Characterization of BaSnO -based ceramics Part 1 ...aazad/pdf/BaSnO3.pdfPart 1. Synthesis,...

LJournal of Alloys and Compounds 270 (1998) 95–106

Characterization of BaSnO -based ceramics3

Part 1. Synthesis, processing and microstructural development*Abdul-Majeed Azad , Nee Chen Hon

Department of Physics, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

Received 24 November 1997; received in revised form 31 January 1998

Abstract

The compound BaSnO together with its Ca- and Sr-analogs, has recently been projected as potential electronic ceramic material3

(thermally stable capacitor, chemical sensor for humidity, CO and NO , etc.). In order to fill the information gaps in the reported research,x

a vigorous and systematic investigation on these exotic materials has been initiated. A thorough study of BaSnO with respect to its3

synthesis, processing and microstructural characterization has been made. In order to establish a standard methodology for low-costmass-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 inthe ceramics, was closely and systematically followed in terms of sintering over a wide range of temperatures and soak time. Thiscommunication forms the first of two parts in a series of investigations on MSnO systems, where results on the synthesis and processing3

of ‘‘phase pure’’ barium stannate (BaSnO ) and development of interesting microstructure are presented. 1998 Elsevier Science S.A.3

Keywords: Barium stannate; Capacitor; Gas sensor; Electronic ceramics; Solid-state; Self-heat-sustained; Microstructure

1. Introduction thermally stable capacitors in electronic industries. In pureas well as in doped forms these stannates have also been

The double oxides of the general formula, (AE)BO , investigated as potential sensor materials for a host of3

formed between the oxides of alkaline-earth metals (AE5 gases, including CO, HC, H , Cl , NO and humidity2 2 x

Ca, Sr and Ba) and those of some of the group IV elements [5–9]. There is great scope for exploiting heterojunctionsare of great industrial and technological importance. For of these stannates with other suitable oxides as capacitiveinstance, the AE carbonates are the well-known precursors sensors for carbon dioxide detection and metering [10].to innumerable inorganic syntheses and reactions, while Recently, Ostrick et al. [11] have reported results of HallAE silicates are of relevance and direct bearing in the slag measurements on BaSnO at high temperatures to eluci-3

chemistry of industrial production of iron and steels. date the nature of defects prevailing in the material. It hasSimilarly, the discovery of superconductivity in ‘‘copper- been reported that barium stannate has a band gap of 3.4free’’ cubic perovskite systems such as Ba Pb SbO eV [12], which is well within the range of 3–3.5 eV,12x x 3

(T 53.5 K at x50.25) and BaPb Bi O (T 513 K at generally desired for gas sensor materials [13]. Sugges-c 12x x 3 c

x50.3) [1] has triggered much activity in the pseudobinary tions have also been made that, by combining BaTiO with3

alkaline-earth oxide–PbO(PbO ) systems. In addition, the BaSnO (BTS), multifunctional ceramic sensors can be2 3

technological impact of closely structure-related titanates developed which can detect temperature, relative humidity,of the alkaline-earth metals is too great to be ignored, of and gases such as prozylene, acetylene and ethylene atwhich BaTiO (piezoelectric) and SrTiO (varistor) are the ambient temperatures and pressures [14].3 3

most important electroceramics [2–4]. Despite such technological importance, these stannatesBarium stannate, BaSnO , belongs to the family of have not been as thoroughly and systematically studied as3

analogous alkaline-earth stannates (MSnO where M5Ca, the corresponding titanates. For example, no sound and3

Sr and Ba) which are currently being pursued for their reliable data is available on the thermodynamic stability ofattractive dielectric characteristics, finding application as these compounds or on other compounds in the AO–SnO2

pseudobinary systems. Such data is invaluable since it will*Corresponding author. E-mail: [email protected] assist in detecting and delineating any phase-field shifts

0925-8388/98/$19.00 1998 Elsevier Science S.A. All rights reserved.PI I : S0925-8388( 98 )00370-3

96 A. Azad, N.C. Hon / Journal of Alloys and Compounds 270 (1998) 95 –106

that could occur at moderate to high temperatures owing to processing, microstructure and electrical behavior of thisthe pronounced volatility of SnO (disproportionation prod- material; such correlation is inevitable in understanding theuct of SnO ). Surprisingly, only a partial phase diagram is underlying working mechanisms in electronic components2

available on the BaO–SnO system in the literature [15]; fabricated from them. For example, in the case of BTS2

no phase diagrams have been reported for the Ca and Sr solid solutions [14], grains are interconnected and theanalogs. In addition, while the BaO–SnO phase diagram pores are channeled at the grain edges, giving a cylindrical2

shows the existence of only one compound, viz. BaSnO pore connection. Development of such a tailored micro-3

(Ba:Sn51:1), reliable JCPDS cards are available for structure definitely requires intelligent processing with acompounds such as Ba Sn O and Ba SnO (with Ba:Sn5 precise control over the processing parameters such as,3 2 7 2 4

1:0.67 and 1:0.50, respectively). Moreover, any infor- temperature, atmosphere, grain size, grain orientation,mation on processing and evolution of microstructure in length of grain boundaries, pore diameter, etc..these materials and their impact on the electrical charac- Thus, throughout the literature, there is a noticeable lackteristics is also lacking in the literature. Smith et al. [16] of correlation of synthesis, processing and microstructuralhave prepared BaSnO by solid-state method using stoi- aspects of the BaSnO compound with its electrical3 3

chiometric amounts of BaCO and SnO and heating at characteristics. Hence, it was relevant to initiate a study to3 2

12008C for 16 h. The compound was then reground, fill in this information gap on this material of technologicalpelletized and annealed in air at 13008C for 43 h. The importance. The ceramic oxide was thoroughly studiedcompound was indexed as having a cubic perovskite with respect to its synthesis, processing and characteriza-

˚structure with the lattice parameter a54.119 A. Antimony tion – physical, microstructural and electrical. A novelsesquioxide doped-BaSnO with the formula preparative method such as the self-heat-sustained (SHS)3

BaSn Sb O were also synthesized by Smith et al. in technique, in addition to the conventionally much-used0.85 0.15 3

a similar fashion using BaCO , SnO and Sb O . The solid-state synthesis technique, was employed. Characteri-3 2 2 3

mixtures were heated at 12008C for 16 h followed by zation methods such as XRD and electron microscopyannealing at 15008C for 65 h. However, antimony ses- (using scanning electron microscope with EDX) were usedquioxide is a low melting compound (T 56568C) and is to ascertain the reaction pathways leading to the formationm

known to have pronounced volatility even at moderate of the targeted compound, particle size and their dis-temperatures (cf. 16.1 mm Hg at 5008C). In the light of tribution and, to systematically follow the development ofreliable thermodynamic data, Sb O has a vapor pressure microstructure in the sintered bodies. Similar investiga-2 3

of 308.6 mm Hg above the liquid at 12008C [17]. With this tions on these aspects of CaSnO and SrSnO have3 3

rather significant magnitude of vaporization of Sb O at recently been reported [20,21]. In this paper, details of2 3

the working temperature, synthesis of the targeted com- synthesis, processing and microstructural evolution in thepound is not likely to be achieved. Bao et al. [18] had BaSnO system are presented. In a subsequent communi-3

prepared BaSnO via coprecipitation from solution con- cation, the results of the electrical measurements using the3

taining BaCl and SnCl , using oxalic acid as the precipi- a.c. immittance (impedance and/or admittance) spectro-2 2

tating agent in the presence of water. However, a sintering scopic technique over a range of temperatures will betemperature of 10008C only was employed. Recently, reported and, a correlation among processing–microstruc-Upadhyay et al. [19] have reported solid-state synthesis of ture–electrical properties will be made [22].BaSnO , using BaCO and SnO as the starting material.3 3 2

The calcination was done at 12008C for 6 h and sinteringat 1250–14008C for times ranging between 6 to 12 h. The 2. Experimentalonly micrograph shown by these authors exhibited asignificant amount of porosity with poor grain-to-grain 2.1. Materials synthesis and characterizationconnectivity. It is also difficult to make out the presence ofgrains with cubic morphology in the micrograph as Metallic Sn (99.999% powder, Pi-Kem, Surrey, UK),claimed by the authors. SnO (99.995% powder, Aldrich, USA), and Ba(NO )2 3 2

Contrary to these investigations [16,18,19], it was found (99.5%, Hopkins and Williams, UK), were used as thein the present work (see the subsequent sections) that starting materials. Commercial grade nitric and hydrochlo-sintering in the range of 1200–14008C was inadequate for ric acids (16 M), liquid ammonia (14 M) and glacial aceticproper grain growth, intergranular connectivity and acid, used as reagents and solvents in various synthesesadequate densification of BaSnO . This aspect is important were procured from BDH, India, while isopropyl alcohol3

since, in order to make a capacitor component out of (AR 95% minimum) was from James Burrough Ltd.,BaSnO , the material is required to be essentially ‘‘pore- (UK). Two different preparative techniques (solid-state and3

free’’, since pores would act as sink to the electrical charge self-heat-sustained) were employed for the synthesis of thecarriers and would be the source of poor grain-to-grain targeted compounds BaSnO (Ba/Sn51:1 molar). It3

connectivity. Consequently, no correlation has been estab- should be pointed out that the idea of adopting variouslished among the key parameters such as, synthesis, synthesis processes was two-fold: first, in the literature

A. Azad, N.C. Hon / Journal of Alloys and Compounds 270 (1998) 95 –106 97

available on this system, only the solid-state reaction route was slightly modified and one sample from the batch ofwas adopted for sample preparation (see the details in the solid-state synthesis was sintered as follows:Section 2.2) and second, it was intended to find the most

RT–13508C 5 9 h; 1350–16008C 5 2.5 hfavorable synthesis technique in terms of the phase purityand benign microstructure in the sintered samples with

soaking @ 16008C 5 12 h; 16008C–RT 5 4 h.most favorable electrical characteristics.

Prior to sintering, the calcined powder was blended with10 wt.% polyvinyl alcohol (PVA) as binder, dried under a2.2. Solid-state reaction routeUV lamp and pressed into green pellets by double-endcompaction at pressure not exceeding 100 MPa. The use ofStoichiometric amounts (1:1) of the compoundpolyfunctional organics such as polyvinyl alcohol orBa(NO ) crystals and SnO powder of stated purity were3 2 2polyethylene glycol (PEG) is a common practice in theweighed in a Mettler high precision electronic balance andsintering of ceramic bodies, which plays an important roledry mixed in an agate mortar. The mixture was thenin the microstructural development; the organic binders areball-milled for 4 h in isopropyl alcohol medium in airtightbelieved to provide strength and lubrication effect therebypolystyrene bottles using clean zirconia balls as the millingbringing the particles in the green body closer to onemedium. Room temperature drying under a UV lamp wasanother. This assists in achieving higher densities inused to remove isopropyl alcohol in a ventilated fumesintered bodies than those sintered without the binder. Thehood. Subsequently, the powder was pressed into discs 6choice of PVA in this work was due to its relatively easieror 12 mm in diameter and 2–3 mm in thickness usingcombustion kinetics compared to those of PEG. Micro-stainless steel die at a pressure of about 100 MPa. Thestructural features of the starting ‘‘green’’ BaSnO powdermixture was calcined first at 8008C for 8 h, crushed, 3

as well as the sintered discs were determined by using arepelletized and fired again at 10008C for 24 h in air. TheJEOL-6400SM scanning electron microscope (Japan).pellets were then pulverized and ground to a fine homoge-Elemental identification and quantification in differentneous powder which was subjected to phase analysisregions of the sintered samples were carried out by usingpowder X-ray diffraction on a Scintag X-ray machinethe EDX analyzer (Link eXL, UK) attached to the above(USA) at room temperature, using monochromatic Cu Ka

˚ SEM machine (Table 1). Owing to the strong susceptibilityradiation (l51.5406 A) in the range 10–908 (2u ). Theof compounds in the Ba–Sn–O systems towards moisture,resulting XRD pattern was also used to detect the presencethe calcined powder as well as the sintered discs wereof, if any, unreacted starting materials and/or new phases.always stored in a humidity-free desiccator containingAfter the formation of ‘‘phase’’ pure compound wasanhydrous CaCl , unless required for microscopic /electri-confirmed by XRD, the calcined powder was subjected to 2

cal measurements.sintering (in order to follow the evolution of microstruc-ture and its effect on the measured electrical properties) atthree different temperatures, viz. at 1200, 1350 and 2.3. Self-heat-sustained (SHS) route16008C, for duration ranging from 2 to 72 h in ambient air.The sintering schedule adopted in this work is shown Metallic tin powder was intimately mixed with anhydr-schematically in Fig. 1. In one case, however, the schedule ous Ba(NO ) crystals in a 1:1 molar ratio. The mixture3 2

was placed in a platinum boat and first heated slowly toand maintained at 2508C for 4 h so as to facilitate completemelting of metallic tin (m.p.52328C) and its uniformdispersion under gravitational flow in the liquid state. Thetemperature was then raised gradually to 8008C andmaintained for another 4 h to cause the reaction betweenmolten and free flowing tin and Ba(NO ) . The mixture3 2

was next calcined at 11008C for 12 h. XRD analysis of the

Table 1Elemental analysis by EDAX in the sintered BaSnO bodies3

Synthesis route Sintered at Element (at.%) % Standarderror

Ba Sn

Solid-state 12008C/60 h 46.274 53.726 62.0SHS 13508C/24 h 47.222 52.778 62.0

Fig. 1. Temperature–time (T–t) schedule for the sintering of BaSnO3 SHS 13508C/36 h 46.851 53.149 62.0powder.


resulting powder at this stage revealed the presence of a was repeated with the calcined powder or the sinteredtwo-phase (Ba SnO 1BaSnO ) mixture. This necessitated material.2 4 3

further heating of the mixture at 12008C for 24 h. Phase The microstructural features of the solid-state derivedidentification was carried out by X-ray as mentioned in the samples sintered at 12008C as a function of soak timeabove subsection, both after 1100 and 12008C calcination between 24 to 72 h is shown in Fig. 3(a–e). From thesesteps, the reason for which will be discussed in Section 3. SEM micrographs, it can be seen that sintering at 12008CA schematic of the experimental setup for the synthesis of for 24 h led to a microstructure with abnormal initial grainBaSnO via SHS technique has been shown and described growth, having a broad grain size distribution, ranging3

elsewhere [20]. The mass thus obtained was crushed, from submicron to as large as |6 mm in some pockets.pulverized, mixed with PVA and sintered according to the This evidently left a significant amount of porosity in the(T–t) schedule adopted in the case of the solid-state sample. As the soak time was increased to 48, 60 and 72 h,derived sample. The details of microscopic examination of very shapely and characteristic ‘‘sugar cube’’ type grainsthe SHS derived sample remain identical to those de- with 3–7 mm in edge dimension evolved (Fig. 3(b–d))scribed above. with a clearly visible narrowing of the grain size dis-

tribution. Taking into account that BaSnO crystallizes in3

cubic perovskite structure having a cell constant of 4.119A, it appears that the grains, on average, had grown to an3. Results and discussionextent of about 10 000 times the unit cell dimension. Thisis about 200 times that reported by Uphadhyay et al. [19]3.1. Solid-state reaction routein their sample sintered at 14008C for 6 h. It can also beseen from Fig. 4(a–d), that even with such phenomenalFig. 2(a) shows the X-ray diffraction pattern of thegrain growth and preferred grain orientation, a significantsample prepared by solid-state reaction between Ba(NO )3 2 amount of porosity was left in the material. The mecha-and SnO , which matches with the standard reported2 nism of formation of such large grains seemed to follow(JCPDS card No. 15-0780), indicating the formation of thethe sequence shown in Fig. 3(e) (microstructure of aintended compound, viz. BaSnO in a single phase. The3 sample sintered at 12008C for 72 h at higher magnifica-appearance of very sharp diffraction peaks further indicatestion). Smaller grains (on the top), cubic in shape and |1quite small crystallite size in the powder. The absence ofmm in size, seem to join together and grow into largerdiffraction peaks due either to the starting materials orones, as seen in the background.second phases in the Ba–Sn–O system, showed the

In order to cause sufficient grain-to-grain connectivitypowder obtained, to be of high quality. The formation ofand reduced porosity, sintering at 16008C for 2 and 12 hBaSnO via solid-state reaction takes place as follows:3 was resorted to, and the resulting microstructure is shownin Fig. 4. The microstructures marked a and b are for2SnO (s) 1 2Ba(NO ) → 2BaSnO 1 4NO (g) 1 O (g).2 3 2 3 2 2samples sintered for 2 h, at two different magnifications

(1)(3500 and 32000, respectively), while c and d are for

It should be noted that, while the sample preparation via those sintered at 16008C for 12 h at the same magnifica-solid-state route was done by following the conventional tion. At lower magnification (Fig. 5(a,c)), almost mono-route, the compound formation according to Eq. (1) sized grains (|3–4 mm, estimated from higher magnifica-occurred at relatively lower calcination temperature tion micrographs) can be seen for both 2 and 12 h sintering(10008C/24 h). This is rather encouraging in the case of at 16008C. Fig. 4(c,d), however, clearly show that thethe solid-state technique where usually larger grains are sample is still porous and exhibits ‘‘sugar cube’’ features,formed due to the use of high temperature and several with edges rather smoothened. Nevertheless, the grain-to-repetitions of the ‘‘heat and beat’’ steps. We attribute this grain connectivity in these samples appeared to haveimprovement in the present case to the use of metal nitrate improved. Density measurements on the sintered bodies,as one of the precursors rather than the conventionally by making use of the pellet dimensions and mass, revealedemployed carbonate or oxide; the metal nitrates have more that the relative density was only up to 70–80% offavorable decomposition kinetics compared to their car- theoretical value even in samples sintered at 16008C for 12bonate counterparts. This is a definite improvement over h; this fact is corroborated very well by the microstructuralthe procedure of compound formation reported by Smith et features discussed above.al. and Upadhyay et al. [16,19]. While Smith et al. reported In the light of the foregoing discussion, it appears that inthe synthesis of BaSnO by heating stoichiometric the solid-state derived BaSnO samples, a high degree of3 3

amounts of BaCO and SnO at 12008C for 16 h, densification cannot be achieved in the temperature range3 2

calcination at 12008C/6 h and ‘‘repeated grinding, pelletiz- used for sintering in this study. It is envisaged, however,ing and firing several times’’ has been reported by Up- that soaking at 16008C for a duration longer than 12 hadhyay et al. [19]. Moreover, it is not clear from the latter might result in a microstructure with improved intergranu-report whether the cycle of grinding, pelletizing and firing lar connectivity and reduced porosity.


Fig. 2. Powder X-ray diffraction patterns of BaSnO samples synthesized via (a) the solid-state reaction route and (b) the SHS technique. XRD signature in3

(b) consists of a mixture of those due to Ba SnO and BaSnO .2 4 3

At this juncture, it is worth comparing the sintering 12008C was found to be inadequate even up to 72 hbehavior of BaSnO with that of CaSnO and SrSnO soaking, as evidenced by the presence of cubic agglomer-3 3 3

obtained via a similar solid-state route [20,21], in order to ates, consisting of 20–30 grains of varying size lumpedfollow the systematic, if any, among the three stannates. In together. Microstructures with 10–15% porosity and uni-the case of CaSnO , it was observed that a highly dense form grain size (|1 mm) could be developed by sintering3

microstructure with near zero porosity and uniform grain SrSnO at 13508C up to 24 h. Sintering at 16008C even for3

size (without much growth) could be developed by sinter- a short time (2 h) was found to be too severe, as it led toing at 12008C for up to 48 h. Sintering at 13508C led to increased but well-connected porosity, without causing anynonuniform grain growth in addition to somewhat in- significant grain growth. All along, very small grain size ofcreased porosity. In the case of SrSnO , sintering at the order of 1 mm or less, was a very characteristic feature3


Fig. 3. Microstructure of solid-state derived BaSnO sintered at 12008C for: (a) 24, (b) 48, (c) 60, (d) 72 h and (e) 72 h at higher magnification.3

of solid-state derived SrSnO samples. In comparison to native to the conventional methods of advanced materials3

these, the present work showed that sintering of BaSnO production, which is gaining rapid popularity in the field of3

proved to be the most difficult. Sintering even up to ceramic– and metal–matrix composites (CMC and MMC)16008C for 12 h could not eliminate porosity in order to [23]. The most attractive feature of the SHS technique isyield a typically dense ceramic structure. the ability of highly exothermic reactions to be self-

sustained and therefore, energetically efficient. The3.2. Self-heat-sustained (SHS) reaction route rationale behind using metallic tin in the present case, was

its rather low temperature of melting (kSnl5hSnj 2328C;21The self-propagating high-temperature or self-heat-sus- DH 57.056 kJ g atom ) [24], affecting a homogeneousm

tained (SHS) synthesis is an attractive and viable alter- mixing and reactivity with the metal nitrate, yielding a


Fig. 4. Microstructure of solid-state derived BaSnO sintered at 16008C for: (a–b) 2 h and (c–d) for 12 h; magnification is 3500 (a, c) and 32000 (b, d).3

‘‘phase’’ pure compound under less demanding conditions Ba SnO 1 Sn(l) 1 O (g) → 2BaSnO (4)2 4 2 3

than those reported hitherto.Fig. 3(b) shows the X-ray diffraction pattern of the [or] Ba SnO 1 SnO (s) → 2BaSnO . (5)2 4 2 3

BaSnO sample obtained by the SHS technique after3

11008C/12 h. In this case, the compound formation was This reaction scheme is corroborated by the XRD signaturefound to have initiated at about 8008C. The XRD signature of the 1:1 molar mixture of metal nitrate and tin, obtainedof the sample after the 11008C calcination step (Fig. 3(b)), on samples heated at 8008C/8 h111008C/12 h and athowever, shows a mixture of peaks corresponding to a 12008C/24 h, respectively. Evolution of heat due to theBa-rich phase, viz. Ba SnO (JCPDS card No. 12-0665), fusion of tin at rather low temperature is believed to have2 4

in addition to those belonging to the intended compound aided the above scheme of the reaction pathway. More-BaSnO . Nevertheless, the XRD pattern of the sample3 over, this reaction sequence is identical to those observedcalcined further at 12008C conformed to that reported for in the formation of Ca- and Sr-stannate via the SHSpure BaSnO and was identical to that obtained on the3 technique [20,21]. This is the first report in the literaturesolid-state derived one. For the sake of obviating repeti- where a technique other than the solid-state method for thetion, the XRD pattern of the later sample has not been synthesis of BaSnO has been adopted. As mentioned in3shown. The scheme of BaSnO formation in this case,3 Section 3.1, the preparative details reported by othertherefore, seems to be via an intermediate Ba-rich phase, workers on this and analogous systems (Ca- and Sr-Ba SnO . It is likely that in the initial stages, molten2 4 stannate) are rather incomplete and obscure. For example,metallic tin reacted in the following way: Mandal et al. [25] Parkash et al. [26] and Upadhyay et al.

[19] have reported the solid-state synthesis of Ca–, Sr–Sn(l) 1 2Ba(NO ) → Ba SnO 1 4NO (g)↑ (2)3 2 2 4 2and BaSnO , respectively, by mixing respective metal3

1 carbonates and tin dioxide in acetone medium, calcining at]Sn(l) 1 Ba(NO ) 1 O (g) → BaSnO 1 2NO (g)↑.3 2 2 3 2212008C/4 h (6 h in the case of BaSnO ) and, sintering at3(3)13508C for 12 h (in the case of BaSnO , first at 12508C for3

The Ba-rich phase subsequently reacted with more Sn or 6 h followed by 12 h at 1350, 1375 and 14008C each),SnO to form BaSnO as shown below: repeating the process ‘‘several’’ times. Shimizu et al. [7]2 3


for 24 and 36 h is shown in Fig. 6. As can be clearly seen,the microstructural features (‘‘sugar cube’’ grains) in thiscase are almost identical to those observed in the case ofsolid-state derived samples, albeit with much larger dis-tribution of the particle size; in this case, the average sizeranged from 1 to about 20 mm in some overly growngrains. While going from 24 to 36 h soak-time (Fig. 6(a,b)respectively), there was a noticeable reduction in porosity,the broad grain size distribution, nevertheless, still remain-ing. Measurements on SHS derived samples sintered at13508C for 36 h showed improved desity (|78% theoret-ical). It is well-known, however, that one of the majorlimitations of SHS synthesis is the presence of a relativelyhigher degree of porosity in the final product. Neverthe-less, since about 95% of this porosity is ‘‘open’’ in nature,it can be eliminated during the sintering step, therebyleaving a dense and compact body. In can be suggestedtherefore, that a longer firing at 13508C, might result in afurther reduction in porosity, but whether the nonuniformi-ty of the grain size could also be eliminated, is ques-tionable and needs more experiments.

For reasons mentioned in Section 3.1 with respect tosufficient grain-to-grain connectivity with reduced porosityand uniform particle size, sintering at 16008C was resortedto, and the resulting microstructure is shown in Fig. 7. The

Fig. 5. Morphological features of SHS derived BaSnO raw powder after3

calcination at: (a) 11008C/12 h and (b) 12008C/24 h.

used an identical technique for making doped-SrSnO , ball3

milling the SrCO and SnO mixture overnight, calcining3 2

at 10008C for 2 h and, finally sintering at 1100–12008C for‘‘several’’ hours. Similarly, Smith et al. [16] synthesizedthe compound by heating the carbonate–oxide mixture at12008C for 16 h and annealing at 13008C for 43 h, whileBao et al. [18] used sintering at 10008C only. As discussedin Section 3.1, solid-state derived BaSnO samples could3

only be sintered partially, even after soaking for 12 h at16008C.

Fig. 5 shows the morphology of the raw powder derivedvia the SHS route; Fig. 5(a) is the SEM picture of thepowder after the 11008C/12 h calcination step, while Fig.5(b) is taken of the powder after the 12008C/24 h. WhileFig. 5(a) revealed that the raw powder was submicron insize, mostly in the form of agglomerates, the morphologyseemed to have undergone significant change after 12008Ccalcination, in terms of homogeneity of the powder andshape and size of the grains. This microstructural feature isevidence that the formation of BaSnO in the case of the3

self-heat-sustained reaction might have proceeded by thesteps shown by reactions (2–5) above.

In the case of the SHS derived BaSnO , sintering began Fig. 6. Microstructural evolution in SHS derived BaSnO samples3 3

at 13508C and the microstructure resulting after sintering sintered at 13508C for (a) 24 and (b) 36 h.


Fig. 7. Microstructural development in SHS derived: (a) BaSnO samples sintered at 16008C for 2 h, (b) CaSnO samples sintered at 16008C for 2 h, (c)3 3

SrSnO samples sintered at 13508C for 12 h and, (d) SrSnO samples sintered at 16008C for 2 h.3 3

drastic morphological and structural changes subsequent to minimal porosity could also be obtained by sintering atfiring at 16008C for 2 h, can be compared to those sintered 16008C for 2 h (Fig. 7(b)); sintering for longer durationat lower temperatures and also with those of samples at this temperature, however, led to abnormal grainderived from solid-state technique (Figs. 3 and 4). The growth and severe morphological variation [20].resulting features included: highly densified material with 2. In SrSnO samples, well-densified microstructure with3

near zero porosity, proper grain orientation and, good small grain size and zero or near zero porosity can beintergranular connectivity. The ‘‘sugar cube’’ features have obtained by choosing a sintering schedule of 13508C/xbeen replaced by the spherical grains which are common in h (12 h,x#24 h) (Fig. 7(c)). However, the micro-ceramic microstructures (average size 1–2 mm). This also structure changed drastically in SrSnO sintered at3

resulted in a significant improvement in the density of the 16008C for 2 h, resulting in highly oriented hexagonal-sintered body. shaped 3-D grains, 3–5 mm in size (Fig. 7(d)).

Comparison of the evolution of microstructure inBaSnO samples obtained by the self-heat-sustained re- That there was no materials degradation in terms of3

action technique with that in CaSnO and SrSnO from the compositional variation and formation of new phases rich3 3

same synthesis route, brings out the following observa- in either Sn or Ba, as a result of sintering at temperaturestions: in the range 1200–16008C, was confirmed by performing

elemental analyses and quantification of segregation by1. A well-densified microstructure with small grain size energy dispersive X-ray (EDX) analyses. The EDX analy-

(|1 mm) and zero or near zero porosity can be obtained ses (in terms of concentration profiles of Ba and Sn) wereby choosing a sintering schedule of 13508C/x h (48 performed in different regions of each of the sinteredh,x#60 h) for CaSnO samples. Very well sintered samples and some typical results are shown in Fig. 8. Such3

samples with relatively larger grains (3–5 mm) and an exercise showed the ratio of Ba:Sn to be 1:1 in all the


Fig. 8. Concentration profiles of Ba and Sn by energy dispersive X-ray (EDX) analysis in randomly chosen grains of BaSnO pellets sintered at: (a)3

12008C/60 h (solid-state), (b) 13508C/24 h (SHS) and, (c) 13508C/36 h (SHS).

samples. Thus, there was no degradation of material, nor state and the novel self-heat-reaction (SHS) techniqueswas there noticed any loss of material due to preferential were employed and reaction pathways leading to theevaporation of the sample. compound formation were closely followed. X-Ray dif-

fraction and electron microscopic techniques were exten-sively used to understand the evolution of phases and

4. Conclusions microstructures as a result of sintering cycles. On the basisof this, the following conclusions could be drawn:

The compound BaSnO has been investigated in much3

detail with respect to its synthesis by different routes, 1. Synthesis by the modified solid-state route using nitrateprocessing under different sintering conditions and ex- (instead of carbonate) precursor helped the calcinationamining the resulting microstructural features. Two differ- and compound formation temperature to be lowered byent methods of material synthesis, viz. conventional solid- a significant 200 degrees margin. Microstructure con-


taining characteristic ‘‘sugar cube’’ shaped grains with spect to metal or oxygen. This is an important factor,3–7 mm in edge dimension evolved with sintering at which otherwise contributes to the sintering via cationic12008C up to 72 h; even with such phenomenal grain or anionic defect formation. Such behavior has beengrowth and preferred grain orientation, significant well documented in the case of solid-state sintering ofamount of porosity was left in the material. Sintering barium titanate ceramics [2–4].even up to 16008C for 12 h could not eliminate porosityin order to yield a typically dense ceramic structure.Thus, solid-state synthesis does not appear to be an Acknowledgementsattractive route for BaSnO as long as the micro-3

structural features are concerned (in the temperature– The authors wish to express their sincere gratitude to Dr.time range employed for sintering in the present Mansor Hashim, for his immense materials support in theinvestigation). However, in this case, liquid phase initial stages of this work, to Steven Baptist for hissintering could be attempted, using low melting com- cooperation during the preliminary stages, to Prof. Sheikhpounds such as Bi O (T 58258C) and/or V O (T 52 3 m 2 5 m Akbar, Department of Materials Science and Engineering,6708C). In such cases, the driving force for densifica- Ohio State University, Columbus, Ohio (USA) and, totion is provided by the capillary pressure of the liquid Prof. S. Radhakrishna, Institute of Postgraduate Studiesphase located between the fine solid particles [27]. Such and Research, Universiti Malaya, Kuala Lumpur, fora treatment might lower the temperature of sintering as carrying out X-ray analyses. Our heartfelt thanks are alsowell. due to Dr. M.K. Vidyadharan (Dr. ‘‘Menon’’), Ms. Azilah

2. As in the case of CaSnO and SrSnO synthesized via3 3 Abdul Jalil, Ms. Aminah Jusoh and Mr. Ho Oi Kuan of thethe SHS technique [20,21], compound formation seems Veterinary Science Department for their tremendous sup-to have initiated at temperatures as low as 8008C in the port and generosity in allowing us to use the SEM andcase of BaSnO ; the formation of BaSnO was com-3 3 EDX facilities. One of the authors (AMA) wishes to thankplete at 12008C and appeared to be facilitated via a UPM for awarding a short-term grant to initiate researchtwo-stage process involving the formation of an inter- on the MSnO system.3mediate Ba-rich phase, Ba SnO , as evidenced by2 4

XRD. Small grain size and a narrower particle sizedistribution was an interesting feature of the SHS

Referencesderived raw powder. Sintering at 13508C for soak-timein the range 24–36 h was found to yield microstructural

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21 Soc. 138 (1991) 173.diffusion. A smaller ion such as Ca can diffuse faster[11] R. Ostrick, M. Fleischer, H. Meixner, J. Am. Ceram. Soc. 80 (1997)21than Ba . Hence, the sintering in CaSnO is favored3 2153.

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[13] M.J. Madou, S.R. Morrison, Chemical Sensing with Solid StateBaSnO , where a relatively high temperature (16008C)3Devices, Academic Press, New York, 1989, Ch. 3.is effective to result in well-densified, pore-free micro-

[14] L.M. Sheppard, Adv. Mater. Proc. 2 (1986) 19.structure, where the starting material is obtained via the [15] G. Wagner, H. Binder, Z. Anorg. U. Allgem. Chem. 297 (1958) 331.SHS technique. In addition, these stannates are line [16] M.G. Smith, J.B. Goodenough, A. Manthiram, R.D. Taylor, W.compounds with no nonstoichiometry either with re- Peng, C.W. Kimball, J. Solid State Chem. 98 (1992) 181.


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