Chiang Mai J. Sci. 2010; 37(1) 165

Chiang Mai J. Sci. 2010; 37(1) : 165-169www.science.cmu.ac.th/journal-science/josci.htmlShort Communication

Effects of Manganese Addition on Phase Formation,and Microstructure of Barium Titanate CeramicsJirapa Tangsritrakul*[a] and Rattikorn Yimnirun [b][a] Department of Physics, Faculty of Science and Technology, Thammasat University,

Pathum Thani 12120, Thailand.[b] School of Physics, Institute of Science, Suranaree University of Technology,

Nakhon Ratchasima 30000, Thailand.*Author for correspondence; e-mail: tang_ji@yahoo.com

Received: 15 May 2009Accepted: 25 June 2009

ABSTRACTThe influence of manganese (Mn) addition on phase formation and microstructure

of Ba(Ti1-xMnx)O3 ceramics (where x = 0.01 to 0.2) was investigated. The XRD analysis showedthat the crystal structure of Ba(Ti1-xMnx)O3 at room temperature changed from tetragonal tohexagonal as Mn concentration increased. The microstructure of Ba(Ti1-xMnx)O3 ceramics atlow Mn concentration revealed a bimodal feature consisting of well-grown plate like grainslarger than 100 μm with low aspect ratios embedded in a fine-grained (about 3–5 μm) matrix.As the x value increased, the well-grown grains decreased with the extent of fine grains increased.

Keywords: Mn-doped BaTiO3, hexagonal, dielectric properties, ferroelectric properties.

1. INTRODUCTIONBarium titanate, BaTiO3 or BT, based

materials are well known electroceramics thatfind applications as dielectric materials incapacitors and as positive temperaturecoefficient of resistance (PTCR) thermistors[1-3]. At temperature above 130oC, bariumtitanate has a paraelectric cubic phase. In thetemperature range of 130 to 0oC theferroelectric tetragonal phase with a c/a ratioof ~ 1.01 is stable. The spontaneouspolarization is along one of the [001]directions in the original cubic structure.Between 0 and -90oC, the ferroelectricorthorhombic phase is stable with thepolarization along one of the [110] directionsin the original cubic structure. On decreasingthe temperature below -90oC, the ferroelectric

phase transition from the orthorhombic torhombohedral phase leads to polarizationalong one of the [111] cubic directions. Mostresearch to date has focused on materials basedon the ferroelectric tetragonal polymorph, t-BaTiO3, due to its high permittivity andbecause its electrical properties can be tailoredto exhibit the PTCR effect [1-3]. By contrary,the high temperature hexagonal polymorphhas not been widely investigated. Thehexagonal polymorph was first reported byBourgeois in 1883 and crystallizes in the spacegroup P63/mmc with lattice parametersa=5.7238 and c=13.9649 [4-9]. Undoped6H-BaTiO3 is reported to be thermodyna-mically stable above 1,460oC, but can bekinetically stabilized at room temperature by

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doping with various transition metal ions onthe Ti-sites, e.g. Fe, Mn, Co, Ni, Cu that canalso lead to the formation of the hexagonalpolymorph at room temperature [4-14].Previously, Langhammer et al. have reportedthat in Cu-doped BaTiO3 ceramics sinteredat 1400oC in air the hexagonal phase reachesa maximum percentage of 92% at 1.5 mol%Cu but does not replace the tetragonal phasecompletely [10,13]. Glaister and Kay showedthat Ni-doped BaTiO3 adopted the hexagonalphase (a fraction of 50%) when sintered at1,400oC and with a doping level of 10 mol%,while a 2 mol% Ni-doped material sinteredat 1,400oC for 5 h showed no hexagonalstructure [14]. On the other hand, Mn wasreported to be the most effective dopant forstabilizing h-BT [5-9]. Hence, in this presentstudy, the influences of Mn addition on phaseformation and microstructure of Ba(Ti1-xMnx)O3 were investigated.

2. EXPERIMENTALBa(Ti1-xMnx)O3 powders with x = 0.0 to

0.2 were prepared by a conventional mixed-oxide method. Raw oxide powders ofBaCO3 (Fluka,99.0% purity), TiO2 (Riedel-deHa•n, 99.0% purity) and MnO2 (Sigma-ALDRICH, Inc., 99.999%) were mixed andvibro-milled in ethyl alcohol for 1 h and thenoven dried overnight at 120oC. After drying,the powders were calcined at 1,250oC for 2 hwith 5oC/min heating and cooling rates [7].The calcined powders were pressed into disk-shaped by uniaxial hydraulic press at pressureof 0.5 tons to form the green pellet of 1 cmdiameter. The green pellets were placed onthe alumina powder inside an alumina cruciblethen heat treated at 500oC for 1 h to eliminatethe PVA, followed by sintering at temperatures1,450oC for 2 h with 5oC/min heating andcooling rates [7]. Phase identification of Ba(Ti1-xMnx)O3 powders and ceramics wasperformed by X-ray diffraction (XRD). The

densities of the sintered ceramics weremeasured by Archimedes method. Micro-structural characterization of sinteredspecimens was examined by scanning electronmicroscopy (SEM).

3. RESULTS AND DISCUSSION3.1 Phase Investigation

The XRD patterns of Ba(Ti1-xMnx)O3powder at different Mn concentrations areshown in Figure 1. For the undoped andBa(Ti0.99Mn0.01)O3, the powders werecharacterized as tetragonal phase (t-BaTiO3).The small amount of hexagonal phase is alsopresent with increasing Mn substitution witha complete transformation to hexagonal phasewhen x reaches 10 mol%, as shown in Figure1. This observation contradicts somewhatwith the results of Langhammer et al. [5] andWang et al. [8] who reported that theformation of pure hexagonal phase wasobserved as Mn concentration in Ba(Ti1-xMnx)O3 exceeded 18 and 15 mol%,respectively. This is possibly due to uses ofdifferent milling methods. In those eitherworks, a conventional ball-milling wasemployed, while the vibro-milling used in thispresent work results in finer powders withapparently more reactivity, hence the purehexagonal phase is formed more easily atlower Mn concentration in the present study.However, in this work the presence ofBa2Ti5O12 is also observed in Ba(Ti1-xMnx)O3powders, similar to the reported by Wanget al. [8,9]. For Ba(Ti1-xMnx)O3 ceramics, theXRD studies show that the crystal structure atroom temperature also changes fromtetragonal to hexagonal as Mn concentrationincreases. The sample with x = 0 shows thetetragonal phase, while x = 0.01, the hexagonalBT phase and small amount of Ba2Ti5O12

occurs. When x increases from 0.02 to 0.2, itis found that Ba2Ti5O12 disappears and theextent of hexagonal phase increases with

Chiang Mai J. Sci. 2010; 37(1) 167

increasing Mn substitution with a completehexagonal phase when x reaches 0.1. Withcomparison to copper doping, Langhammeret al. [10,13] have reported that the phasechanged nearly completely to hexagonal at only

2.0 mol% Cu, if a Ba:Ti ratio of 1.03 wasapplied. This indicated the effectiveness of Cudoping in stabilizing hexagonal phase inBaTiO3. Moreover, the presence of Ba2Ti5O12was not observed in that study.

Figure 1. XRD patterns of Ba(Ti1-xMnx)O3 powders with x = 0 to 0.2.

3.2 Microstructural AnalysisThe SEM micrographs of the Ba

(Ti1-xMnx)O3 ceramics with x=0.01 to 0.2sintered at 1,450oC for 2 h are shown inFigure 2. For Ba(Ti1-xMnx)O3 ceramics withx<0.10, the micrographs reveal that a bimodalmicrostructure consisting of well-grown platelike grains larger than 100 μm with low aspectratios embedded in a fine-grained (about 3-5μm) matrix. The extensive grain growthsuggested the occurrence of a liquid phaseassisted sintering process accompanied by asecondary recrystallization, at which somegrains grow rapidly at the expense of smallerones by a mechanism similar to Ostwaldripening. These growth mechanisms are thesame type described by Langhammer et al. [4]and Wang et al. [8, 9] As x increases, the well

grown grains decrease with the extent of fine-grains increases. Moreover, the micrographsalso show pores at grain boundaries,intragrains (close pore) and cracks. Thebimodal microstructure with distinct sizesvanishes as the x value exceeds 0.1, and themicrostructure becomes more homogeneouswith an average grain size of 1-3 μm.Interestingly, Keith et al. [7] have reported thatthe microstructure of Ni, Co and Ga-dopedBT ceramics formed by sintering at 1,450oCfor 2 h with x=0.05 (single-phase materialsof the hexagonal polymorph were formed)consist of small, uniform grains. Therefore,the exaggerated grain growth with longplate-like grains in excess of 100 μm with lowaspect ratios was observed only in the caseof Mn-doped samples.

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Figure 2. SEM micrographs of the Ba(Ti1"xMnx)O3 ceramics with (a) x = 0.01,(b) x = 0.02, (c) x = 0.05, (d) x = 0.10, (e) x = 0.15, (f) x = 0.20 sintered at 1,450oC.

TABLE 1. List of the phases present and density of Ba(Ti1"xMnx)O3 powders and ceramics.

Composition Phasespowder Phasesceramic Density (g/cm3)

0 t-BaTiO3 t-BaTiO3 5.81

0.01 t-BaTiO3 t-BaTiO3Ba2Ti5O12 h-BaTiO3 5.72

Ba2Ti5O12

0.02 t-BaTiO3 t-BaTiO3h-BaTiO3 h-BaTiO3 5.63Ba2Ti5O12

0.05 t-BaTiO3 t-BaTiO3h-BaTiO3 h-BaTiO3 5.65Ba2Ti5O12

0.10 h-BaTiO3 h-BaTiO3 5.73Ba2Ti5O12

0.15 h-BaTiO3 h-BaTiO3 5.72Ba2Ti5O12

0.20 h-BaTiO3 h-BaTiO3 5.73Ba2Ti5O12

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4. CONCLUSIONSAddition of Mn into BaTiO3 can stabilize

hexagonal polymorph at room temperature.The XRD analysis showed that the crystalstructure of Ba(Ti1-xMnx)O3 at roomtemperature changed from tetragonal tohexagonal as Mn concentration increased,which caused the decrease in the dielectricconstant and loss. The microstructure ofBa(Ti1-xMnx)O3 ceramics at low concentrationrevealed a bimodal feature consisting of well-grown plate like grains larger than 100 μmwith low aspect ratios embedded in a fine-grained (about 3–5 μm) matrix. As the xvalue increased, the well grown grainsdecreased with the extent of fine-grainsincreased.

ACKNOWLEDGEMENTSThis work was jointly supported by the

National Synchrotron Research Centre(NSRC), Thailand under contact No. 2550/02 and PS04/2550.

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