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1.
Growth kinetics of gold nanocrystals: a combined small-angle X-ray scattering and calorimetric study
The growth of gold nanocrystals prepared by the reduction of tetrachloroauric acid by tetrakis(hydroxymethyl)phosphonium chloride, which allows slow reduction, is investigated by small-angle X-ray scattering and isothermal titration calorimetry in combination with transmission electron microscopy. The growth of the nanocrystals does not follow the diffusion-limited Ostwald ripening but instead follows a sigmoidal rate curve. The activation energy obtained from the temperature-dependent growth study is very small. The heat change associated with the growth is determined for the first time as approximately 10 kcal mol(-1) per 1 nm increase in the nanocrystals' diameter. 相似文献
2.
Study of Al2O3 effect on structural change and phase separation in Na2O-B2O3-SiO2 glass by NMR 总被引:1,自引:0,他引:1
The effect of Al2O3 on the structure change and the phase separation in Na2O-B2O3-SiO2 glass was investigated using 11B nuclear magnetic resonance (NMR), 29Si MAS NMR, and 27Al MAS NMR together with infrared absorption spectroscopy and field emission scanning electron microscopy (FE-SEM). The results show that the structure change from the introduction of Al2O3 contributes greatly to the inhibition of phase separation. First, the introduction of Al2O3 imparts an ionic character to the boron-oxygen network, resulting in the formation of B-O-Al-O-Si bonds and thus increases the compatibility of the silicon network with the boron-oxygen network. Second, the addition of Al2O3 causes the sodium ion to transfer from the boron-oxygen network to AlO4 tetradedra, changing a number of four-coordinated borons into three-coordinated borons. As the bond energy of the four-coordinated boron is weaker than that of the three-coordinated boron, the -B-O-Si- bond with the four-coordinated boron in Na2O-B2O3-SiO2 glasses is easily broken and results in severe phase separation during heat treatment. However, the -B-O-Al- bond with the three-coordinated boron formed in Na2O-B2O3-SiO2-Al2O3 glasses is difficult to be broken due to the high bond energy. In addition, the silicon network in Na2O-B2O3-SiO2-Al2O3 glasses is also strengthened by the addition of Al2O3, which prevents [BO] groups from further aggregation. As a result, the tendency of the glass towards phase separation is greatly suppressed in the Na2O-B2O3-SiO2-Al2O3 system. 相似文献
3.
S. F. Horvath M. P. Harmer D. B. Williams M. R. Notis 《Journal of Materials Science》1989,24(3):863-872
Analytical electron microscopy has been used to study the precipitation reactions in sintered samples of 9 mol% La2O3-Y2O3 samples upquenched from the single phase cubic region into the cubic and hexagonal phase field. Samples annealed just inside the two-phase cubic-cubic and hexagonal solvus exhibited predominantly grain boundary precipitation. Small La2O3 rich second phases formed within the first ten minutes and developed into strained, facetted precipitates after 300 min. Intergranular and intragranular precipitation occurred in samples annealed further into the two-phase field. Strained, lathlike La2O3-rich monoclinic precipitates, exhibiting a preferrred orientation in the matrix, appeared as the dominant morphology for long times at temperature. Chemical microanalyses of the strained structures obtained in samples annealed for 300 min revealed La2O3 matrix concentrations in agreement with phase diagram predictions. However, the La2O3 concentrations in the second-phase precipitates were found to be far in excess of the cubic and hexagonal-hexagonal solvus. This discrepancy is believed to arise from a re-equilibration of the second phase in the cubic and monoclinic phase field during quenching. 相似文献
4.
Hollow glass microspheres (HGS) for laser fusion targets were fabricated in the system Na2O-B2O3-SiO2 from NaOCH3, B(OCH3)3 and Si(OC2H5)4. Gel powders prepared from metal alkoxides and urea liberate H2O, CO2 and NH3 gases, evolution of which takes place completely at about 500° C. The precursor of HGS is formed by the encapsulation of these gas components in the glass layer formed at the surface of the powder. HGS are produced from the gel powders having both a melting temperature lower than about 1000° C and a viscosity at that temperature lower than 105 P. In the Na2O-B2O3-SiO2 system, the compositions from which HGS are produced are those containing 55–75 wt% SiO2 and 0–20 wt% B2O3. HGS ranging from 100–500m diameter and 0.5–7.0m wall thickness are obtained by change of urea content. 相似文献
5.
The melting, dissolution and crystallization behaviour of the lamellar overgrowth in polyethylene shish-kebab fibres have been studied by small-angle X-ray scattering and electron microscopy. SAXS experiments in which fibres were heatedin situ demonstrated the irreversible reorganization of the lamellar overgrowth already at 60° C. Reorganization continued in an inhomogeneous manner until the fibre lost its porosity around 140° C. The morphology that developed upon cooling depended on the crystallization temperature as well as on the maximum temperature attained before crystallization. Transmission electron microscopy observations showed the influence of molecular weight on aggregation. The SAXS patterns obtained duringin situ dissolution experiments disclosed that the lamellae dissolved in dodecane above 115° C. Recrystallization of the lamellae on to the backbones was almost complete within 10 min at 110° C. These high temperatures, as compared with single-crystal behaviour, indicate that the cilia nucleated with exceptionally high stem lengths on to the backbones. SAXS of fibres elongated to a maximum ratio of 1.5 at 90° C demonstrated the role of the lamellar overgrowth as a matrix between the elementary fibrils. The shish-kebab morphology could be restored after elongation by selective dissolution of the lamellae and recrystallization. 相似文献
6.
Substitution of SiO2 in the ternary sodium borosilicate system with Ta2O5 was found to produce glasses, which after heat treatment separated into immiscible microphases, one of which was water soluble.
The structure of the leached material after heat treatment was a well developed low temperature form of Ta2O5. After firing at temperatures between 1100 and 1550°C X-ray diffraction analysis showed the presence of low and high temperature
forms of Ta2O5 and of orthorhombic Na2Ta5O21. The high solubility of up to 40 wt% Ta2O5 in the sodium-borate matrix resulting in clear glasses is of practical interest. The specific surface areas of the leached
materials ranged between 5.54 and 35.57 m2g−1 while in an additionally Al2O3 doped material the value of 307 m2g−1 was measured. Mean pore radii of interconnected pores were calculated to be between 18.63 to 41.12 nm in the Ta2O5-rich materials while the additional Al2O3 doping decreased the value to 2.71 nm. A sintering temperature between 1500 and 1550°C is estimated from void volume measurements
after a series of firing steps at temperatures between 1100 and 1550°C were undertaken. 相似文献
7.
The optical absorption and EPR spectra of Cr3+ in binary sodium borate glasses have been studied as functions of chromium concentration and Na2O/B2O3 ratio in the glass; the ligand field and EPR parameters have been calculated and were found to be independent of Cr3+ concentration in any particular glass. In low-alkali borate glasses (Na2O = 11 or 14 mol %) a single symmetrical EPR line was observed with g
¦=g =1.984±0.001 corresponding to perfect octahedral symmetry of the Cr3+ ion in these glasses. With increasing Na2O content of the glass, the EPR line becomes more asymmetric (characteristic two-peaked pattern); this has been explained as being due to axial elongation of the six co-ordinated Cr3+-complex in these glasses. 相似文献
8.
9.
A. Paul 《Journal of Materials Science》1975,10(4):692-696
The optical absorption and the esr spectra of titanium(III) in binary Na2O-B2O3 and Na2O-P2O5 glasses have been studied. Titanium(III) produces two optical absorption bands around 20 000 and 14 000 cm–1 which are assigned to the2B2g
2B1g and2B2g
2A1g transitions respectively of Ti3+ in a tetragonally distorted octahedral environment. The absorption bands in phosphate glasses are narrower and absorption coefficients higher than those in borate glasses. The esr spectrum of titanium(III) in all the glasses consists of a broad asymmetric line withg 1.94 in borate glasses, andg 1.92 in phosphate glasses; no hyperfine structure has been observed. 相似文献
10.
Thomas Höche 《Journal of Materials Science》2010,45(14):3683-3696
Glass ceramics, i.e. intentionally devitrified glasses, are used in manifold applications. This is due to the capability to
precisely control the precipitation of crystalline phases possessing useful properties related to optics, mechanics, biocompatibility,
piezoelectricity, machinability, etc. In order to control the nano and microstructure, transmission electron microscope imaging
proves particularly useful. Illustrated by numerous examples, the diverse precipitate shapes and mechanisms at work upon crystallization
of glasses are discussed. In addition, it is shown how useful cutting-edge transmission electron microscopy is in the field
of nanoanalytics. The investigation of nanoscale processes controlling the precipitation of functional crystals evidences
the need for more sophisticated theories describing the changes of composition accompanying crystallization in non-isochemical
systems (precipitates possess compositions different from those of the host glasses). 相似文献
11.
The sol-gel procedure has been used to prepare vitreous materials whose compositions are situated in the liquid-liquid immiscibility area of the Na2O-B2O3 SiO2 phase equilibrium diagram. Gels were prepared from different precursors and under different experimental conditions. After determining optimum conditions for gelling and heat-treatment, the gels were characterized with the aid of thermal analysis, X-ray diffraction, density and specific surface area measurements, as well as infrared and near-infrared spectroscopy. The textures of the samples and their microstructures were studied by means of scanning electron microscopy and transmission electron microscopy, respectively. The phase separation in the gels was compared to the same phenomenon in one of the glasses of identical composition, but prepared by conventional melting. 相似文献
12.
Hot stage transmission electron microscopy has allowed the nucleation sequence in a 30 mole % Li2O-70 mole % SiO2 glass to be monitored in situ. This result agrees with previous work, in that the sequence of phases observed is identical but small temperature changes give rise to pronounced morphology changes. Hot stage electron microscopy has been proven to be a very useful technique for observing glass crystallisation processes, but the results must be carefully interpreted because of possible surface effects.Commercial masking lacquer. 相似文献
13.
14.
Paul W Wang 《Materials Chemistry and Physics》1997,50(3):221
The thermal effect on silver in ion-exchanged glasses was investigated in situ by X-ray photoelectron spectroscopy (XPS) in an ultra-high vacuum environment. Each XPS signal of Ag 3d3/2 and 3d5/2 consists of two components, the metallic state (Ag0) and the oxidized state (Ag+), resolved after curve fitting. The toward-surface diffusion of silver was observed by monitoring the changes in concentration on the surface during sample annealing between 20 and 450°C. Judging from the variations in line shape and binding energy and from the enhancement of surface silver under annealing, both metallic and oxidized silver are accumulated on the surface. By applying the diffusion theory in a semi-infinite system to the experimental data, the thermal activation energy of the oxidized silver in ion-exchanged glass, 0.16 eV, was estimated. The activation energy of metallic Ag precipitated during heating, 0.23 eV, was estimated as well. 相似文献
15.
《Materials Letters》2005,59(2-3):355-360
The Al2O3–ZrO2(Y2O3) composite powder was synthesized through a sol–gel process using aluminum sec-butoxide and zirconium butoxide as precursors. The as-received powders in an amorphous phase were crystallized with c-ZrO2 at around 980 °C. As the calcination temperature increased, the c-ZrO2 crystalline phase was transformed to t-ZrO2 at about 1200 °C. However, the Al2O3 phase in the Al2O3–ZrO2(Y2O3) composite powders still existed in an amorphous phase up to 1050 °C. In the sintered body using the calcined powders at 400 °C, the Al2O3 phase was crystallized in an α-phase at 1200 °C during the sintering for 2 h. Using the sol–gel Al2O3–ZrO2(Y2O3) powder, a typical nano-composite having a nano-crystalline phase (less than 20 nm) can be successfully obtained by a pressureless-sintering process even at 1200 °C for 2 h.Using the sol–gel Al2O3–ZrO2(Y2O3) powder, a typical nano-composite having a nano-crystalline phase (less than 20 nm) can be successfully obtained by a pressureless-sintering process even at 1200 °C for 2 h. The values of relative density and Vickers hardness were comparatively high value with about 96.2% and 1100 Hv, respectively, even though it was made at low temperature. In the composite sintered at 1400 °C, the hardness value was saturated with 1570 Hv and the values of fracture toughness were almost same with about 6 MPa m1/2. 相似文献
16.
H. Vincent F. Chassagneux C. Vincent B. Bonnetot M. P. Berthet A. Vuillermoz J. Bouix 《Materials Science and Engineering: A》2003,340(1-2):181-192
Continuous boron nitride fibres have been fabricated by melt spinning and pyrolysis of poly[2,4,6-tris(methylamino)borazine]. The longitudinal mechanical properties depend on mechanical stress and temperature applied during the conversion process. High-performance and low-performance fibres were characterized in order to find relationship between structure and physical properties. In all the cases, photoelectron spectroscopy (XPS) analysis proves that the chemical composition of the fibre is close to stoichiometric BN. The crystallite sizes were measured by means of X-ray diffraction (XRD) and Raman techniques. Cross-sections of separated fibres were investigated by high-resolution electron microscopy (HREM) and transmission electron microscopy (TEM). All the BN fibres have a hexagonal turbostratic structure. With increasing stress and temperature, the tensile strength and the elastic modulus increase. In the high-performance fibres, the 002 layers with an increased distance (about 0.35 nm) showed a mean stacking sequence near to graphite and a preferred orientation of the 002 layers parallel to the fibre axis. 相似文献
17.
18.
N. Srinivasa Rao Shashidhar Bale M. Purnima K. Siva Kumar Syed Rahman 《Bulletin of Materials Science》2005,28(6):589-592
The local structure around Cu2+ ion has been examined by means of electron spin resonance and optical absorption measurements in xLi2O-(40-x)Na2O-50B2O3-10As2O3 glasses. The site symmetry around Cu2+ ions is tetragonally distorted octahedral. The ground state of Cu2+ isd
x
2−y
2.The glass exhibited broad absorption band near infrared region and small absorption band around 548 nm, which was assigned
to the
2B1g →2Eg
transition. 相似文献
19.
The thicknesses of the hydration layers on the surfaces of 2 silicate glasses have been assessed using a) a combination of focused ion beam milling and transmission electron microscopy and b) nanoindentation; the two approaches give consistent layer thicknesses. Lighter contrast of the hydrated layers in TEM suggests that the layers have reduced density when compared to the bulk glasses; this is consistent with the reduced near surface modulus and hardness of hydrated glasses observed in nanoindentation. 相似文献
20.
The microstructure and microanalysis study of two glasses containing AgCl precipitated particles have been carried out by transmission electron microscopy (TEM, replica method), scanning electron microscopy (SEM) and SEM/EDX (energy dispersive X-ray spectrometry). The composition of these glasses doped with silver halide and CuO was formulated from the Al2O3-B2O3-SiO2 and Na2O-CaO-SiO2 systems respectively. In both glasses the seeds, nuclei, crystals and matrix were analysed, and the mean size and number of crystals were evaluated from the TEM and SEM observations. The microstructure in both glasses is different because of the different shape of the silver halide particles; the particles of the Al2O3-B2O3-SiO2 glass are rounded while Na2O-CaO-SiO2 shows square precipitated particles. Likewise, the darkening behaviour is basically different; the Al2O3-B2O3-SiO2 glass shows a higher darkening velocity than the Na2O-CaO-SiO2 glass, showing for this glass a very small slope value. 相似文献