Synthesis And Pyroelectric Performance Of Graphene-sodium Bismuth Titanate Heterostructure

Thermal-electricity conversion devices based on the pyroelectric effect of ferroelectric materials,due to their wide application temperature range,fast thermal-electric conversion rate,low energy consumption,and environmental friendliness,were considered as one of the most effective ways of industrial medium-low temperature waste heat recovery.It is the key to prepare ferroelectric thin films with high pyroelectric coefficients for realizing their large-scale application.Compared with the lead-based ferroelectric materials,lead-free ferroelectric ceramic material Bi_0.5Na_0.5TiO₃（BNT）has attracted much attention due to its environmental friendliness,but there are still some features such as low pyroelectric coefficient and large coercive field,which greatly limit the application of lead-free ferroelectric ceramics in the field of waste heat recovery.Therefore,the improvement of BNT-based ferroelectric materials and the optimization of thermoelectric devices have a great significance for the development of lead-free ferroelectric ceramics in the field of waste heat recovery applications.The unique crystal structure of graphene determines its excellent thermal conductivity and electron mobility,making it an excellent choice for electrode materials for pyroelectric devices.Graphene oxide（GO）,partially retaining the excellent properties of graphene,also shows great potential in the study of ferroelectric properties,and the thermal transparency for it could absorb heat radiation quickly.Therefore,the study of the piezoelectric properties of GO films and their synergistic effects with the pyroelectric effect of BNT materials was expected to increase the thermal-electrical conversion energy densities of the heterostructures.In this study,a pyroelectric device based on lead-free pyroelectric thin film/graphene oxide heterostructure was designed.The lead-free ferroelectric ceramic film BNT was used as the thermoelectric conversion unit,and related pyroelectric performance was detected and its thermoelectric conversion energy density was also calculated.The main research is summarized as below.（1）The ferroelectric properties of graphene oxide paper were studied for the first time using experiments based on previous simulations.Briefly,graphene oxide thin films with different thicknesses were prepared by vacuum filtration and Langmuir-Blodgget（LB）membrane techniques,respectively.Piezoelectricity of GO papers was detected by PFM,polar domains were observed,and the domain switching behaviors under external electric fields were also investigated.It was demonstrated that the piezoelectricity of GO could be induced by the hydrogen bonds,which were dominated by the functional groups and water contents in GO papers.Most importantly,the piezoelectric effect in GO papers can be tuned effectively by the water-mediated H-bond contents.Being light-weight,mechanically robust,flexible and low-cost,GO papers are excellent candidates for all kinds of ferroelectric and piezoelectric devices.Results showed that graphene oxide film exhibited a piezoelectric effect and its piezoelectric coefficient could be adjusted by the number of inter-layer hydrogen bonds.The d₃₃ piezoelectric coefficient of the dried graphene oxide film was only about 3 pm·V^-1,when the relative humidity reached up to75%,the d₃₃ was increased to 12.6 pm^-1.（2）Based on the research of the GO films relying on hydrogen bonding to control their ferroelectric properties,a GO-P（VDF-TrFE）-GO layered film heterostructure was further constructed using the P（VDF-TrFE）.GO-P（VDF-TrFE）-GO films with free-standing structure were prepared by an interface self-assembly method,using the graphene oxide film as structural material and an organic ferroelectric material P（VDF-TrFE）as polar fillers.The pyroelectric coefficients and pyroelectric energy conversion densities of GO-P（VDF-TrFE）-GO films were systematically studied at different electric fields and temperatures.Results showed that when the mass ratio of P（VDF-TrFE）filler was at 10%,uniform intercalation between GO layers could be performed.The pyroelectric pyroelectric coefficient was 16.10μC·m^-2·K^-1.（3）The preparation parameters and piezoelectric properties of BNT nanostructures were systematically studied.Controllable BNT nanostructures were prepared via a stirring hydrothermal method by controlling the mechanical stirring rate in the hydrothermal process.Ultra-long BNT nanowires with a diameter of 20-200 nm and a length of more than 10μm were obtained and its piezoelectric coefficient in the d₃₃direction was detected by PFM,and the relationship between the piezoelectric coefficient and the size was also calculated.Results showed that during the stirring heating process,the structure of sodium barium titanate wires began to generate when the temperature reached up to 120°C,but the crystallinity was poor.However,at the temperature of 180°C with stirring at 1000 r/min for 24 h,sodium barium titanate nanowires with high-purity were obtained.The amplitude of the BNT nanowires under the alternating electric field was systematically studied.The maximum piezoelectric coefficient in the d₃₃ direction was 172 pm·V^-1 with size of 16 nm.（4）The parameters for preparation of BNT thin films were systematically studied and the pyroelectric properties of BNT-GO heterostructures were designed and studied.BNT_0.94BT_0.06 ferroelectric thin films,which the composition x is close to the rhombohedral–tetragonal morphotropic phase boundary（MPB）at x=0.06,were prepared by a sol-gel method.The parameters such as coating times,annealing temperature,and annealing time were systematically studied,and the particle size,crystallinity,surface roughness,piezoelectric properties,ferroelectric properties were also detected.Results showed that when the 0.2 M precursor solution has been coated for 8 times,and annealed at 450°C for 5 min,and then at 750°C for 30 min,the obtained film exhibited a good ferroelectric performance,and its maximum piezoelectric coefficient was 73.30 pm·V^-1.Thermal-electricity conversion devices based on BNT-GO film was obtained and systematically tested at different electric fields and temperatures.Pyroelectric energy conversion cycle generates electrical work from temperature and electric field driven charges in the electric displacement of the BNT thin film.Pyroelectric energy harvesting densities was calculated by an Olsen cycle equation.Results showed that the pyroelectric energy conversion performance was greatly improved by the graphene oxide.The pyroelectric energy conversion density was 1 352.66 mJ·cm^-3.