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1.
The technique for the fabrication of Si3N4 which was investigated involves the nitridation of Si:Si3N4 powder compacts containing additions of sintering aids (e.g. Y2O3 and Al2O3) followed by pressureless sintering. The development of microstructure during fabrication by this method has been followed by X-ray diffraction and analytical electron microscopy. As well as being important for the sintering process, it was found that the sintering aids promote nitridation through reaction with the surface silica on the powder particles. During nitridation extremely fine grained Si3N4 forms at silicon powder particle surfaces and at tunnel walls extending into the interior of these powder particles. Secondary crystalline phases which form during nitridation are eliminated from the microstructure during sintering. The- to-Si3N4 phase transformation is completed early in the sintering process, but despite this the fully sintered product contains fine-Si3N4 grains. The grains are surrounded by a thin intergranular amorphous film. 相似文献
2.
The Al2O3-ZrO2 eutectic composition was rapidly solidified, forming amorphous and crystalline structures. The as-quenched material was crushed and pressed into pellets which were sintered conventionally or with microwaves. Conventional and microwave sintering at temperatures up to 1600 °C resulted in a microstructure where 100–200 nm ZrO2 grains were present intergranularly in the -Al2O3 grains. Larger ZrO2 grains (1 m) were found intergranularly. The as-quenched lamellar structure spheroidized during sintering at high temperatures. Boron contamination of the powders resulted in more homogeneous and dense as-fired samples but promoted the ZrO2 tetragonal-to-monoclinic transformation, which was attributed to increased grain boundary diffusivity. Conventional sintering at low temperatures resulted in the formation of rods of an Al2O3-rich phase which grew from a low-melting B2O3-rich liquid. 相似文献
3.
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
4.
Ceramic foams are made of zinc oxide using different amounts of Sb2O3 and Bi2O3 as sintering aids. The effect of a ball milling processing of the starting powders and the sintering temperature on the microstructure and the properties of the ZnO foams is investigated. The focus is set on the evolution of the secondary phases formed within the microstructure of ZnO. A determining effect is identified in the amount of an Al2O3 impurity which is introduced by abrasion of the milling vessels during ball milling. Alumina is partially dissolved in a spinel α–Zn7Sb2O12 secondary phase which is stabilized by a reduction of the unit cell volume. Remaining Al2O3 is incorporated into zinc oxide under formation of a defect wurtzite phase. The phase evolution is a complex function of the content of sintering aids, the Al2O3 impurity level and the sintering temperature. The shrinkage during sintering and the porosity evolution are correlated to the phase composition within the ZnO material. The thermal conductivity and the compressive strength of the foams are determined, normalized with respect to their porosity, and correlated to the microstructure and phase composition of the ZnO strut material. 相似文献
5.
Liquid-phase sintering (LPS) of ceramics issued from powders of bismuth-titanium-doped zinc oxide, made of spherical grains constituted of small crystallites, were studied by dilatometry. A correlation can be established between the shape of the shrinkage curves and the microstructure of the ceramics. During sintering the spherical grains are destroyed as soon as the liquid phase appears and the rearrangement proceeds directly between crystallites. Systematic studies were carried out on bismuth-titanium-doped ZnO. For small Bi contents, the beginning of a solid-phase sintering process occurs. For 1 at % Bi in the binary system Bi-doped ZnO, rapid shrinkage leads to a well-densified ceramic with a grain size of 70 m on average. The influence of the amount of titanium iny% Ti-1% Bi-doped ZnO is discussed. Whatever the amount of Ti, LPS is observed. The shrinkage curves depend greatly on the amount of Ti: only a Ti content ofy=0.6 % leads to an increase in grain size (100 m on average). 相似文献
6.
S. M. Wiederhorn B. J. Hockey D. C. Cranmer R. Yeckley 《Journal of Materials Science》1993,28(2):445-453
Transient creep is shown to dominate the high-temperature behaviour of a grade of hot isostatically pressed silicon nitride containing only 4 wt% Y2O3 as a sintering aid. Contributing factors to transient creep are discussed and it is concluded that the most likely cause of longterm transient creep in the present study is intergranular sliding and interlocking of silicon nitride grains. In early stages of creep, devitrification of the intergranular phase, and intergranular flow of that phase may also contribute to the transient creep process. The occurrence of transient creep precluded the determination of an activation energy on the as-received material. However, after creep in the temperature range 1330–1430°C for times exceeding approximately 1100 h, an apparent activation energy of 1260 kJ mol–1 was measured. It is suggested that the apparent activation energy for creep is determined by the mobility and concentration of diffusing species in the intergranular glassy phase. The time-to-rupture was found to be a power function of the minimum strain rate, independent of applied stress or temperature. Hence, creep-rupture behaviour followed a Monkman-Grant relation. A strain rate exponent of – 1.12 was determined. 相似文献
7.
The sintering behaviours of four kinds of Si3N4 powders were investigated by dilatometry in 10 atm N2 at 1890, 1930 and 2050° C. The sinterabilities of powders were compared and discussed in relation to the powder characteristics. A large size distribution in the powder accelerated grain and pore growth at <1800° C, which resulted in the inhibition of further densification at >1800° C. The presence of carbon in a powder prevented densification. A powder with a uniform grain size kept the microstructure of the sintered material uniform during sintering at <1800° C and gave a high degree of shrinkage at >1800° C. Densification at >1800° C was accompanied by the dissolution of equi-axial -Si3N4 grains and reprecipitation as elongated -Si3N4 grains from the oxynitride liquid. The relation between the densification and microstructure is discussed in terms of the relative rates of densification and grain growth. 相似文献
8.
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
9.
Jae-Yeon Kim Young-Wook Kim June-Gunn Lee Kyeong-Sik Cho 《Journal of Materials Science》1999,34(10):2325-2330
Alpha-SIC powder containing 7.2 wt % Y3Al5O12 (YAG, yttrium aluminum garnet) and 4.8 wt % SiO2 as sintering aids were hot-pressed (SC0) at 1820°C for 1 h and subsequently annealed at 1920°C for 2 h (SC2), 4 h (SC4) and 8 h (SC8). When the annealing time was increased, the microstructure changed from equiaxed to elongated grains and resulted in self-reinforced microstructure consisted of large elongated grains and small equiaxed grains. Development of self-reinforced microstructure, consisted of mostly 6H phase, resulted in significant improvements in toughness. However, the improved toughness was offset by a significant reduction in strength as in the materials consisted of 4H originated from -SiC. The fracture toughness and strength of the 8-h annealed materials were 5.5MPa · m1/2 and 490 MPa, respectively. 相似文献
10.
Guillaume Bernard-Granger Amandine Néri Christelle Navone Mathieu Soulier Julia Simon Maya Marinova-Atanassova 《Journal of Materials Science》2012,47(10):4313-4325
Spark plasma sintering of a p-type Si0.795Ge0.200B0.005 alloy has been investigated in vacuum, in the 400–1200 °C temperature range. The densification mechanism has been determined
using isothermal and anisothermal methods. In spite of a slight material degradation for the highest sintering temperatures
(occurrence of cristobalite nodules homogeneously dispersed in intergranular and intragranular positions), it is proposed
that densification proceeds by grain boundary sliding accommodated most probably by silicon volume diffusion. The microstructure
observation of several sintered samples using transmission electron microscopy supports the densification mechanism advanced.
Because the elemental grains remain mostly equiaxe whatever the sintering conditions, a grain intercalation mechanism may
be also implicated during densification. 相似文献
11.
The effects of small compositional variations on the densification and microstructural evolution of bismuth titanate (Bi4Ti3O12) powder compacts were investigated during sintering and during hot forging. For a nominally stoichiometric Bi4Ti3O12 composition, sintering commenced at 870°C, leading to a relatively dense microstructure (relative density >97% of the theoretical value) with randomly aligned elongated grains after 1 h at 1100°C. Small additions (1 weight percent) of Bi2O3 or TiO2 to the nominally stoichiometric Bi4Ti3O12 composition shifted the onset of sintering to lower or higher temperatures, respectively, but did not significantly alter the final density. Hot forging produced a microstructure of aligned, elongated grains. The small compositional variations did not seriously influence the ability to develop the elongated grain alignment. However, subsequent annealing of the hot forged materials produced significant changes in the aligned grain microstructure. The elongated grain alignment in the nominally stoichiometric Bi4Ti3O12 composition was destroyed during subsequent annealing for less than 2 h at 1100°C. 相似文献
12.
J. S. Vetrano H. -J. Kleebe E. Hampp M. J. Hoffmann M. Rühle R. M. Cannon 《Journal of Materials Science》1993,28(13):3529-3538
Microstructural development and crystallization behaviour of Yb2O3-fluxed sintered silicon nitride materials was investigated using CTEM and HREM. The materials contained 5 and 10 vol% Yb2O3 as sintering additives. After densification, both compositions were subsequently heat treated to crystallize the residual amorphous secondary phases present at triple-grain regions. In the material doped with 5 vol% Yb2O3, only an amorphous secondary phase was observed after sintering, which was about 80% crystalline (Yb2Si2O7) after the post-sintering heat treatment. A metastable phase was formed in the material with 10 vol% additives after sintering, with about 70% crystallinity in the triple-point pockets. Upon postsintering heat treatment, the material could be completely crystallized. During heat treating, the metastable phase combined with the remaining glass to form Yb2SiO5 plus Yb2Si2O7 and a small amount of Si3N4 which deposited epitaxially on pre-existing Si3N4 grains in areas of low-energy within the triple-point pockets. All materials contained thin amorphous films separating the grains. The amorphous intergranular films along grain boundaries (homophase boundaries) revealed excess ytterbium and oxygen. The thickness of the intergranular films was about 1.0 and 2.5 nm for the grain boundaries and the phase boundaries, respectively, independent of additive content and heat-treatment history. 相似文献
13.
TZP ceramic of 99.7% theoretical relative final density was obtained by pressureless sintering a commercial co-precipitated 3 mol % Y2O3-ZrO2 powder at 1400° C for 10 h. Fracture surfaces of the aged material revealed that the fracture of TZP ceramic was typified by an intergranular mode in areas where the phase was mainly tetragonal, whereas the transgranular mode was found predominantly in the area containing more monoclinic phase. Microcracks induced by the (t) (m) transformation provided short paths for water to accelerate the property degradation of TZP upon low-temperature ageing in a humid atmosphere. 相似文献
14.
A nanocomposite material fabricated by hot pressing in the form of nanometre-sized Si3N4 particles dispersed in an Al2O3 matrix has been shown to exhibit enhanced mechanical properties compared with monolithic matrix material. It was observed by transmission electron microscopy (TEM) for the first time that the alumina grains were in the shape of elongated columns with aspect ratios in the range 2.5–4. The presence of liquid phase during sintering was found to be responsible for the appearance of columnar grains. Regular hexagon-shaped larger -Sialon grains formed during sintering were mainly situated at grain boundaries of the matrix material while irregular smaller dispersoids were trapped within the alumina grains. The improvement in the mechanical properties of the nanocomposite is attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the self-reinforcement effect arising from the elongated columnar grains of the matrix, as well as the pinning effect due to the existence of intergranular -sialon particles. It was revealed that the trapped particles have an -Al2O3 structure with partial sites of aluminium and oxygen atoms substituted by silicon and nitrogen atoms, which is also likely to lead to the strengthening of the composite. 相似文献
15.
Investigations were made of varistor microstructure, the morphology of Bi2O3 at multiple ZnO grain junctions, Bi2O3/ZnO grain boundaries and ZnO/ZnO grain boundaries (especially whether Bi2O3 is present or not at the ZnO/ZnO grain boundary) by means of high-resolution transmission electron microscopy and X-ray microanalysis in the scanning transmission electron microscope. Bi2O3 at multiple ZnO grain junctions consists of small particles of 0.1m in diameter, and they are vitrified to some extent. It is suggested that bismuth ions dissolve into ZnO grains over a 30 nm range from a Bi2O3/ZnO grain boundary; however, there is no bismuth at ZnO/ZnO grain boundaries. 相似文献
16.
Ying Yuan Shuren Zhang Changmin Li 《Journal of Materials Science: Materials in Electronics》2004,15(9):601-606
The effect of doping process on the dielectric properties, sintering behavior and microstructure were investigated on the BaTiO3–Nb2O5–Co3O4 ternary system ceramic. Temperature stable dielectric ceramics were obtained by different doping processes if only appropriate Nb5++Co3+ amount and Nb5+/Co3+ ratio were adopted. The dielectric constant was enhanced to the largest extent by nanometer oxide doping and the temperature characteristic satisfied the X7R specification. Two kinds of grains were observed in all the samples: matrix grains (BaTiO3) and the secondary phase grains (Ba6Ti17O40) formed by the incorporation of Nb5+ and Co3+ into BaTiO3 lattice and Ti4+ segregation. The matrix grains were about 1 m in diameter and showed little grain growth with increasing temperature in all the doped samples, whereas the sizes of the secondary phase grains were strongly dependent on the doping process. The secondary phase formed liquid phase during firing, but the liquid phase contributed little to the densification of ceramics. 相似文献
17.
M. V. Vlasova N. G. Kakazey O. Milosevic D. P. Uskokovic D. Poleti D. Vasovc 《Journal of Materials Science》1990,25(10):4324-4330
The structure of ZnO varistors prepared by different chemical methods was studied by the electronic paramagnetic resonance (EPR) method. The presence of Mn2+ ions in both ZnO lattice and electroconductive phase was used as a sensitive probe for the analyses of structural changes which occur during sintering of ZnO varistors. Potential mechanisms which can contribute to the formation of resonant lines were considered. The concentration of paramagnetic centres was quantitatively analysed. The variation of EPR signals of Mn2+ ions in ZnO phase was registered as a function of the chemical methods used for the preparation of powders only when samples were sintered at lower temperatures and non-linear characteristics of varistor ceramics had not yet been reached. At higher sintering temperatures EPR signals of Mn2+ ions in electroconductive phase differed only in the case of powders obtained by NaOH coprecipitation. 相似文献
18.
Silicon nitride ceramics with celsian as an additive 总被引:5,自引:0,他引:5
Four different Si3N4 ceramics using 10 wt% celsian (BaAl2Si2O8) as an additive, have been prepared by hot pressing. Different celsian sources and hot-pressing conditions were used and their effects on densification, in situ crystallization of celsian, to phase transformation, microstructures and properties, were examined. The use of a presynthesized celsian as a raw material was found to enhance the rates of densification and phase transformation due to the excess silica from raw Si3N4 powders which, however, did not change the microstructure and crystallization behaviour of celsian. Increases in hot-pressing temperatures and times increased the total number of large elongated grains and resulted in coincidental increases in strength and toughness until the to transformation was complete. An intermediate quench-and-reheat step during hot pressing made the microstructure finer and more uniform by greatly reducing the large-sized elongated grains. The quenching step also disrupted complete crystallization of celsian which led to more grain-boundary glassy phase and compromised the material's high-temperature properties. 相似文献
19.
Yu. F. Kargin A. S. Lysenkov S. N. Ivicheva A. I. Zakharov N. A. Popova K. A. Solntsev 《Inorganic Materials》2010,46(7):799-803
Silicon nitride ceramics containing calcium aluminates as sintering aids have been prepared by hot pressing at 1650°C in a
nitrogen atmosphere, and the effect of sintering aid content on their microstructure, phase composition, mechanical strength,
and air oxidation resistance has been studied. The results demonstrate that the Si3N4 ceramic containing 10 wt % calcium aluminates has a uniform distribution of intergranular multicomponent oxide phases and
consists of densely packed silicon nitride grains. Owing to this, it offers the maximum mechanical strength (850 MPa) and
is stable to air oxidation up to 1300°C. 相似文献
20.
The sintering behaviour of -Si3N4 powder was investigated in 980 kPa (10 atm) nitrogen at 1800–2000 °C. It is shown that -Si3N4 has a higher sinterability than the finer -Si3N4. The solution of small grains and reprecipitation on large grains occurred during sintering at >1600 °C. The rate-determining step in the liquid-phase sintering is believed to be the diffusion of material through the liquid phase at grain boundaries. There was no abnormal grain growth during gas pressure sintering of -Si3N4. The microstructures of gas pressure sintered materials from -Si3N4 were more uniform than those from -Si3N4. The densification mechanism of -Si3N4 is discussed in relation to that of -Si3N4. 相似文献