Abdul-Majeed Azad

Characterization of BaSnO3-based ceramics: Part 1. Synthesis, processing and microstructural development

The compound BaSnO3 together with its Ca- and Sr-analogs, has recently been projected as potential electronic ceramic material (thermally stable capacitor, chemical sensor for humidity, CO and NOx, etc.). In order to fill the information gaps in the reported research, a vigorous and systematic investigation on these exotic materials has been initiated. A thorough study of BaSnO3 with respect to its synthesis, processing and microstructural characterization has been made. In order to establish a standard methodology for low-cost mass-manufacturing with identical and beneficial microstructure and reproducible electrical characteristics, different synthesis routes (solid-state and self-heat-sustained) were adopted. Evolution of microstructure which is intimately related to the envisaged properties in the ceramics, was closely and systematically followed in terms of sintering over a wide range of temperatures and soak time. This communication forms the first of two parts in a series of investigations on MSnO3 systems, where results on the synthesis and processing of “phase pure” barium stannate (BaSnO3) and development of interesting microstructure are presented.

Synthesis, processing and microstructural characterization of CaSnO3 and SrSnO3 ceramics

Abstract A thorough study of CaSnO 3 and SrSnO 3 with respect to their synthesis, processing and microstructural characterization has been made. In order to establish a standard methodology for mass manufacturing with identical and beneficial microstructure and reproducible electrical characteristics, different synthesis routes were adopted. Evolution of microstructure which is intimately related to the envisaged properties in the ceramics, was closely and systematically followed in terms of sintering over a wide range of temperatures and soak time. By using an alternative precursor (nitrate instead of carbonate) with favorable decomposition kinetics, the temperature of compound formation by solid-state route was lowered by a significant margin of 200°. In the case of the self-heat-sustained (SHS) method, the final compound was found to have formed via two-step reaction between molten tin and metal nitrate. In CaSnO 3 , a dense microstructure with near zero porosity and uniform grain size could be developed by sintering at 1200°C up to 48 h, while the solid-state derived SrSnO 3 could be sintered to about 85–90% density relative to that of the green compact at 1350°C/24 h. In the case of SHS technique, small grain size and a narrower particle size distribution was an interesting feature of the sintered samples. Sintering at 1350°C for soak-time 48 t ≤60 h was found to be the most suitable schedule to obtain dense microstructure in CaSnO 3 with average grain size ∼1 μm and, theoretical densification with larger grain size (3–5 μm) could be achieved by sintering at 1600°C for 2 h. The SHS-derived SrSnO 3 had very dense (relative density ∼95%) microstructure after sintering at 1350°C for soak-time in the range 12 h t ≤24 h; sintering at higher temperature such as 1600°C even for 2 h was found to be deleterious. Citrate-complex synthesis route yielded very fine, homogeneous and reactive submicron sized CaSnO 3 powder, with microstructure benign for making gas sensing devices.

Microstructural evolution in MSnO3 ceramics derived via self-heat-sustained (SHS) reaction technique

Abstract A thorough study of MSnO 3 (M=Ca, Sr and Ba) compounds with respect to their synthesis, processing and microstructural characterization has been made. In order to establish a standard methodology with identical and beneficial microstructure and reproducible electrical characteristics, a novel preparative method called self-heat-sustained (SHS) reaction technique was employed. Evolution of microstructure which is intimately related to the envisaged properties in the ceramics, was closely and systematically followed in terms of wide temperature-soak time ( T-t ) profiles. The results showed that while a well-densified microstructure with small grain size (∼1 μm) and near zero porosity could be obtained by selecting a sintering schedule of 1350°C/ x h (48 h x ⩽ 60 h) for CaSnO 3 samples, very well sintered samples with relatively larger grains (3–5 μm) and minimal porosity could also be obtained by sintering at 1600°C for 2 h. Well-densified microstructure with small grain size and zero or near zero porosity could be obtained by a sintering schedule of 1350°C/ x h (12 h x ⩽ 24 h) in SrSnO 3 samples. Sintering of BaSnO 3 proved to be the most difficult. The BaSnO 3 samples could only be densified to the desired level by soaking the powder compacts for 2 h at 1600°C. The “sugar cube” features were replaced by the spherical grains (average size 1–2 μm).

Immittance response of CaSnO3 prepared by self-heat-sustained reaction

The potential of CaSnO3 for application as a capacitor component possessing a small temperature coefficient of capacitance has been examined by ac small-signal measurements at elevated temperatures (25–300°C) in the frequency range 5–13 MHz. The samples were synthesized by a novel technique called self-heat-sustained (SHS) reaction. The ac data were acquired for the CaSnO3 samples sintered at various temperatures with varying soak temperatures, T, and some times, t (1200°C ≤ T ≤ 1600°C 2 h ≤ t ≤ 48 h). An analysis of the electrical data in more than one complex-plane formalism indicated relaxation processes. The resistance of these sintered samples was dominated by the grain boundaries, and the capacitance exhibited near-linear behavior at elevated temperature for several decades of measurement frequency. The electrical behavior has been correlated with the evolved microstructure in these samples in conjunction with the results obtained in a previous study for solid-state reaction (SSR) derived sintered bodies. The multi-plane analytical criteria provided a meaning for the lumped equivalent circuit representation including the origin and purpose of the contributing elements extracted from each complex plane formalism.

The AC electrical characterization of the solid-state reaction derived CaSnO3

The potential of CaSnO3 (calcium metastannate) for its application as a capacitor component possessing small temperature coefficient of capacitance (TCC) in electrical systems, is examined via the ac small-signal measurements. The ac electrical data were acquired on these samples sintered at various combinations of temperature-time frames (1200°C ≤ T ≤ 1350°C; 24 h ≤ t ≤ 60 h) in the frequency range from 5 Hz to 13 MHz. The measurements were carried out over the temperature range 25–300°C. The electrical response was found to exhibit relaxation processes in more than one complex plane formalism in a simultaneous fashion. The resistance of the sintered samples was dominated by the grain boundaries. The capacitance showed almost linear behavior in this measurement temperature range. The resulting electrical behavior has been discussed with the evolved microstructure in the sintered bodies.

Phase evolution and microstructural development in sol-gel derived MSnO3 (M = Ca, Sr and Ba)

Alkaline-earth stannates having the general chemical formula MSnO3 (M = Ca, Sr and Ba) have been projected as potential electronic ceramics. In view of the information gaps in the reported research, a vigorous and systematic investigation on these exotic materials has been carried out. In this communication, the synthesis of CaSnO3, SrSnO3 and BaSnO3 via sol-gel technique is reported. Infrared spectroscopy and X-ray analyses of various gel samples with different thermal history helped in identifying the reaction pathways and the stage where amorphous gel to crystalline phase transition occurred. Grains of submicron size with narrow size distribution and spherical morphology, were the most noticeable characteristics of sintered calcium metastannate derived by sol-gel method. In the case of barium analogue, a fascinating ‘sugar cube’ structure (akin to that observed in solid-state reaction and the self-heat-sustained reaction derived samples) having improved density characteristics evolved at low sintering temperatures. This gradually transformed into a more familiar spherical granular motif with improved density characteristics as the sintering profiles were varied from 1200 °C/24 h to 1500 °C/2 h. This seems to be an inherent feature of this system, irrespective of the method of synthesis.

ULTRA-LOW TEMPERATURE COEFFICIENT OF CAPACITANCE (TCC) OF THE SrSnO3-BASED ELECTRICAL COMPONENTS

The perovskite-structured SrSnO3 possessing steady capacitance over the temperature range between 27 � C and 300 � C in a frequency domain spanning nearly four decades has been evaluated. The samples investigated in this study were synthesized by using solid-state reaction (SSR) and self-heat-sustained (SHS) techniques. These samples were sintered at a temperature (T ) ranging between 1200 � C and 1600 � C with a soak-time (t) ranging between 2 h and 60 h. The ac immittance (impedance or admittance) measurements were conducted on these sintered bodies in the frequency range 5 Hz to 13 MHz. These ac electrical data were found to exhibit relaxation in more than one complex plane formalisms in a simultaneous manner. The magnitude of the terminal capacitance was found to be in a narrow window of 3 pF to 6 pF possessing very weak temperature dependence. Further analysis also revealed that this material system possessed low dielectric constant and ultra-low temperature coefficient of capacitance (TCC) or dielectric constant (TCK). The electrical behavior of these sintered bodies has been systematically correlated with the evolved microstructures. Plausible equivalent circuit elements were extracted using the lumped parameter=complex plane analysis (LP=CPA) and evaluated at various situations.

Mg2SnO4 ceramics. I. Synthesis-processing-microstructure correlation

Abstract Phase stability of magnesium meta- and orthostannate has been examined in samples synthesized via the traditional solid-state reaction (SSR) and a novel self-heat-sustained (SHS) technique with two molar ratios of magnesium to tin (viz., 1:1 and 2:1). The powder mixtures were calcined over a wide temperature–time (T–t) span ranging from 600 to 1300°C and 3 to 72 h. The powders obtained from the two preparative methods have been processed and sintered under identical conditions. In the 2:1 molar mixtures, Mg2SnO4 has been formed as a single phase up on calcination in both SRR and SHS methods. This phase remained the only compound in the sintered bodies as well. In the 1:1 composition, the ultimate reaction product was a mixture of Mg2SnO4 and SnO2. Both SSR and SHS techniques with 2:1 molar mixture yielded a single phase Mg2SnO4 in the sintered compacts. Compacts with near zero porosity could be achieved in SSR derived samples up on sintering up to 1600°C, while some significant porosity was an interesting feature of the SHS derived samples. Systematic microstructural evolution with the variation of sintering conditions has been discussed.

Microstructural evolution in B-site Mg-substituted La0.9Sr0.1GaO3−δ oxide solid solutions

Abstract Compositions with substitution both at A and B site of the host LaGaO3 system were synthesized via conventional solid-state reaction route. The substitution at A (La3+) site was fixed at 10 mole % Sr2+, while that at B (Ga3+) site was varied between 20 mole % and 50 mole % Mg2+. Cylindrical compacts were sintered using various temperature (T)–soak-time (t) profiles in the range 1200–1400°C and 2–72 h. X-ray analyses showed that up to 40 mole % MgO can go into the host lattice without resulting in new phases. Sintered bodies with higher amount of the dopant showed microstructures with mixed morphology, indicating that solid-solubility limit could have been exceeded. Microstructural examination also revealed that a soak-time of about 6 h at 1400°C was adequate to achieve benign microstructure in 20 mole % MgO-doped LaGaO3.

Electrical conductivity-microstructure correlation in AIN-CuO composites

Water vapor is an important constituent of any gas and in many applications is regarded as a contaminant that needs to be monitored and controlled. AIN-CuO composites (2 % ≤ CuO ≤ 50 % by weight) have been studied to exploit them as novel humidity sensors over wide ranges of moisture levels and temperature. Development of benign microstructure with open porosity has been attempted by varying the composition and firing conditions. The impedance data acquired on the composites over the frequency range 5 Hz to 13 MHz, revealed a bulk response in terms of a single semicircular relaxation in the complex Z*-plane. A systematic variation of electrical conductivity with CuO content in the composites has been explained in the light of percolation theory.