Yasunobu Mizutani

Aging and Raman scattering study of scandia and yttria doped zirconia

Abstract The changes of electrical conductivity during annealing at 1273 K in stabilized zirconia with 8 mol% Y 2 O 3 (8YSZ), 8 mol% Sc 2 O 3 (8ScSZ) and 11 mol% Sc 2 O 3 (11ScSZ) have been examined. 8YSZ and 8ScSZ showed significant decrease in conductivity during the initial stage of annealing, whereas 11ScSZ showed no conductivity decrease. The Raman scattering technique was used to analyze the phase change in the annealed samples. The observed Raman spectrum suggested that the conductivity decrease in 8YSZ and 8ScSZ was caused by a local ordering toward the formation of tetragonal phase in the cubic matrix by annealing at 1273 K.

Development of high-performance electrolyte in SOFC

Abstract The electrical conductivity and mechanical properties of Sc2O3−ZrO2(ScSZ) Prepared by a sol-gel method were investigated. To improve mechanical strength, the composite materials of ScSZ and Al2O3 were investigated. A flat-plate type SOFC, with 11 mol% ScSZ used as an electrolyte, was fabricated. In a cell test a maximum power density of 1.61 W/cm2 was obtained at 1273 K.

Effect of Bi2O3 additives in Sc stabilized zirconia electrolyte on a stability of crystal phase and electrolyte properties

Sintering properties, microstructure, crystal phase transformation, electrical conductivity, mechanical strength, and electrolyte performance of Bi2O3-doped Sc2O3-stabilized zirconia prepared by homogeneous precipitation using hydrolysis were investigated. Adding 1 mol% Bi2O3 decreased the sintering temperature of 10 mol% Sc2O3-doped zirconia (10ScSZ) above 300 °C, while accelerating the growth of zirconia grains, so dense sintered ceramics in the cubic phase were formed in a temperature range of 1000–1100 °C. The addition of 1 mol% Bi2O3 to the 10ScSZ inhibited the cubic–rhombohedral phase transformation. Sufficient conductivity of 0.33 S/cm at 1000 °C and 0.12 S/cm at 800 °C for an electrolyte of solid oxide fuel cells (SOFC) was attained for 1 mol% Bi2O3–10 mol% Sc2O3–codoped cubic zirconia (1Bi10ScSZ) sintered at 1200 °C. A maximum power density of 1.61 W/cm2 was obtained at 1000 °C using 1Bi10ScSZ as the electrolyte in SOFC.

Nonstoichiometry and electrical transport in Sc-doped zirconia

Abstract The results of electrical conductivity measurements on ZrO 2 :Sc (ScSZ) ceramics as a function of oxygen activity and temperature are presented. The influence of the microstructure on electrical transport is discussed and the experimental data for dense microcrystalline specimens have been compared with those obtained for dense nanocrystalline thin films. From these results the electronic and ionic contribution to the electrical transport has been determined and correlated with microstructure. It has been found that nanocrystalline ScSZ films possess quite different nonstoichiometry compared with that observed for microcrystalline specimens in the fact that the electronic conductivity is enhanced so that it dominates the total conductivity in reducing atmospheres.

Changes of Internal Stress in Solid-Oxide Fuel Cell During Red-Ox Cycle Evaluated by In Situ Measurement With Synchrotron Radiation

The internal stress in anode-supported solid-oxide fuel cells (SOFCs) was evaluated by in situ measurement using high-energy x-ray synchrotron radiation. The oxidized cell had a compression of ∼400 MPa in the c-ScSZ electrolyte thin film and a tension of 50-100 MPa in the NiO-YSZ anode substrate at room temperature. The internal stress decreased with increasing temperature, becoming approximately zero at 1000 K. Although the internal stress returned to its initial value after the thermal cycle, the stress decreased to ∼200 MPa in the electrolyte after the reduction cycle because of the decrease of the coefficient of thermal expansion mismatch between the electrolyte and anode. The red-ox cycle would be detrimental for anode-supported SOFC.

In Situ Synchrotron Measurement of Internal Stresses in Solid-Oxide Fuel Cell during Red-Ox Cycle

The internal stress in solid-oxide fuel cells (SOFCs) was evaluated during the thermal, reduction and re-oxidation cycles by using high-energy X-ray synchrotron radiation of about 70 keV at Beam line BL02B1 of SPring-8. The oxidized cell has a compression of about 400 MPa in the c-ScSZ electrolyte and a tension of 50-100 MPa in the NiO-YSZ anode at room temperature. In-situ measurement during the thermal cycle in an air atmosphere, the internal stress decreased with increasing temperature, becoming approximately zero at 1000 K. After the thermal cycle, the internal stress returned to its initial value. In the measurement during the reduction cycle, the internal stress was smaller than that measured during the cooling cycle after the anode was reduced from NiO-YSZ to Ni-YSZ. In the re-oxidation cycle of a reduced cell, the internal stress in the electrolyte went into tension above 800 K when the anode was re-oxidized from Ni-YSZ to NiO-YSZ. This tensile stress is responsible for possible fracture of unit cells in SOFCs.

Development of Planar Type SOFC Stacks Operable Under Rapid Starting

Toho Gas Co. Ltd. and Sumitomo Precision Products Co. Ltd. have been jointly developing a SOFC system using scandia-stabilized zirconia (ScSZ) electrolyte cells. Especially, we focused the scandia tetragonal zirconia polycrystalline (Sc-TZP) electrolyte, because the Sc-TZP electrolyte has good mechanical and electrical properties, therefore high reliability and power generation characteristics are expected. We have been developing the 1kW SOFC system using Sc-TZP electrolyte cells as proof of concept since 2002. The 1kW SOFC combined heat and power (CHP) system was installed in The 2005 World Exposition, Aichi, Japan (EXPO2005), and the system successfully operated during about six months. During the demonstration, some troubles caused by balance of plant (BOP) system and controlling system, and these experiences are useful to our system development. The target of our developing system is a small-scale commercial CHP application and target power range is below 10kW class. To apply such a small-scale commercial use, the rapid start up is very attractive for customers in Japanese market. In this study, we have been developing the rapid starting system. To shorten the start up time, reducing the volume of cell stack and strengthening the cell are developed in parallel. Because heating capacity is very affected factor to determine the start up time. To reduce the volume of cell stack, the improvement of cell performance is very attractive. For the electrolyte-supported type cell, the electrical conductivity of electrolyte material is very important factor on the cell performance. On the other hand, to realize the rapid start up system, the mechanical strength of electrolyte is also important factor, because in the rapid start up conditions, large temperature distribution may be easily occurred, and it leads the cell broken. The relation between electrical conductivity and mechanical strength is trade off in the electrolyte material, and then we focused the electrolyte in the range from 4mol% to 7mol%, and demonstrated that these materials have good combination of electrical and mechanical properties. To estimate the suitable composition, the mechanical strength of electrolyte from room temperature to 1073K that is the maximum operating temperature of our system were investigated. And piston on ring (POR) method was also investigated to estimate the strength of actual electrolyte sheets. Part of this work was performed as R & D program of New Energy and Industrial Technology Development Organization (NEDO).Copyright

Correlative Equation of Temprature Distribution and Calculation of Thermal Stress in Rapid Heating Thermal Fracture Test by Measuring Repulsive Load(Special Issue on Synthesis and Evaluation of Ceramic Materials)

In the thermal fracture testing technique proposed previously by the present authors, a one-side heated slab specimen is held by three points and the repulsive load appearing during heating is measured by the load cell placed at the upper point of the heated side. By this way, the thermal fracture stress of a ceramic specimen can be estimated from the repulsive load without knowing the heat transfer coefficient unlike the water quenching thermal shock test. However, it needs numerical calculation for every specimen at each experimental condition.In this paper, the numerical calculation of non-steady-state temperature distribution in an infinite plate, heated from one side with a constant heat flux were conducted in order to calculate the magnitude of thermal stress from the repulsive load data in the above-mentioned thermal fracture test. The temperature dependences of thermal conductivity, λ*=eAT and thermal diffusivity κ*=eBT were introduced in the calculation to realize a practical heat conductive condition. The time dependent temperature distributions were unified into a correlative equation that includes the coefficients of temperature dependence of material properties A, B, supplied heat flux Qi and Fouriers number ηi=κit/h2. By using this equation, the repulsive load and thermal stress at fracture were calculated for several ceramics, and the influences of material properties were discussed. The correlative equation is also useful to define the experimental conditions in thermal fracture tests.