温稠密氮在部分离解电离区物性及金属化相变机理研究

Warm dense matter is an important matter state generated in nuclear fusion, Z-pinch, and detonation processes, and is also widely found in the interiors of giant planets and the earth. Currently, a variety of compression tools (such as light-gas-gun, laser, z-pinch) are applied to generate warm dense matter, at the same time, its equations of state and the associated transport properties (such as electrical conductivity, thermal conductivity) need to be characterized. However, the diagnostic of a transport properties (i.e., electrical conductivity) in a dynamic compression experiment is possible only when a good estimation of the result is provided by a theoretical method, which is currently absent. The difficulty in the theoretical method is mainly associated with the complexity of the interaction processes among various particles in the warm dense matter system, which is known to be strongly coupled, partially degenerate, and non-ideal. This proposal aims to build a general thermodynamic model of warm dense nitrogen providing both the equations of state and the transport properties, which take into account the effects of the dissociation and ionization processes, and to establish a new numerical calculation method with the computer codes developed, and then to analyze the mechanism of metallization transition based on the calculations above. The validity and applicability of the model will be checked in comparison with the available experimental data and with the available results of quantum molecular dynamic simulations. The model, hopefully, can produce reliable results of the equation of state and the transport properties of warm dense nitrogen, providing a practical guide for the design of diagnostics in a dynamic compression experiment.

温稠密物质是核聚变、Z箍缩、爆轰过程中的重要物质形态，也广泛存在于天体和地球内部。当今，在实验室条件下用各种高压加载手段（如气炮、激光、Z箍缩等）产生温稠密物质，需对其状态和性质诊断和描述，尤其是电导率和热导率等输运性质，但目前缺乏能为高压实验电导率测量提供在工程上实用的理论预测方法。这主要是由于温稠密物质中各种粒子间相互作用非常复杂，且处于非理想、强耦合、部分简并的状态，导致温稠密物质特性的理论预测十分困难。本项目拟选取典型双原子分子氮作为研究对象，考虑温稠密条件下氮的离解和电离反应、由多元混合体系中各种粒子相互作用引起的离解和电离能降低的自洽变分修正，建立温稠密氮的状态方程及输运性质综合热动力学模型和数值计算方法，分析金属化相变机理；并结合量子分子动力学模拟和实验数据研究，评估模型的可靠性、适用性与普适性。为实验室条件下开展极端条件下温稠密物质状态方程和输运性质的测量提供理论预测方法。

温稠密物质存在于天体和惯性约束核聚变等领域，温稠密物质状态方程和输运性质是分析界定物质在高温高压的性质变化的重要参数。本项目对氮在部分离解电离区状态方程和输运性质的模拟。考虑氮的离解和电离对应的多种粒子之间的相互作用，以及氮分子离解、氮分子电离、氮原子电离等多种反应，构建模型和进行程序设计，计算获得氮在部分离解电离区的各种粒子组分分布和电导率等输运性质信息，并通过分析温稠密氮的状态方程和电导率结果给出氮在高温高压下的相图和一阶液液相变边界，得到氮发生金属化的压力。通过自洽流体变分理论构建模型能够合理且较为快速得模拟温度介于300-60000K，密度在0.1-5.0 g cm-3、压力在0-500 GPa范围内的粒子组分分布和状态方程，能够为满足动高压实验、实验室天体物理等领域实验状态理论预估以及建设实际工程中需要的较宽区域的氮的状态方程和粒子组分等性质的数据库。并结合量子分子动力学模拟温稠密氮的状态方程和输运性质。通过电导率和压力数据分析得到氮发生金属化相变的压力约为60 GPa，但这与动高压实验冲击加载金属化压力高于100 GPa相比偏小，可能的原因是理论模拟过程是准静态加压过程与动高压实验加载的相变路径不同。最终通过计算模拟构建了温稠密氮在部分离解电离区宽域的温度-密度-压力-粒子分布-电导率等状态方程和输运性质数据库，这些数据能为动高压、天体物理等领域实验提供理论预估和工程应用检验。