【作者】 张晓泳；

【导师】 周科朝； 李志友；

【作者基本信息】 中南大学 ， 材料学， 2008， 博士

【摘要】 铌镁酸铅系陶瓷Pb(Mg_1/3Nb_2/3)O₃是一类具有优良介电、压电性能的陶瓷体系。如Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃（PMN-PT）压电单晶因其在<001>方向具有优异压电性能而被认为是制作高效水声换能器、大应变驱动器、智能结构传感器等器件核心部件的理想材料。但制备单晶压电陶瓷目前普遍存在周期长、尺寸小、品质稳定性不佳以及难以加工成复杂形状等问题,严重影响其规模化应用。传统烧结型PMN-PT多晶压电陶瓷虽制备简单且性能较稳定,但其压电性能却显著低于对应单晶。采用模板晶粒生长技术（TGG）制备织构型多晶陶瓷既能继承由单晶体各向异性带来的优异性能,又可采用常规陶瓷成形与烧结制备手段,并且对于多种不同晶体结构的陶瓷体系均有较好的制备适用性,是一种高性能陶瓷的先进制备工艺。TGG技术制备织构型PMN-PT多晶压电陶瓷常采用SrTiO₃异质模板。片状SrTiO₃模板在熔盐合成过程中的形貌可控合成与改性、以及低温烧结条件下烧结体内织构组织的快速形成,是其中两个技术关键,将直接影响织构陶瓷的织构取向度和压电性能。本文作者选择PMN-32.5mol%PT压电陶瓷体系作为织构化研究对象,围绕SrTiO₃模板的熔盐可控合成和Ba²⁺掺杂改性、以及TGG技术制备相应织构陶瓷过程中的基体和模板生长动力学行为与织构组织演变过程开展相关研究,并完成以下几方面工作:1)在SrCO₃和TiO₂熔盐合成Sr₃Ti₂O₇的研究中发现,形成片状Sr₃Ti₂O₇产物需经历物相演变和形貌发育两个过程。物相演变过程中先形成中间产物SrTiO₃,进而转变成Sr₃Ti₂O₇;Sr₃Ti₂O₇则通过溶解-析出模式在近似平衡的熔盐环境中发育成片状形貌。基于上述发现建立了熔盐产物先在Sr₃Ti₂O₇颗粒（001）表面外延形核、后沿<100>、<010>台阶式生长的二维生长模型。熔盐种类和添加量以及合成条件主要通过调控反应和传质过程来影响Sr₃Ti₂O₇产物形貌尺寸。如采用KCl/NaCl二元熔盐并改变其中组分,进而调整其对反应物/产物的溶解传质能力,可有效调控片状Sr₃Ti₂O₇颗粒形貌尺寸。缓慢升/降温条件则有助于熔盐产物根据Sr₃Ti₂O₇层状结构高度各向异性特点选择性析出到Sr₃Ti₂O₇颗粒各表面,进而形成高度各向异性的规整片状形貌。2)在片状Sr₃Ti₂O₇与TiO₂熔盐合成SrTiO₃的研究中,验证并分析了生成SrTiO₃物相并形成片状形貌的两种反应模式:一是Sr₃Ti₂O₇层状钙钛矿结构经Sr-O层迁出后切变成SrTiO₃完整钙钛矿结构;二是迁出的Sr-O层与TiO₂熔盐反应所得SrTiO₃产物在Sr₃Ti₂O₇表面外延生长。熔盐种类和添加量及合成条件主要通过改变物质溶解和析出过程来影响SrTiO₃的形貌各向异性程度。如增加熔盐助剂含量、延长保温时间及缓慢降温等措施均有助于SrTiO₃产物Ostwald熟化生长过程进行,经SrTiO₃小颗粒先溶解、进而在片状SrTiO₃颗粒无取向析出后,不利于获得形貌高度各向异性的SrTiO₃颗粒。另外KCl较NaCl更适合作为熔盐助剂合成片状SrTiO₃颗粒。3)在熔盐合成（Sr,Ba）TiO₃模板的研究中发现,所用前驱体类型和Ba2+置换方式直接影响（Sr,Ba）TiO₃产物形貌和Ba²⁺置换程度。片状SrTiO₃前驱体在与BaO反应过程中,其片状形貌会受破坏,（Sr,Ba）TiO₃产物在此受破坏表面析出后形成无规则形貌,最终发育成高Ba²⁺置换程度的片状多晶团聚体,且其形貌无规程度随BaO含量的增加而增加。片状Sr₃Ti₂O₇前驱体与BaO、TiO₂同时熔盐反应时,Sr₃Ti₂O₇经Sr-O层迁出和Ba²⁺在层内置换出Sr²⁺后转变成低Ba²⁺含量的片状（Sr,Ba）TiO₃,Sr-O迁出后与BaO、TiO₂熔盐反应得到高Ba²⁺含量的非片状（Sr,Ba）TiO₃,且其数量随熔盐中Ba²⁺浓度增加而增多,合成后期片状（Sr,Ba）TiO₃经各表面无取向生长后逐渐增厚。对于二步熔盐合成,Sr₃Ti₂O₇先与BaO熔盐反应时,其层状结构有助于Ba²⁺在Sr-O层间扩散并置换出Sr²⁺,在维持原有片状形貌的同时引入更多Ba²⁺,所得产物与TiO₂二次熔盐反应后得到形貌度高各向异性的规整片状（Sr,Ba）TiO₃颗粒,且非片状（Sr,Ba）TiO₃颗粒数量明显少于前两种合成方式。但增加Ba²⁺浓度会降低片状（Sr,Ba）TiO₃表面平整程度。4)在反应-烧结技术制备铌镁酸铅陶瓷的研究中发现,所用镁源反应物对烧结体的物相组成、相对密度以及显微结构有较大影响。将（MgCO₃）₄·Mg（OH）₂·5H₂O代替传统的氧化镁MgO作为镁源反应物后,前者热分解产物的高反应活性有助于反应物之间反应活性互相匹配,进而可促进合成反应和基体烧结致密充分进行,850℃烧结4h后即可得到单一钙钛矿相组成的PMN陶瓷,相对密度约95%。5)在以片状SrTiO₃为模板对PMN-32.5PT进行TGG织构化制备的研究中发现,PbO熔化并形成液相烧结后,基体平均晶粒度和模板平均外延尺度均随保温时间t的1/3次方形式增长,并分别遵循Lay模型和由此衍生得到的模板外延生长模型。基体晶粒初始状态、PbO液相含量和烧结制度可调控两类晶粒的生长动力学系数。PbO液相-方面有助于基体晶粒溶解-析出过程进行,进而促进织构组织形成和基体烧结致密;另一方面也会溶解破坏SrTiO₃模板规整形貌。因此以PMN-32.5PT粉体为基体时,需在PbO熔化前适当保温,通过新生外延层保护作用使SrTiO₃模板继续保持规整片状形貌。其中含3wt%PbO液相和8vol%SrTiO₃模板时,1150℃烧结6h可制得取向因子约59.3%的织构陶瓷,其压电常数d₃₃（10KV/mm）约740pC/N,室温介电常数（1KHz）约3280。而以改进的一次固相合成PMN-32.5PT所用反应混合物为基体时,升温过程中反应所得高烧结活性的PMN-32.5PT可直接在SrTiO₃模板表面析出,有助于低温下迅速形成织构组织并烧结致密。其中基体内过量3wt%PbO并添加8vol%SrTiO₃模板时,1050℃烧结6h即可制得取向因子约53.8%的织构陶瓷,其d₃₃约690 pC/N,室温介电常数约3040。更多还原

【Abstract】 Lead magnesium niobate,Pb(Mg_1/3Nb_2/3)O₃, is one of the electric ceramics with excellent piezoelectric and dielectric properties. For example,Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ （PMN-PT） single crystal has been regarded as a kind of ideal materials to be used in high-performance acoustic transducers, high-strain drivers and intelligent actuators, due to its excellent piezoelectric properties along <001> orientation. However, preparing single crystal always meets some problems, such as intrinsically high cost, small growth size, and low property stabilization and machinability. Although the sintering PMN-PT polycrystalline ceramics with good property stabilization can be easily fabricated, its properties are significantly lower than that of corresponding single-crystal. Templated grain growth method （TGG） is a kind of advanced technique to prepare textured polycrystalline ceramics with high performances that owe to the anisotropy properties of single-crystal. During TGG process, some conventional shaping and sintering processes can be referred, and many ceramics with different crystal structures can also be textured.SrTiO₃ hetero-template has been used widely to texture PMN-PT polycrystalline ceramics by TGG process. The morphology-controlled synthesis and modification of SrTiO₃ template by molten salt method, and the quick formation of textured microstructure at low sintering temperature are two critical factors, which may influence the textured degree and properties of ceramics greatly. During the research by choosing PMN-32.5mol%PT as a textured subject in this paper, the morphology-controlled synthesis and Ba²⁺-doped modification of SrTiO₃ by molten salt method were investigated, and the matrix and template growth kinetics, and the development of textured microstructure during TGG process were also studied.The synthesis of tabular Sr₃Ti₂O₇ by reacting SrCO₃ with TiO₂ in flux can be divided into two stages: the phase formation and morphology development. During the phase formation, SrTiO₃ formed first, followed by the synthesis of Sr₃Ti₂O₇.The tabular Sr₃Ti₂O₇ can form through the dissolution-precipitation process in an approximate-equilibrium growth environment. Based on these observations, a two-dimensional growth process to form tabular Sr₃Ti₂O₇ particles was modeled: the formation of Sr₃Ti₂O₇ nucleuses on the （001） surface of product, and then the stepped growth along <100> and <010> orientations. The amount and type of salts, and the synthesis conditions influence the size and morphology of Sr₃Ti₂O₇ by adjusting the reaction and mass transfer process. The product morphology can be controlled effectively by using KCl/NaCl binary salt and adjusting the KCl/NaCl weight ratio. The slow heating and cooling conditions benifit the formation of tabular morphology with high shape anisotropy through the sufficient selective-prepitation of reaction product on the Sr₃Ti₂O₇ particle surfaces according to the high anisotropy of its layered crystal structure.Two reaction patterns to form tabular SrTiO₃ product by reacting tabular Sr₃Ti₂O₇ with TiO₂ in flux were investigated: the structure transition from layered perovskite structure of Sr₃Ti₂O₇ to perovskite structure of SrTiO₃ by outmigrating Sr-O layers, and the epitaxial growth of SrTiO₃ particles by the precipitation of product synthesized by reacting Sr-O with TiO₂ in flux. The amount and type of salts, and synthesis conditions influence the shape anisotropy of SrTiO₃ particles by adjusting the dissolution-precipitation process. Some synthesis conditions, such as increasing salt amount, prolonging synthesis time, and cooling slowly, would promote the Ostwald ripening growth of SrTiO₃ particles in flux. The small SrTiO₃ particles would be dissolved first, followed by the sufficiently non-selective precipitation on the tabular SrTiO₃ surfaces according to the high symmetry of its perovskite structure, which is disadvantageous to obtain the SrTiO₃ particles with high shape anisotropy. Additionally, KC1 is more suitable than NaCl to synthesis tabular SrTiO₃.As for synthesizing （Sr, Ba）TiO₃ in KC1 flux, the precursors and reactive modes influence the phase composition and product morphology greatly. The perfect tabular morphology of SrTiO₃ precursor would be damaged when reacting with BaO. After the precipitation and irregular growth of （Sr, Ba）TiO₃ on these destroyed surfaces, the tabular aggregates that composed of many irregular （Sr, Ba）TiO₃ particles with high Ba²⁺ content formed. The irregular degree of tabular aggregates decreased by increasing BaO amount in reactants. When Sr₃Ti₂O₇ reacted with BaO and TiO₂ simultaneously, tabular （Sr, Ba）TiO₃ with relatively low Ba²⁺ content formed by outmigrating Sr-0 layers from layered structure of Sr₃Ti₂O₇ and substituting Ba²⁺ for Sr²⁺ in layers. The Sr-O layers reacted with BaO and TiO₂ in flux to form non-tabular （Sr, Ba）TiO₃ with relatively high Ba²⁺ content. After non-oriented growing, the tabular （Sr, Ba）TiO₃ thickened. The amount of non-tabular （Sr, Ba）TiO₃ increased by increasing BaO amount in reactants. As for two-step molten salt synthesis, when Sr₃Ti₂O₇ reacted with BaO first, its layered structure benefits the diffusion and substitution of Ba for Sr in layers, and the perfect tabular morphology can be maintained after doping more Ba²⁺.Further reacting with TiO₂,the （Sr, Ba）TiO₃ particles with perfect tabular morphology and high shape anisotropy can be obtained, and the amount of non-tabular （Sr, Ba）TiO₃ particles also decreased compared with the previous synthesis processes. However, the increase of BaO may lead to decrease smooth degree of particle surface.As for preparing PMN ceramics by reactive sintering, the magnesium precursors influence the phase composition and microstructure of sintering ceramics greatly. By substituting （MgCO₃）₄·Mg（OH）₂·5H₂O for MgO that widely used as a magnesium precursor, the high reactivity of product decomposed from the former benefits the match of reactivity among reactants, leading to promote the completion of reaction and densification. The PMN ceramics with perovskite single-phase composition and relative density of-95% can be prepared by sintering at 850℃for 4 h.During TGG process to texture PMN-32.5mol%PT polycrystalline ceramics by using SrTiO₃ templates, it was found that the experimental datas displayed the t^1/3 relation between sintering time t and average matrix grain size or template growth distance, which followed the Lay model for matrix growth and the kinetic model for template growth that derivated from the former. The initial matrix status, PbO liquid phase content and sintering conditions can adjust the kinetic parameters for the growth of matrix and template. PbO liquid phse benefits the textured microstructure development and densification by promoting the dissolution-precipitation process, but it also dissolved and damaged SrTiO₃ templates. For this reason, when PMN-32.5PT matrix was used, a long-time aneal before forming PbO-based liquid phase is needed. After anealling,SrTiO₃ templates would be protected by a thin epitaxial growth layer. The textured ceramic with Lotgering factor of 59.3% can be obtained by containing PbO of 3 wt% and SrTiO₃ templates of 8 vol%, and sintering at 1150℃for 6 h, which has a piezoelectric coefficient d₃₃ of -740 pC/N （10KV/mm） and room temperature dielectric constant of 3280 at 1 KHz. When the modified mixed reacntants for synthesizing PMN-32.5PT by solid-state synthesis were used as a matrix, the PMN-32.5PT with high reactivity synthesized from in situ reaction can be precipitated on the template surfaces directly, leading to the quick formation of textured microstructure and densification at low sintering temperature. By adding excess PbO of 3wt% and SrTiO₃ templates of 8 vol%, the textured ceramic with Lotgering factor of 53.8% can be prepared by sintering at 1050℃for 6 h, which has a piezoelectric coefficient d₃₃ of -690 pC/N and room temperature dielectric constant of 3040.更多还原