Effect of calcium zirconate nanofiller on Polyethylene productivity and properties

This work reports the preparation of ordered arrays of gold nanoparticles by sonochemically enhanced borohydride reduction of precursor lithium tetrachloroaurate (LiAuCl₄) incorporated into the core of polymeric micelles formed from amphiphilic copolymers of polystyrene (PS) and poly(ethylene oxide) (PEO). The copolymers were prepared with varying styrene block lengths from a PEO macroinitiator by atom transfer radical polymerization (ATRP). UV/visible spectroscopy was used to confirm the formation of elemental gold. The effect of sonication time on the appearance of the gold nanoparticles was determined and showed that the absorbance first increased as the nanoparticles formed but decreased at longer times, presumably as a result of a degree of agglomeration. Transmission electron microscopy revealed the morphology of the nanocomposites which confirmed that micellar polystyrene-block-polyethylene oxide is an excellent vehicle for the formation of well-defined films containing nanoparticulate gold. However, we report for the first time that care must be taken to optimize the preparation time to obtain the desired particle sizes because this parameter is very sensitive to the duration over which sonication is carried out.     

Mechanical properties of structural ceramic materials based on silicon nitride

The creation of structural ceramic materials based on silicon nitride is considered. The results of a study of the properties of these materials in a wide temperature range are presented, including data on the short-term strength, elastic characteristics, and parameters of fracture mechanics and results of long-term static and dynamic tests of some silicon nitride materials.Translated from Ogneupory i Tekhnicheskaya Keramika, No. 11, pp. 14–17, November, 1996.     

Effect of monomer content on physical properties of silicon nitride ceramic green body prepared by gelcasting

The gelcasting technique was employed to prepare Si₃N₄ green body. The monomers used in the research were acrylamide (AM) and N,N′-methylenebisacrylamide (MBAM). The influences of the monomer content (AM and MBAM) and the ratio of monomers (AM/MBAM) on the warpage rate, shrinkage rate, and the flexural strength of Si₃N₄ ceramics green body were investigated. Both warpage rate and shrinkage rate of green body were found to decrease with the increase of monomer content, and monotonically increase with the ratio of monomers after drying. The variation of warpage rate with the ratio of monomers is evident when monomer content is 20 wt.%, but the variations are not evident when monomer contents are 40 and 55 wt.%. The flexural strength of the green body is highest at an optimum value of the monomers ratio, and increases with increasing monomer content, reaching 50–90 MPa when monomer contents are 40 and 55 wt.%.     

Effect of the Y2O3 additive concentration on the properties of a silicon nitride ceramic substrate

Silicon nitride (Si₃N₄) ceramics offer excellent thermal, mechanical and dielectric properties, which make Si₃N₄ a good candidate material for an application as electronic packaging material. For an application as a heat dissipation substrate, most studies focused on achieving a high thermal conductivity through long-time heat preservation and different kinds of heat treatments. Very few studies also considered the mechanical and dielectric properties. In addition, there have not been systematic researches about influence of additives concentration and type on the combination properties of Si₃N₄. Therefore, in this study, Si₃N₄ ceramic samples were prepared via hot pressing at 1800 °C with a relatively short heat preservation step (2 h), with different amounts of Y₂O₃ added as sintering additive. The effect of the initial concentration of the rare earth oxide on the chemical composition, microstructure, thermal conductivity, as well as the mechanical and dielectric properties of the Si₃N₄ ceramic samples was systematically studied.     

Characterization of a silicon nitride ceramic material for ceramic springs

Under extreme working conditions such as high temperature, strong electric and magnetic fields and acidic or basic environments, ceramic springs offer a clear advantage over conventional steel springs. In this study, a tailored grade of silicon nitride ceramic was characterized as spring material. The basic characterization was complemented with component tests. Bend bars, helical springs and conical disk springs were manufactured and tested under various loading scenarios.Manifested by the smallest effective volume of the three tested geometries, helical springs showed the highest fatigue strength. Nevertheless, the complexity involved in manufacturing helical springs pertaining to their geometrical features resulted in a relatively large scatter in fatigue data. The results pointed out the importance of proper design and machining of the contact surface edges in disk springs, which bear the highest stresses.This work demonstrates the potential of producing ceramic springs with broad applicability and sufficient strength and fatigue resistance.     

Effect of temperature on the pre-creep mechanical properties of silicon nitride

The elasto-plastic properties and contact damage evolution of a commercial polycrystalline silicon nitride are evaluated as a function of temperature up to 1000 °C, using a recently developed method combining Hertzian indentation and FEM simulation. The results of the study are compared to existing data for other ceramic materials such as alumina and zirconia. Silicon nitride is found to exhibit an excellent combination of elasto-plastic properties in the pre-creep temperature range and good contact damage resistance. These qualities make this material ideal for high temperature applications in general, and in particular to be used in spherical indenters for the evaluation of mechanical properties of other materials at elevated temperature using the procedure applied in this work.     

Effect of rare earth additives on properties of silicon nitride ceramics

Abstract

Silicon nitride ceramics with rare earth (Re) compound (5 wt-%) and MgO (3 wt-%) additives were fabricated by spark plasma sintering and following heat treatment. The Re compounds included two groups: ReF₃ ((Re?=?La,Nd,Gd) and Re₂O₃ (Re?=?La,Nd,Gd). Specimens show the same tendency in the sintering shrinkage rate, relative density, grain size and bending strength with the increasing Re cation (Re³⁺) radius both in ReF₃ and Re₂O₃ added samples. However, as to aspect ratios and thermal conductivity, the change rules are completely opposite between the two groups of specimens.     

Technology for machining high-refractory ceramic parts based on silicon nitride

The results of the experimental study of mechanical treatment of highly refractory ceramic parts made of various compositions based on silicon nitride are described. The revealed regularities of the surface layer and defect formation have been used to develop a two-stage process for working these parts, which ensures high machining efficiency and a minimum level of surface layer defects. The proposed technology has been used in making cutting plates from nitride ceramics, which has increased their average resistance by 15–20%. __________ Translated from Novye Ogneupory, No. 8, pp. 19–24, August, 2006.     

Interaction of encapsulation glass and silicon nitride ceramic during HIPing

Interaction between ceramic compacts and the encapsulation glass during the HIP process has been studied in a model system of silicon nitride and borosilicate glass. Attention has been focused on what happens when the pressure is first applied in the HIP-cycle, i.e. between about 1200 and 1500°C. At this stage the pore system of the ceramic green body is still rather unaffected by sintering. The model system was characterised to evaluate a possible viscous flow of glass into the green body. Two glass compositions, one with high and one with low viscosity, were used, measurements being made of their viscosity and their contact angle on the nitride. Applying Darcy's law it was predicted that the encapsulation glass with the lowest viscosity should penetrate about 1200 microns into the still open pore structure at 1450°C, but this was not observed experimentally. In the calculations no chemical reactions were assumed to take place. However, increases in hardness of heat-treated mixture of glass and silicon nitride powder indicates that nitrogen dissolves in the glass. It is known that nitrogen increases the viscosity of the glass and this would result in a more limited glass intrusion. After HIP the surface region of the dense ceramic exhibited a phase composition gradient of silicon oxynitride, down to approximately 100–200 microns into the bulk. ©     

Mechanical behaviour of a silicon nitride particulate ceramic composite

Mechanical behaviour of a particulate ceramic composite (Si₃N₄ + SiC) was investigated. Its strength and fracture toughness on heating up to 1300 °C were determined as well as stress–strain curves plotted for this temperature range were analyzed. It is emphasized that this material is not only heterogeneous but also inelastic, and its deformation and fracture behaviour differ considerably from those of conventional ceramics. It was established that SENB fracture toughness measurements on notched specimens in flexure were quite reliable. Thus, there is no need in employing sophisticated standard test methods for this purpose. Fracture resistance estimates by the edge fracture (EF) method demonstrated that this material exhibited a lower barrier to the onset of fracture and a nonlinearly rising R-line, i.e., it displayed the ability to resist crack propagation (R-curve effect). The fracture resistance F_R and initial fracture toughness K_Ii were also determined. This information is rather useful for analysis of its actual performance under mechanical loading. The model of a nozzle vane of the gas turbine was employed to illustrate that the EF method was appropriate for evaluating the uniformity of ceramic items by their fracture resistance.     

Features of adhesive technology for silicon nitride ceramic components

A method is provided for joining (gluing) individual silicon nitride components used in the preparation of structural objects of different types of silicon nitride ceramic with a broad range of porosity (from zero to ~23%). The method includes preparation of an adhesive mixture based on silicon nitride, successive application of the mixture to component surfaces being joined, and firing of the joined object at 1000°C with exposure for 2 h. The glued joint is distinguished by high adhesive strength, which makes it possible to prepare objects of complex structure by this method.     

Ultrasonic monitoring of slip casting kinetics of silicon nitride ceramic

An ultrasonic technique has been used to perform insitu monitoring of ceramics formed by the slip casting of silicon nitride. The technique can monitor the cast thickness as a function of time. Experiments have been performed to study the casting kinetics as a function of slip pH and viscosity, and the results correlated with the resulting cast green density.     

Fabrication and mechanical properties of boron nitride nanotube reinforced silicon nitride ceramics

In this study, silicon nitride (Si₃N₄) ceramics added with and without boron nitride nanotubes (BNNTs) were fabricated by hot-pressing method. The influence of sintering temperature and BNNTs content on the microstructures and mechanical properties of Si₃N₄ ceramics were investigated. It was found that both flexural strength and fracture toughness of Si₃N₄ were improved when sintering temperature increases. Moreover, α-Si₃N₄ phase could transform into β-Si₃N₄ phase completely when sintering temperature rises to 1800 °C and above. BNNTs can enhance the fracture toughness of Si₃N₄ dramatically, which increases from 7.2 MPa m^1/2 (no BNNTs) to 10.4 MPa m^1/2 (0.8 wt% BNNTs). However, excessive addition of BNNTs would reduce the fracture toughness of Si₃N₄. Meanwhile, the flexural strength and relative density of Si₃N₄ decreased slightly when BNNTs were added. The related toughening mechanism was also discussed.     

Hertzian contact damage in a highly porous silicon nitride ceramic

Hertzian indentation tests were performed to evaluate the contact damage behavior of a highly porous Si₃N₄ ceramic. Using a bonded-interface technique, the Hertzian contact damage patterns were examined. As a result of intragranular microfracture under Hertzian contact, a distributed subsurface damage region is developed beneath the indenter. It was found that the damage region extends progressively with increasing contact load. In strength tests, failures were observed to originate from Hertzian indentation sites, giving rise to a gradual strength degradation.     

Effect of MgF2 addition on mechanical properties and thermal conductivity of silicon nitride ceramics

Dense silicon nitride (Si₃N₄) ceramics were prepared using Y₂O₃ and MgF₂ as sintering aids by spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering annealing at 1900 °C for 4 h. Effects of MgF₂ contents on densification, phase transformation, microstructure, mechanical properties, and thermal conductivity of the Si₃N₄ ceramics before and after heat treatment were investigated. Results indicated that the initial temperature of liquid phase was effectively decreased, whereas phase transformation was improved as increasing the content of MgF₂. For optimized mechanical properties and thermal conductivity of Si₃N₄, optimum value for MgF₂ content existed. Sample with 3 mol.% Y₂O₃ and 2 mol.% MgF₂ obtained optimum flexural strength, fracture toughness and thermal conductivity (857 MPa, 7.4 MPa m^1/2 and 76 W m⁻¹ K⁻¹, respectively). It was observed that excessive MgF₂ reduced the performance of the ceramic, which was caused by the presence of excessive volatiles.     

Sintered silicon nitride/nano-silicon carbide materials based on preceramic polymers and ceramic powder

A flexible method is presented, which enables the fabrication of porous as well as dense Si₃N₄/nano-SiC components by using Si₃N₄ powder and a preceramic polymer (polycarbosilazane) as alternative ceramic forming binder. The SiCN polymer benefits consolidation as well as shaping of the green body and partially fills the interstices between the Si₃N₄ particles. Cross-linking of the precursor at 300 °C increases the mechanical stability of the green bodies and facilitates near net shape machining. At first, pyrolysis leads to porous ceramic bodies. Finally, subsequent gas pressure sintering results in dense Si₃N₄/nano-SiC ceramics. Due to the high ceramic yield of the polycarbosilazane binder, the shrinkage during sintering is significantly reduced from 20 to 15 lin.%. Investigations of the sintered ceramics reveal, that the microstructure of the Si₃N₄ ceramic contains approx. 6 vol.% nano-scaled SiC segregations, which are located both at the grain boundaries and as inclusions in the Si₃N₄ grains.     

Mechanical and dielectric properties of functionalized boron nitride nanosheets/silicon nitride composites

Uniformly dispersed boron nitride nanosheets (BNNSs) reinforced silicon nitride (Si₃N₄) composites were prepared by surface modification assisted flocculation combined with SPS sintering. In order to improve the dispersibility of the BNNSs in the composites, the liquid phase stripped BNNSs are surface functionalized by a two-step covalently modification. The amino-modified BNNSs (NH₂-BNNSs) and Si₃N₄ powders have opposite surface potential, mixed evenly by electrostatic interaction during flocculation. The results showed that mechanical properties of Si₃N₄ composites were obviously enhanced by adding NH₂-BNNSs. The fracture toughness and bending strength of Si₃N₄ composites added 0.75 wt% NH₂-BNNSs were increased by 34% and 28%, respectively, compared with monolithic Si₃N₄. Toughening mechanisms are synergistic action of the torn, pull-out or bridging of BNNSs and crack deflection mechanisms with microstructural analyzes. The dielectric properties of the Si₃N₄ ceramics are also improved after the addition of NH₂-BNNSs.     

Effect of Si addition on the mechanical and thermal properties of sintered reaction bonded silicon nitride

Advanced silicon nitride (Si₃N₄) ceramics were fabricated using a mixture of Si₃N₄ and silicon (Si) powders via conventional processing and sintering method. These Si₃N₄ ceramics with sintering additives of ZrO₂ + Gd₂O₃ + MgO were sintered at 1800 °C and 0.1 MPa in N₂ atmosphere for 2 h. The effects of added Si content on density, phases, microstructure, flexural strength, and thermal conductivity of the sintered Si₃N₄ samples were investigated in this study. The results showed that with the increase of Si content added, the density of the samples decreased from 3.39 g/cm³ to 2.92 g/cm³ except for the sample without initial Si₃N₄ powder addition, while the thermal diffusivity of the samples decreased slightly. This study suggested that addition of Si powder, which varied from 0 to 100%, in the starting materials might provide a promising route to fabricate cost-effective Si₃N₄ ceramics with a good combination of mechanical and thermal properties.