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1.
The effects of oxidation on changes in the secondary phases of two Si3N4 ceramics were investigated by transmission electron microscopy. The Si3N4 materials were oxidized at 1400 °C for 168 h in laboratory air. One material, sintered with 5 vol% Yb2O3+0.5 vol% Al2O3, containing a Yb2Si2O7 crystalline secondary phase, displayed no gross changes following oxidation. However, the thickness of the amorphous intergranular film was observed to have decreased by 20% from its initial thickness of 1.0 nm. The second Si3N4 material, sintered with 5 wt% Y2O3+1 wt% MgO, had a completely amorphous secondary phase. Devitrification of the secondary phase at multiple-grain junctions to -Y2Si2O7 accompanied the outward diffusion of additive and impurity cations occurring in the residual amorphous intergranulàr films during oxidation. Substantial cavitation and intergranular phase depletion was observed at both multiple-grain junctions and two-grain boundaries. The equilibrium thickness of the amorphous intergranular film consequently decreased from 1.2 to 0.9 nm following oxidation. Purification of the amorphous intergranular films by diffusion of cations to the surface led to a reduction in impurity concentration, resulting in the observed thinning of grain-boundary films. 相似文献
2.
It is shown that post-sintering heat treatments in air in the temperature range 1100 to 1400° C result in substantial crystallization of the glassy phase in an Si3N4 material which was produced by the nitridation pressureless sintering (NPS) method using Y2O3 and Al2O3 as sintering aids. X-ray diffraction combined with analytical electron microscopy showed that the secondary crystalline phases which form are strongly dependent upon time and temperature of heat treatment as well @S depth below the oxide scale. This effect is primarily due to the outward diffusion of cations (yttrium, aluminium and impurities) as well as the inward diffusion of oxygen. Small glassy pockets and thin amorphous intergranular films remain in the microstructure after heat treatment. 相似文献
3.
L. K. L. Falk 《Journal of Materials Science》2004,39(22):6655-6673
This review paper is focussed on the characterization of the microstructural development during liquid phase sintering and post-densification crystallisation heat treatment of ceramic materials based on the Si3N4 or SiC structures. Grain shape and size distributions, assessed by quantitative microscopy in combination with stereological methods, and fine scale microstructures, investigated by electron diffraction and high resolution imaging and microanalysis in the TEM, are discussed and related to the fabrication process and the overall composition of the ceramic material. It is demonstrated that combined high resolution analytical and spatial information from chemically and structurally distinct fine scale features, such as grain boundary films of residual glass, is obtained by electron spectroscopic imaging and subsequent computation of elemental distribution images. These images reveal that residual glassy grain boundary films are rich in oxygen and cations originating from the metal oxide/nitride additives, consistent with fine probe EDX analysis in the FEGTEM. Elemental analysis with high spatial resolution has also shown that grain growth into pockets of residual liquid/glass is associated with diffusion profiles in the glass in front of the growing grain. High resolution imaging in the TEM and elemental maps computed from electron energy filtered images show that the intergranular film thickness, in general, varies within a particular silicon nitride or sialon microstructure. Furthermore, grain boundaries, apparently free from residual glass may co-exist with glass-containing grain boundaries in some silicon nitride microstructures. In addition to the choice and weight fraction of sintering additives, factors such as the ionic radius of the cations originating from the additives, the local nano-scale chemistry and the relative grain orientation have an effect on the volume fraction and morphology of the intergranular microstructure. 相似文献
4.
Liquid-phase sintered Si3N4 doped with Yb2O3 as a sintering aid was characterized by both transmission electron microscopy and extended X-ray absorption fine structure (EXAFS) measurements. Structural information about the secondary phases was obtained with an emphasis being placed on the evaluation of EXAFS data. Two Si3N4 samples were processed which contained either 5 vol% or 10 vol% Yb2O3 as sintering additive. After sintering, only an amorphous secondary phase was observed in the material doped with 5 vol% Yb2O3. The material with the higher Yb2O3 volume fraction underwent a further heat treatment after the densification, in order to crystallize pockets of the secondary phase. This heat treatment resulted in the formation of Yb2Si2O7 at multi-grain junctions, with however, amorphous phases remaining along the grain and phase boundaries. The EXAFS data obtained from the two doped Si3N4 materials were compared with reference spectra obtained on pure Yb2O3 and synthetic Yb2Si2O7. No residual Yb2O3 was determined in the doped Si3N4 materials, independent of the Yb2O3 volume fraction. Compared to synthetic Yb2Si2O7, the secondary phase formed in the 10 vol% Yb2O3 containing material showed only subtle changes in the EXAFS data. A clear distinction between purely amorphous and a combination of crystalline plus amorphous Yb secondary phases was possible, when both doped Si3N4 materials were compared. However, no distinction between the glass phase present at triple junctions and the amorphous residue along grain/phase boundaries was feasible, since a full numerical data evaluation could not be performed. 相似文献
5.
Bando Yoshio Mitomo Mamoru Kurashima Keiji 《Journal of Materials Synthesis and Processing》1998,6(5):359-365
The chemical compositions of the grain boundary phases of silicon nitride (Si3N4) ceramics containing additives of 1 mole% and 10 mole% of an equi-molar mixture of Y2O3 and Nd2O3 have been studied by 300 kV field emission analytical electron microscopy. The energy dispersive x-ray spectra (EDS) are obtained from both two-grains and triple-grain junctions, where an electron beam of about 0.5 nm in diameter is focused. The thickness of the intergranular thin film is found to be about 1 nm, whose value is almost the same between two samples. The sintering additives are highly enriched at the triple-grain junctions, while they are less concentrated at the two-grain junctions. It is also shown that the additives are distributed inhomogeneously within the triple-grain junctions. Based on the composition analysis among the grain boundaries, an inhomogeneous grain boundary composition model for the Si3N4 ceramics is proposed. 相似文献
6.
The microstructure of liquid phase sintered SiC ceramics was characterised by means of high resolution transmission electron microscopy (HRTEM). The SiC ceramics were pressureless sintered with the additions of Al2O3 and Y2O3 at sintering temperatures of 1800 and 1950°C, respectively. At a sintering temperature of 1800°C the microstructure of the SiC ceramics has no crystallised secondary phase and the SiC grains are separated by an intergranular amorphous film. In contrast, in the case of the microstructure of SiC ceramics sintered at 1950°C a clean interface without any amorphous layer between the SiC grains was observed. The secondary phase is crystallised into the Y3Al5O12 phase and exhibits a clean interface between the SiC grains. An explanation for the existence or the absence of the intergranular glass films are given by an extended Clarke's model of the force balance of attractive van der Waals forces and repulsive steric forces. The chemical decomposition of the intergranular glass film at elevated temperature was considered. 相似文献
7.
Bo Wang Jun Yang Rui Guo Jiqiang Gao Jianfeng Yang 《Materials Science and Engineering: A》2009,500(1-2):79-83
Various silicon nitride materials were fabricated by pressureless sintering using lutetia and alumina as sintering additives. Densification behavior, microstructure, strength and formation of secondary crystalline phases were investigated. The combination of Lu2O3/Al2O3 sintering aids can promote the densification and evolution of a fine grain microstructure of Lu–Al-doped silicon nitride because of the low viscosity of the liquid. The J′ phase given by Lu4Si2−xAlxO7+xN2−x was considered to be secondary crystalline phase at the grain pockets. The composition with a Lu2O3/Al2O3 weight ratio 10/10 had highest strength of 690 ± 50 MPa. 相似文献
8.
Sintering behaviour and microstructure of Al2O3 ceramics without additives and with 0.02–0.25 mol% CaO + SiO2 (CaO/SiO2 = 1) were investigated. When Al2O3 bodies were sintered at 1400 °C, the sinterability and the grain size decreased as the content of CaO + Si2 increased. When Al2O3 ceramics with 0.05 – 0.25 mol% CaO + SiO2 were sintered at higher sintering temperature, both CaO and SiO2 reacted with Al2O3 to produce the liquid phase along grain boundaries, and exaggerated platelet Al2O3 grains, with an aspect ratio of about 4.5, were formed. Because the size of platelet grains decreased as the content of CaO + SiO2 increased, the distribution of either SiO2 particles or this intergranular phase of CaO – Al2O3 – SiO2 might control the microstructure. 相似文献
9.
E. Olsson L. K. L. Falk G. L. Dunlop R. Österlund 《Journal of Materials Science》1985,20(11):4091-4098
The microstructure of a ZnO varistor material has been investigated by a combination of X-ray diffractometry and analytical electron microscopy (SEM, TEM, STEM, EDX). The material was found to consist of: ZnO grains (doped with manganese, cobalt and nickel); smaller spinel grains which hinder the growth of ZnO grains during sintering; intergranular Bi-rich phases (namely -Bi2O3, pyrochlore and an amorphous phase); and a small proportion of ZnO-ZnO interfaces which did not have any intergranular film but to which bismuth had segregated. The intergranular microstructure is largely a result of processes which occur during liquid phase sintering and subsequent cooling to room temperature. 相似文献
10.
Hot-pressing was used to prepare a dense (97% relative density) cubic Al substituted Li7La3Zr2O12 material at temperatures lower than typically used for solid-state and/or liquid phase sintering. Electron microscopy analysis
revealed equiaxed grains, grain boundaries, and triple junctions free of amorphous and second phases and no Al segregation
at grain boundaries. These results suggest that Al2O3 and/or Al cannot act as a sintering aid by reducing grain boundary mobility. If Al2O3 acts as a sintering aid its main function is to enter the lattice as Al to increase the point defect concentration of the
slowest moving species. 相似文献
11.
AbstractYb2O3 is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si3N4 ceramics. Here we report the fabrication of dense Si3N4 ceramics with high thermal conductivity by the gas pressure sintering of α-Si3N4 powder compacts, using only Yb2O3 as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb2O3 content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb2O3 content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb2O3 and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb2O3 content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m-1 K?1 upon 1 mol% Yb2O3 addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m?1 K?1) at 7 mol% Yb2O3. The thermal conductivity value was strongly related to the microstructure. 相似文献
12.
Whisker/matrix interfaces between -SiC whiskers and -Si3N4 or -Al2O3 matrices in composites were examined by high-resolution electron microscopy (HREM), and electron energy loss (ELS) and energy dispersive X-ray (EDS) spectroscopies. Most whisker/matrix interfaces were crystalline, with whiskers directly bonded to matrix crystals. Some whisker/matrix interface regions contained amorphous thin films and these occurred more often in the Si3N4 composite, which contained sintering additives, than in the Al2O3 matrix composite, which did not. No evidence for light element segregation at crystalline whisker/matrix interfaces was detected by ELS or EDS at 5 nm spatial resolution. Impurities were concentrated in glassy regions in matrix grain boundaries, triple junctions, or at infrequent whisker/matrix interfaces containing amorphous films. 相似文献
13.
Xinwen Zhu You Zhou Kiyoshi Hirao Takamasa Ishigaki Yoshio Sakka 《Science and Technology of Advanced Materials》2010,11(6)
Yb2O3 is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si3N4 ceramics. Here we report the fabrication of dense Si3N4 ceramics with high thermal conductivity by the gas pressure sintering of α-Si3N4 powder compacts, using only Yb2O3 as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb2O3 content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb2O3 content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb2O3 and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb2O3 content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m-1 K−1 upon 1 mol% Yb2O3 addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m−1 K−1) at 7 mol% Yb2O3. The thermal conductivity value was strongly related to the microstructure. 相似文献
14.
R. Larker L. -Y. Wei B. Loberg M. Olsson S. Johansson 《Journal of Materials Science》1994,29(16):4404-4414
Diffusion bonding by hot isostatic pressing (HIP) was performed between Incoloy 909 and five different ceramics. Two of the ceramics were composites made from powder mixtures of Si3N4 and either 60 vol% TiN or 50 vol% TiB2, while three were monolithic materials, namely Si3N4 with 2.5 wt% Y2O3 as a sintering additive, Si3N4 without additives, and Si2 N2O without additives. A diffusion couple geometry was developed to facilitate the preparation of thin-foil specimens for examination by analytical electron microscopy (AEM). Diffusion bonding was performed by HIP at 927°C (1200K) and 200 MPa for 4 h. The formation of reaction layers was very limited, being less than 1 m in total layer thickness. Two reaction products were found by AEM; a continuous, very thin, (100 nm) layer of fine TiN crystals at the initial ceramic/metal interface, and larger grains extending about 100–500 nm into the superalloy and forming a semi-continuous layer of a G-phase suicide containing mainly nickel, silicon and niobium. 相似文献
15.
Modified lead titanate ceramics with the composition Pb0.70 Ca0.30 Ti0.94 (Co1/2W1/2)0.06 O3 with 1 mol% MnO were prepared by the mixed oxide route. By varying the sintering temperature, ceramics with average grain sizes between 2.8 and 5m were obtained. An increase in grain size resulted in an increased electromechanical anisotropy and a decreased dielectric constant. Transmission electron microscopy was used to examine the ferroelectric domain boundaries and intergranular phases. Indications are that during poling favourably oriented domains approximately doubled in size by 90 ° type domain wall switching. Evidence was found for the existence of a vitreous intergranular phase at multiple grain junctions. EDS microanalysis indicated that the amorphous grain boundary phase had a lower lead content than the bulk material. 相似文献
16.
R. R. Wills R. W. Stewart J. A. Cunningham J. M. Wimmer 《Journal of Materials Science》1976,11(4):749-759
A new group of materials called the silicon lanthanide oxynitrides has been prepared by the reaction between Si3N4 and several oxides of the lanthanide series (La2O3, Sm2O3, Dy2O3, Er2O3, and Yb2O3). These oxides formed compounds of the type Si3N4 · R2O3 and R4Si2O7N2 (R being lanthanide). In addition La2O3 and Yb2O3 formed the compounds 2Si3N4 · La2O3 and Yb2Si3O5N2, respectively. Certain similarities in the unit cells of these compounds have been noted, and their structures are discussed in terms of similarity to known minerals. It is suggested that this group of materials contains a large number of compounds. 相似文献
17.
Jinhui Dai Jianbao LiYongjun Chen Liu YangGeliang Sun 《Materials Research Bulletin》2003,38(4):609-615
The β-Si3N4 particles were prepared by heating original α-Si3N4 powder with rare earth oxide Nd2O3 or Yb2O3 additives at 1600-1700 °C for 1.5 h. The transformation ratio of α-Si3N4 was also investigated by XRD. The results showed that Yb2O3 could accelerate the transformation of Si3N4 more effectively than Nd2O3 and the powder heated at 1700 °C with over 4 wt.% Yb2O3 has a high transformation ratio of over 98%. The morphologies of the heated powders were observed by scanning electron microscopy. The results showed that the powder heated at 1700 °C with 4 wt.% Yb2O3 had ideal β-Si3N4 rod-like morphology particles. This heated powder was used as a seed by adding it to the original α-Si3N4 powder to prepare self-reinforced Si3N4 ceramic by hot-pressed sintering. The fracture toughness of the seeded Si3N4 ceramics increased to 9.1 MPa m1/2 from 7.6 MPa m1/2 of the unseeded Si3N4 ceramics, while the high value of strength was still kept at 1200 °C. 相似文献
18.
The effect of attrition milling and post-sintering heat treatment on the fabrication, phase relations, microstructure and properties of ZrO2 (+2.3vol% Y2O3) powder used to produce a transformation toughened material was examined. Powder used to fabricate the unmilled material was treated and consolidated by a colloidal method. The same powder, treated and consolidated by the same method, but ball milled in a commercial alumina mill before consolidation, was used to fabricate the milled material. Both materials were sintered at 1400° C for 1 h and then heat treated at higher temperatures. Milling introduced Al2O3 inclusions (< 1 vol%) and a glass phase (7 to 10 vol%). The milled powder was more difficult to sinter and exhibited more bloating (density decrease) during subsequent heat treatment. Transmission electron microscopy observations indicated that the larger glass content of the milled material beneficially reduced residual stresses that arose due to thermal contraction anistropy. Post-sintering heat treatment at temperatures > 1450° C produced detectable amounts of cubic ZrO2 consistent with previously reported phase studies of the ZrO2-Y2O3 system. The development of a bimodal grain structure was concurrent with the formation of detectable cubic phase. The larger grains in this bimodal distribution were primarily observed on the external surface and co-ordinating pores produced during the post-sintering heat treatments which were responsible for the bloating phenomenon. It is hypothesized that the pores were produced by the release of high pressure oxygen during cubic phase formation. Both fracture toughness (K
c) and hardness of the as-sintered materials were unaffected by milling. Hardness decreased with bloating and the decrease was more pronounced for the milled material which exhibited more bloating. 相似文献
19.
T. Sato Y. Tokunaga T. Endo M. Shimada K. Komeya K. Nishida M. Komatsu T. Kameda 《Journal of Materials Science》1988,23(10):3440-3446
The leaching behaviours of hot-pressed Si3N4 ceramics containing Y2O3, Al2O3 and AIN as additives and hot isostatically pressed Si3N4 without additives were studied in 0.1 to 10 M HF aqueous solutions at 50 to 80° C. Silicon and aluminium ions were dissolved into the HF solutions, but yttrium ion did not dissolve at all and formed insoluble YF3. The dissolution of silicon and aluminium ions was controlled by the surface chemical reaction and the apparent activation energies were 70.5 to 87.6 kJ mol–1, respectively. The corrosion rate increased with increasing degree of crystallization of the grain boundary phases. The corrosion resulted in roughness of the surface and degradation of the fracture strength. Si3N4 ceramics containing an amorphous phase at the grain boundaries showed the most excellent resistance to corrosion with HF solution, and kept a fracture strength of above 400 M Pa even after leaching 40% of the silicon ions. 相似文献
20.
Usually, injection moulded SiC green parts are debound in inert atmosphere or vacuum, which induces the residual carbon and increases forming cycle and production cost. In this paper, injection moulded SiC with Al2O3 and Y2O3 as sintering assistant was thermal debound in air and Ar, respectively. The paper investigates the effects of pre-oxidation during debinding stage on the sintering and mechanical property of SiC material. During sintering, the oxide SiO2 is in favour of the shrinkage of debound samples at lower temperature. After sintering, the linear shrinkage of sintered samples with pre-oxidation is bigger than the sample without pre-oxidation. Test results by TEM and XRD indicate that SiO2 disappear from the inside of the sintered samples. The loss of SiO2 decreases the content of Al2O3, which affects the formation of YAG (Y3Al5O12). Sintered Sic samples contain α-SiC phase and intergranular phase. There is no hetero-phase between the boundaries of α-SiC phase and intergranular phase. The bending and compression strength values of sintered samples with pre-oxidation reach to 537 MPa and 2.89 GPa, respectively. These values approach the strength of sintered samples without pre-oxidation (594 MPa and 3.0 GPa). 相似文献