Crystal Structures, Phase Stabilities, and Hydrogen Storage Properties of Metal Amidoboranes

Metal amidoboranes, M­(NH₂BH₃)_<i>n</i> (M = alkali metal or alkaline-earth metal), are candidates for on-board hydrogen storage materials with high gravimetric capacity, low H₂ release temperature, and the ability to suppress toxic borazine emission. We have used a first-principles density functional theory (DFT) combination with Monte Carlo method to search for crystal structures for a wide array of metal amidoboranes (M = Li, Na, K, Be, Mg, Ca, Sr, and Sc). In cases where the experimental structures are known, the DFT energies of the theoretically predicted LiNH₂BH₃, NaNH₂BH₃, KNH₂BH₃, and Ca­(NH₂BH₃)₂ structures are degenerate with the DFT energies computed for the experimental structures [to within 4 kJ/(mol f.u.)], confirming the accuracy of our approach. On the basis of the decomposition reaction pathway, M­(NH₂BH₃)_<i>n</i> → MH_<i>n</i> + <i>n</i>BN + 2<i>n</i>H₂, we compute the H₂ release reaction enthalpies and show that the stability of metal amidoboranes obeys the following trend: The metal amidoborane becomes more stable (the decomposition reaction becomes less exothermic) as the metal cation becomes more electropositive, that is, as the metal cation goes down in the periodic table along a given column or as the metal moves to the left along a given row. The only exception to this rule is Mg­(NH₂BH₃)₂, which is more stable than Ca­(NH₂BH₃)₂. Introducing vibrational entropy effects does not change this exceptional behavior of Mg amidoborane: the phonon contribution serves to shift all reaction enthalpies down by a roughly constant amount, ∼22 kJ/(mol H₂) at <i>T</i> = 300 K

QSPRs for estimating nematic transition temperatures of pyridine-containing liquid crystalline compounds

<p>Quantitative structure–property relationships were developed to predict the nematic–isotropic transition temperatures of 92 pyridine-containing liquid crystalline compounds using molecular descriptors calculated by CODESSA software and DRAGON software. The descriptors were also analysed by using principal component analysis. Essentials accounting for a reliable model were all considered carefully during model construction and assessment process. Five variables were selected out by stepwise forward regression analysis and were used as inputs to perform the multiple linear regression, support vector machine and projection pursuit regression (PPR) study. All models were validated through two ways, that is, internal cross-validation combined with a test set. Comparatively, the PPR model performs best both in the fitness and in the prediction capacity. For the test set, it gave a predictive correlation coefficient (<i>R</i>) of 0.991, root mean square error of 11.799 and absolute average relative deviation of 5.456, respectively. The relationships between the descriptors and the nematic–isotropic transition temperature of compounds were also discussed. The odd–even effect in the transition temperatures of mesogens in the same homologous series was also discussed.</p

First-Principles Prediction of Intermediate Products in the Decomposition of Metal Amidoboranes

The nonvolatile products remaining after the thermal decomposition of metal amidoboranes (MAB, M = metal) are amorphous and incompletely characterized, increasing the complexity of devising regeneration strategies for these potential hydrogen storage materials. Utilizing the combined prototype electrostatic ground state search and density-functional theory (PEGS+DFT), we find that potential reaction products ([NHBH₂]⁻, [NBH]⁻, [N₃H₂B₃H₃]⁻, and polymer-M­[NHBH₂] anion groups) in the decomposition of LiAB and CaAB are calculated to be significantly endothermic, in contrast to the experimentally measured nearly thermoneutral values [∼−4 kJ/(mol H₂) in LiAB and 3.5 kJ/(mol H₂) in CaAB], suggesting that there are alternative products formed. The dianion group [NHBHNHBH₃]^2– has recently been suggested to form in the decomposition of a calcium amidoborane complex in solution. In LiAB and CaAB, we use PEGS+DFT to predict intermediate metal–dianion compounds, and the static H₂ release enthalpy is 27.4 and 27.3 kJ/(mol H₂) in LiAB and CaAB, respectively. Introducing vibrational effects by phonon calculations, the enthalpies are shifted down by a roughly constant amount, ∼25 and ∼22 kJ/(mol H₂) at 0 and 300 K. Thus, our theoretical H₂ release enthalpies agree with the experimentally measured nearly thermoneutral data in the decomposition of LiAB and CaAB. This agreement supports the existence of the dianion phases as products in the decomposition of metal amidoboranes. Then, using the dianion compound as an intermediate in the decomposition of MAB, we further study the stability trends of a series of MAB (M = Li, Na, K, Ca)

Multicollinearity statistics of model variables.

<p>Multicollinearity statistics of model variables.</p

Metro passengers’ route choice model and its application considering perceived transfer threshold

<div><p>With the rapid development of the Metro network in China, the greatly increased route alternatives make passengers’ route choice behavior and passenger flow assignment more complicated, which presents challenges to the operation management. In this paper, a path sized logit model is adopted to analyze passengers’ route choice preferences considering such parameters as in-vehicle time, number of transfers, and transfer time. Moreover, the “perceived transfer threshold” is defined and included in the utility function to reflect the penalty difference caused by transfer time on passengers’ perceived utility under various numbers of transfers. Next, based on the revealed preference data collected in the Guangzhou Metro, the proposed model is calibrated. The appropriate perceived transfer threshold value and the route choice preferences are analyzed. Finally, the model is applied to a personalized route planning case to demonstrate the engineering practicability of route choice behavior analysis. The results show that the introduction of the perceived transfer threshold is helpful to improve the model’s explanatory abilities. In addition, personalized route planning based on route choice preferences can meet passengers’ diversified travel demands.</p></div

A constrained multinomial Probit route choice model in the metro network: Formulation, estimation and application

<div><p>Considering that metro network expansion brings us with more alternative routes, it is attractive to integrate the impacts of routes set and the interdependency among alternative routes on route choice probability into route choice modeling. Therefore, the formulation, estimation and application of a constrained multinomial probit (CMNP) route choice model in the metro network are carried out in this paper. The utility function is formulated as three components: the compensatory component is a function of influencing factors; the non-compensatory component measures the impacts of routes set on utility; following a multivariate normal distribution, the covariance of error component is structured into three parts, representing the correlation among routes, the transfer variance of route, and the unobserved variance respectively. Considering multidimensional integrals of the multivariate normal probability density function, the CMNP model is rewritten as Hierarchical Bayes formula and M-H sampling algorithm based Monte Carlo Markov Chain approach is constructed to estimate all parameters. Based on Guangzhou Metro data, reliable estimation results are gained. Furthermore, the proposed CMNP model also shows a good forecasting performance for the route choice probabilities calculation and a good application performance for transfer flow volume prediction.</p></div

Statistics of the distribution of alternative routes in OD pairs.

<p>Statistics of the distribution of alternative routes in OD pairs.</p

Schematic diagram of the planned routes.

<p>Schematic diagram of the planned routes.</p

The transfer flow volume forecasting performance.

<p>For each spot, it has two values, including the testing data corresponding to horizontal axis and forecasting value corresponding to vertical axis.</p

Prediction of New Stable Compounds and Promising Thermoelectrics in the Cu–Sb–Se System

We study the phase stability and predict as-yet-unreported compounds in the thermoelectric Cu–Sb–Se ternary system. We use a combination of total energies obtained from density-functional-theory-based (DFT) calculations with vibrational entropies from phonon calculations (within the harmonic approximation) and configurational entropies, treated with cluster expansions (CE). The Cu–Sb–Se ternary phase diagram is determined (treating all phases as line compounds) using the grand-canonical linear programming method. We find the following results: (1) we predict the stability of a new previously unknown, zinc blende-based Cu₄SbSe₅ compound but find that it is thermodynamically stable up to only ∼300 K; (2) we also predict that a Cu₁₂Sb₄Se₁₃ phase (isostructural with Cu₁₂Sb₄S₁₃, but unreported in the Cu–Sb–Se system) appears in the phase diagram at high temperatures (but below the temperatures where the observed Cu₃SbSe₃ phase is stable); (3) based on quasi-harmonic phonon and band structure calculations, we find that Cu₁₂Sb₄Se₁₃ has thermal conductivity and an electronic structure that suggests it as a promising thermoelectric material