Effect Of Sintering Techniques On The Texture Degree And Electrical Properties Of Ca₃Co₄O₉ Thermoelectric Ceramics

Nowadays as the situation of non-renewable energy shortage becomes increasingly severe,thermoelectric materials have become one of the most common new energy materials under research.By realizing the direct transformation between thermal energy and electric energy,thermoelectric materials can greatly improve the energy efficiency and reduce environmental pollution.Among all the thermoelectric materials,Ca₃Co₄O₉ layered ceramics,which is mainly utilized as aerospace thermoelectric materials at high temperatures above 600?,shows good application prospects due to their high temperature resistance,high chemical stability and non-toxicity.However,as a new thermoelectric material,although Ca₃Co₄O₉has many advantages,compared with traditional thermoelectric materials,its ZT value still has a lot of room to improve.At present,the modification of bulk Ca₃Co₄O₉ polycrystalline ceramics mainly concentrates on two aspects.One is to increase the ZT value of samples by doping.However,only by doping precious metals or rare earth can the properties of samples be significantly improved,which undoubtedly increases the environmental burden.The other way is to form texture orientation to enhance the properties of samples,basing on the unique layered structure and anisotropy of Ca₃Co₄O₉.Thermoelectric properties in specific directions are mainly achieved by using new sintering technologies such as hot pressing?HP?and spark plasma sintering?SPS?.In order to optimize the electrical properties of Ca₃Co₄O₉ ceramics,the idea of improving the electrical properties of samples by improving the texture degree was utilized in this paper.Among them,the powders needed for preparing bulk polycrystalline Ca₃Co₄O₉ ceramics are synthesized by sol-gel method.In order to improve the texture degree of samples,a new oscillatory pressure sintering?OPS?technique is adopted in this paper.Considering that pressure sintering is not beneficial for the improvement of thermal properties of samples,porous bulk Ca₃Co₄O₉ ceramics were prepared by sacrificial template method to reduce the thermal conductivity of samples.The main research contents and conclusions are as follows:?1?In the study of the preparation of ceramic powders,the combustion process of precursor gel was studied.The combustion products of Co₃O₄ and CaCO₃ mesophase were gradually transformed into Ca₃Co₄O₉ phase at 500?,and the weight of the obtained powders was about 10%of that of the gels.Besides,long time dwelling at 900?would contribute to the further crystallization and growth of Ca₃Co₄O₉ grains.?2?In terms of electrical properties,the samples sintered under oscillatory pressure have higher Seebeck coefficient,conductivity and power factor(1023 K,S=190?V/K,?=100?^-1·cm^-1,PF=356?W/m·K²)than those sintered under constant pressure.The conduction mechanism of the sample at high temperature conforms to the small polaron hopping conductive?SPHC?model,and the coupling oscillatory pressure can reduce the activation energy required for the hopping process(E_{a HP}=101.76 meV,E_{a OPS}=91.00 meV).?3?In terms of texture degree,the samples sintered under oscillatory pressure?0.68?have higher texture than those sintered under constant pressure?0.54?.SEM results also show that the orientation of the OPS samples is more obvious.In addition,on the incised cross-section parallel to pressure before annealing process,the precipitation of cubic particles was observed by SEM and then the particles were confirmed to be CaO by EDS method.After annealing in air,the precipitated particles regenerated into disorderly oriented flake particles.The precipitation and secondary growth of these particles may be one of the reasons why the texture degree of pressure sintered samples could not further be improved.?4?On the experimental method of preparing porous bulk Ca₃Co₄O₉ ceramics by sacrificial template method,carbon nanotubes?CNTs?was used as the sacrificial template.The stirring and ultrasonic dispersion method was first utilized for the preparation of porous ceramic samples,and it turns out that the purpose of reducing thermal conductivity was successfully achieved(923 K,?_x=5=0.71 W/m·K).It was unexpectedly found that this method could also obtain samples with high texture(f _x=0=0.91).However,compared with pressure sintering,this method is not helpful in increasing the conductivity of samples(1023K,?_x=0=43.27?^-1·cm^-1,?_x=5=18.35?^-1·cm^-1).