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1.
Jun-Youp Choi Suk-Joong L. Kang O. Fukunaga Jong-Ku Park K. Y. Eun 《Journal of the American Ceramic Society》1993,76(10):2525-2528
Four kinds of BN powders—amorphous BN with B2 O3 , partially crystallized BN without B2 O3 , well-crystallized hBN with B2 O3 , and well-crystallized hBN without B2 O3 —were prepared to determine the effect of B2 O3 on the crystallization of amorphous BN and the effect of BN crystallinity on the formation of cBN under high pressure (4–5 GPa) and at high temperature (1350–1450°C). The amorphous BN with B2 O3 easily crystallized and transformed to cBN in the presence of A1N catalyst, while the partially crystallized BN without B2 O3 did not. The well-crystallized hBN transformed very slowly to cBN without B2 O3 , in contrast to fast transformation with B2 O3 . It is thus found that the transformation from hBN to cBN in the presence of AIN catalyst is determined by the degree of BN crystallinity as well as the presence of B2 O3 . Cubic BN can be synthesized only from crystallized hBN under the experimental conditions used. The formation of cBN from amorphous BN is possible through its prior crystallization, which can occur in the presence of B2 O3 . 相似文献
2.
Si3 N4 test bars containing additions of BN, B4 C, and C, were hot isostatically pressed in Ta cladding at 1900° and 2050°C to 98.9% to 99.5% theoretical density. Room-temperature strength data on specimens containing 2 wt% BN and 0.5 wt% C were comparable to data obtained for Si3 N4 sintered with Y2 O3 , Y2 O3 and Al2 O3 , or ZrO2 . The 1370°C strengths were less than those obtained for additions of Y2 O3 or ZrO2 but greater than those obtained from a combination of Y2 O3 and Al2 O3 . Scanning electron microscope fractography indicated that, as with other types of Si3 N4 , roomtemperature strength was controlled by processing flaws. The decrease in strength at 1370°C was typical of Si3 N4 having an amorphous grainboundary phase. The primary advantage of non-oxide additions appears to be in facilitating specimen removal from the Ta cladding. 相似文献
3.
Young-Hag Koh Hae-Won Kim Hyoun-Ee Kim John W. Halloran 《Journal of the American Ceramic Society》2002,85(12):3123-3125
The oxidation behavior and its effect on the mechanical properties of fibrous monolith Si3 N4 /BN after exposure to air at temperatures ranging from 1000° to 1400°C for up to 20 h were investigated. After exposure at 1000°C, only the BN cell boundary was oxidized, forming a B2 O3 liquid phase. With increasing exposure temperature, the Si3 N4 cells began to oxidize, forming crystalline Y2 Si2 O7 , SiO2 , and silicate glass. However, in this case, a weight loss was observed due to extensive vaporization of the B2 O3 liquid. After exposure at 1400°C, large Y2 Si2 O7 crystals with a glassy phase formed near the BN cell boundaries. The oxidation behavior significantly affected the mechanical properties of the fibrous monolith. The flexural strength and work-of-fracture decreased with increasing exposure temperature, while the noncatastrophic failure was maintained. 相似文献
4.
Beata Zalinska Mehdi Mirsaneh Ian M. Reaney 《Journal of the American Ceramic Society》2009,92(9):1981-1985
The effect of a bespoke glass sintering aid, 0.3Bi2 O3 –0.3Nb2 O5 –0.3B2 O3 –0.1SiO2 (BN1), developed from the base ceramic composition, BiNbO4 (BN), on the sinterability, microstructure, and microwave (MW) dielectric properties of BN ceramics has been investigated. Densities >97% theoretical could be achieved at 1020°C for samples with up to 15% BN1 additions. The resulting microstructure was composed of BN laths surrounded by a residual glass phase that contained small fibrous crystals. Some evidence of dissolution of BN crystals was observed. Optimum properties were exhibited for samples with 15 wt% of glass addition sintered for 4 h at 1020°C with a relative permittivity ɛr =38, a MW quality factor Q × f 0 =17 353 at 5.6 GHz, and a temperature coefficient of resonant frequency τf =−10 ppm/°C. The high Q × f 0 , ɛr , and low τf , coupled with a relatively low sintering temperature, suggest that the use of bespoke glass sintering aids of this type may have great potential for the fabrication of MW ceramics. 相似文献
5.
Calcium aluminosulfate (Ca4 Al6 O16 S or C4 A3 ̄) was prepared by direct synthesis from calcium and aluminum nitrates, and aluminum sulfate. CaAl4 O7 (CA2 ) formed as an intermediate at 900°C, and C4 A3 ̄ was the main phase after calcination at 1100°C. The specific surface areas after calcination at 1100° and 1300°C were ∼2.5 and 1 m2 /g, respectively. Hydration was investigated using XRD, DSC, SEM, conduction calorimetry, and solid-state 27 Al MAS-NMR spectroscopy. Calorimetry showed that the induction period was longer than that of a sample prepared using conventional solid-state sintering, and this was attributed to the formation of amorphous coatings. Crystalline hydration products, principally calcium monoaluminosulfate hydrate and aluminum hydroxide, appeared subsequently. Although the induction period was very long, complete hydration occurred as early as 3 d in the sample calcined at 1100°C and was 91% complete in the sample calcined at 1300°C. 相似文献
6.
Calcium hexa-aluminate (CaO·6Al2 O3 ) has been prepared from calcium nitrate and aluminum sulfate solutions in the temperature range of 1000°–1400°C. A 0.3 mol/L solution of aluminum sulfate was prepared, and calcium nitrate was dissolved in it in a ratio that produced 6 mol of Al2 (SO4 )3 ·16H2 O for each mole of Ca(NO3 )2 ·4H2 O. It was dried over a hot magnetic stirrer at ∼70°C and fired at 1000°–1400°C for 30–360 min. The phases formed were determined by XRD. It was observed that CaO·Al2 O3 and CaO·2Al2 O3 were also formed as reaction intermediates in the reaction mix of CaO·6Al2 O3 . The kinetics of the formation of CaO·6Al2 O3 have been studied using the phase-boundary-controlled equation 1 − (1 − x )1/3 = K log t and the Arrhenius plot. The activation energy for the low-temperature synthesis of CaO·6Al2 O3 was 40 kJ/mol. 相似文献
7.
Volume crystallization of this glass is nucleated by Li3 PO4 . On heating from room temperature, Li2 SiO3 appears around 650°C and then converts to Li2 Si2 O5 around 850°C by reaction with SiO2 from the melt. Preheating the glass at 1000°C forms larger Li3 PO4 nuclei that promote additional crystallization of cristobalite in the 650° to 850°C range. Crystallization activation energies calculated from scan-rate dependence of DTA peaks are 270 kJ/mol for Li2 SiO3 and 360 to 570 kJ/mol for Li2 Si2 O5 . 相似文献
8.
Wenhai Huang Delbert E. Day Mohamed N. Rahaman 《Journal of the American Ceramic Society》2008,91(6):1898-1904
In our earlier work, it was found that particles of a ternary alkali-borate glass, containing either CaO or BaO, converted completely to a crystalline phosphate of calcium or barium when reacted in an aqueous phosphate solution at 37°C. The present work is an extension of our earlier work to investigate the conversion of tetranary borate glass with the composition 10Li2 O·10CaO·10(AeO or T2 O3 )·60B2 O3 (weight percent), where Ae is the alkai-earth metal Mg or Ba, and T is the transition metal La, Sm, or Dy. In the experiments, particles of each glass (150–300 μm) were reacted in 0.25 M K2 HPO4 solution with a starting pH of ∼9.0 at 37°C. Weight loss and pH measurements indicated that the reaction was complete after 30–50 h, yielding an amorphous product. X-ray fluorescence showed that the as-formed product consisted of a calcium phosphate phase that contained the alkali-earth metal or transition metal present in the starting glass. Heating the as-formed material for 8 h at 600°–700°C produced a mixture of two crystalline phosphates: calcium phosphate and an alkali-earth or transition metal phosphate. The kinetics and mechanism of converting tetranary borate glass to phosphate materials are discussed and compared with data from earlier work for the conversion of ternary borate glass. 相似文献
9.
Glasses were prepared from sintered powders ofSi3 N4 , Al2 O3 , Y2 O3 AlN, andSiO2 to study their crystallization on subsequent heat treatment. Appreciable crystallization was efected only after the glasses were doped with up to 5 wt% ZrO2 . Electron microscopy and microanalysis showed that the crystalline phase was Y2 O3 ·2SiO2 without detectable Zr. The sofening temperature is in the range 850° to 1020°C. In-situ heating in a high-voltage electron microscope at ∼12OO°C produced renucleation and growth of the crystalline phase; at higher temperatures, however, the glass phase volatilized. 相似文献
10.
Ian M. Reaney Peter F. James William E. Lee 《Journal of the American Ceramic Society》1996,79(7):1934-1944
Phase evolution in calcium phosphate-based glass ceramics has been examined. Pure CaO:P2 O5 readily formed a glass which surface nucleated upon annealing, but volume nucleation at 680°C was observed only after the addition of the nucleating agents, TiO2 and A12 O3 . Phase separation of Ti and Al occurred along with the nucleation and growth of a calcium phosphate phase, similar to β-Ca2 P2 O7 . Heat treatments at higher temperatures and/or for longer times resulted in crystallization of A1- and Ti-rich, phase-separated regions. A glass with a higher CaO:P2 O5 ratio (approximately 2:1) could be prepared only when a total of 25-35 mol% of TiO2 , A12 O3 , and SiO2 were present in the batch. The glass phase-separated into respective SiO2 - and CaO/P2 O5 -rich regions on cooling. The SiO2 -rich regions did not influence crystallization and remained amorphous throughout the heat treatments. In the CaO/P2 O5 -rich regions, homogeneous volume nucleation of a Ti-rich phase readily occurred followed by the heterogeneous nucleation and growth on these nuclei of a calcium phosphate phase. Although this phase was macroscopically composed of spherulites, TEM revealed that they consisted of intertwined nanodendrites whose individual arms were approximately 20 nm wide and 50 nm long. 相似文献
11.
Alamanda V. Prasadarao Ulagaraj Selvaraj Sridhar Komarneni Amar S. Bhalla 《Journal of the American Ceramic Society》1992,75(10):2697-2701
Strontium and calcium pyroniobates were prepared by a sol–gel process, using strontium/calcium metal and niobium ethoxide as precursors. The formation of Sr2 Nb2 O7 occurred at 750°C via an intermediate perovskite phase of composition close to Sr0.82 NbO3 . The crystallization of Ca2 Nb2 O7 occurred at 600°C directly without any intermediate phases. Sintered Sr2 Nb2 O7 and Ca2 Nb2 O7 pellets showed a preferred grain orientation. Microstructural studies revealed an increase in grain growth and associated orientation with sintering temperature. 相似文献
12.
Nucleation and Crystallization of a Lead Halide Phosphate Glass by Differential Thermal Analysis 总被引:1,自引:0,他引:1
Hongsheng Zhao Wancheng Zhou Yanqing Li Jingbo Wu 《Journal of the American Ceramic Society》2002,85(4):903-908
The nucleation and crystallization mechanisms of a lead halide phosphate glass [40P2 O5 ·30PbBr2 ·30PbF2 (mol%)] were investigated by differential thermal analysis (DTA) and X-ray diffraction analysis. There were two crystalline phases in the crystallized samples: the major phase was PbP2 O4 , and the minor phase was PbP2 O6 . The average activation energy for crystallization, E , for two different particle sizes of this glass was determined to be 119 ± 4 kJ/mol by the Kissinger method and 124 ± 4 kJ/mol by the Augis–Bennett method. The Avrami constants were determined to be 1.6 and 2.5 for particle sizes of 203 and 1040 μm, respectively, by the Ozawa equation, and 1.7 and 2.4 for particle sizes of 203 and 1040 μm, respectively, by the Augis–Bennett equation. The decrease in the crystallization peak height in the DTA curve with increasing particle size suggested that the particles crystallize primarily by surface crystallization. A nucleation-rate type curve was determined by plotting either the reciprocal of the temperature corresponding to the crystallization peak maximum, 1/ T p , or the height of the crystallization peak, (δ T )p , as a function of nucleation temperature, T n . The temperature where nucleation can occur for this glass ranges from 360°–450°C and the maximum nucleation rate is at 420°± 10°C. 相似文献
13.
High Quality and Yield in Potassium Titanate Whiskers Synthesized by Calcination from Hydrous Titania 总被引:2,自引:0,他引:2
Ningzhong Bao Liming Shen Xin Feng Xiaohua Lu 《Journal of the American Ceramic Society》2004,87(3):326-330
Hydrous titanium dioxide (TiO2 · n H2 O) was used to prepare K2 Ti2 O5 single crystals, K2 Ti4 O9 whiskers and K2 Ti6 O13 whiskers at 820°, 940°, and 1110°C, respectively, by calcination. At T < 820°C, dehydration of hydrous titania, decomposition of potassium carbonate, and reaction between titanate and potassium oxide occurred simultaneously, ending with a crystallization reaction of K2 Ti2 O5 single crystals at 820°C. Subsequently, K2 Ti2 O5 single crystals convert into K2 Ti4 O9 whiskers at 940°C, and K2 Ti4 O9 whiskers further convert into K2 Ti6 O13 whiskers at 1110°C. The reaction temperatures for the generations of these types of potassium titanates were all 10°–40°C lower than the corresponding temperatures when anatase was used as the reactant. The whiskers synthesized in the present study exhibited uniform size, good morphology, and a high yield. 相似文献
14.
P. L. ROEDER F. P. GLASSER† E. F. OSBORN 《Journal of the American Ceramic Society》1968,51(10):585-593
Liquidus phase equilibrium data are presented for the system Al2 O3 -Cr2 O3 -SiO2 . The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al2 O3 -ICr2 O3 -93SiO2 ) coexists with a mullite solid solution (61Al2 O3 -10Cr2 O3 -29SiO2 ), a corundum solid solution (19Al2 O3 -81Cr2 O3 ), and cristobalite (SO2 ). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases. 相似文献
15.
Glass in the MgO-Li2 O-A12 O3 -SiO2 system was observed to crystallize readily at temperatures from 700° to 900°C. The primary crystalline phase evolved was Li2 Si2 O5 , and the secondary phase evolved was Li2 SiO3 . The glass was amorphous after heating in air at 1050°C for 30 min. The addition of 0.5 wt% SiC powder resulted in the crystallization of Li2 SiO3 during heating in air at 1050°C for 30 min. It was suggested that the difference in crystallization behavior with Sic addition was due to dissolution of Sic into the oxide glass. 相似文献
16.
Toshifumi Sugama Marita Allan Janet M. Hill 《Journal of the American Ceramic Society》1992,75(8):2076-2087
We investigated the characteristics of calcium phosphate cements (CPC) prepared by an exothermic acid–base reaction between NH4 H2 PO4 -based fertilizer (Poly-N) and calcium aluminate compounds (CAC), such as 3CaO · Al2 O3 (C3 A), CaO · Al2 O3 (CA), and CaO · 2Al2 O3 (CA2 ), in a series of integrated studies of reaction kinetics, interfacial reactions, in-situ phase transformations, and microstructure development. Two groups were compared: untreated and hydrothermally treated CPC specimens. The extent of reactivity of CAC with Poly-N at 25°C was in the following order: CA > C3 A ≫ CA2 . The formation of a NH4 CaPO4 · x H2 O salt during this reaction was responsible for the development of strength in the CPC specimens. The in-situ phase transformation of amorphous NH4 CaPO4 · x H2 O into crystalline Ca5 (PO4 )3 (OH) and the conversion of hydrous Al2 O3 gel →γ-AIOOH occur in cement bodies during exposure in an autoclave to temperatures up to 300°C. This phase transformation significantly improved mechanical strength. 相似文献
17.
Kazuhiro Narita Yoshihiro Takahashi Yasuhiko Benino Takumi Fujiwara Takayuki Komatsu Takeshi Hanada Yoshihiko Hirotsu 《Journal of the American Ceramic Society》2004,87(1):113-118
Some K2 O-Nb2 O5 -GeO2 glasses are prepared, and their crystallization behaviors are examined. 25K2 O·25Nb2 O5 ·50GeO2 glass with the glass transition temperature T g = 622°3C and crystallization onset temperature T x = 668°3C shows a prominent nanocrystallization. The crystalline phase is K3,8 Nb5 Ge3 O20,4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat-treated at 630° and 720°3C for 1 h are °10 and 20–30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620°-850°3C for 1 h maintain good transparency. The density of crystallized glasses increases gradually with increasing heat-treatment temperature, and the volume fraction of crystals in the sample heat-treated at 630°3C for 1 h is estimated to be ∼35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K2 O·25Nb2 O5 ·50GeO2 glass change steeply by heat treatment at T g , i.e., at around 35% volume fraction of nanocrystals. The present study demonstrates that the composite of nanocrystals and the glassy phase has a strong resistance against deformation during Vickers indenter loading in crystallized glasses. 相似文献
18.
Si3 N4 /BN fibrous monoliths were prepared with 4 wt% Y2 O3 added as a sintering aid to the Si3 N4 . Residual carbon, present in the billet before hot-pressing, was shown to influence the final microstructure. The sintering aid glass, known to migrate into the BN cell boundaries during hot-pressing, was not sufficient in quantity to prevent premature shear failure when samples were tested in flexure. Increasing the hot-pressing temperature alleviated this problem. For flexure samples tested at 1400°C, fibrous monoliths fabricated with 4 wt% Y2 O3 demonstrated linear-elastic loading behavior at a greater stress than fibrous monoliths fabricated with 6-wt%-Y2 O3 /2-wt%-Al2 O3 sintering aids. 相似文献
19.
Xiao Yuan Yebin Xu Guohua Huang Chunlian Zeng 《Journal of the American Ceramic Society》2007,90(7):2283-2286
Strontium dialuminate (SrAl4 O7 ) can be synthesized by crystallization of amorphous spray-dried precursors or solidification from the liquid. In this paper, SrAl4 O7 was prepared via the ethylenediaminetetraacetic acid precursor route. Differential thermal analysis, thermogravimetric, fourier transform infrared spectroscopy, and X-ray diffraction (XRD) were used to characterize the precursors and the oxide powders derived. XRD analysis showed that single-phase SrAl4 O7 was obtained from pulverized resin at a temperature of 1050°C for 2 h and SrAl4 O7 can be stable at least up to 1200°C. Single-phase SrAl4 O7 was not obtained from the citrate acid route. 相似文献
20.
The effect of P2 OS on the devitrification of binary lead silicate glasses containing 64 and 59 mol% PbO was studied. Glasses underwent isothermal crystallization treatments at 400°, 450°, 500°, and 550°C. A polymorph of 3PbO2 SiO2 was the major product of crystallization for all compositions of glasses. Secondary products of crystallization were found to be a polymorph of PbOSiO2 in the 59 mol% and a low-temperature polymorph of 2PbOSiO2 in the 64 mol% PbO glasses. Dominant mode of crystallization in both binary glasses was surface devitrification at all temperatures studied. Addition of P2 O5 promoted internal crystallization in the form of spherulites. 400°C was found to be the most effective temperature for nucleating spherulitic growth. Crystallization at 400°C led to high concentrations of spherulites in all glasses containing at least 0.5 mol% P2 O5 . Concentrations of 0.5 mol% P2 O5 were needed to produce detectable levels of spherulitic nucleation at r<400°C. 相似文献