Oxide Nanopowders Prepared Via Hydrothermal/solvothermal Method And Their Applications In Piezoelectric Ceramics

Transition metal oxides(ZrO₂,TiO₂,etc.)and rare earth metal oxides have attracted extensively interests in the fields of materials,energy and environmental protection,etc.because of their unique chemical and physical properties. Nevertheless,the performances of these oxides depend on their structure features, such as phase form,morphology and size,and surface status,etc.Recently, preparation,properties and potential applications of nanometer oxides have been widely studied because of their excellent performances.Many approaches, including gaseous method,precipitation method and sol-gel method,etc.can be used to prepare above mentioned nano-materials.However,upon these approaches,one will obtain difficultly nano-materials with controllable structure, morphology and particle size.In addition,most of these preparation protocols require the calcination procedure to obtain the materials with high crystallinity, which will result in high cost.In general,the sintering temperature of ceramics can be significantly decreased by addition of nanomaterials due to their high specific surface area and sintering activity.However,heavy volatilization of PbO from sintering of PZT ceramics at high temperatures will lead to the problems of environmental pollution and performance deterioration.In this dissertation,based on the investigations about the preparation of nanomaterials,we investigated the effects of the nanomaterials on lowering the sintering temperature and improving the preparation techniques and performance of PZT ceramics in an industrial scale.Nano-ZrO₂ and nano-TiO₂ with controllable phase and morphology were prepared via hydrothermal and solvothermal methods,respectively.And their effects on the sintering and electric properties of PZT ceramics(P612 Composition,Zhejiang Jiakang Electronics Co.Ltd.)were investigated and discussed as well for the aim of obtaining practical industrial technique.The main contents are as follows.1.Nano-ZrO₂ was synthesized via the hydrothermal method.The sol precursor was characterized by X-ray diffraction(XRD),thermogravimetric analysis(TG), FT-IR,and the solubility of xerogel of the precursor in nitric acid,etc. Furthermore,phase transition activation energy was obtained by Johnson-Mehl-Avrami(JMA)equation according to the DTA results of the gel precursors at various heating-up rates.Results show that the values of activation energy calculated according to Owaza method and Kissinger method are 173.49 kJ·mol^-1and 159.49 kJ·mol^-1,respectively.Results from XRD patterns and TEM images indicate that the phase composition can be effectively adjusted in a large range by changing the initial molar ratio of Zr⁴⁺/OH^- and the hydrothermal reaction time.With reaction condition at 180℃for 10 hours,the volume fraction of the tetragonal ZrO₂ increases from 18.63%to 60.06%as the initial molar ratio of Zr⁴⁺/OH^- decreases from 1:4 to 1:12.5.Also,the volume fraction of the monoclinic ZrO₂,which was prepared at 180℃with 1:12.5 molar ratios of Zr⁴⁺/OH^-,increases with the hydrothermal reaction time.However,in the water medium we failed to prepare ZrO₂ with fully changeable phase composition by adjusting the reaction condition such as molar ratio,reaction temperature or reaction time,etc.2.Nano-ZrO₂ was prepared via the solvothermal method using ethanol as the solvent,and the effects of precipitators(KOH,NaOH and NH₄OH)on phase composition and morphology were investigated.It is found that the product prepared at 140℃for 3 hours with NH₄OH as the precipitator is mainly amorphous,while the products prepared with either NaOH or KOH as the precipitator in the same condition are in the crystalline form of tetragonal and(or) monoclinic phase,and the volume fraction of tetragonal ZrO₂ is 48.26%and 100%,respectively.The effects of precipitator concentration,reaction time and temperature,and surfactant,etc.on the preparation of ZrO₂ were investigated when KOH was used as the precipitator.It is showed that both reaction time and temperature have significantly effects on the preparation while the effect of the surfactant is negligible.As for the NH₄OH precipitator system,the effects of reaction temperature and time on the phase composition of the prepared product were also investigated.It is found that the prepared products are mainly in the form of tetragonal phase and contain little monoclinic phase even at a relatively high temperature(220℃for 10 hours)or after a long heat-treatment(180℃for 36 hours).Summarily,the phase composition of products can be effectively controlled to any ratio from pure tetragonal phase to pure monoclinic phase by adjusting the reaction time when KOH is used as the precipitator.In addition,the luminescent property of nano-ZrO₂ prepared via the solvothermal method was also investigated.The photoluminescence spectrum of nano-ZrO₂ shows that,under 250 nm excitation,there is a strong emission peak at 402 nm.Unexpectedly,the nano-ZrO₂ shows good infrared emission performance.3.Rare earth metal ions and transition metal ions doped ZrO₂ occupy a large fraction in the based-ZrO₂ structure materials and functional materials.Herein,we reported the preparation of ZrO₂:Nd for the first time and investigated the effects of the precipitators(NH₄OH,KOH and NaOH)on the preparation of ZrO₂:Nd in the water medium,and also the effects of Nd³⁺concentration and reaction temperature on the phase composition using KOH as the precipitator in the water medium.The nano-ZrO₂:Nd with nominal doping content of Nd/Zr of 5mol%prepared via the hydrothermal method are calcined at various temperatures for 2 hours,and results show that the nanoparticles calcined at 875℃below are mainly tetragonal, and that the fraction of the monoclinic phase increases remarkably after calcination at 1000℃,indicating that introduction of Nd³⁺into ZrO₂ can improve the thermal stability of tetragonal ZrO₂.The stabilization mechanism of Nd³⁺in ZrO₂ is also preliminarily discussed.4.Nano-TiO₂ was prepared via the hydrothermal method using titanium n-butoxide as the starting material,and the effect of addition of nano-TiO₂ on the sintering performance,electric performance and mechanical performance of PZT ceramics were investigated.XRD results show that the prepared products contain 26.7%rutile and 73.3%anatase.Alter partial replacement of micro-TiO₂ by nano-TiO₂,the sintering performance,electric performance and mechanical performance of the PZT ceramics and the performance of the corresponding frequency device were also investigated.It is found that 20%replacement of micro-TiO₂ by nano-TiO₂ can benefit the sintering of PZT ceramics to a large extent.The modified PZT ceramics have the dielectric and piezoelectric performances equivalent to the formerly unmodified ones but the curie temperature rises from 300℃to 400℃, which will favor the higher work temperature of PZT piezoelectric ceramics. Generally,doping is a common way to adjust effectively the curie temperature of the piezoelectric ceramics.Present work offers a novel approach and technique to adjust the curie temperature without the modification of nominal composition of PZT ceramics.The change of band width for ceramic frequency device,which is a parameter requiring as small as possible for practical application,decreases from 49.9%of un-modified ceramic to 2.4%for nano-modified one after 200 times shock cycles between -55℃and 85℃.At the same time,the bending strength and tenacity of the nano-modified PZT ceramics are improved remarkably from 81.53 MPa to 97.9 MPa and from 1.15 MPa.m^1/2to 1.42 MPa.m^1/2,respectively.5.Above results have been verified in Zhejiang Jiakang Electronics Co.Ltd. under mass production level,and the obtained technique is applied to produce frequency devices.It is found that the resistance ability to temperature shock can be improved for both the nano-modified PZT ceramic and the frequency devices while the overall cost of nano-modified frequency devices only increases by 0.2%. Therefore,the overall performance of frequency devices is significantly improved, and the ratio of performance to the cost is promoted significantly with present technique.