Various factors determine the applicability of rice husk ash (RHA) as a pozzolanic material. The amount and accessibility of reactive sites is thought to be a key factor. A structural study of RHA samples in relation to their reactivity has been performed; Silica in RHA formed by burning rice husk in a laboratory furnace under continuous supply of air have been characterized as a function of incineration temperature, time and cooling regime. The characterization methods included chemical analyses, conductivity measurements, microscopic analysis, X-ray diffraction (XRD) and 29 Si magic-angle spinning (MAS) nuclear magnetic resonance (NMR). In line with earlier observations, the analyses show that the highest amounts of amorphous silica occur in samples burnt in the range of 500°C-700°C. The 29 Si NMR data allow direct identification of the reactive silanol sites in the RHA samples. De-convolution of the NMR spectra clearly shows that the quickly cooled RHA resulting from burning rice husk for 12 h at 500°C has the highest amount of silanol groups. This sample also induced the largest drop in conductivity when added to a saturated calcium hydroxide solution giving an indication of its reactivity towards lime. Therefore, this RHA is the favorable sample to be used as pozzolanic cement additive.

The epitaxial lift-off process allows the separation of a thin layer of III/V material from the substrate by selective etching of an intermediate AlAs layer with HF. In a theory proposed for this process, it was assumed that for every mole of AlAs dissolved three moles of H 2 gas are formed. In order to verify this assumption the reaction mechanism and stoichiometry were investigated in the present work. The solid, solution and gaseous reaction products of the etch process have been examined by a number of techniques. It was found that aluminum fluoride is formed, both in the solid form as well as in solution. Furthermore, instead of H 2 arsine (AsH 3 ) is formed in the etch process. Some oxygen-related arsenic compounds like AsO, AsOH, and AsO 2 have also been detected with gas chromatography/mass spectroscopy. The presence of oxygen in the etching environment accelerates the etching process, while a total absence of oxygen resulted in the process coming to a premature halt. It is argued that, in the absence of oxygen, the etching surface is stabilized, possibly by the sparingly soluble AlF 3 or by solid arsenic. The epitaxial lift-off ͑ELO͒ process allows the production of single-crystalline thin films of III/V materials. The technique is interesting for the optoelectronics industry, because the use of thin film devices results in a more efficient transfer of generated heat from device to carrier or heat sink and significantly reduces the amount of material needed by reuse of the substrates. Furthermore, ELO allows the integration of III/V-based components with, e.g., silicon-based devices.In 1978, Konagai et al. 1 first reported on peeled-film technology ͑PFT͒; they separated a Ϯ5 m thick GaAs epilayer from the GaAs substrate by etching a thin intermediate AlGaAs release layer with aqueous HF solution. It was found that this process stopped at certain depths, because etchant and reaction products could not be exchanged sufficiently fast through the narrow etch slit.2 In 1987, Yablonovitch et al. 3 reported that for thinner epilayers with a thickness in the order of 1 m this problem could be overcome by placing a droplet of black wax on top of the GaAs layer. The GaAs epilayers experience some stress due to the wax and curl up, thereby forcing open the small crevice between substrate and epilayer. As a result, the etch process, now referred to as ELO, no longer stopped at a certain depth. In a model to describe this process, Yablonovitch et al.3 assumed that in etching AlAs release layers with HF solution in water each mole of AlAs forms three moles of H 2 gas and that the out-diffusion of this H 2 gas through the etch crevice is the limiting factor for the lateral etch rate. By assuming the rate of diffusion of H 2 out of the etch slit to be equal to the rate of production at the etch front, the maximum attainable etch rate was found to bewhere N and n are the molar concentrations of AlAs and dissolved H 2 , respectively, D the diffusion constant of H 2 in the solution, R the radius of curvature of the fi...

Vanadium bromoperoxidase from the brown seaweed Ascophyllum nodosum was studied with electron spin echo envelope modulation (ESEEM) spectroscopy. After comparing the Fourier transformed (FT) ESEEM spectra with those of a number of vanadyl model compounds, it could be concluded that nitrogen is present in the equatorial plane of the vanadyl cation of reduced bromoperoxidase (14N frequencies occurred at 3.1, 4.2, 5.3 and 8.1 MHz). Furthermore, the FT‐ESEEM spectra of reduced bromoperoxidase exhibited an intense 1H modulation (13.8 MHz), which was completely replaced by a deuterium modulation at ∼2 MHz when bromoperoxidase was dissolved in D2O, instead of H2O. These latter data confirm earlier EPR experiments on reduced bromoperoxidase [(1988) Biochemistry 27, 1629–1635], showing that the oxo‐vanadium (IV) ion is coupled to exchangeable protons.

In this contribution we describe a high-frequency high-field EPR facility which has been developed at the University of Nijmegen. We present the design of a heterodyne quasi-optical bridge based on a millimeter-wave vector network analyzer as source and detection system. The mm-waves are transported in free-space through Gaussian beam optic elements and through a corrugated guide inside the resonator insert. The Fabri-Perot (TEMoo.) resonator is coupled through a metallic mesh and because of its bimodal property it can be operated using orthogonal detection leading to substantial improvement in sensitivity. In the first stage of the project, a multifrequency CW-facility is realized covering the 100-500 GHz range. In our initial explorative experiments we demonstrate the advantages of HF-EPR of high-spin systems: Due to the large microwave quantum, transitions which would be undetectable at X-band due to the large zerofield splitting can now be observed in good sensitivity. As a model for biological high-spin systems a sample of metmyoglobin was measured at D-band (130 GHz). The g = 5.9 perpendicular line from the S = 5/2 ferric heme was detected and its line-width was compared to data previously obtained at Q-, X-, S-and L-band. As a model for biological integer spin systems the S = 1 signal of Ni(II) in nickel Tutton salt (Ni(NH 4 ) 2 (SO4)2) was studied at 35 and 130 GHz.

This contribution presents a novel design of a double-resonance structure for high-field dynamic nuclear polarization operating at 95 GHz and 144 MHz, in which a miniaturized radiofrequency coil is integrated within a single-mode nonradiative dielectric resonator. After a detailed discussion of the design principles, the conversion factors of this system are determined by means of microwave and radiofrequency measurements. The obtained results, 1.68 mT/W 1/2 for the microwave conversion factor and 0.8 mT/W 1/2 for the radiofrequency conversion factor, represent the state-of-the-art among the double-resonance structures. Simultaneous electron paramagnetic resonance and liquid-state 1 H nuclear magnetic resonance experiments are performed on samples of nitroxide radical 2,2,6,6-tetramethylpiperidine-1-oxyl dissolved in a mixture of water and dioxane. A maximum dynamic nuclear polarization enhancement of about -16 is obtained at a microwave power of 70 mW with a radical concentration of 10 mM in nanoliter-sized sample volumes. These results are discussed in view of further improvements and applications of the proposed double-resonance structure. G. Annino (&)Istituto per i Processi Chimico-Fisici, CNR,