Concentration Of Available Sites Research Articles

Mg(OH)2 Nucleation and Growth Parameters Applicable for the Development of MgO‐Based Refractory Castables

MgO is a very attractive raw material for refractory applications. However, its use has been mainly aimed at developing high‐magnesia and magnesia–carbon bricks due to the marked hydration likelihood of this oxide and the related drawbacks (volumetric expansion, crack formation) associated with this transformation. This work aims to evaluate some critical aspects that affect the MgO reaction with water (magnesia source, particle size, concentration of available sites for brucite nucleation, influence of a hydrating additive – acetic acid, and others) during the curing and drying steps of Al2O3–MgO binder‐free refractory castables. The attained experimental results were associated with the boundary nucleation and growth model proposed in the literature. According to the in situ elastic modulus measurements carried out at 110°C, the MgO particle size and reactivity present an important effect on the nucleation and growth of brucite crystals, highlighting that a proper site activation should be induced during the castables' curing process in order to effectively allow the use of magnesia as a binder source for refractories. Faster Mg(OH)2 nuclei generation also helps to minimize further growth of these crystals, leading to a decrease in the samples' porosity and, consequently, a continuous increase in the overall refractory stiffness (E values).

Origin of the Anisotropy in the Anodic Dissolution of Silicon

We report on an in situ, real-time, electrochemical, polarized, attenuated total reflection Fourier transform infrared spectroscopic investigation of Si(100), (111), and (110) surfaces in diluted HF solution (, pH 3). Prior to the IR measurements, cyclic voltammetry was carried out on the samples in a standard three-electrode arrangement, then IR spectra were recorded as a function of potential using 50-mV potential increments. The results give evidence for potential-induced absorption changes of the chemical surface species (Si–O, Si–H, Si–OH), which depend somewhat on substrate orientation. The potential-induced changes regard the shape and magnitude of all the above vibration bands and include a shift for some vibrations. In addition, the SiOH contribution appears about twice as large on (100) as on (110) and (111). The SiOH species, present just above the onset of anodic current flowing, at potentials around 0 V, appear as a key factor in the anisotropic dissolution of silicon. These observations are accounted for semiquantitatively in the framework of a kinetic model. The main factor responsible for the anisotropic effects appears to be the ability of surface silanols to couple and form siloxane bridges, a reaction governed by the surface concentration of available sites and the geometry of their neighborhood.

GNRH Interaction with Anterior Pituitary. I. Determination of the Affinity and Number of Receptors for GNRH in Ovine Anterior Pituitary1

The specificity of the interaction of 152 �11 -GNRH and its receptor in ovine anterior pituitary tissue was evaluated. Hormone and receptor were found to be stable during incubation for at least 4 h at 4#{176}C in the presence of Aprotinin (100 KIU/ml). The association rate constant (k1) was 1.3 X 10' Msecand the dissociation rate (k1 ) was 3 X 10' sec� . Of the substances tested only synthetic GNRH and (D-Leu)6-(des-Gly-NH,)' #{176}-GNRHethylamide were capable of inhibit- ing the binding of 1' 2.511 -GNRH to the anterior pituitary preparation. Vasopressin, melatonin, thyrotropin releasing hormone, LH, FSH, prolactin and inactive analogs of GNRH did not inhibit binding of 1 2 �i) -GNRH to its receptor. Scatchard plot analysis of saturation experiments indi- cated that a single class of high affinity sites (KA = 2.33 ± 0.31 X 10'#{176} M� ) were present. It should be emphasized that the relative affinity of GNRH for receptors in anterior pituitary tissue is only 3% that of 1' 2511 -GNRH. A constant concentration of available sites (4.85 ± 0.27 X 10_I molcs/g fresh anterior pituitary tissue) were present in ewes 10-12 days after superovulation. The number of receptors present in anterior pituitaries of anestrous ewes was not different at 0, 5, 10 or 20 h after injection of 100 �ig estradiol-1713 even though serum levels of LH increased -100- fold 13 h after treatment.

Study of Electron Hopping in Eu3S4 by Combined Electrical Conductivity and Thermal EMF Measurements

Combined electrical conductivity and thermoelectric power measurements have been made on Eu3S4 at temperatures from 100° to 1000°K. These measurements show that the charge carrier concentration in this temperature range is essentially constant. The activation energy for electrical conduction arises, therefore, from the temperature dependence of the drift mobility, and is interpreted as an energy required for the hopping motion of the electronic charge carriers. An abrupt change in resistivity, characteristic of a transition, occurs at about 175°K. The activation energy for conduction above and below the transition is 0.163±0.004 eV and 0.21±0.01 eV, respectively. The Seebeck coefficient data obtained are interpreted in terms of the contribution of the kinetic energy term and the influence of the high charge carrier density on the concentration of available sites.