Yoshihiko Sadaoka

Ionic Conductivity of Solid Electrolytes Based on Lithium Titanium Phosphate

Solid electrolytes based on lithium titanium phosphate were prepared, and their phase, porosity of the sintered pellets, and electrical conductivity were studied. The conductivity was increased and the porosity decreased greatly by partially replacing Ti4+ and P5+ in with M3+ and Si4+ions, respectively. The maximum conductivity at 298 K is for . The conductivity was considerably increased by the mixing of binders such as or with . The main reason for the conductivity enhancement of these electrolytes seems to be attributable to the increase of the sintered pellet density with the enhancement of the lithium concentration at the grain boundaries.

Humidity sensors based on polymer thin films

Studies on humidity sensors fabricated with organic polymers for the last 10 years are reviewed. Several useful methods for improving the characteristics of humidity sensors based on polymers are proposed. In the case of a resistive-type sensor, cross-linking of hydrophilic polymers or formation of interpenetrated polymer networks with a hydrophobic polymer makes the hydrophilic polymers durable at high humidities. Graft polymerization is another method of preparing water-resistive humidity sensors. In the case of capacitive-type sensors, cross-linking is also useful to modify the hydrophobic polymer to produce a sensor with small hysteresis, high selectivity and high sensitivity.

Ionic conductivity of the lithium titanium phosphate (Li/sub 1+x/M/sub x/Ti/sub 2-x/(PO/sub 4/)/sub 3/, M=Al, Sc, Y, and La) systems

High lithium ionic conductivity was obtained in Li/sub 1+X/M/sub X/Ti/sub 2-X/(PO/sub 4/)/sub 3/ (M=Al, Sc, Y, and La) systems. Lithium titanium phosphate, LiTi/sub 2/(PO/sub 4/)/sub 3/, is composed of both TiO/sub 6/ octahedra and PO/sub 4/ tetrahedra, which are linked by corners to form a three dimensional network, with a space group R3-barC. Some workers have already described that the conductivity increased considerably if Ti/sup 4+/ in LiTi/sub 2/(PO/sub 4/)/sub 3/ was substituted by slightly larger cations such as Ga/sup 3+/(1),Sc/sup 3+/(2), and In/sup 3+/(3,4). These results are similar to each other because of their close ionic radii. In this communication, substitution effects of Ti/sup 4+/ in LiTi/sub 2/(PO/sub 4/)/sub 3/ by various ions (Al/sup 3+/, Sc/sup 3+/, Y/sup 3+/, and La/sup 3+/) on their conductivities are reported.

Ionic conductivity of lanthanoid silicates, Ln10(SiO4)6O3(Ln = La, Nd, Sm, Gd, Dy, Y, Ho, Er and Yb)

Ionic conductivities have been investigated for lanthanoid silicates of the Ln10(SiO4)6O3 solid solution series and related compounds. The activation energy and conductivity at 500 °C were estimated to be 69 kJ mol–1 and 1.8 × 10–4 S cm–1 for La10(SiO4)6O3 and 71 kJ mol–1 and 1.5 × 10–4 S cm–1 for Nd10(SiO4)6O3. The a and c lattice constants of the hexagonal phase decreased with decreasing radius of the Ln3+ ion for Ln10(SiO4)6O3(Ln = La, Nd, Sm, Gd and Dy). The activation energy increased and the conductivity decreased when Ln3+ ions with smaller ionic radii were used. The sole carrier in these materials is the O2– ion.

Screen-printed perovskite-type thick films as gas sensors for environmental monitoring

Abstract Thick films of LaFeO 3 and SmFeO 3 were fabricated by screen-printing technology on alumina substrates with comb-type Au electrodes. The perovskite-type oxide powders used for the preparation of the thick films have been prepared by the thermal decomposition at 700°C of hexacyanocomplexes, Ln[Fe(CN) 6 ] ·  n H 2 O. These powders are ultrafine, homogeneous, and free of intragranular pores. The films have been fired at different temperatures in the 750–1000°C range, in N 2 and air atmospheres. The gas-sensitive electrical response of the thick films have been tested in laboratory, in environments with different gases (CO and NO 2 ) in dry and wet air. For field tests, the prototype sensors have been placed beside a conventional station for environmental monitoring. The electrical response of the thick films has been compared with the results of the analytical instruments for environmental monitoring. The same trend was observed for both systems, with very promising results.

Electrical property and sinterability of LiTi2(PO4)3 mixed with lithium salt (Li3PO4 or Li3BO3)

Abstract A lithium salt (Li 3 PO 4 or Li 3 BO 3 ) was added to LiTi 2 (PO 4 ) 3 to obtain a dense pellet of the phosphate. The porosity of the sintered pellets decreased and the conductivity was enhanced by the utilization of a binder. A maximum conductivity of 3.0 × 10 −4 S·cm −1 at 298 K was obtained for a sample of LiTi 2 (PO 4 ) 3 −0.2Li 3 BO 3 . The activation energy for the lithium migration at grain boundaries was decreased by the addition of lithium salt. The reason for the conductivity enhancement was attributed to a decrease in the activation energy for the lithium migration at the grain boundary and an increase in the contact area between grains. The conductivity of the bulk component was also increased by the enhancement of Li + -ion migration at grain boundaries.

Crystallographic characterization and NO2 gas sensing property of LnFeO3 prepared by thermal decomposition of LnFe hexacyanocomplexes, Ln[Fe(CN)6]·nH2O, Ln = La, Nd, Sm, Gd, and Dy

Finer perovskite-type LnFeO3 (Ln=La, Nd, Sm, Gd, and Dy) powders were prepared by the thermal decomposition of heteronuclear complexes, Ln[Fe(CN)6]·nH2O. The prepared LnFeO3 showed a p-type semiconductive behavior and the highest enhancement of conductance due to NO2 exposure was observed for SmFeO3 sensor. The atomic ratio of adsorbed oxygen Oad increased with the surface coverage of Ln, expressed as Ln/(Ln+Fe). Experimentally, the atomic ratio of Ln/(Ln+Fe) was estimated to around 0.6 and the largest value, 0.65, was observed for SmFeO3. For SmFeO3, the distance between the central Sm ion and nearest four ions of oxygen is comparable with the sum of crystal radius of Sm3+ (C.N.=6) and O2−, and the distance between Sm ion and fifth and higher oxygen is longer than the expected length form Shannon’s crystal radius. The observed longer length for C.N.=5 or more is comparable to the length for Sm2+O bond. The possible valence of Ln ion is directly related with the electron configuration of Ln species and only for Sm the existence of the divalent cation is expected. The divalency of the Sm3+ in the surface layer is suggested by the configurations of coordinated oxygen. The highest sensitivity for NO2 observed for SmFeO3 would be attributed to the formation of Fe defects due to the higher coverage of Ln and the divalency of Sm3+ in SmFeO3.

NO2 sensitive LaFeO3 thin films prepared by r.f. sputtering

Abstract LeFeO 3 thin films with different thicknesses have been fabricated by the r.f. magnetron sputtering method on Al 2 O 3 substrates with comb-type Au electrodes. The influence of annealing temperatures and times on the NO 2 sensitivity of the thin films has been investigated. The thin films are annealed at temperatures from 600 to 1000 °C in flowing air for 30 or 60 min. The conductivity of thin films increases noticeably when NO 2 gas is introduced into the measuring chamber, and returns to its original level after NO 2 is removed. The NO 2 response curve of LaFeO 3 thin films shows two distinct phases: an initial fast and large conductivity change, followed by a much slower change. These phases are attributed to the surface chemisorption process of NO 2 and the oxidation process of the bulk, respectively. The materials structure plays a fundamental role in its NO 2 response, while the influence of film thickness is less important. The annealing temperature and time of LaFeO 3 sputtered thin films determine their NO 2 sensitivity. The thinner films show higher resistivities, slightly greater NO 2 sensitivities and less stable results.

Ionic conductivity and sinterability of lithium titanium phosphate system

Abstract Lithium titanium phosphates mixed with various metal ions of M 3+ (M=Al, Cr, Ga, Fe, Sc, In, Lu, Y, or La), Li 1+ x M x Ti 2− x (PO 4 ) 3 systems, were prepared, and their properties were investigated. The conductiv ity was enhanced and the porosity of the sintered pellets decreased by the partial replacement of Ti 4+ with the M 3+ ion. The porosity was considerably influenced by the ionic radius of the M 3+ ion. The sinterability was greatly related to the increase of lithium concentrations at the grain boundary. The conductivity enhancement by the substitution mainly resulted from the densification of the sintered pellets.

The Electrical Properties of Ceramic Electrolytes for LiM x Ti2 − x ( PO 4 ) 3 + yLi2 O , M = Ge , Sn , Hf , and Zr Systems

The electrical properties of systems of , were examined in detail. The conductivity and the sinterability increased with the amount of excess lithium oxide in the phosphate. The secondary phase acts as a flux to accelerate the sintering process and to obtain high conductivity grain boundaries. The conductivity decreased and the activation energy of the bulk component for Li+ migration increased by the partial substitution of Tr4+for M4+ in systems of . A minimum activation energy of 0.28–0.30 eV, was obtained for the sample with ca. 1310 A3 in the cell volume. has the most suitable tunnel size for a Li+ migration through the NASICON‐type network structure.