D.K. Agrawal

Fabrication of transparent hydroxyapatite ceramics by ambient-pressure sintering

Abstract For the first time, transparent hydroxyapatite ceramics were fabricated by microwave processing as well as by conventional sintering at ambient pressure. This was achieved essentially using a fine crystalline hydroxyapatite powder synthesized hydrothermally as a starting material. The sintered hydroxyapatite ceramic was phase pure and the average grain size was around 0.2 μm.

Transparent mullite ceramics from diphasic aerogels by microwave and conventional processings

Transparent mullite ceramics were developed by both microwave and conventional sintering of compacts starting with a diphasic aerogel near 1300°C. Both sintering processes were carried out in air at ambient pressure. The conventionally sintered sample was essentially non-crystalline, whereas the microwave sintered sample was highly crystalline mullite. Using a xerogel of the same composition, no transparency was achieved under the same conditions. The results indicate that the agglomeration-free microstructure of the starting aerogel was the key for achieving the transparency. The achievement of transparent mullite ceramics by microwave processing is attributed to the rapid-heating, accelerated-mullitization, enhanced densification, and limited grain-growth of the diphasic mullite gel in the microwave field.

Microwave synthesis and sintering of Ba0.95Sr0.05TiO3

Abstract Paper reports synthesis and structural properties of Ba 0.95 Sr 0.05 TiO 3 (BST) system. The material powder was prepared by solid state route and sintering was carried out using conventional furnace and microwave oven. XRD, SEM and thermal expansion studies were carried out for both the samples. It is found that the microwave sintered BST shows better densification than conventional furnace sintering, fine and uniform grain size, higher linear thermal expansion and improved dielectric properties. Possible mechanisms are explained.

Microwave synthesis and sintering of Ba(Mg1/3Ta2/3)O3

Abstract Ba(Mg 1/3 Ta 2/3 )O 3 , BMT, was synthesized and sintered by microwave processing at 2.45 GHz. Appropriate amounts of BaCO 3 , MgO, and non-stoichiometric Ta 2 O 5− x were used as starting materials for the synthesis. BMT was obtained by heating the precursors in a multimode microwave cavity of 2 kW between 900°C and 1300°C for just 20 min. The synthesis was carried out in N 2 atmosphere to prevent the oxidation of Ta 2 O 5− x . The perovskite BMT starts forming at 900°C and becomes single phase material at 1300°C. The sintering of BMT was carried out in a single mode microwave cavity for 5 to 30 min at 1400°C to 1600°C without using any sintering aid. A relative density of 97% was obtained in 30 min of microwave sintering at 1600°C. In contrast, it was found that to achieve the same densification, it requires 6 h of soaking at the same temperature in conventional sintering process.

A simple laser speckle dilatometer for thermal expansion measurements

Abstract A simple laser speckle method utilizing a 4 mW HeNe laser for thermal expansion measurements is described. The method utilizes the rotation of a probe kept on the sample and reference rods. For 5 cm long sample and reference rods kept 0.5 cm apart, a measurement sensitivity of 5 × 10−6 for ΔL L can easily be obtained.

Phase, Dielectric and Ferroelectric Properties of Microwave Sintered La and Ca Modified BaTiO{sub 3} Ceramics

Lanthanum and calcium modified barium titanate, Ba(1−x)LaxTi(1−x/4)O3/BLT and Ba(1−x)CaxTiO3/BCT, where xu2009=u20090.02, ferroelectric ceramic samples were synthesized in single perovskite phase by microwave processing technique. Sintering temperature was optimized at 1100u2009°C for 1h. Presence of pore free micron size uniform grains suggested the advantage of using microwave sintering process. Transition temperature (Tc) decreases with the substitution of La content in BT system. Temperature coefficient of capacitance is negligible from RT temperature to 75u2009°C for BLT ceramic samples. Polarization vs. electric field (P‐E) hysteresis loop study confirms the ferroelectric nature of the modified BT ceramic samples.

Application of dial-gage and laser speckles for ultra-low thermal-expansion measurements using push-rod dilatometers

Abstract Conversion of conventional push-rod dilatometers for ultra-low thermal-expansion measurements is described. The method is based on the determination of the displacement of the probe-rod by ultra-precise dial-gage measurements by means of a laser-speckle technique. The movement of speckles created in space by the laser beam incident on an aluminum foil attached to the needle of the gage is observed for the measurements. It has been found that the sensitivity of the modified dilatometer is orders of magnitude higher than the existing LVDT approach.

Measurement of thermal expansion using a variable area slit method

Abstract The light reflected from a mirror on a probe, which is positioned on both sample and reference rods, is rotated by uneven thermal expansions of the two materials. A simple method is described to measure this rotation and hence the thermal expansion using a variable area slit and photodetector combination. The method is capable of determining very low thermal expansions of ceramic materials with high precision and accuracy. A measurement sensitivity of 10 -6 for Δ L / L can easily be obtained.