• Journal of the Korean Ceramic Society
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• v.32 no.5
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• pp.582-586
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• 1995

The crystal structures of (Na0.3Sr0.7)(Ti0.7M0.3)O3 (M=Ta, Nb) compounds were determined using the Rietveld method. Due to the tilting of a oxygen octahedron, (Na0.3Sr0.7)(Ti0.7Nb0.3)O3 had a superlattice of doubled a, b and c of simple perovskite. The crystal structure of (Na0.3Sr0.7)(Ti0.7M0.3)O3 was tetragonal with a space group 14/mmm. The crystal structure of (Na0.3Sr0.7)(Ti0.7M0.3)O3 was a cubic with space group Pm3m, in which no tilting of oxygen octahedron was observed. The difference in the oxygen tilting of these two materials was due to the larger covalency of Nb-O bond than that of Ta-O bond, which induced a strong $\pi$Nb0 bonding in (Na0.3Sr0.7)(Ti0.7M0.3)O3. Therefore, the higher transition temperature of (Na0.3Sr0.7)(Ti0.7M0.3)O3 could be related to the larger tilting of oxygen octahedron.

• Journal of Ceramic Processing Research
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• v.13 no.spc2
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• pp.341-345
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• 2012

[Li0.055(K0.5Na0.5)0.945](Nb1-xTax)O3 (0.05 ≤ x ≤ 0.25) ceramics were prepared by the partial sol-gel (PSG) method to improve the microstructure homogeneity of Ta5+ ion and were compared to those prepared by the conventional mixed oxide (CMO) method. For the PSG method, Ta(OC2H5)5 was directly reacted with calcined [Li0.055(K0.5Na0.5)0.945]NbO3 powders and the specimens sintered at 1100 ℃ for 5 hrs showed a single phase with a perovskite structure. Compared to the specimens prepared by conventional mixed oxide powders, the relative ratio of tetragonal phase to orthorhombic phase of the sintered specimens prepared by Ta(OC2H5)5 was larger than that of the sintered specimens prepared by Ta2O5. The electromechanical coupling factor (kp), piezoelectric constant (d33) and dielectric constant (εr) of the sintered specimens were increased with Ta5+ content. These results could be attributed to the decrease of the orthorhombic-tetragonal polymorphic phase transition temperature (To-t), which could be evaluated by oxygen octahedral distortion. Strain of the sintered specimens prepared by the PSG method was higher than that of specimens prepared by the CMO method due to the increase of relative density. The effects of crystal structure on the strain characteristics of the specimens were also discussed.

• Journal of the Korean Ceramic Society
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• v.40 no.12
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• pp.1177-1182
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• 2003

The porous sensing materials for BaTiO$_3$ gas sensors were fabricated by adding the graphite powders. The crystalline structure and microstructure of the porous BaTiO$_3$-based ceramics were studied. All the sintered bodies showed a tetragonal perovskite structure. The porosity increased with increasing graphite contents. This is mainly due to an enhanced evolution of CO and $CO_2$ gases resulting from the exothermic reactions of graphite and oxygen during the sintering. It was found that the discrepancy in the resistivities measured in air and CO atmospheres at high temperatures (>∼20$0^{\circ}C$) became remarkable with increasing temperature. The sensitivity of CO gas increased with porosity, since the reactions between CO gas and $O_2$$^{[-10]}$ and between CO gas and $O^{[-10]}$ are active due to the formation of many reaction sites. The porous BaTiO$_3$-based ceramics could be promising as a sensing material for CO gas sensors.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.11a
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• pp.32-32
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• 2008

Microstructure and positive temperature coefficient of resistivity (PTCR) characteristics of $0.9BaTiO_3-0.1(Bi_{0.5}Na_{0.5})TiO_3$ [BaBiNT] ceramics doped with $Nb_2O_5$ were investigated in order to develop the Pb-free high Curie temperature ($T_c$)(>$160^{\circ}C$) PTC thermistor. The BaBiNT ceramics showed a tetragonal perovskite structure, irrespective of the added amount of $Nb_2O_5$. They also have a homogeneous microstructure. The resistivity of BaBiNT ceramics was gradually decreased by doping $Nb_2O_5$, which might be due to $Nb^{+5}$ ions substituting for $Ti^{+4}$ sites. The PTCR characteristics of BaBiNT ceramics appeared when the amount of doped $Nb_2O_5$ exceeded 0.0025mol%. Moreover, the abrupt grain growth was observed for the 0.03mol% $Nb_2O_5$added BaBiNT ceramics. It showed an especially high $T_c$ of approximately $172^{\circ}C$ and good PTCR characteristics of a high $\rho_{max}/\rho_{min}$ ratio ($2.96\times10^3$), a high resistivity temperature factor (11.40/$^{\circ}C$) along with a relatively low resistivity ($3.5\times10^4\Omega{\cdot}cm$).

• The Korean Journal of Ceramics
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• v.4 no.2
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• pp.83-89
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• 1998

The structure refinements and the electrical and magnetoelectric measurements were performed for BIT.1BF and BIT.2BT. The tetragonal distortion of the ab plane became lessened with the addition of $4BiFeO_3 into Bi_4Ti_3O_{12}$ significantly. However, the tilting of the outer-oxygen octahedra of the perovskite unit and the elongatin of the $(Bi_2O_2)^{2+}$ layers became more pronounced. For the both phases, the bariations of dielectric properties and electrical conductivities at high temperatures showed that the ferroelectic I-rerroelectric II phase transition existed before reaching the Curie temperature. The electrical conductivity became higher with the increase of $Fe^{3+}$ ions, implying that the electron transfer increased correspondingly. The magnetoelectric effect was observed linear up to ~8 kOe, which was stronger in BIT.1BF than BIT.2BF. This behavior indicates that the distortion of the ab plane may affect the induced polarization as well as magnetic moment.

• Korean Journal of Materials Research
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• v.25 no.7
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• pp.317-321
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• 2015

The effects of Nb doping on the crystal structure, microstructure, and dielectric ferroelectric and piezoelectric properties of $(Bi_{0.5}Na_{0.5})_{0.935}Ba_{0.065}Ti_{(1-x)}Nb_xO_3-0.01SrZrO_3$ (BNBTNb-SZ, with ${\chi}=0$, 0.01 and 0.02) ceramics have been investigated. X-ray diffraction patterns revealed that all ceramics have a pure perovskite structure with tetragonal symmetry. The grain size of the ceramics slightly decreased and a change in grain morphology from square to spherical shape was observed in the Nb-doped samples. The maximum dielectric constant temperature ($T_m$) increases with increasing amount of Nb; however, ferroelectric-relaxor transition temperature ($T_{F-R}$) and maximum dielectric constant (${\varepsilon}_m$) values decrease gradually. Nb addition disrupted the polarization hysteresis loops of the BNBT-SZ ceramics by leading a reduction in the remnant polarization coercive field and piezoelectric constant.

• Bulletin of the Korean Chemical Society
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• v.16 no.7
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• pp.600-603
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• 1995

A series of samples in the Sm1-xSrxCoO3-y(x=0.00, 0.25, 0.50, 0.75 and 1.00) system has been prepared at 1200 ℃ under ambient atmosphere. The X-ray diffraction patterns of the samples with x=0.00 and 0.25 are indexed with orthorhombic symmetry like GdFeO3 and x=0.50 appears to be perfectly cubic. In the tetragonal system (x=0.75), the structure is similar to that of SrCoO2.80. The composition of x=1.00, SrCoO2.52, shows the brownmillerite-type structure. The reduced lattice volume is increased with x value in this system. The chemical analysis shows the τ value (the amount of the Co4+ ions in the system) is maximized at the composition of x=0.50. Nonstoichiometric chemical formulas are determined by the x, τ and y values. The electrical conductivity has been measured in the temperature range of 78 to 1000 K. The activation energy is minimum for those of x=0.25 and x=0.50 with metallic behavior. First-order semiconductor-to-metal transition of SmCoO3 is not observed. Instead, a broad, high-order semiconductor-to-metal transition is observed. In general, the effective magnetic moment is increased with increasing τ values at low temperature. At high temperature, the magnetic moment is maximum for that of x=0.00. The 3d-electrons are collective and give ferromagnetism in x=0.50.

• Journal of the Mineralogical Society of Korea
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• v.1 no.2
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• pp.94-103
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• 1988

Diamond anvil cell (DAC) interfaced with a YAG laser heating system has been used to study the phase transformations on perovskite structured titanates ($BaTiO_3$, and $PbTiO_3$) and to synthesize the silicate perovskite phase from the orthopyroxenes of $MgSiO_3$ and $(Mg_{0.87},\;Fe_{0.13})SiO_3$. $BaTiO_3$ and $PbTiO_3$ transform from tetragonal phase to cubic at the pressures of approximately 2.6 GPa and 4.0 GPa at room temperature, respectively. Cubic phases of the both show wide range of stability in the extended in-situ high pressures and high temperature regions. Starting orthoenstatite of $MgSiO_3$ has yielded the perovskite phase as the major structure with ilmenite, gamma-spinel, betta-spinel and stishovite phases at ~38 GPa and ${\sim}1,000^{\circ}C$. $(Mg_{0.87},\;Fe_{0.13})SiO_3$ has shown the perovskite as the major phase with betta-spinel, stishovite and enstatite phases at ~35 GPa and ${\sim}1,000^{\circ}C$. The ilmenite phase does not occur at this condition.

• Journal of the Korean Ceramic Society
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• v.49 no.3
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• pp.253-259
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• 2012

In this study, crystal structural changes and dielectric properties of $(1-x)Pb(Yb_{1/2}Nb_{1/2})O_3-xPbTiO_3$ ((1-x)PYN-xPT) solid solutions were measured and analyzed with respect to the $PbTiO_3$ (PT) contents ($0.40{\leq}x{\leq}0.60$). From X-ray diffraction (XRD) measurements, (1-x)PYN-xPT solid solutions showed changes of the crystal structure from pseudocubic ($0.40{\leq}x{\leq}0.44$) to tetragonal ($0.52{\leq}x{\leq}0.60$) on increasing PT contents and exhibited the coexistence of pseudocubic and tetragonal phase near the morphotropic phase boundary (MPB) composition ($0.46{\leq}x{\leq}0.50$). The dielectric constant showed a maximum value at x = 0.46 and the maximum values in (1-x)PYN-xPT decreased with higher PT contents. The phase transition temperatures of (1-x)PYN-xPT solid solutions increased over the whole composition ranges tested ($0.40{\leq}x{\leq}0.60$).

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.25 no.3
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• pp.198-203
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• 2012

In this study, $[Li_{0.04}(Na_{0.5}K_{0.5})_{0.96}](Nb_{0.86}Ta_{0.10}Sb_{0.04})O_3+xSrO$ (x=0, 0.0025, 0.005, 0.0075) ceramics were synthesized by the conventional mixed oxide method. The X-ray diffraction patterns demonstrated that ceramics possessed single perovskite structure. The SEM images indicate that microstructure can be obviously affected by a small amount of added SrO. The phase transition temperature tetragonal-cubic($T_c$) and orthorhombic-tetragonal($T_{o-t}$) shifts downward and upward with the increase of Sr addition, respectively. The excellent piezoelectric properties of $d_{33}=170[pC/N]$, $k_p=0.37$, $Q_m=64.12$, $T_{o-t}=153^{\circ}C$ and $T_c=370^{\circ}C$ were obtained from the 0.25 mol% Sr added ceramics sintered at $1,120^{\circ}C$ for 1 h.