Investigation on heat transfer performance of fluoride-free and titanium-bearing mold fluxes

Fluorine erodes continuous casters and pollutes the environment. In order to reduce the damage caused by fluorine, it is necessary and urgent to carry out research on fluoride-free mold fluxes. There has been little research done on the heat transfer performance of fluoride-free mold fluxes either domestically or abroad. The present work adopted TiO₂ to take the place of fluorine in mold fluxes and studies its heat transfer performance. Heat flux simulation equipment was developed and the heat flux density of titanium-bearing mold fluxes containing TiO₂ was measured; in addition, a solid slag film was obtained. The crystallization behavior and the change of activation energy for crystallization of the slag film was analyzed. Our results show that when the TiO₂ content is increased, the heat flux density of fluoride-free mold fluxes decreases, the crystallization activation energy of mold fluxes is diminished and the crystallization ratio of mold fluxes increases, and the mineral phase of the slag film turns from akermanite into perovskite. When the basicity is increased, the heat flux of fluoride-free mold fluxes is reduced, the crystallization ratio of mold fluxes increases and the mineral phase of the slag film turns from unitary akermanite into the coexistence of two phases of akermanite and perovskite. Furthermore, the capability of fluoride-free and titanium-bearing mold fluxes to control heat transfer is better, so it can be expected to replace industrial slag containing fluorine completely.     

Crystallization kinetics in Cu46Zr45Al7Y2 bulk metallic glass by differential scanning calorimetry (DSC)

Crystallization transformation kinetics in isothermal and non-isothermal (continuous heating) modes were investigated in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass by differential scanning calorimetry (DSC). In isochronal heating process, activation energy for crystallization at different crystallized volume fraction is analyzed by Kissinger method. Average value for crystallization in Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass is 361 kJ/mol in isochronal process. Isothermal transformation kinetics was described by the Johnson-Mehl-Avrami (JMA) model. Avrami exponent n ranges from 2.4 to 2.8. The average value, around 2.5, indicates that crystallization mechanism is mainly three-dimensional diffusion-controlled. Activation energy is 484 kJ/mol in isothermal transformation for Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass. These different results were discussed using kinetic models. In addition, average activation energy of Cu₄₆Zr₄₅Al₇Y₂ bulk metallic glass calculated using Arrhenius equation is larger than the value calculated by the Kissinger method in non-isothermal conditions. The reason lies in the nucleation determinant in the non-isothermal mode, since crystallization begins at low temperature. Moreover, both nucleation and growth are involved with the same significance during isothermal crystallization. Therefore, the energy barrier in isothermal annealing mode is higher than that of isochronal conditions.     

Homogeneous versus heterogeneous crystal nucleation in Li2O · 2SiO2 glass

When crystal nucleation occurs in glasses, it is difficult to ascertain whether the crystallization has initiated homogeneously or by a heterogeneous mechanism. Here we suggest a method which might enable one to make this choice, and it is applied to the bulk crystal nucleation observed in Li₂O · 2SiO₂. It is concluded that crystal nucleation in the latter system most probably occurs homogeneously.     

Effect of V2O5 addition on the crystallisation of glasses belonging to the CaO–ZrO2–SiO2 system

The crystallisation of CaO–ZrO₂–SiO₂ glasses doped with V₂O₅ (0.1–5 mol%) has been investigated in terms of microstructure and thermal parameters. Results indicate that crystallisation is predominantly controlled by a surface nucleation mechanism, even though a partial bulk nucleation has been encountered in compositions containing more than 2 mol% of doping oxide. As detected from differential thermal analysis curves, glass transition temperature and crystallisation temperature, are strongly dependent upon V₂O₅ content varying from 0.0 to 2.0 mol%, while the crystallisation activation energy values decrease with a parabolic trend from B-glass (0.0 mol% V₂O₅ content, 495±7) to V-0.7 (0.7 mol% V₂O₅ content, 420±6) composition, increasing again to 442±5 kJ/mol K with higher amount of V₂O₅. The microstructure of the glass-ceramic materials clearly showed a marked dependence upon the amount of V₂O₅, also due to the presence of phase separation for content higher than 0.7 mol%. Wollastonite, CaO·SiO₂, and a calcia–zirconia–silicate, 2CaO·4SiO₂·ZrO₂, are the main crystalline phases whose ratio slightly varies with vanadium oxide content. The glass ceramics obtained from the studied materials are greenish and bluish coloured, so it is possible to use the studied glasses as coloured frits for tile glazes.     

Precipitation selectivity of perovskite phase from Ti-bearing blast furnace slag under dynamic oxidation conditions

The effects of dynamic oxidations on the crystal morphology and the precipitation behavior of the perovskite phase from Ti-bearing blast furnace slag were investigated. Air was blown into the molten slag as an oxygen source through a lance during the dynamic oxidation process. It was found that the dispersed Ti components were selectively taken into the perovskite phase (CaTiO₃), and the perovskite phase could be selectively precipitated and grown. Oxidation of molten slag produced a large amount of heat, which not only increased the temperature of the slag, but also promoted the precipitation and growth of the perovskite phase. It was confirmed by isothermal experiments and non-isothermal pilot experiments that the precipitation of the perovskite (CaTiO₃) in molten slag is obviously affected by operation factors such as oxidizing time, and slag temperature. The perovskite phase precipitation kinetics and mechanisms from molten slag during the dynamic oxidation processes were also investigated.     

Investigation of the devitrification and microwave penetrating properties of fused silica

Four kinds of fused silica (SiO₂) bulk materials were prepared by the hot-pressing method. Bulk SiO₂ made from sol-gel powders was optically transparent and exhibited the best microwave penetrating properties. Commercial SiO₂ obtained from melted quartz increased crystallization. The addition of polyvinyl alcohol (PVA) (aqueous solution) blocked the crystallization of commercial SiO₂ to a limited extent. In contrast, PVA powders accelerated the heterogeneous nucleation and growth of crystalline silica. In this case, the bulk material of SiO₂-PVA was visibly darkened and exhibited the worst microwave penetrating properties. X-ray diffraction patterns and electron probe X-ray microanalysis confirmed that crystallization was correlated with the optical transmission loss. Crystallized areas randomly dispersed in the continuous amorphous body of fused SiO₂, served as scattering centers which affected the optical transparency and also served as microwave absorbing or scattering centers which limited the microwave penetration. The connection between devitrification and microwave penetrating properties of fused SiO₂ could therefore be used as a reference to produce other microwave penetrating materials with better performance.     

Crystallization of blast furnace slag glass melted in SnO2 crucible

The scope of this work was to study the possibility of obtaining glass–ceramics from Brazilian blast furnace slag. Glass melting on laboratory scale is usually carried out in platinum crucibles, which are normally not attacked by molten silicates and can be easily cleaned and reused. However, the use of platinum-coated furnaces is not economically viable for producing these materials on industrial scale. As an alternative, SnO₂ crucibles were used to melt glasses. Crucible corrosion was observed and thus the effect of SnO₂ on the crystallization behavior of the resulting glass was studied. For this purpose differential scanning calorimetry (DSC), X-ray diffraction, optical microscopy, scanning electron microscopy and micro-Raman spectroscopy were used. Besides merwinite, melilte, larnite and wollastonite, a crystalline phase containing tin was only possible to be determined by Raman spectroscopy, due to the presence of a band at 570 cm⁻¹, attributed to a calcium tin silicate.     

A kinetic treatment of glass formation V: Surface and bulk heterogeneous nucleation

An analytical method is described for calculating the detailed distribution of crystallite sizes in a supercooled liquid, and the changes in this distribution as a function of temperature (time) while the liquid is cooled from above the melting point.This method, termed the analysis of crystallization statistics, is applied to the calculation of continuous cooling curves for anorthite and o-terphenyl as representative of inorganic and organic systems. In addition to homogeneous nucleation, bulk as well as surface heterogeneous nucleation are considered. The effects of distributions of heterogeneities with contact angles between 40 and 100° as well as overall concentrations of heterogeneities between 10³ and 10⁹ cm^?3 are considered. Heterogeneities with contact angles higher than about 100° are shown not to have an effect on the critical cooling rate for typical concentrations of heterogeneities.For liquids containing distributions of heterogeneities, the nucleation behavior is dominated by small concentrations of heterogeneities having small contact angles. Theoretical log (I_vη) versus (T_r³ΔT_r²)^?1 curves have been constructed for homogeneous nucleation + heterogeneous nucleation with a single type of heterogeneity and for homogeneous nucleation + heterogeneous nucleation with the heterogeneities distributed with regard to contact angle. In the former case, the curve is composed of two linear portions; and in the latter case, the curve shows pronounced curvature. The curvature reflects a continous change in the frequency of heterogeneous nucleation.Surface heterogeneous nucleation was assumed to originate at discrete surface heterogeneities and was shown to give rise to continuous cooling curves similar to those calculated for bulk heterogeneous nucleation.     

高炉熔渣微晶玻璃的结构与性能研究

高温熔渣具有大量显热与渣体.采用熔融法制备微晶玻璃可以更好地利用其热和渣,达到高效利用的目的.通过高温条件下混熔的方式制备性能稳定的基础玻璃.利用差示扫描量热仪(DSC)确定基础玻璃的热处理工艺制度.结合高分辨透射电子显微镜(TEM)、X射线衍射分析(XRD)、场发射扫描式电子显微镜(FE-SEM)对微晶玻璃的结构进行了研究.研究表明:基础玻璃中氟含量的增加,有利于促进微晶玻璃成核和晶体长大,降低微晶玻璃的形核结晶温度.在低温热处理得到微晶玻璃的主晶相为萤石,高温热处理得到的微晶玻璃析出了霞石和萤石两个微晶相.高炉渣微晶玻璃中,微晶相的出现可有效提高微晶玻璃的力学性能,试样的显微硬度最高可达585.68 MPa,抗折强度最高可达126.21 MPa.     

The influence of the cooling rate of A 2PbO · SiO2 melt on the constitution of silicate anions

The kinetics of crystallization in a 2Pbo · SiO₂ melt have been investigated. A TTT-diagram was constructed, which describes the kinetic parameters for the formation of crystalline phases in the system. By means of silicate anion analysis the relationship between the cooling rates of the melt and the structure of silicate units in solid 2PbO · SiO₂ has been studied. Substantial differences in the thermal treatment of the melt lead to alterations of the silicate anion constitution, which cause changes in the crystallization behavior of glassy 2PbO · SiO₂.     

Non-isothermal crystallization kinetics of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>–CaO–SiO<Subscript>2</Subscript> glass containing nucleation agent P<Subscript>2</Subscript>O<Subscript>5</Subscript>/TiO<Subscript>2</Subscript>

Fe₂O₃–CaO–SiO₂ glass ceramics containing nucleation agent P₂O₅/TiO₂ were prepared by sol-gel method. The samples were characterized by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The activation energy and kinetic parameters for crystallization of the samples were calculated by the Johnson-Mehi-Avrami (JMA) model and Augis-Bennett method according to the results of DSC. The results showed that the crystallization mechanism of Fe₂O₃–CaO–SiO₂ glass, whose non-isothermal kinetic parameter n = 2.3, was consistent with surface crystallization of the JMA model. The kinetics model function of Fe₂O₃–CaO–SiO₂ glass, f(α) = 2.3(1–α)[–ln(1–α)]^0.57, was also obtained. The addition of nucleation agent P₂O₅/TiO₂ could reduce the activation energy, which made the crystal growth modes change from onedimensional to three-dimensional.     

On the possibility of devitrification of ancient glass

The question whether some crystallization will take place in old glass during long-time storage (up to 1000 years) at low temperatures well below T_g is considered theoretically. Evaluations for the systems Na₂O · SiO₂ and Li₂O · 2SiO₂ are carried out using the model of homogeneous and heterogeneous nucleation theory. For these systems a barely observable crystallized volume fraction will be reached at 300 K only after storage for astronomical periods (10⁶ ? 10¹³ years), depending on the number of heterogeneities and the contact angle of heterogeneity. If the observed strong influence of H₂O content in glass on crystallization is also taken into account, crystallization will not take place for at least 1000 years of storage at 300 K. This limiting value is close to archaeological periods and will therefore merit careful attention in further investigations.     

晶核剂P2O5对CaO-Al2O3-SiO2系黄磷炉渣微晶玻璃析晶的影响

采用熔融法以自然冷却黄磷炉渣为主要原料,P2O5为晶核剂,制备了黄磷炉渣微晶玻璃.利用X射线衍射仪(XRD)、差热分析(DTA)和扫描电子显微镜(SEM)等分析技术手段,探究了晶核剂P2O5(以KH2PO4的形式引入)对黄磷炉渣微晶玻璃晶化行为及物化性能的影响规律.结果表明:黄磷炉渣基础玻璃的析晶峰温度Tp及析晶活化能E随着P2O5晶核剂的增加呈现先减小后增大的趋势;当晶核剂P2O5加入量达到4wt;时,黄磷炉渣基础玻璃的析晶峰温度Tp及析晶活化能E最小,析晶效果最优,物化性能最优;主晶相硅灰石(CaSiO3)并不随着晶核剂P2O5加入量的增大而发生改变,同时能够促进晶相氟磷灰石(Ca5(PQ)3F)生成.     

Chemical durability and weathering resistance of canasite based glass and glass-ceramics

Classic composition 8.4Na₂O·5K₂O·10.8CaO·64SiO₂·10.5CaF₂·1.3Al₂O₃ (G1/GC1) and high silicon composition 7.6Na₂O·4K₂O·8.4CaO·71SiO₂·8CaF₂·1.0Al₂O₃ (G2/GC2) canasite-based glass and glass-ceramics were prepared, and the chemical durability and weathering of samples were studied with XRD, ICP-AES, SEM and optical microscopy. Interestingly, a kind of color fringe pattern caused by the acid leaching was directly observed on the glass ceramic surface under optical microscopy. The 20 day weight losses of glass and glass ceramic in acid (1 M HCl) and alkali (1 M NaOH/1 M Na₂CO₃) solution were measured. Accelerated weathering was used to demonstrate that increasing silicon content contributes to the weathering performance of glass and glass-ceramics. For different micro-structures and compositions, the weight loss of each glass and glass-ceramic is quite different. In general, through increasing the network interconnectivity of residual glass network and suppressing the crystallization of the less durable canasite phase, the addition of SiO₂ (from 60 mol% to 71 mol%) enhanced the chemical durability of canasite-based glass and glass ceramic relatively under acid, alkali and weathering conditions.     

Effect of perovskite phase precipitation on viscosity of Ti-bearing blast furnace slag under the dynamic oxidation condition

The effects of the dynamic oxidation on the viscosity of Ti-bearing blast furnace slag, on a selective enrichment, a precipitation and a growth of the Ti component were studied. The oxygen source was the air that was blown into the molten slag through a lance during the dynamic oxidation process. It was found that the oxidation of TiC, metallic Fe, TiO and Ti₂O₃ in the slag was not only in favor of the decrease of the viscosity of the slag, but also for the selective enrichment of the Ti component into the perovskite phase as well as the precipitation and growth of the perovskite phase. As oxidation time increases, the precipitation and growth of the perovskite phase rapidly increased, and the slag changed from the melt to a mixture of the melt and crystalline phase(s), that caused the viscosity of the oxidized slag for 12 min increased rapidly.     

Continuous compositional changes of crystal and liquid during crystallization of a sodium calcium silicate glass

This paper deals with a systematic study of crystal nucleation and growth kinetics in a 14.6Na₂O–34.0CaO–51.4SiO₂ mol% glass, which is close to the CaO · SiO₂–Na₂O · SiO₂ pseudo-binary section, just left of the stoichiometric Na₂O · 2CaO · 3SiO₂ (N₁C₂S₃) compound. We show that crystallization begins with nucleation of a Na_4+2xCa_4−x[Si₆O₁₈] (0 < x < 1) solid solution that is enriched in sodium as compared with both parent glass and the N₁C₂S₃ compound; while a fully crystallized sample is composed only by a solid solution that is stable at very high temperatures, but is metastable in the temperatures under investigation. We thus confirm a continuous compositional change of the crystals during the course of crystallization.     

Transparent phosphosilicate glasses containing crystals formed during cooling of melts

Effect of P₂O₅–SiO₂ substitution on spontaneous crystallization of SiO₂–Al₂O₃–P₂O₅–Na₂O–MgO melts during cooling was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and rotation viscometry. Results show that addition of P₂O₅ leads to amorphous phase separation (APS), i.e., phosphate- and silicate-rich phases. It is due to the tendency of Mg²⁺ to form [MgO₄] linking with [SiO₄]. Molar substitution of P₂O₅ for SiO₂ enhances the network polymerization of silicate-rich phase in the melts, and thereby the spontaneous crystallization of cubic Na₂MgSiO₄ is also enhanced during cooling of the melts. In addition, the sizes of the local crystalline and separated glassy domains are smaller than the wavelength of the visible light, and this leads to the transparency of the obtained glasses containing crystals.     

Crystallization Kinetics of Silicalite-1: Use of Tetrapropylammonium Bromide and Chloride as Templates

Silicalite-1, the aluminum-free end number of the ZSM-5 zeolites, was synthesized from a batch composition of 3.25 Na₂O · 40.0 SiO₂ · 552 H₂O · 2.00 TPA by using tetrapropylammonium bromide (TPA-Br) or chloride (TPA-Cl) as templates in the temperature range of 100 to 175 °C in a batch system. Conversion of silica in the starting batch composition into silicalite-1 in the product was followed quantitatively. The activation energies of nucleation and crystallization were determined as 37.2 and 66.5 kJ/mol, respectively. The use of TPA-Cl as the template instead of TPA-Br results in longer induction period for crystallization to start and a larger crystal size in product.     

Rate of homogeneous nucleation in alkali disilicate glasses

Rates of crystal nucleation in alkali disilicate glasses were measured by counting the number of crystals under an optical microscope. The viscosities of these glasses were measured by the method of beam-bending and penetration. Using the data of rate of nucleation and viscosity obtained in the present study and the data of free energy measured by Takahashi and Yoshio, crystal-glass interfacial energies for alkali disilicate systems were estimated on the basis of homogeneous nucleation theory as follows: 196 erg/cm² for Li₂O·2SiO₂, 126–144 erg/cm² for Na₂O·2SiO₂ and 88–104 erg/cm² for K₂O·2SiO₂. The effects of the viscosity of glass, the free energy difference between crystal and glass and crystal-glass interfacial energy on the rate of nucleation were discussed, and the remarkably higher rate of crystal nucleation in the Li₂O·2SiO₂ glass was attributed to the larger free energy difference.