Li_3N在生成BN反应中的作用

研究了常压高温下Li3N在B4 C与含氮化合物生成BN反应中的作用。实验结果表明 ,在 950℃高温下 ,B4 C与Si3N4 反应不生成hBN ,B4 C与NH4 Cl反应只生成少量hBN。在该两种原料中加入Li3N后 ,反应产物中hBN的生成量都明显增多。但Li3N本身没有与B4 C生成hBN的反应。由此推断 ,Li3N在上述B4 C与含氮化合物生成hBN的反应中表现出了催化作用。此外 ,在以hBN为原料 ,以Li3N为催化剂合成出cBN的温度压力区域内 ,对B4 C Si3N4 Li3N体系所做的高温高压实验没有合成出hBN或cBN。还讨论了在低压条件下原位合成cBN的探索实验中 ,应如何选择硼源和氮源的问题     

六方氮化硼的振动光谱与立方氮化硼的合成

 以X射线衍射分析作参比，分析了高度三维有序到近乱层结构的9种六方氮化硼的红外和拉曼光谱，并进行了立方氮化硼的高温高压合成。光谱分析表明，随着晶性的降低，六方氮化硼的低频红外吸收峰的位置及拉曼谱线等基本振动光谱发生明显的特征性的变化，并伴随出现各自不同的次级光谱结构。合成结果表明，在触媒作用下，立方氮化硼的形成需要六方氮化硼原料有一定的结晶度，但立方氮化硼合成效果与六方氮化硼结晶度并非是简单的单调关系。对振动光谱和合成试验的结果进行了讨论。     

N NMR in AlN and BN

A characterisation by ¹⁴N NMR of the binary nitrides AlN and BN is presented. Both the static and magic angle spinning (MAS) lineshapes have been investigated in order to determine, or set upper limits on, the nuclear quadrupole coupling (C_q) at the nitrogen site. Additional data are given for the C_q values at the Al and B sites. A comparison is made with other similar (mainly wurtzite) binary compounds for which C_q is known at each atomic site.     

β NMR on8Li in the superiqnic conductor Li3N

Polarized⁸Li nuclei were produced in a Li₃N single crystal by irradiation with polarized neutrons, Β-ray detected NMR signals and spin-lattice relaxation of⁸Li were observed between B and 300 K. In Li₃N there are two non-equivalent Li sites. The corresponding two quadrupole split NMR spectra could be resolved. From the measured relaxation rates activation enthalpies for two diffusion processes were deduced.     

Reversible hydrogen-storage functions for mixtures of Li3N and Mg3N2

The hydriding and dehydriding reactions occurring in mixtures of lithium and magnesium nitrides (Li₃N and Mg₃N₂) were investigated for the first time in fixed composition ratios. A mixture of Li₃N–20 at.%Mg₃N₂ after heat treatment at 833 K was hydrogenated at 523 K in 35 MPa according to the following overall reaction: 4Li₃N+Mg₃N₂+12H₂ 12LiH+3Mg(NH₂)₂. This reaction is one of the candidates for reversible hydrogen-storage functions in which 9.1 mass% of hydrogen can be stored in the solid state. PACS 81.05.Zx; 71.20.Ps; 82.30.-b     

Li3N、Mg3N2、Ca3N2催化作用的比较和分析

 Li₃N、Mg₃N₂和Ca₃N₂是高温高压下以hBN为原料合成cBN的催化剂。在实验中发现它们对常压高温下生成hBN的反应也表现出催化作用。对比了三种氮化物催化效果的差异，发现三种氮化物都只有在熔融状态下才能表现出催化效果，以及三种氮化物对生成hBN反应的催化效果与它们在高温高压下合成cBN反应的催化效果次序相似。提出了对合成hBN有催化作用的化合物也将对合成cBN表现出催化作用的观点。     

Energy bands and electron densities of Li3N

Self-consistent band-structure calculations on the fast ionic conductor lithium nitride are performed by means of the linearized augmented plane wave (LAPW) method within the local density formalism. The corresponding density of states is decomposed into local (inside spheres), partial (l-like) and symmetry (p _x p _y ;p _z ) components from which an only very small covalent contribution to the bonding in Li₃N can be deduced. Electron density maps reveal Li₃N to be highly ionic (near Li⁺ and N^3–). A simple Watson model, although a good first approximation, cannot account for all details. For instance a remarkable non-spherical electrons density is found around N which may explain the high electric field gradient experimentally observed on this site; furthermore a reduced electron density around the Li-sites appears in contrast to a simple supper-possition of N^3– ionic densities. However, calculated x-ray structure factors and difference electron densities are in good qualitative agreement with recent x-ray diffraction experiments.This work has been supported by the Hochschuljubiläumsstiftung der Stadt Wien. All calculations were carried out at the Rechenzentrum der Technischen Universität Wien     

β-Radiation Detected Li Diffusion in the Fast Ionic Conductor Li3N

The method of β-radiation detected nuclear magnetic resonance (β-NMR) was applied to ⁸Li in a ⁷Li₃N single crystal. From NMR signals and spin-lattice relaxation rates the activation enthalpies for two distinct Li⁺ diffusion processes were deduced. Ultraslow diffusion corresponding to ionic jump rates down to 0.1 s^?1 was observed. It could be confirmed that the static electric field gradients at the two inequivalent Li sites have opposite signs.     

Thermal conductivity and heat capacity of Si3N4/BN fiber monoliths

This paper reports on measurements within the 5–300-K temperature interval of the thermal conductivity of Si₃N₄ and BN polycrystalline ceramic samples and Si₃N₄/BN fiber monoliths (FM) with different fiber arrangement architecture, [0], [90], and [0/90], with fibers arranged, accordingly, along and across the sample axis and the [0] and [90] layers stacked alternately. In the 3.5–300-K interval, the heat capacity at constant pressure, and at 77 K, the sound velocity have been measured in polycrystalline Si₃N₄ and BN samples and in Si₃N₄/BN [0] fiber monoliths. Our studies suggest that, with a high enough degree of confidence, but for some compositions—with minor assumptions, it can be maintained that, in the case of the Si₃N₄/BN fiber monoliths, one can use for calculation of their thermal conductivities and heat capacities within certain temperature intervals simple models considering mixtures of the Si₃N₄ and BN components with due account of their contributions to formation of the Si₃N₄/BN FM. It has been established that in the low-temperature domain (5–25 K), phonons in Si₃N₄/BN [0], [90], and [0/90] fiber monoliths scatter primarily from dislocations. This effect is not observed in ceramic Si₃N₄ and BN samples. The experimental data obtained on the thermal conductivity, heat capacity, and sound velocity have been used to calculate phonon mean free path lengths in polycrystalline Si₃N₄ and BN samples and the effective mean free path length in the Si₃N₄/BN [0] FM.     

Chemical bonding and lithium ion conductions in Li3N

Electronic states of Li ions during the ionic jumps in the Li₃N crystal was discussed on the basis of the first principle molecular orbital calculations. The movements of the Li ions were simulated by several model clusters with different positions of the moving cation. The net charge of the moving Li ion and the total bond overlap population between the moving Li ion and the other ions were used for discussion of chemical bonding of the moving Li ion. Furthermore we have estimated the local cluster energy (LCE) to compare the energy change in the different moving path of the Li ion. The total bond overlap population of the moving Li ion along the conduction path changed smaller than those of the other paths. On the other hand, the changes of the net charges of the moving Li ions were similar in any paths. The change of the LCE in the model cluster of the conduction path was much smaller than that in another model cluster. As the results, the smaller change of the total bond overlap population of the moving Li ions played an important role for the fast ion movement in the Li ion conductors, rather than the change of the net charge of the moving Li ions. This bonding state of the moving Li ions is one of the characteristics of the electronic state in Li ion conductors.