BET Data Research Articles

Permittivity and the form factor of BaTiO 3 and related powders

Capacitance measurements of ferro- and paraelectric oxide powders in the frequency range of 1 kHz–10 MHz have been performed in order to deduce a form factor which is related to the morphology of the particles and their arrangement in the powder. Using a capacitor measuring setup the frequency independent real part of the complex dielectric constant was measured in the range of 1 kHz–2 MHz as a function of the volume fraction. The interpretation follows a model derived for inhomogeneous dielectrics, which makes it possible to calculate a morphology dependent parameter provided that the permittivity of the solid is known. The results are verified for various BaTiO 3 powders and for aggregated and stepwise ground Ba 1 − x Sr x TiO 3 powders and are related to other particle shape parameters derived from SEM studies indicating the suitability of this method to describe the morphology of ferroelectric powders. For comparision the specific surface area BET data and the d 50 values from grain size distribution measurements of the powders are also reported. The applicability of the method for checking the reproducibility of different lots of ferroelectric capacitor raw materials is shown for Z5U and X7R powders.

Characterization of dielectric powders by a new defined form factor

Capacitance measurements of oxide powders in the frequency range of 1 kHz to 10 MHz have been performed in order to deduce a form factor which is related to the morphology of the particles and their arrangement in the powder. Using a capacitor measuring setup the frequency independent real part of the complex dielectric constant was measured in the range of 1 kHz to 2 MHz in dependence on the volume fraction. The interpretation follows the models derived for inhomogeneous dielectrics, which allow to calculate a morphology dependent parameter provided that the permittivity of the solid is known. A new defined form dependent weighting factor introduced in Bruggeman’s model of geometrical averaging between lamellar and spherical constituents is shown to supply the better approximation to measuring data than the aspect ratio of ellipsoids determining the depolarization factor. The results are verified for aggregated and partially stepwise ground powders of Mg 2TiO 4, MgAl 2O 4, SrTiO 3, CaTiO 3, Al 2O 3, and ZnO and related to other particle shape parameters derived from SEM studies indicating very clearly the suitability to describe the morphology of powders by this new form factor. The frequently applied Lichtenecker rule does not allow to consider particle morphology. For comparison the specific surface area BET data and the d 50 values from grain size distribution measurements of the powders are also reported. The applicability of the method for checking the reproducibility of different lots of capacitor raw materials is shown for two COG powders.

The point of zero charge of Co 3O 4 prepared by thermal decomposition of basic cobalt carbonate

Co 3O 4 has been prepared by thermal decomposition of basic CoCO 3 in the temperature range 200 to 800 °C. The pzc has been measured by potentiometric titration and has been found to coincide with the iep determined by microelectrophoresis. The pzc is constant at ca. 7.3 between 200 and 450 °C. The GCSG model of the electrical double layer has been applied to the experimental data to derive the real surface area of the oxide and the inner layer capacitance. The electrochemical surface area coincides with BET data. The results of this work have been compared with those obtained previously with Co 3O 4 prepared by thermal decomposition of the nitrate.

Mechanically Activated MoO3. 1. Particle Size, Crystallinity, and Morphology

Physicochemical changes induced in MoO 3 by mechanical activation in a planetary mill were investigated. During the milling process, the BET surface area increases from about 1.3 to 32 m 2 /g. Energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS) of the powdered MoO 3 samples reveal that no agate is abrased during the milling process. An estimation of the mean particle size using the BET data or scanning electron microscopic (SEM) images indicates a decrease from about 1 μm to about 50 nm. The primary crystallite size calculated from X-ray diffraction (XRD) line broadening also shows a decreasing size from about 160 nm to about 80 nm. The difference between the particle size of unmilled MoO 3 , as determined from the BET surface area or by scanning electron microscopy (SEM), and the calculated primary crystallite size using X-ray line broadening is explained by MoO 3 particles consisting of smaller primary crystallites. The smaller average particle size of MoO 3 milled for 600 min calculated from BET data, on the other hand, as compared to the XRD primary crystallite size is ascribed to the presence of ultrafine amorphous material which is X-ray amorphous and, therefore, does not contribute to the X-ray line broadening. This formation of amorphous material is also indicated by an increasing amorphous scattering background in the X-ray diffraction patterns. In SEM pictures, these particles appear to have an amorphous overlayer. The strange behavior of both the X-ray diffraction pattern quality and the diffuse scattering background, which do not coincide, during mechanical activation probably indicates a complex process of particle size reduction. Variation of X-ray reflection profiles, intensity ratios, and additional X-ray reflections may point toward the formation of shear defects during this process. A Warren-Averbach analysis of the most intense X-ray reflections of milled MoO 3 reveals that internal strain is only marginally enhanced by mechanical activation.

Highly efficient heterogeneous photooxidation of 2-propanol to acetone with amorphous manganese oxide catalysts

Amorphous manganese oxides, potassium oxalatomanganate complexes, and crystalline tunnel structure manganese oxides (hollandite, todorokite, pyrolusite) have been prepared and used as photocatalysts for the oxidation of 2-propanol to acetone. Amorphous mixed valent manganese oxides, a new class of photocatalytic materials, were the most active catalysts studied at room temperature under visible light illumination. High activities correlate with higher oxidation state manganese (Mn[sup 3+], Mn[sup 4+]) oxides, large surface oxygen concentrations, and the presence of hydroxyl groups on surfaces of these catalysts. BET (surface area analysis), XRD (X-ray diffraction), EDX (energy dispersive X-ray analysis), SEM (scanning electron microscopy), XPS (X-ray photoelectron spectroscopy), and FTIR (Fourier transform infrared) data support these correlations. A mechanism for these photooxidation flow reactions involving manganese oxide catalysts has been proposed. Photochemical and characterization data suggest that the design of active photocatalysts may rely on preparation of mixed valent amorphous oxides that release oxygen more readily than crystalline materials. Conversions as high as 8% and 100% selectivity to acetone have been realized with such systems. 45 refs., 9 figs., 1 tab.

Phase transition porosimetry and surface area determination

The method of computerized dilatometric Phase Transition Porosimetry (PTP), developed previously, is outlined in comparison with the method of “Thermoporometry” and critically reviewed. The present revised method takes into account extreme cases in which the sample contains relatively large and/or extremely small pores. To prevent supercooling in the former case, this method calls for repeated cooling. Because of the substantial amount of unfrozen pore water at low temperatures in the latter case, the revised data processing takes into account dilation of pore water as temperature is lowered below 0°C. PTP can measure the pore body size in the case of pores having pronounced irregularities consisting of enlargements and constrictions. Thus it gives a unique opportunity for realistic determination of surface area from pore size distribution in such porous materials. Whereas, as can be shown thermodynamically, mercury intrusion porosimetry (MP) is bound to overestimate surface area. To support these statements PTP, MP, and BET data on some samples are presented.

The physical adsorption of 1,1,2-trichloro-1,2,2-trifluoroethane on uranium dioxide

The interaction between 1,1,2-trichloro-1,2,2-trifluoroethane (Freon TF) and uranium dioxide was investigated from a series of gravimetric adsorption measurements in the temperature range −15 to 28°C. All of the isotherms were Type II and completely reversible obeying the BET adsorption equation. Adsorption data also could be fit to the Freundlich equation which suggested a heterogeneous surface. The isosteric heat of adsorption varied from 8.6 to 5.8 kcal/mole as coverage increased and reached a minimum at a coverage of 1.5 molecular layers. The cross-sectional area of the adsorbed molecule was calculated from BET data to be 40Å2. Adsorption of Freon TF was compared to the adsorption of other chlorinated solvents using the potential theory of adsorption.

A Radioactive Tracer Method for Measuring the Relative Surface Areas of Small Samples

A new method for measuring the relative surface area of small samples is described. The method is based on liquid-sorption theory and uses stearic acid tagged with 14C. Relative surface area data on alumina samples deposited under various conditions by the anodic spark process are reported. The method was validated successfully using BET data taken on alumina samples.