Polarization Behavior And Application Research Of Ammonium Perchlorate Ferroelectrics

The spontaneous polarization state of ferroelectric materials can be changed with the external stimulations such as electric-field,temperature,stress and so on,which makes them have the effects of electrocaloric,pyroelectric and piezoelectric.Therefore,ferroelectric materials have great research and application value in the fields of environmental refrigeration,temperature detection,health detection,energy conversion and so on.Molecular ferroelectric is a new type of ferroelectric material.It can be made up of organic groups and inorganic ions through weak chemical bonds,or composed of small organic molecules by hydrogen bondings.Due to its adjustable composition,molecular ferroelectrics have rich polarization characteristics and excellent electrical properties.In addition to its simple preparation process,the research of molecular ferroelectrics has attracted extensive attention in recent years.Among various kinds of molecular ferroelectrics,ammonium perchlorate ferroelectrics have good application potential due to their excellent electrical properties,higher phase transition temperature and stable crystal structure.Therefore,the ammonium perchlorate ferroelectrics was chosen as the research object and carried out the detailed study.Ammonium perchlorate ferroelectric materials with different polarization properties were obtained by changing the type of cationic groups.With the help of detailed experimental analysis and scientific theoretical calculation,the electrocaloric properties,pyroelectric properties and piezoelectric sensing properties of ammonium perchlorate ferroelectrics are studied in this paper.Imidazole perchlorate（Im Cl O₄）prepared by the reaction of perchloric acid and imidazole has the second-order phase transition characteristics,so there is no thermal hysteresis in the process of heating and cooling,which makes its electrocaloric performance stable in the cycle test.The PE loops tested with variable temperatures show that spontaneous polarization decreases with the increase of temperature and reaches zero near the phase transition temperature.Making use of the Maxwell relations,the entropy and temperature changes of Im Cl O₄ at the phase transition temperature were calculated to be～5.4 J kg^-1·K^-1 and～1.26 K,respectively.Due to the low driving electric field,the electrocaloric strength of Im Cl O₄ is as high as 3.6 J·mm K^-1·kg^-1·k V^-1（|ΔS/ΔE|）and 0.84 K·mm k V^-1（|ΔT/ΔE|）,respectively,which is superior to most inorganic ceramics,organic polymers and their composites.The ultra-high electrocaloric strength ensures that Im Cl O₄ can obtain large entropy change and temperature change under low electric field,which makes it possible to play a role in the preparation of high efficiency and low power consumption refrigeration devices.Pyroelectric figure of merits are important parameters to measure the performances of pyroelectric materials.According to the definition of figure of merits,pyroelectric materials should have not only high pyroelectric coefficient,but also low dielectric constant.Because of their low dielectric constant,improper ferroelectrics have more advantages in pyroelectric applications.The N,N-Dimethylethanolamine perchlorate（[Hdabco]Cl O₄）prepared by the reaction of perchloric acid and N,N-Dimethylethanolamine shows obvious improper ferroelectric performances.Testing results indicate that[Hdabco]Cl O₄ is a kind of ferroelectric material with first-order phase transition characteristics.And its pyroelectric coefficient at the phase transition point is as high as 48000μC cm^-2·K^-1,which is comparable to most ferroelectric ceramics.First principles calculation and thermodynamic simulation reveal that the high pyroelectric coefficient of[Hdabco]Cl O₄ is due to the strong coupling between the rotation of anion and the relative displacement between anions and cations.Due to the low dielectric constant,[Hdabco]Cl O₄ exhibits ultra-high pyroelectric figure of merits,that the voltage response F_V is 5.58 m² C^-1,detectivity F_D is 25.2×10^-5Pa^-1/2,and pyroelectric energy harvesting merit F_E is 1.36×10⁷ J m^-3K^-2.The flexible sensors based on piezoelectric materials have the advantages of high sensitivity,strong fatigue resistance and stable electrical performance,which has caused great attention.Guanidinium perchlorate（[C（NH₂）₃]Cl O₄）prepared by the reaction of perchloric acid and guanidine carbonate not only has a high piezoelectric coefficient,but also has multiple polar axies along the diagonal direction of the crystal cell,which ensures a satisfying piezoelectric coefficient of[C（NH₂）₃]Cl O₄ in the polycrystalline state.In addition,the excellent temperature and humidity stability makes[C（NH₂）₃]Cl O₄ a good choice for the fabrication of flexible piezoelectric sensors.To obtain good sensor performances,a novel flexible piezoelectric sensor with three-dimensional network structure was prepared by combining[C（NH₂）₃]Cl O₄ and polyurethane sponge by an immersion evaporation method.The test results show that the hybrid piezoelectric sensor exhibits ultra-high sensitivity(170 m V k Pa^-1)and ultra-low stress detection limit（0.003 k Pa）.The finite element analysis proves that the special three-dimensional skeleton structure of polyurethane sponge not only realizes the effective transmission of stress,but also enlarges the stress in the process of transmission.The amplified stress can change the polarization state of the material better,so that the piezoelectric properties of the material can be effectively exerted.