共查询到20条相似文献,搜索用时 46 毫秒
1.
Long Zhang Ansgar Bögershausen Hellmut Eckert 《Journal of the American Ceramic Society》2005,88(4):897-902
Transparent and colorless AlPO4 gel and glass are prepared via a simple aqueous sol–gel route using aluminum lactate and phosphoric acid as precursors. The stoichiometric AlPO4 glass derived from this sol–gel route has a mesoporous structure with a surface area of 504 m2 /g after calcination to 600°C. With increasing gel-to-glass processing temperature, the average degree of P/Al connectivity increases. After sample calcination at 600°C for 4 h, 27 Al MAS NMR spectra indicate that aluminum is almost completely converted into AlO4 units. 27 Al{31 P} rotational echo double resonance and 31 P{27 Al} rotational echo adiabatic passage double resonance NMR experiments as well as 27 Al and 31 P MAS NMR results further confirm that the prepared AlPO4 glass has a three-dimensional network based on alternating Al(OP)4 and P(OAl)4 tetrahedral units in analogy to the local structure of crystalline AlPO4 . 相似文献
2.
Reactive dicalcium silicate (Ca2 SiO4 ) has been synthesized by the Pechini process, and hydration kinetics studied. With increasing calcination temperature, the amorphous product first crystallizes to α'L -phase and subsequently to the ß- and γ-phases. The specific surface area, ranging from 40 to 1 m2 /g, strongly depends on the calcination temperature of 700°-1200°C for 1 h. Samples with a high surface area have a high water demand; a water/cement ratio >2.0 is required to produce formable pastes in some instances. Hydration kinetics are determined by XRD, 29 Si magic-angle spinning nuclear magnetic resonance (MAS NMR), and differential scanning calorimetry/thermogravimetry (DSG/TG). The hydration rate depends only on the surface area, not on the polymorph. Complete hydration occurs in as early as 7 d. Very little calcium hydroxide (Ca(OH)2 ) is formed in the most reactive specimens (calcined at 700° and 800°C), which indicates the Ca/Si ratio in C-S-H gels is ∼2.0, but more Ca(OH)2 forms from samples calcined at higher temperature. The silicate structure of the hydrated Ca2 SiO4 pastes is investigated using 29 Si MAS NMR spectroscopy and trimethylsilylation analysis. 相似文献
3.
Richard K. Brow R. James Kirkpatrick Gary L. Turner 《Journal of the American Ceramic Society》1990,73(8):2293-2300
A series of x Al2 O3 · (1 − x )NaPO3 glasses (0 ≤ x ≤ 0.275) were prepared and characterized by magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and by X-ray photoelectron spectroscopy (XPS). 27 AI MAS NMR spectra reveal that: at low alumina contents ( x < 0.125), Al is in dominantly octahedrally coordinated Al(OP)6 structural environments; in glasses with x > 0.125, Al(OP)4 environments are increasingly favored; and at x = 0.275, most Al is tetrahedrally coordinated. 5-coordinated Al environments also appear to be present. Variations in the 31 P MAS NMR spectra are consistent with bonding between neighboring P and Al polyhedra. Quantitative changes in the oxygen bonding, determined from the O 1 s spectra obtained by XPS, also reflect these structural changes. The effects of alumina content on the relative concentration of nonbridging oxygen (PO− ) are quantitatively described by a simple structural model derived from the MAS NMR data. The thermal expansion coefficients and glass transition temperatures are shown to be sensitive to these structural changes. 相似文献
4.
Mary W. Barnes Maria Klimkiewicz Paul W. Brown 《Journal of the American Ceramic Society》1992,75(6):1423-1429
Compositions along the Ca2 SiO4 –Ca3 (PO4 )2 join were hydrated at 90°C. Mixtures containing 15, 38, 50, 80, and 100 mol% Ca3 (PO4 )2 were fired at 1500°C, forming nagelschmidtite + a 1 -CaSiO4 , A -phase and silicocarnotite and a -Ca3 (PO4 )2 , respectively. Hydration of these produces hydroxylapatite regardless of composition. Calcium silicate hydrate gel is produced when Ca2 SiO4 ≠ 0 and portlandite when Ca2 SiO4 is >50%. Relative hydration reactivities are a -Ca3 (PO4 )2 > nagelschmidtite > α 1 -Ca2 SiO4 > A -phase > silicocarnotite. Hydration in the presence of silica or lime influences the amount of portlandite produced. Hydration in NaOH solution produces 14-A tobermorite rather than calcium silicate hydrate gel. 相似文献
5.
Hideki Ishida Kaori Sasaki Takeshi Mitsuda 《Journal of the American Ceramic Society》1992,75(2):353-358
The hydration behavior at 25°C of β-dicalcium silicate synthesized from hillebrandite (Ca2 ,(SiO3 )(OH)2 ) at 600°C was studied over a period of 224 d. The hydration rate of the β-dicalcium silicate having fibrous crystals with specific surface area of 7 m2 /g is extremely rapid. For water/solids ratios of 0.5 and 1.0, the hydration reaction is completed in 28 and 14 d, respectively. The hydrate contains almost no Ca(OH)2 , and its Ca/Si ratio is close to 2. SEM observations indicate that the hydrate forms an outer shell on the surface of β-dicalcium silicate and grows inwards. The silicate anion structure is considered to consist of dimers and single-chain structures from 29 Si MAS NMR. Variations of physical properties of press-formed bodies have also been discussed. 相似文献
6.
Young-Min Kim Seong-Hyeon Hong Hwan Kim 《Journal of the American Ceramic Society》2002,85(8):1941-1946
A chlorine-bearing alinite cement was synthesized using reagent-grade chemicals, and the phase evolution and hydration behavior of the alinite clinker were examined. The effects of the MgO content on alinite formation and hydration also were investigated. Alinite began to appear at 1000°C from β-C2 S, C11 A7 CaCl2 , and unreacted raw materials, and an almost single-phase alinite was obtained at 1300°C. The alinite phase also was produced without MgO addition. However, CaO, β-C2 S, and C11 A7 CaCl2 phases were present. Alinite cements hydrated rapidly after a short incubation period, and the hydration products were C-S-H gels, Ca(OH)2 , and a Fridel's saltlike phase. The local environmental changes of silicon and aluminum during the formation and hydration of alinite were determined using magic-angle-spinning nuclear magnetic resonance spectroscopy. The Cl− -ion exsolution from the alinite paste during hydration was measured using ion chromatography. 相似文献
7.
Hideki Ishida Yoshihiko Okada Takeshi Mitsuda 《Journal of the American Ceramic Society》1992,75(2):359-363
The hydration behavior at 25°C of highly reactive β-dicalcium silicate synthesized from hillebrandite (Ca2 (SiO3 )(OH)2 ) was studied over a period of 7 to 224 d using 29 Si magic-angle spinning nuclear magnetic resonance (MAS NMR). The hydration product, C-S-H, contains Q2 and Q1 silicate tetrahedra, the chemical shifts of which are independent of the water/solid (w/s) ratio and curing time. Until the reaction is completed, the amounts of Q1 and Q2 formed are independent of the w/s ratio, being determined only by the degree of reaction. The ratio Q2 /Q1 increases as the reaction progresses and as the curing time becomes longer. From the values of Q2 /Q1 , it appears that the hydrate is a mixture of dimers and short single-chain polymers. The Ca/Si ratio of the hydrate is high, taking values close to 2.0, but the Ca/Si ratio does not influence the Q2 /Q1 ratio. It was also found that the NMR peak intensities allow quantitative assessment similar to XRD. 相似文献
8.
Tomohiro Yamakawa Junichi Tatami Toru Wakihara Katsutoshi Komeya Takeshi Meguro Kenneth J. D. MacKenzie Shinichi Takagi Masahiro Yokouchi 《Journal of the American Ceramic Society》2006,89(1):171-175
Aluminum nitride (AlN) powders were synthesized by gas reduction–nitridation of γ-Al2 O3 using NH3 and C3 H8 as the reactant gases. AlN was identified in the products synthesized at 1100°–1400°C for 120 min in the NH3 –C3 H8 gas flow confirming that AlN can be formed by the gas reduction–nitridation of γ-Al2 O3 . The products synthesized at 1100°C for 120 min contained unreacted γ-Al2 O3 . The 27 A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing reaction temperature, the tetrahedral AlO4 resonance decreasing prior to the disappearance of the octahedral AlO6 resonance. This suggests that the tetrahedral AlO4 sites of the γ-Al2 O3 are preferentially nitrided than the AlO6 sites. AlN nanoparticles were directly formed from γ-Al2 O3 at low temperature because of this preferred nitridation of AlO4 sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of γ-Al2 O3 not only because the reaction temperature is sufficiently low to restrict grain growth, but also because γ-Al2 O3 contains both AlO4 and AlO6 sites, by contrast with α-Al2 O3 which contains only AlO6 . 相似文献
9.
Adrian R. Brough Christopher M. Dobson Ian G. Richardson Geoffrey W. Groves 《Journal of the American Ceramic Society》1994,77(2):593-596
Selective isotopic enrichment of SiO2 with 29 Si in a mixture with tricalcium silicate (C3 S) has allowed the Si from this phase to be effectively labeled during the course of the hydration reaction, thus isolating its contribution to the reaction. A double Q2 signal has been observed in 29 SI solid-state MAS NMR spectroscopy of C-S-H gels of relatively low Ca/Si ratio, prepared by hydration or by carbonation of a C3 S paste. The origin of the weaker, downfield peak is discussed and tentatively attributed to bridging tetrahedra of a dreierkette silicate chain structure. 相似文献
10.
The C3 A compacts were hydrated and the reaction was studied by DTA, X-ray diffraction, mercury porosimetry, and volume change analysis. The hexagonal hydroaluminates C2 AH8 and C4 AH19 formed at 2°, 12°, and 23°C by a direct mechanism between C3 A and H2 O. The hydration reaction at 52° and 80°C was stopped by formation of C3 AH6 around the C3 A grains. The rate of conversion of the hexagonal hydrates to cubic C3 AH6 increased with temperature. Volume change analysis confirmed that C3 AH6 grows epitaxially on the surface of the C3 A grain. The reaction at this surface and the passage of water through the layer of hexagonal hydroaluminates control the overall reaction rate. The conversion of the hexagonal hydrates to C3 AH6 accelerates the reaction by removing the layer of products from around the C3 A grain by a solution mechanism. At 52° and 80°C, C3 AH6 may form without the intermediate formation of the hexagonal hydrate. 相似文献
11.
Sharon E. Ashbrook Karl R. Whittle Laurent Le Pollès Ian Farnan 《Journal of the American Ceramic Society》2005,88(6):1575-1583
Pollucite, CsAlSi2 O6 , a Cs polymorph of leucite (KAlSi2 O6 ), has been proposed for ceramic immobilization of 135 Cs and 137 Cs fission products. 133 Cs NMR of both low- (tetragonal) and high-temperature (cubic) forms of pollucite exhibit a considerable distribution of local Cs environments. 29 Si and 27 Al NMR data from directly prepared pollucite show greater Al/Si disorder than either leucite, or pollucite produced by ion exchange. Little evidence for Cs motion is observed in tetragonal or cubic pollucite, and only at high temperatures (∼850°C) is any substantial dynamic behavior detected. Dynamic NMR lineshape calculations allow a determination of the frequency of Cs motion and diffusivity. 相似文献
12.
Physicochemical Mechanism for the Continuous Reaction of γ-Al2 O3 -Modified Aluminum Powder with Water
Zhen-Yan Deng José M. F. Ferreira Yoshihisa Tanaka Jinhua Ye 《Journal of the American Ceramic Society》2007,90(5):1521-1526
Previous experiments showed that γ-Al2 O3 -modified Al powder could continuously react with water and generate hydrogen at room temperature under atmospheric pressure. In this work, a possible physicochemical mechanism is proposed. It reveals that a passive oxide film on Al particle surfaces is hydrated in water. OH− ions are the main mobile species in the hydrated oxide film. When the hydrated front meets the metal Al surface, OH− ions react with Al and release H2 . Because of the limited H-soluble capacity in small Al particles and the low permeability of the hydrated oxide film toward H+ species, H2 molecules accumulate and form small H2 gas bubbles at the Al:Al2 O3 interface. When the reaction equilibrium pressure in H2 bubbles exceeds a critical gas pressure that the hydrated oxide film can sustain, the film on the Al particle surfaces breaks and the reaction of Al with water continues. As the surface oxide layer on modified Al particles has a lower tensile strength, the critical gas pressure in H2 bubbles is lower so that under an ambient condition, the reaction of modified Al particles with water is continuous. The proposed mechanism was further confirmed by a new experiment showing that the as-received Al powder could continuously react with water at temperatures above 40°C and under low vacuum, because the vacuum makes the critical gas pressure in H2 bubbles decrease as well. 相似文献
13.
Hideki Ishida Satoru Yamazaki Kaori Sasaki Yoshihiko Okada Takeshi Mitsuda 《Journal of the American Ceramic Society》1993,76(7):1707-1712
α-C2 SH can be synthesized by hydrothermal treatment of lime and silicic acid for 2 h at 200°C. When heated to 390–490°C, α-C2 SH dissociates through a two-step process to form an intermediate phase plus some γ-C2 S. This appears to be a new dicalcium silicate different from known dicalcium silicates—α, α'L , α'H , β, and γ phase—and is stable until around 900°C. At 920–960°C, all the phases are transformed to the α'L phase. The intermediate phase has high crystallinity and is stable at room temperature. 29 Si MAS NMR measurements indicate the possibility that it contains both protonated and unprotonated monosilicate anions. The intermediate phase that has passed through the α'phase at higher temperature yields β-C2 S on cooling. The intermediate phase is highly active, and completed its hydration in 1 day ( w/s = 1.0, 25°C). Among the crystalline calcium silicate hydrates with Ca/Si = 2.0, it is hillebrandite that yields β-C2 S at the lowest temperature. 相似文献
14.
Chunfeng Hu Fangzhi Li Lingfeng He Mingyue Liu Jie Zhang Jiemin Wang Yiwang Bao Jingyang Wang Yanchun Zhou 《Journal of the American Ceramic Society》2008,91(7):2258-2263
In this work, a bulk Nb4 AlC3 ceramic was prepared by an in situ reaction/hot pressing method using Nb, Al, and C as the starting materials. The reaction path, microstructure, physical, and mechanical properties of Nb4 AlC3 were systematically investigated. The thermal expansion coefficient was determined as 7.2 × 10−6 K−1 in the temperature range of 200°–1100°C. The thermal conductivity of Nb4 AlC3 increased from 13.5 W·(m·K)−1 at room temperature to 21.2 W·(m·K)−1 at 1227°C, and the electrical conductivity decreased from 3.35 × 106 to 1.13 × 106 Ω−1 ·m−1 in a temperature range of 5–300 K. Nb4 AlC3 possessed a low hardness of 2.6 GPa, high flexural strength of 346 MPa, and high fracture toughness of 7.1 MPa·m1/2 . Most significantly, Nb4 AlC3 could retain high modulus and strength up to very high temperatures. The Young's modulus at 1580°C was 241 GPa (79% of that at room temperature), and the flexural strength could retain the ambient strength value without any degradation up to the maximum measured temperature of 1400°C. 相似文献
15.
SALLY A. RODGER GEOFFREY W. GROVES NIGEL J. CLAYDEN CHRISTOPHER M. DOBSON 《Journal of the American Ceramic Society》1988,71(2):91-96
The use of cross-polarization (CP) NMR in conjunction with magic angle sample spinning (MASS) to examine the hydration reaction of tricalcium silicate (C3 S) is described. In particular the very early stages of the reaction both with and without admixtures has been studied as well as the hydration in a ball mill. The combination of CP and non-CP 29 Si NMR permits the distinction between silicate units associated with protons, i.e., in hydrated material, and those in anhydrous material. It has been found that in paste hydration there is steady formation of a small amount of hydrated monomeric silicate units during the induction period. In ball mill hydration the formation of the crystalline calcium silicate hydrate, afwillite, which contains only hydrated monomeric silicate species, can be monitored. These results are interpreted in terms of possible mechanisms for C3 S hydration. 相似文献
16.
Masato Kakihana Maria M. Milanova † Momoko Arima Toru Okubo Masatomo Yashima Masahiro Yoshimura 《Journal of the American Ceramic Society》1996,79(6):1673-1676
A polymerized complex technique was used to prepare high-purity pyrochlore Y2 Ti2 O7 powders at 750°C. Heating of a mixed solution of citric acid (CA), ethylene glycol (EG), and yttrium and titanium ions, with a molar ratio of CA:EG:Y:Ti = 5:20:1:1, at 130°C produced a yellowish transparent polymeric gel without any precipitation; this material was used subsequently as a precursor for Y2 Ti2 O7 . Based on the results of 13 C-NMR spectroscopy, it was suggested that a mixed-metal (Y,Ti)-CA3 chelate complex formed in a starting CA/EG solution. The formation of pure pyrochlore Y2 Ti2 O7 occurred when the precursor was heat-treated in a furnace set at 750°C in static air for 4 h. 相似文献
17.
Factors influencing the low-temperature formation of AIPO4 and its precursor phases, AIPO4 · x H2 O (1 x 2), were investigated. AIPO4 formed by reaction between 33.3 wt% H3 PO4 solution and alumina. Five aluminas (three anhydrous and two hydrated) were utilized. Each differed in particle size, surface area, and crystallinity. The reaction temperatures investigated were 113°, 123°, and 133°C. The high-surface-area aluminas were sufficiently reactive in the phosphoric acid solution at these temperatures to produce crystalline reaction products. However, only hydrated forms of AIPO4 , AIPO4 · x H2 O (1 x 2), crystallized directly out of solution. x generally decreased as the curing temperature was increased. Upon dehydration of these hydrated reaction products, anhydrous AIPO4 was formed, primarily in the berlinite and/or cristobalite modifications. Both the temperature of reaction and the alumina used influence the hydrates that form. In turn, the hydrates which form, the macroscopic assemblages into which they may crystallize, and the morphologies of the crystallites all affect the polymorphic form and the crystallinity of the anhydrous AIPO4 phase ultimately produced on dehydration. Phase-pure and highly crystalline AIPO4 -cristobalite (the high-temperature modification) was formed by the dehydration of AIPO4 ·H2 O at a temperature as low as 113°C. 相似文献
18.
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
19.
KINJIRO FUJII WAKICHI KONDO HISAHIRO UENO 《Journal of the American Ceramic Society》1986,69(4):361-364
Hydration of CaAl2 O4 (CA) was studied by calorimetry, analysis of the liquid phase, measurement of the combined water, and electron microscopy. During the induction period, the solution remains almost unchanged and is equilibrated temporarily with both superficially intrusion-hydrated CA particles and Al(OH)3 gel formed by dissociation of Al(OH)4 – ions, the solubility of the Al(OH)3 gel being 10–4.24 molkg–1 at 25°C, while the intrusion-hydrated layer on the CA particles grows following a nearly linear law to reach a critical thickness (∼3 nm at 10° to 20°C, or 12 nm at 30°C). At this point destruction of the layer occurs, nuclei of hydrous compounds are generated, and the induction period terminates. Subsequent reaction proceeds in accordance with the rate equation of Schiller based on the dissolution-crystallization mechanism. 相似文献
20.
Formation Kinetics of Calcium Aluminates 总被引:3,自引:0,他引:3
Vipin K. Singh Mohammad M. Ali Upendra K. Mandal 《Journal of the American Ceramic Society》1990,73(4):872-876
The kinetics of formation of calcium aluminates was studied by firing the reaction mixes in the temperature range 12000° to 1460°C for reaction times from 15 to 360 min. Phases formed were determined by taking X-ray diffractograms of the samples. It was observed that all stable calcium aluminates were formed and that monocalcium aluminate (CA) grew with calcium dialuminate (CA2 ) in a 1:2 reaction mix of CaO and Al2 O3 . CA reacted further with Al2 O3 to form CA2 . The formation of CA2 obeyed the rate law equation 1 - (1 - x )1/3 = Kt / r 2 . The activation energy for the system (140 kJ·mol−1 (33.4 kcal · mol−1 )) was determined by the Arrhenius equation. 相似文献