Pranesh B. Aswath

Role of mineralizers on the hexacelsian to celsian transformation in the barium aluminosilicate (BAS) system

Abstract The mechanism of additive-enhanced kinetics of the hexacelsian–celsian transformation in BAS was assessed by doping various mineralizers having different cationic radii and valence. The semi-quantitative data of XRD analysis indicate that an ionic valence of +3 is less efficient and ionic radii ranging from 0.68 to 1.12 A are more effective in promoting celsian formation. Insights about the sites in which the doped cations reside are obtained by examining the possible defect structures developed in the mineralizer-containing hexacelsian. Oxygen- and barium-vacancies defects induced by the addition of mineralizers in BAS could assist the breaking of BaO or (Al, Si)O bonds and promote the polymorphic transformation.

Oxidation of TiAl based intermetallics

The high temperature oxidation behaviour of the binary and ternary alloys of the Ti–48Al system was studied at different temperatures. The primary objectives of this work were the establishment of the activation energies, the migration tendencies of the alloy species, mechanism of oxidation and chemistry of the oxide scales. The ternary additions were Cr (1.5 at 19%), V (2.2 at%), W (0.2 at%) and Mn (1.4 at%). The addition of ternary additions did not play a significant role in the oxidation behaviour at 704°C. At 815°C the alloys with Cr and V exhibited linear oxidation behaviour with large weight gains while the base Ti–48Al alloys exhibited the best behaviour. At 982°C the Mn-containing alloy was the worst, exhibiting a linear oxidation behaviour while the alloy with V and W and the base alloy with 400 p.p.m. oxygen exhibited the best oxidation behaviour. At 982°C the outermost oxide layer in contact with air is always near stoichiometric TiO2. In all the alloys a layer of porosity is created just below the outer TiO2 layer by the Kirkendall mechanism due to the rapid outward diffusion of Ti atoms. The addition of trivalent atoms like Cr in small amounts appear to be detrimental to the oxidation process as they can generate additional oxygen vacancies while the addition of atoms with valence of 5, such as V, and 6, such as W, appear to have beneficial effect on the oxidation behaviour at 982°C by tying up oxygen vacancies.

Combinatorial effect of Si4 +, Ca2 +, and Mg2 +released from bioactive glasses on osteoblast osteocalcin expression and biomineralization

Osteocalcin (OCN) expression is an essential osteogenic marker of successful bone regeneration therapies. This study hypothesizes that Si(4+) and Ca(2+) combinatorial released by bioactive glass enhance osteoblast biomineralization through up-regulation of OCN expression; and Mg(2+) release delays such enhancement. Osteoblasts (MC3T3-E1) were treated with ionic products of bioactive glass dissolution (6P53-b experimental bioactive glass and 45S5 commercial Bioglass™). Results showed that gene expressions, including OCN and its up-stream transcription factors (Runx2, ATF4, MSX1, SP7/OSX), growth factors and signaling proteins (BMP2, BMP6, SMAD3), were enhanced in both 45S5 and 6P53-b glass conditioned mediums (GCMs). This up-regulation led to enhanced mineral formation by 45S5 glass conditioned mediums ([GCM], Si(4+)+Ca(2+)) after 20 days, and by 45S5 GCM and 6P53-b GCM (Si(4+)+Ca(2+)+Mg(2+)) after 30 days. In examining the extracellular matrix generated by cells when exposed to each GCM, it was found that 45S5 GCM had slightly elevated levels of mineral content within ECM as compared to 6P53-b GCM after 30 days while control treatments exhibited no mineral content. The formation of well-defined mineralized nodules (distinct PO4(3-) [960 cm(-1)] and CO3(2-) [1072 cm(-1)] peaks from Raman Spectra) was observed for each GCM as the soluble glass content increased. In examining the individual and combined ion effects between Si(4+), Ca(2+), and Mg(2+), it was found Mg(2+) down-regulates OCN expression. Thus, ions released from both 45S5 and 6P53-b bioactive glasses up-regulate OCN expression and biomineralization while 6P53-b GCM Mg(2+) release down-regulated OCN expression and delayed osteoblast biomineralization. These results indicate that Si(4+), Ca(2+), and Mg(2+) combinatorially regulate osteoblast OCN expression and biomineralization.

Microstructure development and fracture of in-situ reinforced Ti-8.5Al-1B-1Si

The conventional titanium alloys like Ti-6Al-4V are not suitable for application at high temperatures due to their metastable microstructure, high oxidation rates and loss of strength. In comparison, the {alpha} + {alpha}{sub 2} alloys have been studied for elevated temperature properties. Research on titanium alloys have of late included synthesis through the rapid solidification processing (RSP) route. The use of RSP allows the incorporation of B and Si in Ti alloys which allow for the in-situ nucleation of needle shaped borides and spherical silicides as dispersoids. This contrasts with the heterogeneous distribution of coarse dispersoids with conventional melting techniques. It has also been shown that these reinforcements lead to improvements in modulus, high temperature strength and creep resistance. Very limited information is presently available on the elevated temperature fracture properties of in-situ formed titanium composites. It is the object of this study to elucidate the micromechanisms of fracture under monotonic loading.

Microstructural stability, microhardness and oxidation behaviour of in situ reinforced Ti–8.5Al–1B–1Si (wt%)

Microstructural stability, microhardness and oxidation behaviour of an in situ reinforced Ti–8.5Al–1B–1Si (wt%) alloy was examined in both air and argon environments. When exposed for up to 5760 min at temperatures below the α/α+α2 transius, hardening occurred in both air and argon environments. The increase in hardness in the air heat-treated samples is attributed to a combination of solid-solution strengthening due to the oxygen and the precipitation of the α2 phase, while the increase in hardness in the argon heat-treated samples is primarily due to the precipitation of the α2 phase. On the other hand, when heat treated above the α/α+α2 transius, after an initial increase in hardness there is a drop in hardness which is attributed due to elimination of the α2 phase and a decreased contribution of boron and silicon in the matrix towards the solid-solution strengthening by virtue of coarsening of the TiSi2 and TiB reinforcements. Oxidation of the alloys follows a parabolic oxidation law when oxidized both in an environment of flowing air and static air with the primary oxidation product being TiO2. The activation energy for oxidation is 200 kJ mol-1 in an environment of flowing air and 303 kJ mol-1 in static air. The difference in activation energy arises from the difference in the availability of oxygen at the reaction front in the two cases.

Synthesis of polylactic acid–polyglycolic acid blends using microwave radiation

Degradation rates of a copolymeric PLGA can be controlled by varying the constituent amount in the copolymer. In the present study we have made an attempt to utilize microwave irradiation to blend PLLA and PGA in different concentrations. FTIR, NMR and DSC measurements clearly show the blending and cross-linking between the constituents.

Oxidation of ductile particle reinforced Ti-48Al composite

The oxidation behavior of a ductile TiNb particle reinforced Ti-48A1 (at. pct) system was studied at different temperatures. The objective of this work was to establish the role played by the TiNb reinforcement on the oxidation process. The activation energy for oxidation of Ti-48A1 was 324 kJ/mol and that of Ti-48Al-20 vol pct TiNb was 315 kJ/mol. The kinetics of oxidation of both the material systems are similar. At 982 °C, the outermost oxide layer in contact with air was always near stoichiometric TiO2. In all the alloys, a layer of porosity was created just below the outer TiO2 layer by the Kirkendall mechanism as a result of the rapid outward dif-fusion of Ti atoms. TiNb particles are more oxidation resistant than Ti-48A1. The boundary of the TiNb particles was oxidized to form TiO2. However, most of the TiNb particles were un-touched and existed as islands embedded within the Ti and Al oxides formed by oxidation of Ti-48A1. Oxidation in pure oxygen lowers the oxidation kinetics, as compared to oxidation in pure air for identical gas flow rates at 982 °C.

Synthesis and properties of in situ Si3N4-reinforced BaO·Al2O3·2SiO2 ceramic matrix composites

Silicon nitride (94.5% α, 5.5% β), BaCO3, Al2O3, and SiO2 powders were mixed and pressureless sintered to produce a ceramic matrix composite consisting of 30 vol% barium aluminosilicate (BaO·Al2O3·2SiO2 or BAS) matrix reinforced with in situ grown whiskers of β-Si3N4. In situ X-ray studies of the reactions indicated that BaCO3 decomposes first to yield BaO which reacts with SiO2 to yield a series of barium silicates which then react with Al2O3 between 950 and 1300°C to yield hexacelsian BAS. The sintering times were varied in order to develop a material system that combines the favourable properties of BAS with the high strength of Si3N4. In situ high-temperature X-ray studies after composite processing did not reveal any changes in the BAS or Si3N4 up to temperatures of 1300°C. Dilatometry studies of the sintered composite indicated a low-temperature transformation between 230 and 260°C with the temperature of transformation and volume change associated with the hexagonal to orthorhombic transformation decreasing with an increase of sintering time. Room- and high-temperature (1400°C) strengths were evaluated using four-point bend flexural tests. Composites exhibited near theoretical densities and an increase in flexural strength that was primarily dependent on the higher α- to β-Si3N4 transformation.

Al2O3/Al particle-reinforced aluminum matrix composite by displacement reaction

The development of a novel Al matrix composite is described based on a simple displacement reaction when an SiO 2 particulate preform is brought into contact with liquid Al at temperatures between 1273 and 1373 K. This interaction leads to the wetting of the SiO 2 particles by Al and its eventual transformation to a composite with Al 2 O 3 /Al particles in an Al matrix. Infiltration of the preform as induced by this reaction takes place with the simultaneous formation of the Al 2 O 3 /Al particles in situ . Synthesis of engineered multiphase composites, wherein reinforcements of other materials incorporated into the preform and reacted with liquid Al, is also presented.

Kinetics of the hexacelsian to celsian transformation in barium aluminosilicates doped with CaO

Pre-synthesized hexacelsian powders doped with CaO were held isothermally at different temperatures between 1200 and 1275°C to study, using X-ray diffraction, the kinetics of the hexacelsian-to-celsian transformation in barium aluminosilicate. Semi-quantitative analysis was used to determine the extent of the transformation. Values of rate constant, k, and Avrami exponent, n, for the transformation at various temperatures were evaluated using the Johnson–Mehl–Avrami equation. The average Avrami exponent of 1.0 and the lower activation energy of 45.4 kJ/mol for the CaO-enhanced transformation might suggest that the presence of mineralizer like CaO could promote the formation of celsian nuclei with a plate-like shape on basal planes in the hexacelsian structure.