水热法制备ZrO_(2)掺杂钨粉体过程中的粉体形貌与相转变

Morphology and Phase Transition of ZrO_(2) Doped Tungsten Powder Prepared by Hydrothermal Method

为了探究掺杂不同含量、不同晶型ZrO_(2)是否对W合金粉体微观形貌产生影响及相转变,通过水热合成、煅烧、两段氢还原工艺制备了不同含量及不同晶型的ZrO_(2)掺杂W合金粉体,研究了ZrO_(2)对W合金粉体微观形貌、尺寸的影响。结果表明:水热合成的ZrO_(2)掺杂含量W合金粉体前驱粉由仲钨酸铵(APT)和ZrO_(2)两相构成,粉体团聚现象比较严重,粉体颗粒主要呈片状或块状,粒径较分散。煅烧后粉体由WO_(3)和ZrO_(2)两相构成,颗粒细小而分散。经二段氢还原后,粉体转变成W和ZrO_(2)两相,粉体颗粒大部分呈近球形,分散均匀。与稀土氧化物细化粉体颗粒作用机制较类似,ZrO_(2)的存在从促进形核、抑制生长两方面影响W颗粒长大的化学气相沉积反应而细化粉体颗粒,且掺杂量越大粉体越细小。纯W粒径约为15.329μm,掺杂ZrO_(2)可使粉体粒径减小到3.552μm左右。同样掺杂量的条件下,ZrO_(2)晶型对粉体的微观形貌和尺寸无明显影响。

Many researchers have studied how to improve the properties of tungsten alloy to meet the needs of industrial development,and they found that the mechanical properties of tungsten alloy can be greatly improved by doping rare earth oxides.However,due to the limited rare earth resources and strategic vision,it is very important to find materials that can replace rare earth oxides.Therefore,zirconium oxide(ZrO_(2))was used instead of rare earth oxide,and different contents of zirconium oxide were doped in tungsten(W)powders,which was expected to refine W alloy powders and lay a foundation for the subsequent preparation of tungsten alloy with excellent performance.It was known that zirconia had reversible phase transition at different temperatures,and it was a common monoclinic phase(m-ZrO_(2))at room temperature.The monoclinic phase changed into tetragonal phase(t-ZrO_(2))at above 1170℃ and cubic phase(c-ZrO_(2))when the temperature exceeded to 2370℃.In order to find out whether different crystal forms of ZrO_(2) affected the morphology of powders,different proportions of Y_(2)O_(3) were doped as stabilizers,so that t-ZrO_(2) and c-ZrO_(2) could be obtained at room temperature.Ammonium paratungstate(APT),zirconium nitrate,yttrium nitrate and urea were used as raw materials for medium W alloy powders.Firstly,the weighed APT was calcined at 120℃for 18 h in a drying oven,then poured into a roller,and then added with appropriate amount of distilled water,and stirred for 24 h.At the same time,zirconium nitrate,yttrium nitrate and urea were dissolved in proportion,then poured into a high-pressure reaction kettle,and kept at 180℃ for 18 h in the drying oven.Then,the above two liquids were fully stirred and mixed by liquid-liquid doping,dried,crushed by a pulverizer and sieved by a 350μm sieve.Then that prepare powders were calcine at 550℃ for 2 h.Finally,after two-stage hydrogen reduction(reduction conditions:600℃×4 h+950℃×6 h),ZrO_(2) doped W alloy powders were obtained.In addition,the influence of ZrO_(2) on the microstructure and size of W alloy powders were systematically analyzed by means of chemical analysis,such as X-ray diffraction analysis(XRD),microscope analysis,scanning electron microscope(SEM)and energy dispersive analysis(EDS).The results showed that the precursor powders of hydrothermal synthesis consisted of APT and ZrO_(2),and W alloy powders with different crystal forms of ZrO_(2) could be successfully prepared by hydrothermal method:monoclinic phase,tetragonal phase and cubic phase,in which tetragonal powder contains part monoclinic phase.The agglomeration of precursor powder was serious,and the powder particles were mainly flaky or blocky,with dispersed particle size.The calcined powder consisted of WO_(3) and ZrO_(2),and the particles were irregular and agglomerated.After the second-stage hydrogen reduction,the powder was transformed into W and ZrO_(2) phases,and the second-stage reduced powder was dispersed uniformly.Pure W particles were easy to agglomerate into nearly spherical shape,and the particle size distribution was different.However,after ZrO_(2) doping,the agglomeration phenomenon was obviously improved,and with the increase of ZrO_(2) doping content,the particles were obviously closer to spherical shape,and the nano-sized ZrO_(2) particles on the surface of the particles became more and more obvious.In the process of hydrogen reduction,the generated WO_(2) particles were fine and had a large specific surface area,which was easy to combine with water vapor to form hydrated tungsten oxide WO_(2)(OH)_2.Its volatility was higher than that of tungsten oxide,and it was easy to deposit on the surrounding coarse tungsten powder particles.The large particles adsorbed the surrounding smaller particles,and their particle sizes were increasing continuously.However,the dense structure of WO_(3) could be destroyed by WO_(2) obtained by the first step of low-temperature reduction.Therefore,the deposition of volatile hydrated tungsten oxide on W grains was weakened,which was similar to the mechanism of refining powder particles by rare earth oxides.The existence of ZrO_(2) affected the chemical vapor deposition reaction of W alloy powders growth from two aspects:promoting nucleation and inhibiting growth,so that the powder particles were refined and the larger the doping amount,the finer the powder particles were.The particle size of pure W was about 15.329μm,and the particle size of ZrO_(2)-doped W alloy powders was reduced to about 3.552μm.Under the condition of the same doping content,by adjusting the doping of ZrO_(2) with different crystal forms,it was found that the micro-morphology and size of the powder were very close.This overturned the previous speculation that different crystal forms might affect the surface activity of W matrix and thus the micromorphology of powder.In a word,it was feasible to use zirconia instead of rare earth oxide to refine W alloy powders.In the experimental ZrO_(2) doping range(0.1%~1.5%),the larger the doping content was,the finer W alloy powders were.However,doping ZrO_(2) with different crystal forms did not affect the micromorphology of tungsten alloy powder.