Dehydriding and rehydriding reactions of LiBH₄

Structural differences in LiBH₄ before and after the melting reaction at approximately 550 K were investigated to clarify the experimental method for the confirmation of reversible dehydriding and rehydriding reactions. Since the long-range order of LiBH₄ begins to disappear after the melting reaction was achieved, investigation of the atomistic vibrations of the [BH₄]-anion in LiBH₄ was found to be effective for the confirmation of the reversibility. In the present study, LiBH₄ was successively dehydrided (decomposed) into LiH and B under 1 MPa of hydrogen at 873 K, and then rehydrided (recombined) into LiBH₄ under 35 MPa of hydrogen at the same temperature (873 K). The temperatures at the beginning and ending of the dehydriding reaction are lowered, by approximately 30 K, for LiBH₄ substituted (or mixed) with Mg (atomic ratio of Li:Mg=9:1) as compared to those for LiBH₄ alone. This is similar to the tendency exhibited by LiNH₂

First-principles study on the intermediate compounds of LiBH4_4

We report the results of the first-principles calculation on the intermediate compounds of LiBH$_4$. The stability of LiB$_3$H$_8$ and Li$_2$B$_n$H$_n (n=5-12)$ has been examined with the ultrasoft pseudopotential method based on the density functional theory. Theoretical prediction has suggested that monoclinic Li$_2$B$_{12}$H$_{12}$ is the most stable among the candidate materials. We propose the following hydriding/dehydriding process of LiBH$_4$ via this intermediate compound : LiBH$_4 \leftrightarrow {1/12}$Li$_{2}$B$_{12}$H$_{12} + {5/6}$ LiH $+ {13/12}$H$_2 \leftrightarrow$LiH $+$ B $+ {3/2}$H$_2$. The hydrogen content and enthalpy of the first reaction are estimated to be 10 mass% and 56 kJ/mol H$_2$, respectively, and those of the second reaction are 4 mass% and 125 kJ/mol H$_2$. They are in good agreement with experimental results of the thermal desorption spectra of LiBH$_4$. Our calculation has predicted that the bending modes for the $\Gamma$-phonon frequencies of monoclinic Li$_2$B$_{12}$H$_{12}$ are lower than that of LiBH$_4$, while stretching modes are higher. These results are very useful for the experimental search and identification of possible intermediate compounds.Comment: 7 pages, 5 figures, submitted to PR

Magnesium borohydride: A new hydrogen storage material

Magnesium borohydride (Mg(BH₄)₂) is a promising material for hydrogen storage because of its high gravimetric storage density (15.0 mass%). We intended to synthesize Mg(BH₄)₂ by decomposition reaction of LiBH₄ with MgCl₂ by heat treatment without using a solvent, where the product consists of LiCl and a compound of magnesium, boron and hydrogen. Hydrogen desorption temperature of the product is approximately 100 K lower than that of LiBH₄ and the decomposition consists of a two-step reaction. The products of the 1st and 2nd decomposition reactions are MgH₂ and Mg, respectively. This result indicates the following two-step reaction (1st reaction: Mg(BH₄)₂→MgH₂+2B+3H₂, 2nd reaction: MgH₂→Mg+H₂). The first decomposition peak is dominant and is around 563 K. The 2nd decomposition occurs at the temperature greater than 590 K

Materials designing of metal borohydrides: Viewpoints from thermodynamical stabilities

Double-cation borohydrides MLim−n(BH₄)m (M = Zn, n = 2; M = Al, n = 3; M = Zr, n = 4; n≤ m) were expected to be synthesized and their thermodynamical stabilities were also examined experimentally. The samples with the compositions of ZnLi(BH₄)₃ and AlLi(BH₄)₄ disproportionate into Zn(BH₄)₂- (or Al(BH₄)₃-) and LiBH₄-based phases upon heating, respectively. However, no disproportionation reaction is observed in ZrLim−4(BH₄)m (m = 5 and 6). It should be emphasized that hydrogen desorption temperature Td of ZrLim−4(BH₄)m continuously increases from 440 to 650 K as the composition m increases from 4 to 6, and approaches to 740 K (Td of LiBH₄). The experimental results indicate that the combination of appropriate cations is an effective method to adjust the thermodynamical stabilities of metal borohydrides, similar to the conventional “alloying” method for hydrogen storage alloys

MÖSSBAUER STUDY ON RECOVERY OF COLD-WORKED Fe-Al ALLOYS

Le processus de la restauration isotherme de l'ordre atomique dans les alliages Fe-Al écrouis a été étudié par spectrométrie Mössbauer. On a analysé les spectres observés pour obtenir à chaque étape du recuit la distribution du champ interne dans les alliages. La probabilité d'existence des configurations atomiques a pu être obtenue aussi, en supposant une fonction de distribution Gaussienne du champ hyperfin. Les alliages ordonnés de type DO3, magnétiques ou non-magnétiques selon leur composition, ont été désordonnés et rendus magnétiques par laminage. Pour ces alliages, on assiste à une restauration très lente,au cours de recuit, vers l'état initial de type DO3 en passant par un ordre de type B2. Les alliages ordonnés de type B2, non-magnétiques, ont été partiellement désordonnés par laminage et ont donné un spectre Mössbauer comportant des caractéristiques magnétiques et non-magnétiques. Les alliages laminés se sont rapidement réordonnés pour donner les états initiaux par le recuit.The process of atomic reordering of cold-worked Fe-Al alloys on isothermal annealing has been investigated by means of Mössbauer spectroscopy. Observed spectra were analyzed to obtain the distribution function of the internal magnetic field in the alloys at each stage of annealing. The probabilities of the nearest neighbor configurations of constituent atoms could also be obtained by assuming the Gaussian distribution function of hyperfine field acting on iron atoms with various number of iron neighbors. The alloys ordered with DO3 symmetry, which are either magnetic or nonmagnetic depending on their composition, were changed by cold working into disordered and strongly magnetic states. These alloys recovered very slowly in the course of annealing to their initial state by way of B2 type of order. The alloys with B2 type of order, which are nonmagnetic, were partially disordered by cold working, and exhibited the Mössbauer spectra of both magnetic and nonmagnetic characters. The alloys recovered promptly to their initial state by annealing

Hydrogen storage properties of Mg[BH₄]₂

Among the large variety of possible complex hydrides only few exhibit a large gravimetric hydrogen density and stability around 40 kJ mol⁻¹H₂. Mg[BH₄]₂ is based on theoretical approaches a complex hydride with an equilibrium hydrogen pressure of approximately 1 bar at room temperature and a hydrogen content of 14.9 mass%. The reaction of Li[BH₄] with MgCl₂ at elevated temperatures was investigated as a possible route to synthesize Mg[BH₄]₂. Li[BH₄] reacts with MgCl₂ at a temperature >523 K at a pressure of 10 MPa of hydrogen, where the product contains LiCl and Mg[BH₄]₂. The desorption pc-isotherm of the product obtained at 623 K shows two flat plateaus, which indicates that the decomposition of the product consists of a two-step reaction. The products of the first and the second decomposition reaction were analyzed by means of X-ray diffraction and found to be MgH₂ and Mg, respectively. The enthalpy for the first decomposition reaction was determined to be ΔH = −39.3 kJ mol⁻¹H₂ by the Van’t Hoff plot of the equilibrium measurements between 563 K and 623 K, which is significantly lower than that for pure Li[BH₄] (ΔH = −74.9 kJ mol⁻¹H₂). However, only the second reaction step (MgH₂ → Mg) is reversible at the condition up to 623 K at 10 MPa of hydrogen

Study of an acoustic field in a microchannel

Using a standing-wave field, it is possible to trap small objects at nodes of a sound pressure distribution. In the present study, a sound wave was generated by a transducer outside of a microchannel, and propagated into a microchannel on a glass plate, where it generated a standing wave field. When water containing alumina particles was injected into the microchannel, several layers of particles were formed in the sound field. Moreover, when the ultrasound driving frequency was swept, it was possible to control the direction of the particle flow. The sound field was numerically calculated and the experimental results are discussed

Study of an acoustic field in a microchannel

Using a standing-wave field, it is possible to trap small objects at nodes of a sound pressure distribution. In the present study, a sound wave was generated by a transducer outside of a microchannel, and propagated into a microchannel on a glass plate, where it generated a standing wave field. When water containing alumina particles was injected into the microchannel, several layers of particles were formed in the sound field. Moreover, when the ultrasound driving frequency was swept, it was possible to control the direction of the particle flow. The sound field was numerically calculated and the experimental results are discussed