Hai Wen Li

Formation of an intermediate compound with a B12H12 cluster: experimental and theoretical studies on magnesium borohydride Mg(BH4)2.

Experimental and theoretical studies on Mg(BH4)2 were carried out from the viewpoint of the formation of the intermediate compound MgB12H12 with B12H12 cluster. The full dehydriding and partial rehydriding reactions of Mg(BH4)2 occurred according to the following multistep reaction: Mg(BH4)2 -->1/6MgB12H12 + 5/6MgH2 + 13/6H2 <--> MgH2 + 2B + 3H2 <--> Mg + 2B + 4H2. The dehydriding reaction of Mg(BH4)2 starts at approximately 520 K, and 14.4 mass% of hydrogen is released upon heating to 800 K. Furthermore, 6.1 mass% of hydrogen can be rehydrided through the formation of MgB12H12. The mechanism for the formation of MgB12H12 under the present rehydriding condition is also discussed.

Stabilization of lithium superionic conduction phase and enhancement of conductivity of LiBH4 by LiCl addition

LiBH4 exhibits lithium superionic conduction accompanied by structural transition at around 390 K. Addition of LiCl to LiBH4 drastically affects both the transition and electrical conductivity: Transition from low-temperature (LT) to high-temperature (HT) phases in LiBH4 is observed at 370 K upon heating and the HT phase can be retained at 350–330 K upon cooling. Further, the conductivity in the LT phase is more than one or two orders of magnitude higher than that of pure LiBH4. These properties could be attributed to the dissolution of LiCl into LiBH4, suggested by in situ x-ray diffraction measurement.

Ultrafast Formation of Amorphous Bimetallic Hydroxide Films on 3D Conductive Sulfide Nanoarrays for Large‐Current‐Density Oxygen Evolution Electrocatalysis

Developing nonprecious oxygen evolution electrocatalysts that can work well at large current densities is of primary importance in a viable water-splitting technology. Herein, a facile ultrafast (5 s) synthetic approach is reported that produces a novel, efficient, non-noble metal oxygen-evolution nano-electrocatalyst that is composed of amorphous Ni-Fe bimetallic hydroxide film-coated, nickel foam (NF)-supported, Ni3 S2 nanosheet arrays. The composite nanomaterial (denoted as Ni-Fe-OH@Ni3 S2 /NF) shows highly efficient electrocatalytic activity toward oxygen evolution reaction (OER) at large current densities, even in the order of 1000 mA cm-2 . Ni-Fe-OH@Ni3 S2 /NF also gives an excellent catalytic stability toward OER both in 1 m KOH solution and in 30 wt% KOH solution. Further experimental results indicate that the effective integration of high catalytic reactivity, high structural stability, and high electronic conductivity into a single material system makes Ni-Fe-OH@Ni3 S2 /NF a remarkable catalytic ability for OER at large current densities.

Pressure and temperature dependence of the decomposition pathway of LiBH4

The decomposition pathway is crucial for the applicability of LiBH(4) as a hydrogen storage material. We discuss and compare the different decomposition pathways of LiBH(4) according to the thermodynamic parameters and show the experimental ways to realize them. Two pathways, i.e. the direct decomposition into boron and the decomposition via Li(2)B(12)H(12), were realized under appropriate conditions, respectively. By applying a H(2) pressure of 50 bar at 873 K or 10 bar at 700 K, LiBH(4) is forced to decompose into Li(2)B(12)H(12). In a lower pressure range of 0.1 to 10 bar at 873 K and 800 K, the concurrence of both decomposition pathways is observed. Raman spectroscopy and (11)B MAS NMR measurements confirm the formation of an intermediate Li(2)B(12)H(12) phase (mostly Li(2)B(12)H(12) adducts, such as dimers or trimers) and amorphous boron.

Diffuse and doubly split atom occupation in hexagonal LiBH4

A theoretical study has been performed to explain problems in the structural analysis of LiBH4 and its recently discovered superionic conductance. First-principles molecular dynamics simulations for the high temperature (hexagonal) phase show doubly split and diffuse occupation in the c-direction at Li and B sites, respectively. Li hopping within the split sites and libration of H atoms are also found. These dynamics are supported by the Rietveld analysis showing atomic displacement ellipsoids for Li and B atoms.

First-principles studies of complex hydride YMn2H6 and its synthesis from metal hydride YMn2H4.5

First-principles calculations were performed for a complex hydride YMn2H6 to investigate its electronic structure and thermodynamic stability. The results indicated that an Y atom and one of two Mn atoms were ionized as Y3+ and Mn2+, respectively, and another Mn atom bound covalently to H atoms to form a [MnH6]5− complex anion. Based on the enthalpy change of −65 kJ/mol estimated from the calculation, we experimentally verified a possible low-pressure synthesis of YMn2H6 from a metal hydride YMn2H4.5. X-ray diffractometry confirmed the formation of YMn2H6 after hydrogenation below 5 MPa, much lower than the previously reported value of 170 MPa.

Improvement of hydrogen storage property of three-component Mg(NH2)2–LiNH2–LiH composites by additives

The three-component Mg(NH2)2-LiNH2-4LiH composite reversibly stores hydrogen exceeding 5 wt% at a temperature as low as 150 °C. In this work, a number of additives such as CeF4, CeO2, TiCl3, TiH2, NaH, KBH4 and KH are added to the Mg(NH2)2-LiNH2-4LiH composite in order to improve its kinetics, thermodynamics and cycling properties. Addition of 3 wt% of KH reduces the dehydrogenation onset temperature of the Mg(NH2)2-LiNH2-4LiH composite to below 90 °C without emission of NH3 during the whole dehydrogenation process up to 450 °C. Moreover, the dehydrogenation kinetics and cycling ability are remarkably enhanced upon KH-addition. The reaction model of the Mg(NH2)2-LiNH2-4LiH composite is altered upon KH-addition with the active molecule density improved by about 200 times. In addition, by optimization of the ratio of Mg2+ to Li+ in the Mg(NH2)2-LiNH2-LiH system, several novel composites, e.g., Mg(NH2)2-2LiNH2-5.9LiH-0.1KH and Mg(NH2)2-LiNH2-5.9LiH-0.1KH, with the hydrogen storage capacity exceeding 6 wt% without emission of NH3 below 250 °C are developed. Our study demonstrates that there are various undiscovered candidates with promising hydrogen storage properties in the three-component Li-Mg-N-H system.

Facile synthesis of anhydrous alkaline earth metal dodecaborates MB12H12 (M = Mg, Ca) from M(BH4)2.

Metal dodecaborates M2/nB12H12 are among the dehydrogenation intermediates of metal borohydrides M(BH4)n with a high hydrogen density of approximately 10 mass%, the latter is a potential hydrogen storage material. There is therefore a great need to synthesize anhydrous M2/nB12H12 in order to investigate the thermal decomposition of M2/nB12H12 and to understand its role in the dehydrogenation and rehydrogenation of M(BH4)n. In this work, anhydrous alkaline earth metal dodecaborates MB12H12 (M = Mg, Ca) have been successfully synthesized by sintering M(BH4)2 (M = Mg, Ca) and B10H14 in a stoichiometric molar ratio of 1 : 1. Thermal decomposition of MB12H12 shows multistep pathways with the formation of H-deficient monomers MB12H12-x containing icosahedral B12 skeletons and is followed by the formation of (MByHz)n polymers. Comparison of the thermal decomposition of MB12H12 and M(BH4)2 suggests different behaviours of the anhydrous MB12H12 and those formed from the decomposition of M(BH4)n.

Chemical and electrical characterisation of the interaction of BCl3/Cl2 etching and CF4/H2O stripping plasmas with aluminum surfaces

In this work, the effect of BCl 3 /Cl 2 etching and CF 4 /H 2 O stripping plasmas on Al surfaces during patterning is investigated. The purpose is to consider the direct impact of plasma exposure on the intrinsic electrical properties of the interconnect in terms of its effectively conductive cross section. First, the origin of the plasma-transformed sidewall area is clarified. It is demonstrated by X-ray photoelectron spectroscopy that after the final dry stripping sequence, the Al sidewalls are transformed into AlF x or Al(FO) x due to the chemical interaction with the CF 4 plasma, which degrades their metallic, conductive nature. The actual dimension of the plasma-affected region is then quantified electrically to be in the order of 20 nm and is found to increase with decreasing BCl 3 /Cl 2 ratio. The latter indicates that it is the structural integrity of the protecting sidewall polymer formed during the etching process which controls the degree of interaction between the interconnect sidewall and the subsequent stripping plasma. Since the CF 4 plasma interaction seriously degrades the electrical performance of an Al interconnect by reducing its effective conductive area, care should be taken in general that while optimizing a plasma-stripping sequence, both stripping efficiency and sidewall interaction are considered.

Process integration induced thermodesorption from SiO2/SiLK resin dielectric based interconnects

The thermodesorption from SiO2/SiLK resin dielectric assemblies at various stages of interconnect fabrication are studied by mass spectrometry. Species desorption from such an assembly is governed by the intrinsic material properties and the process step history, such as patterning chemistry and by environmental contamination. The thermodesorption of an as-cured SiLK resin film is compared to the desorption of species after different process steps. It is shown that as cured SiLK films contain very low amounts of moisture and volatile organic compounds. The plasma-enhanced chemical vapor deposition SiO2 hardmask is the main source of moisture in the SiO2/SiLK resin dielectric stack. Annealing at T⩾350 °C in both vacuum and nitrogen ambient conditions drastically reduces the species desorption. The origin and the kinetics of the desorbed species are described.