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1.
Niobium pentoxide (T form, orthorhombic system) was utilized to promote devitrification in Li2O · Al2O3· 6SiO2 glasses. Two or more mole percentage of this nucleating dopant enhanced crystallization in these glasses. Glasses containing 4.0 and 8.0 mol% T-Nb2O5 exhibited a high tendency to form dispersed TT-Nb2O5 (monoclinic system) precipitates during the glass quenching process. The crystallization process in glasses containing 2.0 or 4.0 mol% T-Nb2O5 occurred as microphase separation, followed by the formation of dispersed TT-Nb2O5 crystalline precipitates (760°C), followed by β-quartz solid-solution ( ss ) formation (850° to 900°C) heterogeneously nucleated from the precipitates. β-quartz( ss ) transformed to β-spodumene( ss ), along with a polymorphic transition from the TT-Nb2O5 to M-Nb2O5 (tetragonal system) crystalline phase.  相似文献   

2.
The effects of Si and ZrO2 dopants on the crystallization and phase transformation process in Li2O · Al2O3· 6SiO2 glasses were investigated using differential thermal analysis, X-ray powder diffractometry (XRD), and high-resolution transmission electron microscopy (TEM) interactively. Phase separation was observed in the studied glasses prior to substantial crystallization. Elemental Si (1 mol%) significantly aided in glass devitrification. Dropletlike phase-separated regions in the as-quenched or heat-treated glass devitrified at ∼760°C, which in turn provided sites for the heterogeneous nucleation and growth of β-quartz(ss) (solid solution), which transformed to β-spodumene(ss) at higher temperature. Low-temperature surface crystallization in these glasses occurred as low as 760°C. ZrO2 has limited solubility in this glass system. Small ZrO2 crystallites (·5 nm) in the as-quenched glass acted as sites for the heterogeneous nucleation and subsequent growth of large (<5 μm) β-quartz(ss) crystals in glasses containing 1.0 mol% or more ZrO2. The transformation from β-quartz(ss) to β-spodumene(ss) was increasingly inhibited with ZrO2 additions. The nucleating efficiency of Si was significantly greater than that of ZrO2 in this glass system.  相似文献   

3.
The kinetics of secondary grain growth in a Ti02-nucleated β-spodumene solid-solution glass-ceramic was studied. The thermal stability of the grains was excellent. Grain growth followed the cube-root-of-time law. The activation energy of the grain boundary migration was 55 ± 10 kcal/mol. Grain growth inhibition due to Ti02 precipitates and the residual glassy phase was closely examined. The excellent thermal stability of the grains is due to grain growth inhibition by the residual glassy phase, not by rutile precipitates. It is suggested that the diffusion of A2+, and probably the simultaneous diffusion of Li+, through the residual glass is the rate-limiting process for the grain boundary migration.  相似文献   

4.
Sintering, crystallization, microstructure, and thermal expansion of Li2O·Al2O3·4SiO2 glass-ceramics doped with B2O3, P2O5, or (B2O3+ P2O5) have been investigated. On heating the glass powder compacts, the glassy phase first crystallized into high-quartz s.s., which transformed into β-spodumene after the crystallization process was essentially complete. The effects of dopants on the crystallization of glass to high-quartz s.s. and the subsequent transformation of high-quartz s.s. to β-spodumene were discussed. The major densification occurred only in the early stage of sintering time due to the rapid crystallization. All dopants were found to promote the densification of the glass powders. The effect of doping on the densification can fairly well be explained by the crystallization tendency. All samples heated to 950°C exhibited a negative coefficient of thermal expansion ranging from about −4.7 × 10-6 to −0.1 × 10-6 K-1. Codoping of B2O3 and P2O5 resulted in the highest densification and an extremely low coefficient of thermal expansion.  相似文献   

5.
The development of crystalline phases in lithium oxynitride glass-ceramics was examined, with particular emphasis placed on the effect of the nitrogen source (AlN or Si3N4) on the formation and stability of a β-quartz solid-solution ( ss ) phase. Oxynitride glasses derived from the Li-Si-Al-O-N system were heat-treated at temperatures up to 1200°C to yield glass-ceramics in which β-quartz( ss ) and β-spodumene( ss ) of approximate composition Li2OAl2O34SiO2 formed as major phases and in which X-phase (Si3Al6O12N2) and silicon oxynitride (Si2N2O) were present as minor phases. The nitrogen-containing β-quartz( ss ) phase that was prepared with AlN was stable at 1200°C; however, the use of Si3N4 as the nitrogen source was significantly less effective in promoting such thermal stabilization. Lattice parameter measurements revealed that AlN and Si3N4 had different effects on the crystalline structures, and it was proposed that the enhanced thermal stability of the β-quartz( ss ) phase that was prepared with AlN was due to both the replacement of oxygen by nitrogen and the positioning of excess Al3+ ions into interstitial sites within the β-quartz( ss ) crystal lattice.  相似文献   

6.
Metastable solid solutions with the β-quartz structure can be crystallized from most glasses in the system SiO2-Mg(AlO2)2-LiAlO2 as well as from many containing the additional components Zn(AlO2)2 Al(AlO2)3, Li2ZnO2, and Li2BeO2. Internal nucleation is afforded by additions of ZrO2 or TiO2. Either transparent or opaque crystalline materials can be formed from glasses containing about 70% SiO2. The transparency is due to a combination of low birefringence in the major stuffed β-quartz phase and minute crystal size. Thermal expansions vary from -20 to +50 × 10−7/°C. Thermal stability is highly variable. Breakdown products include spinel, cordierite, β-spodumene, willemite, mullite, and cristobalite. Magnesian compositions can be strengthened by a 2Li+⇌ Mg2+ ionexchange reaction. Abraded flexural strengths range from 30,000 to 160,000 psi.  相似文献   

7.
The potassium ions in potassium β-ferrite ((1 + x)K2O ·11Fe2O3) crystals were exchanged with Na+, Rb+, Cs+, Ag+, NH4+, and H3O+ in molten nitrates or in concentrated H2SO4. On the other hand, spinel and hexagonal ferrites were formed by soaking the crystals in the melt of divalent salts. The crystals of K+, Rb+, and Cs+β-ferrites decomposed to form α-Fe2O3 at high temperatures of 800° to 1100°C. In addition, H3O+, NH4+, and Ag+β-ferrites decomposed to form α-Fe2O3 at relatively low temperatures of 350° to 650°C, in accordance with the stabilities of the inserted ions. The electrical properties of some β-ferrites were measured.  相似文献   

8.
Glasses with compositions Li1.2M0.2Ge1.8(PO4)3 (M = Al, Ga, Y, Gd, Dy, and La) were prepared and converted to glass-ceramics by heat treatment. The effects of the M3+ ions on the conductivity of the glasses and glass-ceramics were studied. The main phase present in the glass-ceramics was the conductive phase LiGe2(PO4)3. Al3+ and Ga3+ ions entered the LiGe2(PO4)3 structure by replacing Ge4+ ions, but lanthanide ions did not. The glass-ceramics exhibited much higher conductivity than the glasses. With increased ionic radius of the M3+ ions, the conductivity remained almost unchanged at ∼3 × 10−12 S/cm for the glasses, but it decreased from 1.5 × 10−5 to 8 × 10−9 S/cm for the glass-ceramics at room temperature. The higher conductivity for Al3+- and Ga3+-containing glass-ceramics was suggested to result from the substitutions of Al3+ and Ga3+ ions for Ge4+ ions in the LiGe2(PO4)3 structure.  相似文献   

9.
Porous glass-ceramics with a skeleton of the fast-lithium-conducting crystal Li1+ x Ti2− x Al x (PO4)3 (where x = 0.3–0.5) were prepared by crystallization of glasses in the Li2O─CaO─TiO2─Al2O3–P2O5 system and subsequent acid leaching of the resulting dense glass-ceramics composed of the interlocking of Li1+ x Ti2− x Al x (PO4)3 and β-Ca3(PO4)2 phases. The median pore diameter and surface area of the resulting porous Li1+ x Ti2− x Al x (PO4)3 glass-ceramics were approximately 0.2 μm and 50 m2/g, respectively. The electrical conductivity of the porous glass-ceramics after heating in LiNO3 aqueous solution was 8 × 10−5 S/cm at 300 K or 2 × 10−2 S/cm at 600 K.  相似文献   

10.
Fast lithium ion conducting glass-ceramics have been successfully prepared from the pseudobinary system 2[Li1+ x Ti2Si x P3− x O12]-AlPO4. The major phase present in the glass-ceramics was LiTi2P3O12 in which Ti4+ ions and P5+ ions were partially replaced by Al3+ ions and Si4+ ions, respectively. Increasing x resulted in a considerable enhancement in conductivity, and in a wide composition range extremely high conductivity over 10−3 S/cm was obtained at room temperature.  相似文献   

11.
Complete solid solubility was demonstrated to occur between LiAlGeO4 and the low temperature form of Li AlSiO2 (a-eucryptite). Hydrother-mal preparation was necessary for the silicate-rich compositions. Under atmospheric pressure, about 65 mole % LiAlGeO4 entered the β-eucryβ-tite phase at 1150°C, but solid solutions containing more than 25 mole % LiAlGeO4 exsolved if held at lower temperatures. Directional thermal expansion data were obtained by X-ray diffraction methods on both α- and β-eucryptite and their solid solutions. Substitution of Ge4+ for Si4+ produced no significant difference in the thermal expansion coefficients in the α and β phases. An increase in the lattice parameters in the a and c directions took place as expected when Ge4+ (0.53 A) was substituted for Si4+ (0.39 A).  相似文献   

12.
Crystallization of Li-Al-Si-O-N oxynitride glasses based on the β-spodumene composition and the properties of the resultant glass-ceramics have been studied. The onset of the precipitation of metastable high-quartz solid solution and its transformation to β-spodumene shift to higher temperatures with increasing nitrogen content of the oxynitride glasses. Nitrided glass-ceramics crystallized at 1200°C have negative thermal expansion coefficients, since high-quartz structure is maintained up to 1000° and 1200°C. Knoop hardness and density of the glass-ceramics increase with increasing nitrogen content. There was evidence that part of the nitrogen atoms were incorporated into the high-quartz solid-solution structure and that a small amount of the minor phase of Si2N2O was precipitated in highly nitrided glass-ceramics.  相似文献   

13.
A porous glass-ceramic in the CaO–TiO2—P2O5 system has been prepared by crystallization and subsequent chemical leaching of the corresponding glass. By applying a two-step heat treatment to 45CaO · 25TiO2· 30P2O5 glasses containing a few mol% of Na2O, volume crystallization results in the formation of dense glass-ceramics composed of CaTi4(PO4)6 and β-Ca3(PO4)2 phases. By leaching the resultant glass ceramics with HCI, β-Ca3(PO4)2 is selectively dissolved out, leaving a crystalline CaTi4(PO4)6 skeleton. The surface area and mean pore radius of the porous glass-ceramics were approximately 40 m2/g and 13 nm, respectively.  相似文献   

14.
Phase equilibria along the nonbinary join between cordierite (2MgO · 2Al2O3· 5SiO2) and spodumene (Li2O · Al2O3· 4SiO2) were investigated in the temperature range 800° to 1550°C. using the quench technique on fourteen compositions. The phase diagram at high temperatures is characterized by a very small region of solid solution on the cordierite side, appreciable solid solution on the spodumene side, and regions of three and four phases toward the center of the system, including liquid, α-cordierite, mullite, spinel, corundum, and β-spodumene and its solid solutions. The liquidus has a flat minimum between 40 and 50% cordierite at 1347°, and rises on one side to the congruent melting point of β-spodumene (1421°) and on the other side to the temperature of complete melting of cordierite (1530°). The lowest temperature at which liquid appears is 1325°. At low temperatures a complete series of metastable solid solutions exists between μ-cordierite and β-spodumene. The significance of the data in the preparation of thermal-shock-resisting bodies is discussed.  相似文献   

15.
MAS-osumilite is a metastable double-ring silicate which is attractive for use in glass-ceramics because of its low thermal expansion coefficient. The phase can be crystallized from glass powders of MAS-osumilite composition containing small amounts of BaO. The nucleation and crystallization process has been investigated by means of TEM and SEM. Crystallization starts with the precipitation of β-quartzss of composition MgAl2O4· n SiO2. BaO concentrates in a residual glass phase between the β-quartzss crystals. In a second stage small amounts of Ba-osumilite (BaMg2Al6-Si9O30) nucleate and crystallize from this glass phase, In a third stage BaO-free MAS-osumilite crystals are formed by epitactic growth on the Ba-osumilite surfaces and grow into adjacent β-quartzss grains. Two factors are essential for the suppression of undesired further phases to develop a monophase (besides some Ba-osumilite) MAS-osumilite glassceramic: a residual glass phase with a composition close to Ba-osumilite, and β-quartzss crystals having a composition close to that of MAS-osumite.  相似文献   

16.
The effect of monovalent cation addition on the γ-Al2O3-to-α-Al2O3 phase transition was investigated by differential thermal analysis, powder X-ray diffractometry, and specific-surface-area measurements. The cations Li+, Na+, Ag+, K+, Rb+, and Cs+ were added by an impregnation method, using the appropriate nitrate solution. β-Al2O3 was the crystalline aluminate phase that formed by reaction between these additives and Al2O3 in the vicinity of the γ-to-α-Al2O3 transition temperature, with the exception of Li+. The transition temperature increased as the ionic radii of the additive increased. The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phase-transition temperature. Thus, Cs+ was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of γ-Al2O3. Because the order of the phase-transition temperature coincided with that of the formation temperature of β-Al2O3 in these samples, suppression of ionic diffusion in γ-Al2O3 by the amorphous phase containing the added cations must have played an important role in retarding the transition to α-Al2O3. Larger cations suppressed the diffusion reaction more effectively.  相似文献   

17.
Films of a complete series of solid-solution oxides, Ba1-χSrχWO4 (O ≥ X ≥ 1), have been prepared on tungsten substrates in an electrolytic solution containing Ba2+ and Sr2+ ions by an electrochemical method at room temperature (25°C). The composition X could easily be controlled by the concentrations of Ba and Sr species in the starting solutions. X-ray diffraction and X-ray photoelectron spectroscopy analyses showed that a complete series of well-crystallized solid-solution oxides was formed even at room temperature.  相似文献   

18.
19.
Tm3+-Ho3+- and Tm3+-Ho3+-Eu3+-ion-codoped oxyfluoride transparent glass-ceramics containing PbF2 nanocrystals were prepared, and the near-infrared fluorescence properties of the Tm3+ ions were investigated for their potential use as a 1.4 μm amplifier. For all samples, the lifetime of the Tm3+:3 H 4 level increased with heat treatment because of the decrease of the phonon energy as PbF2 crystals were formed. Moreover, it was revealed that codoping with Ho3+ or Eu3+ was effective in suppressing the lifetime of the Tm3+:3 F 4 level by energy transfer to the Ho3+:5 I 7 or Eu3+:7 F 6 level. For the codoped samples, the heat treatments decreased the Tm3+:3 F 4 lifetime and increased the Tm3+:3 H 4 lifetime. This was attributed to the concentration of rare-earth ions in the fluoride crystallites. These properties improved the population inversion of the 1.4 μm transition.  相似文献   

20.
The green emitting Ca2SiO4:Eu2+ (C2S:Eu) phosphors were synthesized by the polymeric precursor process (Pechini-type), and the effects of calcination temperature and europium (Eu) doping concentration on the luminescent properties were investigated. The crystalline β-C2S was obtained in the calcination temperature of 1100°–1400°C, and Eu was reduced into Eu2+ by annealing in 5% H2/N2 atmosphere. The obtained C2S:Eu2+ phosphors exhibited a strong emission at 504 nm under the excitation of λexc=350 nm. The highest photoluminescence (PL) intensity was observed in the C2S:Eu2+ phosphors either calcined at 1300°C or doped with 3 mol% Eu. The obtained PL properties were discussed in terms of crystal structure, particle size and shape, surface roughness, and effect of concentration quenching.  相似文献   

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