Yaseen Iqbal

Influence of mixing on mullite formation in porcelain

Various forms of mullite occur in porcelains. Primary mullite forms from decomposition of pure clay. Secondary mullite forms from reaction of feldspar and clay and feldpar, clay and quartz. Which of these forms is strongly dependent on the extent of mixing of the body raw materials. Additionally, in aluminous porcelains tertiary mullite may precipitate from alumina-rich liquid obtained by dissolution of alumina filler. The size, shape and composition of the mullite crystals is controlled by the fluidity of the local liquid matrix from which they precipitate and grow, which itself is largely determined by the extent of mixing of the original powder batch.

Metastable phase formation in the early stage crystallisation of lithium disilicate glass

Abstract Early stage crystallisation behaviour of binary lithium disilicate [LS 2 ] glass containing 33.91 mol% Li 2 O has been investigated. Two metastable crystalline phases, α′- and β′-LS 2 precipitated prior to crystallisation of stable lithium disilicate in samples heat treated at 454±1°C for 120–480 h. Metastable α′- and β′-LS 2 phases transformed to stable LS 2 after extended holds (∼551 h) at 454°C. The broader 29 Si MAS-NMR crystalline peaks in samples nucleated for >228 h at 454°C, than those of samples crystallised at 600°C, indicate the presence of multiple polymorphs or a disordered structure in the former samples. The streaking of some of the spots observed in electron diffraction patterns from β′-LS 2 crystals is consistent with this observation.

Crystal nucleation in P2O5-doped lithium disilicate glasses

The bulk (volume) crystallisation of Li2O·2SiO2 (LS2) glasses with additions of 1, 2 and 5 mol % P2O5 (G1P, G2P and G5P) was investigated. In the G1P and G2P glasses heated at 454°C metastable α′- and β′-LS2 crystal phases formed initially and only stable LS2 at later stages. In G1P heated at 463°C transmission electron microscopy (TEM) revealed particles (≥2 μm) of stable LS2 but also smaller (<0.1 μm) crystalline particles which were unstable under the electron beam and could not be identified. Crystalline lithium orthophosphate (Li3PO4) was detected by XRD and 31P magic angle spinning nuclear magnetic resonance (MAS-NMR) in both as-quenched and heat treated G5P but not in G1P and G2P. In G5P heated at 476°C, metastable α′-LS2 phase initially formed and at later times stable LS2. In as-quenched G5P, a trace of lithium metasilicate (LS) was observed. Li3PO4 crystals were not detected in G5P by TEM but a marked increase in nucleation rate due to P2O5 addition was observed. 29Si MAS-NMR revealed amorphous Q2 (LS) and Q4 (SiO2) species in an average Q3 (LS2) environment in the as-quenched glasses. Increased P2O5 concentration caused a greater degree of amorphous phase separation. 31P MAS-NMR showed amorphous lithium phosphate units in the as-quenched and nucleated G1P and G2P. The presence of crystalline Li3PO4 phase in glass G5P suggests that Li3PO4 crystals may act as sites for heterogeneous nucleation of stable lithium disilicate.

Crystallisation of silicate and phosphate glasses

Abstract An overview is given of recent studies of crystal nucleation and growth in silicate and phosphate based glasses, emphasising microstructural characterisation. The following topics are discussed: nucleation kinetics in silicate systems, comparisons with classical theory, and new evidence in the early stages of crystallisation of Li 2 O · 2SiO 2 glass for the appearance of previously unreported metastable precursor phases, probably crystalline polymorphs of lithium disilicate; the role of P 2 O 5 as a nucleating agent in lithia—silica glasses; volume nucleation and glass ceramic formation in CaOP 2 O 5 based glasses, particularly the roles of additions of Al 2 O 3 . TiO 2 , SiO 2 and B 2 O 3 ; crystallisation of aluminosilicate glasses including celsian and barium osumilite compositions, and marked differences in crystallisation between melt-derived and sol—gel derived compositions.

Non-equilibrium microstructure of bone china

Abstract Commercial bone china microstructures are far from equilibrium consisting of distinct regions of lath-like (0·4–10 μm) anorthite and spheroidal (1–3 μm) β-tricalcium phosphate (β-TCP) nodules embedded in a heterogeneous composition glass along with isolated irregular (≤30 μm) α-quartz crystals. The composition and morphology of the phases formed on firing suggest that anorthite crystallised in clay relicts by the reaction of metakaolin with CaO, β-TCP crystallised from decomposition of bone ash, and that the liquid formed on vitrification has variable composition depending on the composition of adjacent phases. P2 O5was never detected in the glass suggesting that any P2 O5 that dissolves in the liquid on firing is transient. The cracks sometimes observed in and around β-TCP clusters arise from thermal expansion mismatch between β-TCP and the surrounding glass and anorthite.

Research trends in microwave dielectrics and factors affecting their properties: A review

Abstract Ceramists are constantly looking for materials to be used as dielectric resonators in the telecommunication industry. These applications require materials with ∊r ∼ 4 – 120, Q × f0 > 10 000 GHz and τf ∼ 0 ppm K−1. Additionally, efforts are also underway to lower the sintering temperatures (e. g. ≤ 800 °C) to reduce processing and electrode costs. The simultaneous achievement of all the three properties mentioned above is difficult; nevertheless, some materials have been synthesized fulfilling the criteria for microwave applications. This study is an overview of various studies on materials for possible applications as microwave dielectrics and factors affecting their microwave properties. These factors include crystal structure, defects, fabrication route, and the type and concentration of substituents and additives.

Microstructural evolution in bone china

Abstract Phase and microstructural evolution in model bone china bodies was determined by XRD and electron microscopy of quenched samples fired for 3 h at 600–1500°C. Unfired but shaped bone china comprised bone ash and clay agglomerates (≤70 μm) in a matrix of smaller (from submicron to 10 μm) mixed clay, feldspar, and bone ash particles. The unfired microstructure and subsequent phase evolution is believed to be strongly dependent on the extent of prior mixing. On firing, the clay component dehydroxylated to metakaolin at ∼550°C. Metastable sanidine formed from decomposition of the feldspar component above 600°C and dissolved at 1100°C. The bone ash com ponent decomposed into β-TCP and lime (and/or Ca2+ and O2- ions) beginning at ∼800°C. CaO from the bone ash reacts with the clay decomposition products forming liquid and anorthite at ∼900°C. Liquid formation is due to reaction of CaO with feldspar and clay relict grains and is discussed in terms of the CaO–P2O5–Al2O3 ternary phase diagram. Above 1200°C pure bone ash relicts contained small (5–10 μm) β-TCP crystals, CaO penetrated clay relicts contained anorthite, and mixed clay–bone–feldspar regions contained both anorthite and larger (>50 μm) β-TCP crystals in calcium aluminosilicate glass. The major phase in the clay relicts was anorthite although a few elongated (∼100 nm) needles resembling mullite in composition and morphology also crystallised in samples fired to 1100°C and grew to ∼30 μm in length at 1300°C.

Unification of the negative electrocaloric effect in Bi1/2Na1/2TiO3-BaTiO3 solid solutions by Ba1/2Sr1/2TiO3 doping

The microscopic mechanisms of the negative electrocaloric effect (ECE) of the single-phase (1−x)(0.94Bi1/2Na1/2TiO3-0.06BaTiO3)-xBa1/2Sr1/2TiO3 (BNT-BT-BST) perovskite solid solutions fabricated via the sol-gel technique are explored in this study. Dielectric and mechanical relaxation analyses are employed to investigate the ferroelectric and structural transitions of the samples. The electrocaloric properties of the samples were measured by thermodynamics Maxwell relations. The difference between the depolarization temperature (Td) and the maximum dielectric constant temperature (Tm) was found to decrease with increasing BST content. Doping with BST stabilized the ferroelectric phase along with unifying the EC temperature changes (ΔT) to only negative values. The origin of the uniform negative ECE of BNT-BT-BST is discussed.

Influence of Cr additions in magnetic properties and crystallization process of amorphous iron based alloys

Amorphous Fe63.5Cr10Si13.5B9Cu1Nb3 ribbon samples have been prepared by melt spinning. The addition of Cr to a FINEMET-type alloy decreases the Curie temperature of the amorphous matrix and also modifies the alloy crystallization temperature. The evolution of the microstructure during crystallization has been analyzed by means of x-ray diffractometry and transmission electron microscopy. An interesting dependence of the intensity of the amorphous diffraction halo was observed for temperatures below 818 K, where a large fraction of glassy phase was still present in the sample. The evolution of the coercivity with annealing temperature for various measurement temperatures has also been studied and, in conjunction with the microstructural observations, is interpreted within the framework of the two-phase theory for nanocrystalline materials.

Early stages of crystallisation of lithium disilicate glasses containing P2O5 : An NMR study

The 29 Si nuclear magnetic resonance (NMR) spectra of glasses with a 1:2 molar ratio of Li 2 O : SiO 2 , containing P 2 O 5 , showed contributions from Q 4 , Q 3 and Q 2 species, consistent with a disproportionated binary distribution and with removal of Li + from the silicate network to form lithium phosphate units. Crystallisation in the 0% P 2 O 5 glass was detected by X-ray diffraction before it was observed by nuclear magnetic resonance. The crystalline Q 3 resonance grew largely at the expense of the glassy Q 3 , the amounts of glassy Q 4 and Q 2 remaining constant. The width of the crystalline Q 3 resonance is related to the formation of metastable lithium disilicate phases which are precursors to the stable lithium disilicate. The presence of glass-in-glass phase separation, undetectable by transmission electron microscopy, was inferred from the chemical shift and relaxation time of the Q 4 species. Lithium disilicate was observed at shorter times, as the P 2 O 5 content was increased, and lithium metasilicate was also formed in glasses containing 2% or more P 2 O 5 . Crystalline Li 3 PO 4 was not observed prior to formation of the lithium disilicate.