toughness of 5.1 ± 0.1 MPa m^1/2 compared with transition metal carbides. The mechanically mixed and low sintering temperature lead to the formation of finer grains. The higher fracture toughness can be attributed to atomic relaxation resulting from carbon vacancies and solid solution strengthening.     

Pressure dependent mechanical properties of calcium carbides

The mechanical properties of newly synthesized Ca₂C₃ and Ca₂C under pressure have been studied by using the first-principles calculations with generalized gradient approximation. The equilibrium geometry, elastic stiffness constants, various moduli, and Pugh's ratio of the C2/m phase of Ca₂C₃ and the C2/m and Pnma phases of CaC₂ are systematically studied. The elastic stiffness constants of C2/m-Ca₂C₃ under 0–30GPa, C2/m-Ca₂C under 0–7.5 GPa, and Pnma-Ca₂C under 7.5–30 GPa satisfy the Born?Huang mechanical criteria. The three phases of calcium carbides exhibit ductile characteristics. The surface constructions of bulk and Young's moduli illustrate the mechanical anisotropy of Ca₂C₃ and Ca₂C. Our results are consistent with previously obtained experimental and theoretical data and have significant implications for the application of calcium carbides.     

Porosity in combustion synthesized titanium carbides

Stoichiometric and non-stoichiometric titanium carbides were obtained by combustion synthesis. Ignition of compacts of titanium containing either graphitic or activated carbons resulted in titanium carbides. Adsorption-desorption isotherms of carbons and combustion synthesized carbides were constructed and compared to determine the pore structural changes resulting from the synthesis. Original porosity of the carbon was lost after combustion synthesis, and the loss of porosity due to combustion synthesis is attributed to the reaction between liquid titanium and carbon.     

Modified Tape Casting Method for Ceramic Joining: Application to Joining of Silicon Carbide

A simple modified tape casting procedure has been developed for application to ceramic joining when the joining materials are in powder form. The method involves preparation of a slurry from the powder, solvent, and thermoplastic binder, and then casting directly onto the joining surface using a moving doctor blade. Handling of the tape prior to joining is not necessary: therefore, binder content is minimized, plasticizers are not required, and viscosity is controlled by solvent content. The utility of this technique for producing joints with thin, uniform interlayers is demonstrated for silicon carbide materials joined with TiC + Ni and SiC + Si.     

Ultrafast high-temperature synthesis and densification of high-entropy carbides

Recently, high-entropy carbides have attracted great attention due to their remarkable component complexity and excellent properties. However, the high melting points and low self-diffusion coefficients of carbides lead to the difficulties in forming solid solution and sintering densification. In this work, six dense multicomponent carbides (containing 5–8 cations) were prepared by a novel ultrafast high-temperature sintering (UHS) technique within a full period of 6 min, and three of them formed a single-phase high-entropy solid solution. The solid solubility of the UHSed multicomponent carbides was highly sensitive to the compositional variation. The presence of Cr₃C₂ liquid had significant contributions to the formation of solid solution and the densification of multicomponent carbides. All UHSed multicomponent carbides exhibited high hardness, which, unexpectedly, did not simply increase with increasing number of the components. The highest nanohardness with a value of 36.6 ± 1.5 GPa was achieved in the (Ti_1/5Cr_1/5Nb_1/5Ta_1/5V_1/5)C_x high-entropy carbide. This work is expected to expedite the development of high-entropy carbides and broaden the application of UHS in the synthesis and densification of advanced ceramics.     

Seamless Joining of Silicon Nitride Ceramics

High-strength joining of Si₃N₄ ceramics has been achieved by developing a process that effectively eliminates the seam, and may allow for fabrication of large or complex silicon nitride bodies. This approach to joining is based on the concept that when sintering aids are effective in bonding individual grains, they could be equally effective in joining bulk pieces of Si₃N₄. Optimization of the process led to Si₃N₄/Si₃N₄ joints with room-temperature bend strengths as high as 950 MPa, corresponding to more than 90% of the bulk strength of the Si₃N₄. At elevated temperatures of 1000° and 1200°C joint strengths of 666 and 330 MPa, respectively, were obtained, which are the highest values reported to date for these temperatures. These bend strengths are also more that 90% of the strength of bulk Si₃N₄ measured at these temperatures.     

聚乙烯管道系统的连接技术问题(1)

介绍了各种聚乙烯管道系统的连接技术。连接常是管道系统成败的关键,各种连接技术都有其适用的范围。聚乙烯压力管道的连接以熔接为主,但有些场合需要机械密封连接。在聚乙烯的机械密封连接中特别要注意聚乙烯容易蠕变的特性。有些适用于聚氯乙烯管道的连接方法和设计不适用于聚乙烯管道。聚乙烯无压力管道的连接也有多种方法,要根据对于密封的要求选择。介绍了国内外为解决聚乙烯双壁波纹管承口刚度问题采用的种种做法。简介了交联聚乙烯管道和增强聚乙烯管道的特殊连接技术。提出在聚乙烯管道系统的连接方面希望国内外科技界关注的几个重要研究课题。     

Electrical Joining of Silicon Nitride Ceramics

The electrical joining of sintered Si₃N₄ ceramics by Joule heating was studied. A mixture of CaF₂/kaolinite (70/30 wt%) with excellent electroheating characteristics and reactivity with Si₃N₄ ceramics was selected as a joining agent. The optimum conditions for electrical joining were determined using this joining agent. Analysis of the joint obtained under optimum conditions revealed that joining was accomplished by the formation of reaction zones and a joining layer through the mutual diffusion of the components in the joining agent and the sintering aids in the Si₃N₄. The joint layer was composed of a glassy substance consisting of Ca─Al─Si─Y─O─(F)─(N) and contained a few particles of β─Si₃N₄. Four-point bend tests indicated that joined bodies could be obtained which maintained a strength of about 300 MPa up to 800°C. Finally, a comparative study was made with a joint obtained using furnace heating. These results indicated that the joints obtained using electrical joining were superior to those produced in the furnace.