1,720,972 research outputs found
A kinetic study of the quartz-cristobalite phase transition
No full textCristobalite is a common silica polymorph in ceramics, as it can crystallize in SiO2-rich systems during high temp. processes. Its occurrence in final traditional ceramic bodies remarkably affects their thermal expansion, thus playing an important role in the shrinkage upon cooling. The quartz-cristobalite transformation kinetics is investigated by in-situ isothermal X-ray powder diffraction expts. and then correlated to the av. particle size (〈d〉) of the starting quartz using a model here developed. An Avrami-like rate equation, i.e. α(t) = 1 - exp(- k × t)n, in which the n-term is assumed to account for the dependence on the av. particle size, has provided the best fitting of theor. to exptl. data, yielding activation energy values that range from 181 to 234 kJ mol-1, and exponential n-coeffs. from 0.9 to 1.5. Ex-situ observations have demonstrated that the formation of cristobalite from quartz after 50 min, 2, 4 and 6 h at 1200 and 1300 °C, exhibits a remarkable dependence on 〈d〉 of quartz, showing comparable behaviors in the case of 〈d〉 equal to 15.8 and 28.4 μm, but significant differences for 〈d〉 of 4.1 μm. The formation of cristobalite is boosted remarkably at temp. higher than 1200 °C, with an increase by wt. even of 500%, with respect to its content at lower temp. The method of sample prepn. (dry powder, wet powder and tablet of compressed dry powder) seems to influence the results only at temp. > 1200 °C and in the case of fine powder
Influence of composition on some industrially relevant properties of traditional sanitary-ware glaze
No full textTwo series of glazes have been produced from different combinations of the same raw materials in the range of interest for sanitary-ware applications: they are designed to allow one to get insight into network-forming and network-modifying species. Fusibility tests and hot stage microscope observations show the influence of even low differences in the starting chemical compositions on the transformation temperatures. X-ray powder diffraction, wavelength dispersion spectrometry and scanning electron microscopy prove that: (i) zircon, the most abundant crystalline phase, is homogeneously distributed and decreases by a 3% from its starting value; (ii) the glass-phase of glaze has a quasi-uniform composition. X-ray synchrotron radiation micro-tomography shows that glaze porosity is 15% by volume, and voids are prevalently not interconnected and with size up to 50 μm. The linear thermal expansion of the glass phase of glaze ranges between 6 and 7 × 10 -6 °C -1, without apparent correlation with composition
Effects of soda-lime-silica waste glass on mullite formation kinetics and micro-structures development in vitreous ceramics
No full textThe effects of soda-lime waste glass, from the recovery of bottle glass cullet, in partial replacement of Na-feldspar for sanitary-ware ceramic production are discussed. Attention is paid to the mullite growth kinetics and to the macroscopic properties of the final output, the latter ones depending on the developed micro-structures and vitrification grade. Measurements have been performed by in situ high temperature X-ray powder diffraction, scanning electron microscopy, thermal dilatometry, water absorption and mechanical testing. Glass substituting feldspar from 30 to 50wt% allows one (i) to accelerate the mullite growth reaction kinetics, and (ii) to achieve macroscopic features of the ceramic output that comply with the latest technical requirements. The introduction of waste glass leads to (i) a general saving of fuel and reduction of the CO2-emissions during the firing stage, (ii) a preservation of mineral resources in terms of feldspars, and (iii) an efficient management of the bottle glass refuse by readdressing a part of it in the sanitary-ware manufacturing. •The replacement of feldspar with glass cullet in the sanitary-ware ceramic sector is considered.•The kinetics of mullite and the study of the microstructure of blends bearing glass are performed.•Glass accelerates the kinetics of mullite growth.•Ceramics bearing glass have mechanical features similar to the traditional ceramics
Control of the amorphous content in traditional ceramics by means of alternative fluxing agents
No full textThe role of KOH and NaOH as mineralisers in kaolinite-quartz (KQ) and kaolinite-quartz-feldspar (KQF) systems has been investigated, in order to determine how they affect the phase composition of ceramic bodies, and if they can replace traditional flux to boost formation of amorphous phases. Attention was paid to the effects induced by mineralisers on (i) the fundamental ceramic equilibrium reaction between crystal phases and amorphous phase, and (ii) on the morphology of the final ceramic product in terms of secondary mullite habit. NaOH and KOH turn out to be amorphous content boosters, but their efficiency to promote amorphous phases is related to quartz particle size, too. The amorphous component appears at the expenses of mullite and quartz, and an analysis of the formation Gibbs energy demonstrates that an increase in mineraliser content shifts the fundamental ceramic equilibrium reaction towards amorphous formation, though Na differs from K in affecting such transformation
Sanitary-ware vitreous body characterization method by optical microscopy, elemental maps, image processing and X-ray powder diffraction
No full textAbstract
The present investigation deals with a characterization method for fired body (cast piece) from a standard sanitary-ware industrial cycle, relying
upon data mainly from optical/electron microscopy and chemical mapping. Image processing techniques have been used to reconstruct the phasedistribution
throughout the body, so as to provide a phase-arrangement-sensitive description of the crystalline and amorphous components. The
results so attained are compared with those from classical X-ray powder diffraction in terms of total phase-contents. The amorphous phase-content
and its distribution can be determined by optical microscopy, and the modest level of precision of the phase quantification is improvable by
accounting for the occurrence of voids. Residual quartz is identified by means of the backscattered electron contrast differences in images, and
thereby its morphologic properties and amount are determined. The use of elemental X-ray maps allows one to attain a description of the local
composition and, on the basis of some assumptions, to quantify microcrystalline mullite and also discriminating between primary versus secondary
mullite
Effects of particle size distribution and starting phase composition in Na-feldspar/kaolinite system at high temperature
No full textMullite-glass Gibbs energy of formation (ΔGeff), micro-texture and phase composition evolution are investigated in the Na-feldspar (F) and kaolinite (K) system, over the 1240-1320 °C interval, as a function of the starting F/K ratio by weight and particle size distribution of F (), using scanning electron microscopy, X-ray diffraction and thermodynamic modeling. Electron microscopy images show that size and aspect ratio of primary and secondary mullite have their largest figures for
the smallest and, in general, monotonically increase upon firing temperature. ΔGeff has been modeled by α()×(F/K)2+ß()×F/K+γ()(α,ß and γ are linear functions of ). The parameters of such a function have been determined by fitting it to the experimental ΔGeffs, inferred from quantitative phase analysis of X-ray diffraction patterns. We have gathered that (i) F/K affects energetics and mullite content more markedly than does, and (ii) the mullite formation is
energetically favored by decreasing F/K and increasing
Na-feldspar (F) and kaolinite (K) system at high temperature: Resulting phase composition, micro-structural features and mullite-glass Gibbs energy of formation, as a function of F/K ratio and kaolinite crystallinity
No full textThe system Na-feldspar (F) and kaolinite (K) was investigated at temperatures of interest in ceramic applications (1200-1280 degrees C) to study the effects of F/K ratios by weight and crystallinity degree of kaolinite on the final product, micro-structural features and mullite-glass Gibbs energy of formation (Delta G(eff)). Mullite and glass are the dominant phases; in general, the higher the temperature, the larger the former. An F/K increase promotes the formation of glass and secondary mullite, appearing along with the primary one. Delta G(eff) was modelled by alpha(T) x (F/K)(2) + beta(T) x F/K + gamma(T), alpha, beta and gamma being linear functions of temperature whose coefficients were determined by fitting the Delta G(eff)-theoretical to the Delta G(eff)-obtained from the measured phase compositions. Delta G(eff) is less affected by temperature than by F/K, whose increase shifts equilibrium towards glass phases. The Delta G(eff)-curves for ordered and disordered kaolinite intersect one another at F/K similar to 0.5, a ratio close to that used in industrial practice
Feldspar and firing cycle effects on the evolution of sanitary-ware vitreous body
No full textThis work is focused on the study of macroscopic and microscopic properties of traditional sanitary-ware vitreous bodies as a function of feldspar flux and firing time-temperature profile, using a fixed slip formulation (50 wt.% clay, 25 wt.% quartz and 25 wt.% feldspar). Two flux particle sizes (45 and 75 μm), three flux compositions (Na-based feldspar, K-based feldspar and a mix of them) and three firing cycles with the same soaking temperature (i.e. 1240 C) have been combined to evaluate their effects on the relevant industrial properties of water absorption and thermal expansion. The micro-scale observables, phase composition and micro-morphology, have also been investigated. Despite a general similarity exhibited by the ceramic samples, qualitative and quantitative differences in terms of feldspar dissociation temperature, phase-composition and densification trends have been observed. In particular, for a fixed firing cycle, the combination of the sodium based feldspar with the smallest flux particle size leads systematically to a water absorption value that is below the 0.5 target value and to a glass amount that approaches 70 wt.%. Thermal expansion coefficients below the quartz α-β transition are found in the 6.2-6.9×10-6 C-1 range; the highest values seem to be favoured by incorporation of potassium based
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