框架结构复合氧化物的物相演变与热膨胀调控

发布时间：2018-03-04 04:26

  本文选题：负热膨胀　切入点：框架结构　出处：《北京科技大学》2017年博士论文　论文类型：学位论文

【摘要】：框架结构负热膨胀(NTE)材料是一个比较庞大的NTE材料家族,其包括ScF_3、A_2M_3O_(12)(A=三价金属离子,M=W或Mo)、ZrM_2O_8(M=W或Mo)、MOF等,其相结构和热膨胀的研究具有重要的学术意义和潜在的应用价值。本文主要选择A_2W_3O_(12)和ZrM_2O_8为研究对象,虽然人们对这两类材料已经进行了大量的研究,但皆是围绕阳离子的替代去调控相变温度和热膨胀性。本文通过在框架结构中引进客体小分子和采用高温淬火手段对其进行改性,进而研究其结构、相转变和热膨胀性的改变。主要内容如下：氨化使小分子(NH_3)进入了ZrW_2O_8晶格内,与W_2形成弱配位键,并与近邻原子发生相互作用,进而改变了相变温度和负热膨胀性。氨化使ZrW_2O_8的相变温度提高了近50℃,使其负热膨胀系数由-7.8×10~(-6)℃~(-1)调节到了-2.1×10~(-6)℃~(-1)。产生这些改变的原因主要包括两方面：一方面,NH_3的进入填充了晶格间隙,减小了材料收缩的空间,使ZrW_2O_8的整体框架结构刚性增强；另一方面,NH_3与近邻原子相互作用,束缚了氧原子的横向热振动。氨化还可以防止ZrW_2O_3水化,提高其稳定性。另外,对ZrMo_2O_8,ZrWMoO_8和Y_2W_3O_(12)也都进行了氨化。ZrMo_2O_8和ZrWMoO_8氨化后,其变化趋势和ZrW_2O_8类似。Y_2W_3O_(12)经过氨化后,晶体结构发生了改变。室温下,Dy_2W_3O_(12)呈单斜相,不具有负热膨胀性；升高到一定温度时,转变为具有负热膨胀性的正交相,相变温度为996℃。通过高温淬火的方式,Dy_2W_3O_(12)的高温正交相被保留至低温。正交相Dy_2W_3O_(12)在500℃以下可以稳定存在,其负热膨胀系数为αv=-2.6×10~(-5)℃~(-1)。到目前为止,Dy_2W_3O_(12)是A_2W_3O_(12)系列负热膨胀材料中负热膨胀系数最大的。另外,对其发光和磁性也进行了研究。Ho_2W_3O_(12)低温时的相结构一直是个有争议的问题。本文通过不同的制备条件(包括高温淬火)成功地解决了这个争议。研究发现：低温时,Ho_2W_3O_(12)的热力学稳定相为单斜相,高温时为正交相。由于正交相向单斜相的转变是一个缓慢的过程,故低温时的相组成与制备时的降温速率紧密相关。当降温速率为10℃/min时,Ho_2W_3O_(12)就可以完全使高温时的正交相保留至低温。正交相Ho_2W_3O_(12)在600℃下可稳定存在,负热膨胀系数为-2.1×10~(-5)℃~(-1)。在Gd_xTm_(2-x)W_3O_(12)(x=0.5、1和1.5)固溶体中,Gd含量较低(x=0.5)时,室温稳定相为正交相；Gd的含量较高(x=1,1.5)时,室温稳定相变为单斜相,并在高温时转变为正交相。采用高温淬火的方法,Gd_xTm_(2-x)W_3O_(12)(x=1,1.5)的高温正交相被保留至室温,并且在500℃以下稳定存在。Gd_(0.5)Tm_(1.5)W_3O_(12)的吸水性较弱,而正交相Gd_xTm_(2-x)W_3O_(12)(x=1.1.5)吸水性则较强。Gd_xTm_(2-x)W_3O_(12)在200～5000C的温度区间内均具有较大的负热膨胀性,并且其负热胀系数随着Gd的含量增加而增大。引进客体小分子和采用高温淬火拓展了A_2W_3O_(12)、ZrM_2O_8等框架结构类负热膨胀材料的研究途径,无论在理论研究上,还是在实际应用中,都有着重要意义。
[Abstract]:Frame structure of negative thermal expansion (NTE) material is a relatively large NTE family of materials, including ScF_3, A_2M_3O_ (12) (A= M=W trivalent metal ions, or Mo), ZrM_2O_8 (M=W or Mo), MOF, to study the phase structure and thermal expansion has important application value and academic significance potential. This paper chooses A_2W_3O_ and ZrM_2O_8 (12) as the research object, although there have been a lot of studies on the two kinds of materials, but are instead around the cation to control the phase transition temperature and thermal expansion. In this paper, through the introduction of small molecule object and it was modified by high temperature quenching method in the frame structure, and then study the structure, the change of phase transformation and thermal expansion. The main contents are as follows: ammonization of the small molecule (NH_3) into the ZrW_2O_8 lattice, forming a weak coordination bond with W_2, and interact with the neighboring atoms, thereby changing the phase transition temperature And the negative thermal expansion. The ammoniation transformation temperature of ZrW_2O_8 increased by nearly 50 degrees, the negative thermal expansion coefficient by -7.8 * 10~ (-6) C ~ (-1) adjusted to -2.1 * 10~ (-6) C ~ (-1). The causes of these changes mainly includes two aspects: on the one hand, enter the lattice filled the gap of NH_3, reduce the shrinkage of space, so that the overall rigid frame structure of ZrW_2O_8 was enhanced; on the other hand, NH_3 and nearest neighbor atoms bound the transverse thermal vibration of oxygen. Ammonification can also prevent ZrW_2O_3 hydration, improve its stability. In addition, the ZrMo_2O_8, ZrWMoO_8 and Y_2W_3O_ (12) were.ZrMo_2O_8 and ZrWMoO_8 after ammoniation ammonification, the trend is similar to ZrW_2O_8.Y_2W_3O_ (12) after ammoniation changed after the crystal structure at room temperature, Dy_2W_3O_ (12) in the monoclinic phase, with negative thermal expansion; increased to a certain temperature, change into Negative thermal expansion of the orthorhombic phase, phase transition temperature of 996 DEG. Through high temperature quenching method, Dy_2W_3O_ (12) orthogonal phase was retained to high temperature low temperature. The orthogonal phase Dy_2W_3O_ (12) at 500 DEG C can exist stably, the negative thermal expansion coefficient alpha v=-2.6 * 10~ (-5) C ~ (-1). So far, Dy_2W_3O_ (12) A_2W_3O_ (12) series of negative thermal expansion materials in negative thermal expansion coefficient of the maximum. In addition, the luminescent and magnetic properties were also studied in.Ho_2W_3O_ (12) phase structure at low temperature has been a controversial issue. This article through different preparation conditions (including high temperature quenching) successfully solved the dispute. It is found that at low temperature, Ho_2W_3O_ (12) phase is thermodynamically stable monoclinic phase, high temperature to orthorhombic phase. Due to the change of orthorhombic to monoclinic phase is a slow process, so the temperature when the phase composition is closely related with the preparation of the cooling rate. 褰撻檷娓╅，

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