Characterization of B2O3-SiO2 glasses prepared via sol-gel

B₂O₃-SiO₂ glasses were prepared by the sol-gel method from boron and silicon alkoxides. The gels were densified by several heat-treatments at temperatures above 800° C. The gel-glass transition was studied with the data obtained from differential thermal analysis, dilatometry and thermal evolution of density. The structural features of the glassy materials prepared were analysed by means of infrared (IR) and near-infrared (NIR) spectroscopy. The IR spectra indicated the existence of mixed Si-O-B bonds, preferential located at the outside of the material particles. The NIR spectra, recorded from gels heat-treated at low temperatures, demonstrated the existence of a great number of OH^– groups varying in nature, as well as their evolution and their positional changes on the silica particle surface as a function of the heat-treatment.     

Preparation and non-linear optical properties of CdS quantum dots in Na2O-B2O3-SiO2 glasses by the sol-gel technique

Non-linear optical properties of CdS quantum dots embedded in the sol-gel derived Na₂O-B₂O₃-SiO₂x³ glass matrix have been studied using nanosecond degenerate four-wave mixing (DFWM), pump-probe experiments, and time-resolved subpicosecond DFWM measurements. The concentration of CdS microcrystallites was varied from 1.4 to 10.2 wt% while the particle size was controlled to be in the range of 3–6 nm in diameter so that the confinement effects can be realized. The third-order susceptibility, ³, was determined to be of the order of 10^–7-10^–6 e.s.u. near the resonant wavelength between 450 and 470 nm by the nanosecond DFWM and pump-probe experiments, and of the order of 10^–11-10^–10 e.s.u. at the off-resonant wavelength, 580 nm, by a time-resolved DFWM measurement with 400 fs laser pulse. The decay time changed from 0.5 to 50 picoseconds as a function of the size, size distribution and number density of CdS microcrystallites in the glasses. Photodarkening reduced the optical non-linearity of the melt-quenched samples by a factor of 20, while it had no appreciable effect on that of the sol-gel derived samples.[/p]     

溶胶凝胶法制备Er3+-Yb3+共掺纳米SiO2粉

利用溶胶-凝胶法合成了Er2O3-Yb2O3-SiO2纳米二氧化硅(SiO2)粉。Er3 和Yb3 直接加入有机酸催化的TEOS反应溶液中,凝胶后经过700~900℃,得到Er3 -Yb3 共掺纳米SiO2粉。样品经过红外光谱、XRD、TEM等分析,结果表明,纳米粉的粒径受有机酸比例和处理温度的影响。在室温PL谱上,样品可观察到较强的1530nm荧光,同时镱的引入对掺铒SiO2纳米粉在1.54μm的荧光发射有增强作用。     

EPR and optical absorption of Cr3+ in binary Na2O-B2O3 glasses

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.     

Study of Al2O3 effect on structural change and phase separation in Na2O-B2O3-SiO2 glass by NMR

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.     

Luminescence of nanostructured SnO2-SiO2 glass-ceramics prepared by sol-gel method

Nanostructured silica based glass-ceramics samples of composition (100 - x)SiO2-xSnO2, with x from 1 to 10, have been synthesized by thermal treatment of precursor sol-gel glasses. The average size of the obtained SnO2 nanocrystals, calculated by using the X-ray diffraction, can be predetermined by using well-controlled concentration of tin precursor. The mean radius ranging from 1.6 to 5.5 nm, is comparable to the exciton Bohr radius, corresponding to wide band-gap semiconductor quantum-dots in an insulator SiO2 glass. A spectroscopy study in terms of optical absorption and photoluminescence spectra has been carried out as a function of SnO2 concentration. Size-dependent red-shifts of excitation and emission bands, with increasing of tin precursor concentration, point to the quantum confinement effect. The nanocrystal sizes have been obtained and compared by using the Brus and Scherrer equations. The band gap increase is in agreement with results, based on the effective mass model. The recombination of conduction band electron with oxygen vacancies is proposed to explain the luminescence red-shift.     

The infrared and Raman spectra of ZrO2-SiO2 glasses prepared by a sol-gel process

Infrared and Raman spectra were measured and interpreted on the basis of structure for various ZrO₂-SiO₂ glasses that were prepared by a sol-gel process involving zirconium nitrate. Interpretation of the spectra indicates the presence of both Zr-O-Si and Si-O-H linkages depending upon heat treatment conditions, and the evidence of zirconium atoms with eight fold coordination for glasses with high ZrO₂ contents. The crystalline products formed during glass devitrification were monitored by vibrational spectra. The formation and detection of tetragonal ZrO₂, zircon, monoclinic ZrO₂ and -cristobalite depended upon the conditions of devitrification.     

Toughened glass-ceramics in the ZrO2-Al2O3-SiO2 system prepared by the sol-gel process

Monolithic glass-ceramics containing Al₂O₃ or TiO₂ were prepared in the ZrO₂-SiO₂ system by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by heat treatment at 900–1200 °C and its crystal growth was increased by adding TiO₂ or Al₂O₃. Further heating at higher temperature resulted in the precipitation of zircon and monoclinic ZrO₂ which was transformed from tetragonal ZrO₂. The addition of Al₂O₃ had less effect on both the tetragonal-to-monoclinic ZrO₂ transformation and the precipitation of zircon. The fracture toughness increased as the size of tetragonal ZrO₂ particles increased and then decreased with the appearance of monoclinic ZrO₂ or zircon. The fracture toughness of the glass-ceramics was measured in the glass-forming regions of the ZrO₂-Al₂O₃-SiO₂ system. The fracture toughness was sensitively dependent on both Al₂O₃ and ZrO₂ content, of which the highest value achieved was 9 MPa m^1/2 for the 50ZrO₂·10Al₂O₃·40SiO₂ composition.     

Fabrication of hollow glass microspheres in the Na2O-B2O3-SiO2 system from metal alkoxides

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.     

Na2O-B2O3-SiO2玻璃焊料连接碳化硅陶瓷接口抗热震性能

采用水淬法对以Na₂O-B₂O₃-SiO₂体系玻璃焊料连接的常压烧结碳化硅试条的抗热震性能进行了研究. 对不同温度下淬火后接口以及断面处微观结构进行了比较与分析, 并对比了不同淬火温度以及固定淬火温度为150℃时多次热循环后连接试条的残余弯曲强度. 结果表明, 对于单次淬火, 当淬火温度为150℃时, 由于热应力引起中间层内部微裂纹扩展, 导致残余弯曲强度迅速降低到(152±28) MPa. 淬火温度在150℃~320℃时, 中间层内部裂纹保持在亚临界状态, 相应弯曲强度基本保持在140 MPa. 继续升高淬火温度至420℃时, 裂纹进一步扩展, 试条残余弯曲强度迅速降低至(32±8) MPa. 对于多次热循环, 当淬火温度固定在150℃时, 经多次热循环, 残余弯曲强度与热循环次数变化不明显, 基本保持在120 MPa左右, 这说明在150℃以下淬火, 连接试条具有较好的抗热循环冲击性能.     

Vaporization of glasses in the R2O-Cs2O-B2O3-SiO2 (R = Li,Na, K and Rb) system

A mass-spectrometric Knudsen effusion method has been used for thermochemical study of the vaporization of alkali borosilicate glasses in the R₂O-Cs₂O-B₂O₃-SiO₂ system, where R = Li, Na, K and Rb. Vapours of RBO₂(g), CsBO₂(g), R₂(BO₂)₂(g) and RCs(BO₂)₂(g) have been identified. Over the glasses in which the molar ratio of (R₂O + Cs₂O)/B₂O₃ is slightly larger than unity, vapours of R(g), except for Li(g), have been further observed in the initial stage of heating; over the glass containing lithium, however, the vaporization of Cs(g) has been found. Neither R(g) nor Cs(g) has been identified over the glasses in which the molar ratio is equal to unity. Partial pressures of CsBO₂(g) over the glasses containing lithium and sodium are lower than those with potassium and rubidium, indicating that chemical activities of the CsBO₂ pseudo-component in the glasses with lithium and sodium are small. Enthalpies of formation as well as dissociation energies for dimeric vapours of alkali metaborates are not very different from one another, except that the dissociation energies for the dimeric vapours containing lithium are larger than for those containing the other elements.     

Optical and esr spectra of titanium (III) in Na2O-B2O3 and Na2O-P2O5 glasses

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 the²B_2g ²B_1g and²B_2g ²A_1g 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.     

Preparation of B2O3-P2O5-SiO2 coating films by the sol-gel method

ThexB₂O₃ · (20-x) P₂O₅ · 80SiO₂ (in mol%) glass films withx=0, 10 and 20 have been prepared from metal alkoxides by carrying out the coating in a dry atmosphere. These coating films have shown a larger value of load at scratch and a smaller shrinkage during heat-treatment by replacing P₂O₅ in the films with B₂O₃. It has been found that B₂O₃ more effectively reduces the glass transition temperature of SiO₂ glass than P₂O₅. The concentrations of sodium ions, which migrated from soda-lime-silica glass substrates during the film formation, were higher in phosphosilicate and borophosphosilicate films than in borosilicate and pure silica films. This finding should be ascribed to the gettering effects of phosphorus for sodium ions.     

Characterization and sintering of a porous glass-ceramic in the system Na2O-B2O3-Ta2O5

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.