气动悬浮无容器激光加热技术的应用:MgTi_(2)O_(5)晶体及其熔体微结构的原位超高温拉曼光谱研究

Application of Aerodynamic Levitator Laser Heating Technique:Microstructures of MgTi_(2)O_(5)Crystal and Melt by in-situ Superhigh Temperature Raman Spectroscopy

搭建气体动力学悬浮无容器激光加热装置耦合皮秒级时间门控拉曼光谱仪,突破常规加热法的温度与坩埚材料的限制的同时,依靠皮秒级脉冲激光极短的测量周期大幅度屏蔽高温极端条件下黑体辐射对拉曼信号的干扰。并利用该平台首次原位测定了高熔点MgTi_(2)O_(5)超高温下(1903、1953和2003 K)的高信噪比熔体拉曼光谱。并通过耦合三代增强型电荷耦合探测器(ICCD)与纳秒级脉冲激光实现测定MgTi_(2)O_(5)晶体样品室温(RT)到1673 K的完整温度范围的原位拉曼光谱。在RT升至1953 K的升温过程中晶体的拉曼光谱出现展宽和红移现象,相对强度降低,当温度升高到熔体(2003 K)成为单一宽泛的包络线,表明此时晶体的长程有序的结构已经被破坏,体系内微结构发生本质改变。运用密度泛函理论(DFT)计算其常温拉曼光谱,比照实验光谱,对主要振动模式进行了归属分析,拉曼光谱位移低于350 cm^(-1)的低波数区的振动主要归属于晶体的晶格振动,中波数区域485 cm^(-1)的振动峰为Ti—O—Ti弯曲振动,主要特征峰648 cm^(-1)处为TiO_(6)八面体内O—Ti伸缩振动;787 cm^(-1)处为TiO_(6)八面体内O—Ti—O的弯曲振动。对熔体结构运用量子化学从头计算法,模拟了系列团簇模型的拉曼光谱,获得了特征振动模式的波数和散射截面,实验拉曼光谱采用散射截面校正后,解谱并定量分析了熔体中团簇结构的分布。定量分析显示,MgTi_(2)O_(5)晶体熔化后,存在TiO_(4)四面体构型(不同构型的Q_(i)相对摩尔分数分别为54.6%Q_(0)、20.1%Q_(1)、5.0%Q_(2)、4.8%Q_(3),Q_(i)为不同桥氧数i的钛氧四面体)和TiO_(6)八面体构型(H_(0)的相对摩尔分数为14.8%,H_(0)为孤立的六配位钛氧八面体)。Ti^(4+)主要以孤立四面体结构Q_(0)、二聚体结构Q_(1)四配位形式存在,少部分以孤立的钛氧八面体H_(0)六配位的形式存在。结果表明:MgTi_(2)O_(5)熔体成分中占较大比例的孤立结构,破坏了体系网络连接性,抑制了玻璃形成能力,因此该高温熔体不具备形成玻璃的条件。在升温过程中MgTi_(2)O_(5)晶体的拉曼光谱显示无相变发生;熔融过程中,晶体微结构中的Ti—O多面体结构由单一TiO_(6)型转变为TiO_(4)与TiO_(6)型共存。

The aerodynamic levitator laser(ADL)heating device coupled with picosecond time-gate Raman spectrometer was built.It breaks through the limitation of temperature and crucible material of conventional heating method.It greatly shields the interference of blackbody radiation on Raman signal under high temperature and extreme conditions by relying on the extremely short measurement cycle of picosecond pulsed laser.In-situ Raman spectra of MgTi_(2)O_(5)melt with high melting points at superhigh temperatures(1903,1953,2003 K)were measured for the first time.In-situ temperature-dependent Raman spectra of MgTi_(2)O_(5)crystal before melting(1673 K)were measured by coupling the third-generation intensified charge-coupled device(ICCD)detector and nanosecond pulsed laser.Raman spectra of the crystal broaden and redshift with increasing temperature from room temperature(RT)to 1953 K,and the relative intensity decreases.A single broad envelop was observed when the temperature was increased to the melt(2003 K).Indicating that the long-range ordered structure of the crystal has been destroyed and the microstructures in the system have changed essentially.The Raman spectrum of MgTi_(2)O_(5)crystal at RT was calculated by density functional theory(DFT),and major vibration modes were thus assigned by comparing the calculated spectrum with the experimental one.The vibration peaks in the low wavenumber region(<350 cm^(-1))can be mainly attributed to the external lattice vibration modes.The peak at 485 cm^(-1)in the medium wavenumber region corresponds to Ti—O—Ti bending vibration,and the main characteristic peak at 648 and 787 cm^(-1)stand for O—Ti stretching vibration and O—Ti—O bending vibration in the TiO_(6)octahedron,respectively.A series of cluster models assumed in melt were constructed and simulated by the quantum chemistry ab initio calculation method.The characteristic Raman active vibration wavenumbers and their scattering cross section were obtained.After the experimental Raman spectra of melt were corrected by scattering cross section,the deconvolution of the molten Raman spectra was carried out,and the concentration distribution of various species was thus described quantitatively.Results show that there are TiO_(4)tetrahedral clusters(The relative mole fractions of respective species Q_(i)in different configurations are 54.6%Q_(0),20.1%Q_(1),5.0%Q_(2)and 4.8%Q_(3),and Q_(i)is the titanium oxide tetrahedron with different bridge oxygen number i)and TiO_(6)octahedral clusters(Hexacoordinated titanium oxide octahedron,whose relative mole fraction is 14.8%H_(0))in MgTi_(2)O_(5)melt.Ti^(4+)mainly exists in isolated tetrahedral structure Q_(0)and dimer structure Q_(1),and a small part exists in the form of isolated titanium oxide octahedral H_(0).The isolated structure accounts for most of the composition of MgTi_(2)O_(5)melt,which destroys the system s network connectivity and inhibits the glass forming ability.No solid-solid phase transitions were observed for MgTi_(2)O_(5)crystal with the increasing temperature before melting.Above the melting point,the Ti—O polyhedron in the crystal changes from a single TiO_(6)species to the coexistence of both TiO_(4)and TiO_(6)species.