Roy Johnson

A new sintering aid for magnesium aluminate spinel

Abstract A dense MgAl 2 O 4 sintered spinel has been prepared following a conventional double stage firing process. A new type of sintering aid, AlCl 3 , can improve the bulk density, apparent porosity and water absorption of MgAl 2 O 4 spinel sintered at 1550°C devoid of any contamination. In order to see the effect of AlCl 3 on densification and formation of a MgAl 2 O 4 spinel, 0.01–0.03 mol% AlCl 3 was added separately prior to calcination, subsequent to calcination as well as simultaneously at both stages. AlCl 3 is found to be equally effective in enhancing spinel formation during calcination as well as in increasing sintered density. The efficacy of AlCl 3 as a “spinelizer” or as a sintering aid is also compared with the conventional mineralizer, AlF 3 , which is generally added only prior to calcination to enhance the spinel formation. AlCl 3 is found to be active as a sintering aid provided the powder contains some free Al 2 O 3 and MgO and probably the hygroscopic nature of AlCl 3 is responsible for its effectiveness as a sintering aid.

Thermal anisotropy in sintered cordierite monoliths

Cordierite honeycombs with oxide compositions of 49.5-51 wt.% SiO 2 , 35.5-36 wt.% Al 2 O 3 and 14-14.5 wt.% MgO were prepared from clay, talc, and alumina using conventional extrusion process. Effects of sintering parameters such as heating rate, soaking time, cooling rate as well as amount of calcined clay in the raw material compositions on the overall coefficient of thermal expansion (CTE) as well as thermal anisotropy in terms of CTE difference between the extrusion direction and across the extrusion direction of the cordierite honeycombs were studied. For this purpose, extruded honeycombs were sintered with various heating and cooling rates varying from 80 to 180°C h -1 up to a peak temperature of 1420°C for various soaking times ranging from 4 to 8 h. All the sintered cordierite honeycombs were characterized with respect to phase composition (XRD), morphology (SEM), bulk density, water absorption, apparent porosity, and CTE (dilatometric studies). XRD studies reveal that all the investigated sintered honeycombs contain >90% cordierite phase with smaller quantities of mullite, spinel and α-Al 2 O 3 . Raw materials compositions containing 60% calcined clay of total clay yielded crack free honeycombs with lower thermal anisotropy. Among the various parameters studied, honeycombs sintered with heating rate of 80°C h -1 , soaking time of 8 h and cooling rate of 180°C h -1 showed minimum thermal anisotropy.

Effect of rubber encapsulation on the comparative mechanical behaviour of ceramic honeycomb and foam

Abstract Cordierite–mullite honeycombs with square cells were prepared by an extrusion technique. Mechanical behaviour of the honeycomb was compared with that of commercial ceramic foam with and without rubber encapsulation. While impact testing, honeycombs as well as foam had shown low energy absorption, which was increased substantially upon rubber encapsulation in both cases. It has been found that the honeycomb absorbs significant amounts of energy under compression when loaded in the direction parallel to channel walls in comparison with when it is loaded in the perpendicular direction. Upon rubber encapsulation, honeycomb had shown substantial decrease in the absorbed energy parallel to channels, whereas, there was modest increase in the perpendicular direction. However, in both the orientations the critical stress was decreased upon rubber encapsulation. The foam on other hand absorbed significantly lower energy under compression in comparison with that of the bare honeycombs. Interestingly, upon rubber encapsulation, the foam had shown increased amounts of energy absorption as well as enhancement in the critical stress, contrary to the behaviour observed in the case of honeycombs. Failure mechanisms for the different types of configurations with and without rubber encapsulation are proposed in this paper.

Mineral-Oxide-Doped Aluminum Titanate Ceramics with Improved Thermomechanical Properties

Investigations were carried out, on the effect of addition of kaolinite (2Al2O3·3SiO2·2H2O) and talc (Mg3Si4O10(OH)2) in terms of bulk density, XRD phases, microstructure, as well as thermal and mechanical properties of the aluminium titanate (AT) ceramics. AT ceramics with additives have shown enhanced sinterability at 1550°C, achieving close to 99% of TD (theoretical density) in comparison to 87% TD, exhibited with pure AT samples sintered at 1600°C, and found to be in agreement with the microstructural observations. XRD phase analysis of samples with maximum densities resulted in pure AT phase with a shift in unit cell parameters suggesting the formation of solid solutions. TG-DSC study indicated a clear shift in AT formation temperature with talc addition. Sintered specimens exhibited significant reduction in linear thermal expansion values by 63% (0.4210−6/C, (30–1000°C)) with talc addition. Thermal hysteresis of talc-doped AT specimens showed a substantial increase in hysteresis area corresponding to enhanced microcrack densities which in turn was responsible to maintain the low expansion values. Microstructural evaluation revealed a sizable decrease in crack lengths and 200% increase in flexural strength with talc addition. Results are encouraging providing a stable formulation with substantially enhanced thermomechanical properties.

Comparative evaluation of electrical conductivity of hydroxyapatite ceramics densified through ramp and hold, spark plasma and post sinter Hot Isostatic Pressing routes

Hydroxyapatite ceramics synthesized through sonochemical route were processed and densified through ramp & hold (R&H) and Spark Plasma Sintering (SPS) routes. The effect of processing route on the relative density and electrical conductivity were studied. Further, the samples were Hot Isostatically Pressed (HIP) under argon pressure at elevated temperature to further densify the sample. All these samples processed under different conditions were characterized by X-ray diffraction, Scanning Electron Microscopy and AC Conductivity. The samples have exhibited hydroxyapatite phase; however, microstructures exhibited distinctly different grain morphologies and grain sizes. AC impedance spectroscopic measurement was carried out on hydroxyapatite samples processed through different routes and the corresponding spectra were analyzed by the analogy to equivalent circuit involving resistors and capacitors. SPS sintered sample after HIPing has exhibited the highest conductivity. This can be attributed to the higher density in combination with finer grain sizes. Activation energy based on Arrhenius equation is calculated and the prominent conduction mechanism is proposed.

Hot Isostatic Pressing of ZnS Powder and CVD ZnS Ceramics: Comparative Evaluation of Physico-chemical, Microstructural and Transmission Properties

Polycrystalline transparent zinc sulphide (ZnS) monoliths were processed through chemical vapour deposition (CVD) followed by hot isostatic pressing (HIP) at 975oC and 135 MPa pressure (samples were designated as CVDHIP). Physico-chemical, microstructural and transmisson properties of the CVDHIP specimens were evaluated. The results were compared with ZnS specimens processed through an alternative route of powder encapsulation and hot isostatic pressing under identical conditions (samples were designated as PHIP). CVDHIP and PHIP ZnS specimens exhibited close to theoretical densities of 4.086 and 4.079 g.cm–3 respectively. HIPing of CVD samples exhibited a highly preferred orientation along (111) in comparison to powder HIPed samples. Microstructures exhibited similar grain sizes for CVDHIP and PHIP ZnS specimens, however, the morphology of grains was different with significant twinning in CVDHIP specimens. Transmission properties of CVDHIP were relatively high in comparison to PHIP samples and also exhibited multispectral transmission properties in the wavelength range of 0.4-10 µm.

Transparent magnesium aluminate spinel: a prospective biomaterial for esthetic orthodontic brackets

Adult orthodontics is recently gaining popularity due to its importance in esthetics, oral and general health. However, none of the currently available alumina or zirconia based ceramic orthodontic brackets meet the esthetic demands of adult patients. Inherent hexagonal lattice structure and associated birefringence limits the visible light transmission in polycrystalline alumina and make them appear white and non transparent. Hence focus of the present study was to assess the feasibility of using magnesium aluminate (MgAl2O4) spinel; a member of the transparent ceramic family for esthetic orthodontic brackets. Transparent spinel specimens were developed from commercially available white spinel powder through colloidal shaping followed by pressureless sintering and hot isostatic pressing at optimum conditions of temperature and pressure. Samples were characterized for chemical composition, phases, density, hardness, flexural strength, fracture toughness and optical transmission. Biocompatibility was evaluated with in-vitro cell line experiments for cytotoxicity, apoptosis and genotoxicity. Results showed that transparent spinel samples had requisite physico-chemical, mechanical, optical and excellent biocompatibility for fabricating orthodontic brackets. Transparent spinel developed through this method demonstrated its possibility as a prospective biomaterial for developing esthetic orthodontic brackets.

Development of Cordierite Based Reticulated Foams with Improved Mechanical Properties for Porous Burner Applications

ABSTRACT Cordierite is a ternary oxide of silica, magnesia and alumina and one of the interesting ceramic materials and a potential material for various thermo-structural applications. The present study is to explore the possibility of enhancing the mechanical properties of cordierite using zirconia as an additive in the range of 0-10 wt% followed by comparative evaluation of properties. Cordierite precursor mix was subjected to calcination to ensure the partial formation of cordierite. A part of calcined cordierite and another two parts were blended with 5 and 10 wt% of zirconia and were planetary ball milled to ensure homogeneity in compositions. The formulations were compacted after granulation. The green compacts were subjected to TG-DSC and XRD investigations were carried out on sintered samples. Mechanical properties such as flexural strength at room and elevated temperatures and thermal expansion behaviour were analyzed for all the samples. Zirconia addition was found to enhance the mechanical properties; however, at higher concentration (10 wt%) segregation of zirconium silicate resulted in the reduction in flexural strength. Reticulated foams of cordierite and cordierite+5 wt% zirconia were fabricated by polymeric sponge replication process and evaluated for their properties. Further, initial trials as a porous combustion media demonstrated the feasibility for burner applications. GRAPHICAL ABSTRACT

Experimental Investigation on Flowability and Compaction Behavior of Spray Granulated Submicron Alumina Granules

Aqueous slurry with various solid loadings (up to 40 wt%) of alumina powder (D50 = 300 nm) with suitable rheological properties were spray dried into granules. Solid loading and feed rate of the slurry are found to have a prominent effect on the shape and size distribution of granules. Powder flow measurements exhibited a cohesive index of 28.45 signifying an extremely cohesive flow due to high surface area and irregular morphology. Finer sizes though it offers high geometrical surface area it leads to more surface contacts and hence, high interparticle friction. Spherical morphology achieved through optimum spray drying parameters significantly reduced the cohesive index to 6.45 indicating free flow behavior. Compaction studies of the spray-dried granules and corresponding plot of relative density versus compaction pressure revealed an agglomerate strength of 500 MPa followed by a plateau-like behavior reaching a maximum in the relative density of 59%-60% of the theoretical values.

Mechanical Behavior of Alumina based Reticulated Foams Encapsulated and Infiltrated with Polymer under Quasistatic and Dynamic Conditions

ABSTRACT Cellular ceramic structures, due to their unique combination of thermal, structural and mechanical properties, are widely explored for the wide range of thermo-structural applications. However, the ceramic based structures have not been well explored for energy absorption properties due to their inherent brittleness. In the present study, alumina based ceramic foams were fabricated by polymeric sponge replication process and further these foams were encapsulated and infiltrated respectively in epoxy resin. All the three foams – bare, encapsulated and infiltrated were evaluated comparatively under quasistatic compression and dynamic impact conditions. The study revealed that the energy absorption properties can be significantly enhanced through encapsulation and infiltration of bare foam with epoxy resin. Extent of enhancement in energy absorption in case of infiltration was superior in comparison to encapsulation in both static and dynamic test conditions. The study demonstrated the possibility of exploring the potential of encapsulated and infiltrated ceramic foams in various shock attenuating applications. GRAPHICAL ABSTRACT