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New density functional theory (DFT+U) calculations on ceria show that nextnearest-neighbor (NNN) configurations for Ce 3+ and oxygen vacancy are more stable than the nearest-neighbor (NN) configuration usually found in such calculations, thus removing a longstanding discrepancy with classical simulations. The calculated chemical expansion due to reduction is now in excellent agreement with the experimental value, in contrast to that predicted by the NN configuration. The results suggest that the reduction mechanism in ceria involves relaxation from metastable NN to stable NNN configurations through charge transfer or oxygen hopping. The energy barrier for oxygen hopping mechanism was found to be 0.43 eV. It also suggests that DFT+U simulations of mixed-valence materials should be conducted with considerable care.

An atomic force microscopy based nanoindentation method was employed to study how the structure of cellulose microfibril packing and matrix polymers affect elastic modulus of fully hydrated primary plant cell walls. The isolated, single-layered abaxial epidermis cell wall of an onion bulb was used as a test system since the cellulose microfibril packing in this cell wall is known to vary systematically from inside to outside scales and the most abundant matrix polymer, pectin, can easily be altered through simple chemical treatments such as ethylenediaminetetraacetic acid and calcium ions. Experimental results showed that the pectin network variation has significant impacts on the cell wall modulus, and not the cellulose microfibril packing.

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Summary This paper proposes a robust algorithm for time‐optimal rigid spacecraft reorientation trajectory generation. Based on the Pontryagin's maximum principle, the first‐order necessary optimality conditions are derived. These optimality conditions are numerically satisfied by adopting a pseudospectral method integrated homotopic approach to solve the associated shooting functions. First, the energy‐optimal reorientation solution is obtained using the Radau pseudospectral method, which has a spectral convergence speed and can give a precise estimation of the initial costates used to start the homotopic approach. Then, a modified homotopy scheme is given to deform the associated energy‐optimal solution to the desired time‐optimal solution continuously. Finally, for the inertially symmetric spacecraft reorientation problem, the newly found time‐optimal solutions are presented. The performance of the algorithm is illustrated by simulating a general asymmetric rigid spacecraft time‐optimal reorientation problem. Copyright © 2014 John Wiley & Sons, Ltd.