Abstract :
[en] Linked to their high chemical stability in air for temperatures from 875 K to higher, oxide compounds appear as required materials for applications in thermoelectric energy conversion at high temperature. Among the different studied oxide compositions, misfit cobaltite Bi2Sr2Co2Ox and doped calcium manganate CaMnO3-d have attracted much attention, as p and n-type oxide thermoelectric elements, respectively. Bi2Sr2Co2Ox (abbreviated BSC-222) material exhibits a layered structure, composed of CoO2 layers which alternate with Bi2Sr2O4 block layers along the c-axis. Reported studies show that a ZT value of 0.27 can be reach in the p-type polycrystalline bulk material of this oxide compound at 973 K. CaMnO3-d compound (abbreviated as CMO) has an orthorhombic perovskite-type structure and exhibit relatively high ZT's, in polycrystalline form, which is 0.3 at 1000 K.
Thermoelectric performances are closely linked to sample’s microstructure, i.e. the shape, size, or alignment of grains, size and distribution of porosity of samples. However, such control on the microstructure of materials is difficult to reach by using conventional solid-state reaction processing way. With the aim of developing a processing method which can be easily transposable to an industrial level in terms of simplicity, speed, large-scale production and cost, a pilot spray-drying processing method has been developed for the production of BSC-222 and CMO thermoelectric powders. Spray-drying is a processing way in which preparing powders is achieved by passing a solution, or a suspension, in a stream of hot air. By using solutions, or suspensions, of precursor compounds, a more homogeneous dispersion of the cations into the material is reaching. Spray-dried powders are so more reactive, which can help reducing the sintering temperature or duration.
This study deals with the preparation of BSC-222 and doped CMO thermoelectric powders by the development of spray-drying method. Contrary to CMO powders, prepared from solution of cations, BSC-222 powders have been prepared from Bi2O3-Co3O4-SrCO3 suspensions. For that purpose, a preliminary study of the stability of the precursors in the suspension’s solvent has been conducted in order to favor a homogeneous distribution of precursors in prepared spray-dried powders. After calcination thermal treatment, spray dried powders with BSC-222 or CMO compositions have been successfully obtained. Polycrystalline BSC-222 and CMO bulk materials have been then prepared by conventional sintering. In this study, we report on the impact of the use of spray-dried BSC-222 and CMO powders on the sintering and the thermoelectric properties, carried out up to 1000 K, of such prepared polycrystalline bulk materials.
