Three-dimensional manganese dioxide-functionalized carbon nanotube electrodes for electrochemical supercapacitors

Three-dimensional manganese dioxide (MnO₂)-functionalized multiwalled carbon nanotube (MWCNT) electrodes have been produced by a simple and scalable thermal decomposition process. The electrodes are prepared by treating planar MWCNT sheets with manganese(II) nitrate (Mn(NO₃)₂) solution and annealing at low temperature (200–300 °C) and ambient pressure. The morphology, chemical composition, and structure of the resulting matrices have been investigated with scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and X-ray diffraction. Supercapacitors assembled with three-dimensional electrodes exhibit a 14-fold increase in specific capacitance (C _sp) in comparison to those containing pristine, two-dimensional MWCNT electrodes. C _sp varies linearly with Mn(NO₃)₂ thermal decomposition temperature (from 100 to 61 F/g at 0.2 A/g), a trend that is discussed in the context of nitrate reaction chemistry and MWCNT structure. This efficient and promising approach allows for simultaneous enhancement of electrode–electrolyte contact area and incorporation of redox-based charge storage within electrochemical capacitors.     

Unique Path Partitions: Characterization and Congruences

We give a complete classification of the unique path partitions and study congruence properties of the function which enumerates such partitions.     

Observation of a very large internal hyperfine field (62.4 T) in the ferromagnetically ordered state of the S = 1 alpha-iron(II) octaethyltetraazaporphyrin

The origins of the extraordinarily large internal hyperfine field (62.4 T) for the three-dimensional (weak) ferromagnetically ordered ground state of alpha-Fe(OETAP) are discussed semiquantitatively in terms of existing physical theory. In particular, the classical Fermi-contact contribution to the internal field is found to be highly enhanced by a very large orbital contribution and a significant dipolar term of the same sign. A rationale for the unexpected ordering of this S = 1 non-Kramers system is also presented.     

A Monte Carlo simulation of energy transfer processes in thermal unimolecular reactions. II. An applications to polyatomic dissociation

The Monte Carlo method has been used to provide a numerical solution to the ro-vibrational master equation for the low pressure unimolecular decomposition of a polyatomic molecule. This type of solution is made possible through the use of a simple exponential transition probability function, that represents the efficiency with which energy transfer takes place between the reactant molecule and an unspecified heat bath gas. The Monte Carlo technique is used to generate random variables that are distributed in a manner prescribed by the transition probability function. In the case of the present simulation, these variables correspond to random energy jumps induced in the molecule through single collision events. In order to account for the energy dependence of the vibrational state densities, we have proposed that vibrational relaxation in the polyatomic takes place from a single vibrational mode. Under equilibrium conditions we are able to show that with this assumption, the Monte Carlo model is capable of reproducing molecular quantities, such as the average vibrational energy per molecule and the vibrational specific heat, that compare favourable with the corresponding values calculated from equilibrium statistical mechanics. The model has been applied to a study of the low pressure unimolecular decomposition of a series of polyatomics. For three of the molecules, CH₄, CD₄, and C₂H₆ the agreement between the calculated and the high temperature experimental rate constants is very good. The calculations indicate that a significant proportion of the molecules that dissociate are rotationally as well as vibrationally excited. Very few of the reactive molecules have a vibrational energy content equal to or greater than E₀, the dissociation energy. The extent of rotational excitation is found to be temperature dependent.     

Arithmetic Properties of Non-Squashing Partitions into Distinct Parts

A partition with is non-squashing if On their way towards the solution of a certain box-stacking problem, Sloane and Sellers were led to consider the number b(n) of non-squashing partitions of n into distinct parts. Sloane and Sellers did briefly consider congruences for b(n) modulo 2. In this paper we show that 2^r-2 is the exact power of 2 dividing the difference b(2^r+1n)–b(2^r-1n) for n odd and r 2.     

Polariton emission in GaN microcavities

We present a detailed experimental study aimed at demonstrating the polariton nature of the photoluminescence emission in a bulk GaN microcavity grown on (111) silicon. The comparison of the photoluminescence with coincident reflectivity measurements at different temperatures shows an anticrossing behaviour with a Rabi splitting of the order of 35 meV up to room temperature. Relevant confirmations of the mixing between excitons and photons are found in the analysis of the spectral linewidth and of the time resolved kinetics.     

Detection of halogenated organic compounds using immobilized thermophilic dehalogenase

Environmental pollutants containing halogenated organic compounds can cause a plethora of health problems. Detection, quantification, and eventual remediation of halogenated pollutants in the environment are important to human well-being. Toward this end, we previously identified a haloacid dehalogenase, L-HAD_ST, from the thermophile Sulfolobus tokodaii. This thermophilic enzyme is extremely stable and catalyzes, stereospecifically, the dehalogenation of l-2-haloacids. In the current study, we covalently linked L-HAD_ST to an N-hydroxysuccinimidyl Sepharose resin to construct a highly specific sensor with long shelf life for the detection of l-2-haloacids. The enzyme-modified resin was packed into disposable columns. Samples containing l-2-haloacids were first incubated in the column, and were then collected to quantify the chloride produced through the breakdown of the substrate. The optimum pH of the immobilized enzyme is around 9.5, similar to that of the soluble protein. Its catalytic activity increased with temperature up to the highest temperature measured (50 °C). The resin could be fully regenerated after multiple reaction cycles and retained 70% of the initial activity after being stored at 4 °C for 6 months. The L-HAD_ST-modified resin could be used to breakdown and quantify l-2-haloacids spiked in the simulated environmental samples, indicating dehalogenases from extremophiles can potentially be employed in the detection and decontamination of l-2-haloacids.     

Force-Free States, Relative Strain, and Soft Elasticity in Nematic Elastomers

The remarkable ability of nematic elastomers to exhibit large deformations under small applied forces is known as soft elasticity. The recently proposed neo-classical free-energy density for nematic elastomers, derived by molecular-statistical arguments, has been used to model soft elasticity. In particular, the neo-classical free-energy density allows for a continuous spectrum of equilibria, which implies that deformations may occur in the complete absence of force and energy cost. Here we study the notion of force-free states in the context of a continuum theory of nematic elastomers that allows for isotropy, uniaxiality, and biaxiality of the polymer microstructure. Within that theory, the neo-classical free-energy density is an example of a free-energy density function that depends on the deformation gradient only through a nonlinear strain measure associated with the deformation of the polymer microstructure relative to the macroscopic continuum. Among the force-free states for a nematic elastomer described by the neo-classical free energy density, there is, in particular, a continuous spectrum of states parameterized by a pair of tensors that allows for soft deformations. In these force-free states the polymer microstructure is material in the sense that it stretches and rotates with the macroscopic continuum. Limitations of and possible improvements upon the neo-classical model are also discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Torsion in biochemical reaction networks

This article starts in Part I with a simple example of two biochemical reaction networks that are indistinguishable at the macroscopic level but are different at the molecular level and are shown to have significantly different kinetic properties. So, if one completely ignores the fact that reactions advance in discrete steps at the molecular level, then one can fail to distinguish between networks with widely different kinetics. In part II biochemical reaction networks are treated in a general way to discover what property of a network, only seen at the molecular level, affects its kinetics. It is shown that every such network has a unique torsion group which can be described numerically and readily determined by a programmable computation. If the group is found to be the singleton {0} (as is most often the case in practice), then the network is said to be torsion-free and its kinetic properties unaffected by ignoring its discrete character. A chemical reaction network has to be represented algebraically to calculate its torsion group. If the network is to be understood only at the macroscopic level, it can be placed in the context of real vector spaces, but to recognize its discrete character and its torsion group, each vector space is replaced by a discrete subset of that space, where each molecule can be recognized as a distinct and indivisible entity. Next, the process of calculating a torsion group is shown in several cases, including the example in part I. In this particular case it is shown to have the torsion group with 2 elements, reflecting the fact that the substrate molecules become product molecules 2 at a time, with the result that the overall macroscopic reaction is R ⇔ T, whereas at the molecular level it is 2R ⇔ 2T. In general, however, the torsion group of a biochemical reaction network can be any finite additive group, which is a property of the network that can only be seen at the molecular level. Finally, this fact is demonstrated by showing how to construct a hypothetical, but plausible, biochemical reaction network that has any given finite additive group as its torsion group.