放电等离子体烧结制备纳米非晶Al_(2)O_(3)-La_(2)O_(3)-ZrO_(2)三元共晶氧化物

Preparation of Nanostructured-Amorphous Al_(2)O_(3)-La_(2)O_(3)-ZrO_(2)Ternary Eutectic Oxides by Spark Plasma Sintering

纳米非晶材料是由纳米非晶颗粒以及颗粒间非晶界面构成的新型材料,表现出很多独特性能,但其制备往往需要>1 GPa的高压来完成致密化。本工作采用溶胶凝胶法制备了较宽过冷液相区(844~914℃)、平均粒径为11.6 nm的Al_(2)O_(3)-La_(2)O_(3)-ZrO_(2)(ALZ)纳米非晶粉体,并通过放电等离子烧结在875~1000℃条件下制备ALZ材料。结果表明,在875、900、927℃和1000℃烧结体相对密度分别达87.6%、98.3%、96.6%和91.5%,其中,在900℃、50 MPa条件下烧结样品具有最大的相对密度和均匀的显微组织,在STEM-BF和HRTEM下观察到了纳米晶相从非晶相基质中析出现象,纳米压痕测试样品的平均纳米硬度和杨氏模量分别为12.0 GPa和159.9 GPa。上述各烧结体相对密度大小主要与ALZ纳米非晶粉末在各温度下结晶度变化,引起粉末黏性流动改变,从而影响致密化进程有关。875℃烧结中La_(0.28)Zr_(0.72)O_(1.86)结晶度较大阻碍了非晶粉末的黏性流动;而在875~900℃会发生La_(0.28)Zr_(0.72)O_(1.86)向结晶度较小La_(2)(Zr_(2)O_(7))的相变,这有利于黏性流动;927℃和1000℃烧结的样品结晶度进一步增大不利于黏性流动。

Introduction Nanostructured amorphous materials represent a novel class of materials characterized by nanostructured amorphous particles and amorphous interfaces between particles.Due to the nanometer Scales,amorphous disorder,controllable atomic and electronic structure,this kind of material exhibits numerous unique properties distinct from conventional materials.Recently,Scholars diScovered that composites composed of nanostructured amorphous materials as the matrix with precipitated nanocrystals exhibited excellent mechanical properties.However,densification of both nanostructured amorphous materials and nanostructured amorphous composites was primarily achieved through ultra-high-pressure forming.During ultra-high-pressure forming,the plastic deformation of nanostructured amorphous powder under high pressure enabled amorphous particles to contact each other,extrude pores,and diffused through interfaces to form new interfaces.However,these studies required high-pressure forming equipment,and the associated costs constrained further applications.In contrast to the ultra-high-pressure molding mentioned earlier,Rosenflanz et al.employed viScous sintering to consolidate amorphous powder within the supercooled liquid region.This method utilized amorphous powder with low viScosity above the glass transition temperature(T_(g))and its viScous flowed under external pressure to fill the pores between particles,thereby achieving densification.This approach only required 34 MPa pressure,effectively circumventing the unrealistically high pressures previously required for amorphous material preparation.However,crystallization during viScous sintering may impede the viScous flow of amorphous powder,necessitating a sufficiently wide kinetic window between the glass transition temperature(T_(g))and crystallization temperature(Tx)to achieve densification.To address this issue,Rosenflanz et al.again employed an alumina-rare earth oxide system(Al_(2)O_(3)-Re_(2)O_(3))to modulate the polyhedral structure of Al-O,regulated the glass formation ability of the system,substantially widen the kinetic window of the oxide system,and fabricated a range of dense amorphous ceramics.Nonetheless,viScous sintering process has never been reported in the realm of nanostructured amorphous materials.Methods Amorphous powder of 57.3%Al_(2)O_(3)-19.7%La_(2)O_(3)-23%ZrO_(2)(ALZ)was synthesized via sol-gel method and subsequently calcined at 800℃for 1 h.The ALZ amorphous powder was then densified using the SPS system under vacuum conditions.Uniaxial compression was applied using graphite punches at the top and bottom,with a loading pressure of 50 MPa.The heating rate from room temperature to sintered temperatures(Ts)was set at 100℃/min.The sintered temperatures ranged between 875℃and 1000℃,with a holding time of 5 min.The obtained sample was cylindrical with a diameter of 15 mm and a thickness of 1 mm.Results and diScussion The powders calcined at 800℃for 1 h was amorphous with an average particle size of 11.6 nm,The glass transition temperature(T_(g))and crystallization onset temperature(Tx)of the obtained powders are 844℃and 914℃,respectively.Numerous pores can be observed in samples sintered at 875℃,indicating an incomplete densification at this temperature,whereas no pores were detected in samples sintered at 900℃.The relative densities of samples sintered at 875,900,927℃and 1000℃were 87.6%,98.3%,96.6%and 91.5%,respectively.The main diffraction peaks in XRD pattern for the sample sintered at 875℃corresponds to La_(0.28)Zr_(0.72)O_(1.8) with a grain size of 14.2 nm,when sintered at 900℃,diffraction peaks of samples corresponded to La_(2)(Zr_(2)O_(7))with a grain size of 9.9 nm,which suggested that high degrees of crystallization of La_(0.28)Zr_(0.72)O_(1.86)sintered at 875℃impeded the viScous flow of the amorphous powder,resulting in a formation of numerous pores.However,the relatively low crystallinity of La_(2)(Zr_(2)O_(7))phase sintered at 900℃facilitated the viScous flow of the amorphous powder,and densification was achieved only under a pressure of 50 MPa.Thus,the phase transition formed La_(0.28)Zr_(0.72)O_(1.86),which owned a higher crystallinity,to La_(2)(Zr_(2)O_(7))with lower crystallinity was advantageous for the densification of the amorphous powder via viScous flow.A further increase of crystallinity of samples sintered at 927℃and 1000℃was not conducive to the viScous flow of the amorphous powder,resulted in the formation of numerous pores.STEM-BF images of sample sintered at 900℃exhibited noticeable light-dark contrast.Upon further magnification,distinct lattice fringes can be observed at the interfaces of dark regions,while no lattice fringes were observed in the bright regions.The precipitated crystals were composed of La(AlO_(3)),La_(0.28)Zr_(0.72)O_(1.86),and La_(2)(Zr_(2)O_(7))phases,being consistent with that of XRD results.Both EDS line Scanning and mapping revealed that the bright areas were composed of Al-enriched materials,whereas the interfaces between dark areas were Zr and La-enriched materials.Furthermore,the HRTEM image revealed that the nanocrystalline phase precipitates from the amorphous phase matrix.Conclusions Nanostructured amorphous ALZ materials with a relative density of 98.3%can be prepared through spark plasma sintering at 900℃.Nanocrystalline phases precipitated from the amorphous matrix were observed via STEM-BF and HRTEM,revealing the presence of La(AlO_(3)),La_(0.28)Zr_(0.72)O_(1.86),and La_(2)(Zr_(2)O_(7))phases.The average hardness and Young's modulus of samples were 12.0 GPa and 159.9 GPa,respectively.Meanwhile,samples sintered at 875,927℃and 1000℃existed notable pores,being originated from the changes in crystallinity of ALZ under different temperatures.The significant crystallization of La_(0.28)Zr_(0.72)O_(1.86)sintered at 875℃impeded the viScous flow of amorphous powder.Concurrently,the phase transition from La_(0.28)Zr_(0.72)O_(1.86)to La_(2)(Zr_(2)O_(7))with lower crystallinity occurred at 875-900℃,which facilitated the viScous flow of amorphous powder.However,a further increase in crystallinity at 927℃and 1000℃was not conducive to the viScous flow of amorphous powder.