Parag Bhargava

Chemical Synthesis, Characterization, and Biocompatibility Study of Hydroxyapatite/Chitosan Phosphate Nanocomposite for Bone Tissue Engineering Applications

A novel bioanalogue hydroxyapatite (HAp)/chitosan phosphate (CSP) nanocomposite has been synthesized by a solution-based chemical methodology with varying HAp contents from 10 to 60% (w/w). The interfacial bonding interaction between HAp and CSP has been investigated through Fourier transform infrared absorption spectra (FTIR) and x-ray diffraction (XRD). The surface morphology of the composite and the homogeneous dispersion of nanoparticles in the polymer matrix have been investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The mechanical properties of the composite are found to be improved significantly with increase in nanoparticle contents. Cytotoxicity test using murine L929 fibroblast confirms that the nanocomposite is cytocompatible. Primary murine osteoblast cell culture study proves that the nanocomposite is osteocompatible and highly in vitro osteogenic. The use of CSP promotes the homogeneous distribution of particles in the polymer matrix through its pendant phosphate groups along with particle-polymer interfacial interactions. The prepared HAp/CSP nanocomposite with uniform microstructure may be used in bone tissue engineering applications.

Shape forming of ceramics via gelcasting of aqueous particulate slurries

Gelcasting is a promising technique for shape forming of bulk dense or porous ceramic, metal structures. The process offers a number of advantages over processes such as slip casting, injection molding in forming complex ceramic shapes. It is shown here that the optimization of slurry rheology, choice of mold material, mold design and the drying conditions have a significant role in the overall success of the process. In this process, components of simple or complex shapes can be produced to near net shape by direct casting. If required complex shapes can also be produced by machining the green gelcast bodies. The process of gelcasting also has a lot of potential in forming highly porous ceramic shapes.

Machining behaviour of green gelcast ceramics

Abstract Green gelcast alumina samples containing binder and samples prepared in bisque fired conditions were subjected to three machining operations—drilling, grinding and milling. Forces were measured during grinding and milling. Drilling of binder-containing samples indicated rapid wear of HSS tools while carbide tools could be used without significant wear. Bisque fired samples required lower force for grinding. Tangential and normal forces during grinding increased rapidly with successive passes due to clogging of the wheel. Increase in forces was much more rapid for binder-containing samples and for higher depth of cut. The rise in grinding force with increase in binder content correlated with the observed increase in Vickers hardness number of the green samples. During milling the tangential force increased with slight wearing of the tool. Material removal during grinding was in the form of large binder-powder aggregates for binder-containing samples, in smaller powder particle clusters for bisque fired samples. During milling of binder-containing samples, the material came out in the form of long chips.

Effect of Additives on Ceramic Foam Microstructure Processed by Direct Foaming of Aqueous Slurries

Ceramic foams with porosity 95% or above could be prepared by controlling slurry rheology through additives in direct casting route. The additives used to control slurry rheology included sucrose and two different dispersants, Darvan 821 A (ammonium polyacrylate) and DBAC (dibasic ammonium citrate). In addition, slurry rheology could also be varied through variation in ceramic loading. Slurry rheology had a significant influence on the foaming behaviour resulting in sintered foams with varying microstructure and total porosity. Lower viscosity slurries foamed to a greater extent, leading to higher porosity in foams. DBAC based slurries foamed to greater extent, resulting in thinner cell walls and increased number of interconnections between the cells. Use of sucrose helped in strengthening of green ceramic foams enabling ease in handling and machining.

Effect of Process Parameters and Binder Concentration on Mechanical Properties of Phosphate Bonded Alumina

Alumina and alumina matrix composites typically require sintering temperature in excess of 1550°C. Utilization of aluminium phosphate bonding provides process flexibility and significantly lowers sintering temperature (500° to 1280°C). While phosphate bonded alumina (PBA) may have a lower sintering temperature, its limitations include the presence of porosity in pressure less sintered samples leading to lowered strength and phase instability above approximately 1200°C. The present study describes the effect of processing routes and binder concentration on density, hardness, Youngs modulus and transverse rupture strength (TRS) of PBA. In this study, ultra fine Al2O3 (particle size ≈0.5 µm) was reacted with H3PO4 to yield phosphate bond in a matrix of tabular Al2O3 (particle size ≈0.5 µm) after final sintering at 1280°C for 6 h. Green samples were compacted at pressures in the range of 96-290 MPa. The effect of aluminium phosphate binder concentration on mechanical properties of the PBA samples was measured at optimized pressure. The presence of different polymorphs of AlPO4 in the sintered product was confirmed by XRD.

Electrophoretic Deposition of 8YSZ on Lanthanum Strontium Manganite Substrates

Suspensions of 8 mol% yttria stabilized zirconia (YSZ) were prepared with ethanol as the dispersing medium. The suspensions were stabilized by a dispersant composed of fixed ratios of citric acid and triethylamine. Different quantities of the dispersants were used for a given amount of solid loading and the optimal amount was determined by measuring their zeta potential. Suspension stabilized by using the optimal dispersant amount was used for deposition on stainless steel substrates at different voltages for varying times. The optimal voltage and time was determined by observing obtained microstructures under SEM. Similar studies were then done for a lanthanum strontium manganite (LSM) substrate. The optimal composition of suspensions and the used parameters made it possible to obtain coatings with uniform thickness.

Tailoring porosity and pore characteristics in oxide ceramic foams through controlled processing

Processing routes to make oxide ceramic foams of porosity more than 70% with pore sizes in the range of 0.1–4 mm are reviewed. Microstructures and mainly characteristics of pores produced by incorporation of fugitives, infiltration through 3D-preforms, direct casting of foams are compared. Emphasis has been given to the versatile processes with use of natural additives, which allow in controlling microstructural features while improving the desired mechanical property. The critical step in setting of ceramic foam through direct casting foaming process, i.e. stabilization of bubbles during consolidation, is being highlighted emphatically with use of water soluble additives.

Simplified Aqueous Gelcasting of Silicon Carbide Ceramics

A new ceramic forming process, protein coagulation casting (PCC), has been applied for consolidation of green silicon carbide bodies. The process utilizes aqueous ceramic slurries based on egg white (ovalbumin). The silicon carbide powder was dispersed in an aqueous medium without any pH adjustment or addition of any dispersing agents. The ovalbumin based slurries could be gelled simply by heating without use of any chemical additives and it provided sufficient strength for handling the bodies. An addition of ovalbumin to aqueous silicon carbide slurries enhanced the viscosity and thus limited the solid loading. The ovalbumin containing slurries were thixotropic in nature. With the use of the present process, defect free near-net shaped silicon carbide bodies were prepared.

Stereological characterization of crack path transitions in ceramic matrix composites

All ceramic composites involve a mismatch in physical properties the extent of which differs from one composite to another. Mismatch in thermal expansion (Δα) and elastic modulus (ΔE) is known to produce stresses that influence the path of a propagating crack. Thus, the relative effect of thermal and elastic mismatch on the crack path is expected to change with change in stress intensity. We propose that the crack path in ceramic composites should undergo a transition with the crack being strongly influenced by the thermal mismatch stresses at low stress intensity and elastic mismatch stresses at high stress intensities. Thus, a material in use under different applications each with its own loading conditions is expected to exhibit different crack propagation tendencies which may be reflected in the υ-K characteristics of the composite material. In the present work several model composites with different combinations of thermal and elastic mismatch have been considered. Cracks propagating at different sub-critical stress intensities (velocities) were generated by a novel indentation technique. Each indentation was performed at a constant displacement rate and a peak load. A range of displacement rates were used to produce cracks propagating at different velocities. The indentations were made using a Vickers indentor fitted in a universal mechanical testing machine. The crack paths in composites were quantified by stereological technique and the proposed theory was verified.