Investigation On Synthesis, Structure And Properties Of Cu Oxides Based Sensitive Materials

With the progress of science and technology and the development of society,semiconductor metal oxide sensitive components have been widely used in special fields such as petroleum,chemical,mineral,automobile and environmental monitoring.CuO,Cu2O and other Cu-containing oxides sensitive materials have also received more attention in recent years.However,there are still some shortcomings,such as sensitivity,selectivity,stability of performance and etc..In addition,there are few literatures on systematically investigating gas-sensitive,I-V nonlinear characteristic and mechanism of Cu-containing oxides ceramics.Therefore,it is necessary to systematically study the relationship between the preparation,structure and properties of Cu-containing oxides,which is important to develop new sensitive materials with excellent properties.In consequence,CuO,Cu2O monobasic system,CuO-TiO2 binary system and CaCu3TiaO12(CCTO)inCaO-CuO-TiO2 ternary system have been studied in this paper.The synthesis methods,preparation processes,phase and microstructure evolution,ion valence,composition segregation,electrical and sensitive properties have investigated for the present ceramics containing Cu.Through the above study of the relationship to reveal the intrinsic properties and mechanism of sensitive ceramics containing Cu oxide,thus it provides important reference and guidance for the research,design,preparation and application of these sensitive materials.The submicron Cu2O powders were prepared by glucose reduction method.The effects of reaction temperature and reductant concentration on particle size,particle growth and agglomeration,surface morphology and gas sensitivity were studied.It is found that although increasing the reaction temperature has little effect on the size of primary particle,it causes the agglomeration of the particles.While the increase of the reducing agent makes the primary particles grow obviously,but the secondary particles can be refined.The Cu2O nanopowder synthesized at 70? and 0.9mol/L reductant has the best gas sensitivity:11.9 for 400ppm ethanol.Using CuSO4,Na2CO3 as raw materials and CTAB as surfactant,CuO powder has been synthesized by wet chemical method.Effects of initial droplet size and concentration on particle size,morphology and gas sensitivity have been investigated.CuO particles with different surface morphologies can be obtained by changing the mixing method of the initial solution.The surface of the particles obtained by pressurized spray is short needles,and the powder prepared by adding CuSO4 solution drop-wise is long rod-shaped and short rod-shaped particles,the powder prepared by direct mixing is a hollow microspheres assembled with nano-spherical particles and nano-layer sheet-shaped crystal beam.CuO powder has a certain selectivity of ethanol.The sensitivity of 400ppm ethanol is up to 5.6.The investigation on CuO-TiO2 binary system shows that a small amount of TiO2 can significantly improve the densification of CuO ceramics,lower sintering temperature.But there is no solid-solution reaction between CuO and TiO2.TiO2 grains distribute in the grain boundary of CuO,inhibite the grain growth.With the increase of sintering temperature,Cu+ content obviously increases and the nonlinear current-voltage effect degrades gradually.The dielectric constant of CuO-2wt%TiO2 ceramics sintered at 800? is higher than 400 and the V-Inonlinear coefficient is 4.83.Single-phase CCTO powder has been treated by molten salt and then sintered to obtain a dense CCTO ceramic.The effects of molten salt treatment on the phase and micro structure evolution,ionic valence,dielectric and V-1 properties have been invesitated systematically.A decomposition and re-synthesis process of CCTO exists in the molten salt treatment process.At the same time,the thermal stability of CCTO phase has been obviously improved.The decomposition endothermic peak shifts from 1033? of pure CCTO phase powder to around 1080? of molten salt treated CCTO powder.Molten salt treatment suppresses the segregation of CCTO ceramics and the semiconducting during sintering.The higher grain activation energy of 0.15-0.21 eV is formed,resulting in the lower dielectric constant(<104).XPS results show the formation of Ti3+,Cu2+,Cu3+ and no detectable Cu+.These variations of valence and structure reduce the dielectric constant,but significantly increases the varistor properties.CCTO ceramics derived from the molten salt treated powder molten salt in Na2SO4-K2SO4 system has the best varistor properties of the highest nonlinear coefficient of-48.7,which is close to the value of the recent applied ZnO varistors.The corresponding process parameters is as following,the ratio of molten salt 1;2,the treatment temperature of 950? and the sintering temperature of 1100?.Comprehensive analysis of a monobasic,binary,ternary copper oxide-sensitive materials,the valence variation of copper is a key factor on the properties in the semiconductor material.The composition and distribution of the grain boundaries ultimately determine the electrical properties of ceramics.There is no obvious secondary phase in the grain boundary of monomorphic ceramics.So the dielectric constant is very low,and the varistor property is not obvious.With adding TiO2,the second phase in the grain boundary of CuO ceramics is TiO2.Therefore,a certain dielectric properties and varistor properties can be obtained through adjusting the process parameters.However,the second phase of the grain boundary in the CuO-TiO2 binary system can not fully cover the crystal grains,which is difficult to form more Schottky barrier,so it restricts the improvement of the electrical properties.In the ternary system,the precipitation and uniform distribution of the copper-rich phase in the grain boundary forms well the internal barrier layer capacitance(IBLC)structure,resulting in a high dielectric constant and excellent varistor properties.Meantime,due to the large leakage loss of pure CCTO ceramic,the heating problem is obvious during the use or testing.The long time,high voltage state result in the considerabletemperature change,will seriously affect the test results and performance.Through the molten salt treatment,changing the relaxation activation energy and valence alternation of Cu ion,can improve the resistivity of the sample with high dielectric properties and excellent varistor properties.Reducing the heat will help to improve the test accuracy and stability of the performance.