Riko Ozao

DSC study of alumina materials — applicability of transient DSC (Tr-DSC) to anodic alumina (AA) and thermoanalytical study of AA

Thermo-analytical studies were performed on two types of porous anodic alumina (AA) membranes prepared from sulfuric acid. Both samples were 150 mm in total thickness; the first sample AA-1 consists of a 50 mm thick layer having 10 nm diameter pores and a 100 mm thick layer of 25 nm diameter pores, and the second sample AA-2 consists only of 25 nm pores. From the high temperature DSC run, the AA membranes as received were found to undergo dehydration up to 3508C where they exhibit a plateau, and at ca. 9708C, they yield a sharp exotherm immediately followed by a distinct endotherm. The apparent Cp values obtained by Tr-DSC at 3508C differed depending on the contact area of the samples with the sample pan or on the impurity content. The contact area depends on the pore diameter that is a function of applied voltage, and the impurity content similarly depends on the applied voltage. It is therefore presumed that Tr-DSC is advantageous in that it is a quick and handy method in obtaining apparent Cp as a parameter to identify samples differing in the properties which depend on the voltage applied at the preparation. In addition, the thermal changes obtained on the present AA membranes were found to be different from those obtained by oxalic acid known in the literature. That is, the present AA membranes exhibit a sharp exothermic reaction followed by a broad endothermic reaction apparently attributed to an amorphous to polycrystalline (AA! g-ad-Al2O3) transition, and, a exothermic reaction at ca. 14008C, presumably due to the final transformation from the metastable ccp polycrystalline alumina to the most stable phase, i.e., the hcp a-Al2O3. The transformation temperatures are higher than those of AA prepared from oxalic acid by 150‐3008C. # 2000 Elsevier Science B.V. All rights reserved.

Preparation of γ-Alumina Membranes From Sulphuric Electrolyte Anodic Alumina and Its Transition to α-Alumina

Gamma-alumina membrane was prepared from anodic (amorphous) alumina (AA) obtained in a sulphuric acid electrolyte. The transformation scheme, i.e., the crystallization to form metastable alumina polymorphs and the final transition to α-Al2O3 with heating was studied by TG-DTA and X-ray diffraction (XRD) using fixed time (FT) method. When heating at a constant rate, the crystallization occurred at 900°C or higher and the final formation of α-Al2O3 occurred at 1250°C or higher, which temperatures were higher than the case of using anodic (amorphous) alumina prepared from oxalic acid electrolyte. Relative content of S of the products was obtained by transmission electron microscope (TEM)-energy dispersive spectroscopy (EDS). The proposed thermal change of anodic alumina membrane prepared from sulphuric acid is as follows:1. At temperatures lower than ca 910°C: Formation of a quasi-crystalline phase or a polycrystalline phase (γ-, δ- and θ-Al2O3);2. 910–960°C: Progressive crystallization by the migration of S toward the surface within the amorphous or the quasi-crystalline phase, forming S-rich region near the surface;3. 960°C: Change of membrane morphology and the quasi-crystalline phase due to the rapid discharge of gaseous SO2;4. 960–1240°C: Crystallization of γ-Al2O3 accompanying δ-Al2O3; and5. 1240°C: Transition from γ-Al2O3 (+tr. δ-Al2O3) into the stable α-Al2O3.The amorphization which occurs by the exothermic and the subsequent endothermic reaction suggests the incorporation of SO3 groups in the quasi-crystalline structure.

Morphological and structural change of nano-pored alumina membrane above 1200 K

The transition and the change in pore morphology of a porous alumina membrane prepared by anodically oxidizing aluminum in sulfuric acid were studied mainly by TG-DTA, TMA, dilatometry and TEM. At ca. 1243 K, TMA showed an expansion followed by contraction; the CO2 and SO2 gases were quickly discharged, and the pore morphology of the as-prepared porous membrane (ca 150 mm-t, with pores ca 25 nm in diameter and containing ca 11% by mass of SO2) showed an abrupt change, but the pores were retained to ca. 1573 K. Sulfur incorporated in the membrane was lost in two stages, i.e., at ca 1243 K and in a range up to 1373 K. Isothermal measurements revealed the complex crystallization of the amorphous phase into polycrystalline phase.

Crystallization of anodic alumina membranes studied by simultaneous TG-DTA/FTIR

The thermal change of anodic alumina (AA), particularly the exothermic peak followed by the endothermic peak at ca 950°C was studied in detail by mainly using simultaneous TG-DTA/FTIR. The gradual loss of mass up to ca 910°C is attributed to dehydration. When heated at a constant rate by using TG-DTA, an exothermic peak with subsequent endothermic peak is observed at ca 950°C, but the exothermic peak becomes less distinct with decreasing heating rate. It has been found that gaseous SO2 accompanying a small amount of CO2 is mainly discharged at this stage. The reaction in this stage can be considered roughly in two schemes. The first scheme can be said collectively as crystallization, in which the migration of S or C trapped inside the crystal lattice of the polycrystalline phase (γ-, δ-, and θ-Al2O3, which presumably accompanies a large amount of amorphous or disordered phase) occurs. In the second scheme, the initial polycrystalline (+amorphous) phase crystallizes into a quasi-crystallineγ-Al2O3-like metastable phase after amorphization. Conclusively,after the distinct exo- and endothermic reactions, the amorphous phase crystallizes intoγ-Al2O3, presumably accompanying small amount of δ-Al2O3. It is also found that, when maintained isothermally, the metastable phases undergo transformation into the stable α-Al2O3 at 912°C.

Sulfur Concentration in Nanoporous Alumina Membrane

Nanoporous alumina membrane prepared by anodic oxidation using sulfuric acid electrolyte was subjected to TG-DTA and X-ray Photoelectron Spectroscopy (XPS or ESCA) to further study the distribution of sulfur. In XPS study, Ar+ ion bombardment was performed on the sample to etch the surface at a rate of 3 nm min-1. As a result, sulfur was found to be concentrated within a depth of 3nm from the surface. The S content of the surface was found to be 2.7±0.5 wt%, and that at a depth of ca. 3 nm and ca. 10 nm was found to be as low as about 0.6±0.11 wt% (5.37±1.0 wt%→ 1.26±0.2wt% SO2). In TG-DTA, the mass loss of 7.3% was in fair agreement with that calculated on XPS results (7.1±1.2%).

DSC of etched aluminum foils for use in electrolytic capacitors

A DSC method for evaluating the surface area of etched Al foils for use in high performance electrolytic capacitors is presented. A linear relationship between the etching degree (effective surface area) and the thermal resistance of the sample is obtained by means of DSC, based on the transient phenomenon. This method using the transient state in DSC measurement is not only novel, but also rapid and simple in evaluating the surface area of an etched aluminum foil. The method is effective even when the Al foil has a naturally oxidized surface.

Thermoanalytical characterization of powder samples I. Wet pretreated samples

Routine wet pretreatment of a powder sample causes a drastic change in the thermoanalytical results. The pretreatment may alter the crystallographic or molecular structure, but more distinguished change occurs in the powder characteristics. Thus, consistent thermoanalytical results can be obtained by subjecting the powder sample to wet pretreatment for classification.