Variation of the specific surface area and porosity of anhydrous zinc oxalate with water vapour pressure

Specific surface areas and pore structure studies were carried out on two sets of samples of anhydrous zinc oxalate prepared from the dihydrate by heating at 130 and 180°C in various atmospheres of differing water vapour pressure. The two sets of samples were freshly prepared and aged samples. The variation of specific surface areas with water vapour pressure was found to behave in accordance with the Smith-Topley effect, with a minimum S_BET value located at 3.0 mmHg of water vapour. Analysis of the nitrogen adsorption isotherms by the t-method revealed the existence of both micropores and mesopores in the freshly prepared samples heated both at 130 and 180°C in 8.0 mmHg water vapour, and also in the aged samples prepared at 130°C in vacuo, and at 180°C in 8.0 mmHg water vapour. The rest of the samples possess only wide pores. Complete pore structure analysis for both micro- and mesopores was carried out for all samples investigated, and an attempt was made to correlate the factors affecting the development of surface area and pore structure, both in the presence and absence of water vapour, with those affecting the dehydration rate, and which occasionally lead to the Smith-Topley effect.     

Variation of the specific surface area and porosity of zinc oxide prepared in various atmospheres

Specific surface area and pore structure studies were carried out on zinc oxide samples prepared from zinc oxalate dihydrate by heating at 330 and 370°C and in various atmospheres of water vapour, oxygen and hydrogen. The variation of S_BET with water vapour pressure for the samples heated at 330°C was found to show a behaviour which can be compared with the Smith-Topley effect. For the samples prepared at 370°C, rapid sintering occurred, and the Smith-Topley effect was not followed in this case. Zinc oxide prepared in an oxygen atmosphere gave higher surface area values than that prepared in a hydrogen atmosphere, which is thought to be due to the oxidation of the carbon impurities on the surface. Analysis of nitrogen adsorption isotherms by the t-method revealed the existence of micropores in the sample prepared at 330°C in vacuo. The rest of the samples have wide pores.     

Variation of the specific surface area and porosity of zinc oxide prepared in various atmospheres

Specific surface area and pore structure studies were carried out on zinc oxide samples prepared from zinc oxalate dihydrate by heating at 330 and 370°C and in various atmospheres of water vapour, oxygen and hydrogen. The variation of S_BET with water vapour pressure for the samples heated at 330°C was found to show a behaviour which can be compared with the Smith-Topley effect. For the samples prepared at 370°C, rapid sintering occurred, and the Smith-Topley effect was not followed in this case. Zinc oxide prepared in an oxygen atmosphere gave higher surface area values than that prepared in a hydrogen atmosphere, which is thought to be due to the oxidation of the carbon impurities on the surface. Analysis of nitrogen adsorption isotherms by the t-method revealed the existence of micropores in the sample prepared at 330°C in vacuo. The rest of the samples have wide pores.     

Surface area and pore structure of stannic oxide gel from organic vapour adsorption

Surface area measurements were performed on stannic oxide gel and its thermal dehydration products obtained both in vacuo and in presence of air in the temperature range 100–600°C by benzene and carbon tetrachloride adsorption at 35°C. The presence or absence of air is found to impose changes in the surface area and consequently vary the pore structure of the solid material. Pore size analyses for both micro- and mesopores are performed. The low temperature samples are characterised by the presence of micropores only. At 350°C widening of pores commences and at higher temperatures the results indicate the existence of pores of “submicro” dimensions together with the mesopores. These changes in pore structure go in parallel with the gradual development of the crystalline cassiterite structure.     

Effects of activation temperature on the properties and structure of PAN‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate, then further oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of activation temperature of a precursor fiber on the microstructure, specific surface, pore‐size distribution, and adsorption properties of PAN‐based activated carbon hollow fibers (PAN‐ACHF) were studied in this work. After the activation process, the BET surface area of the PAN‐ACHF and surface area of mesopores in the PAN‐ACHF increased very remarkably and reached 1422 m² g^?1 and 1234 m² g^?1, respectively, when activation temperature is 1000°C. The adsorptions to creatinine and VB₁₂ of PAN‐ACHF were much high and reached 99 and 84% respectively. In PAN‐ACHF which went through the activation at 700°C and 800°C, the micropore filling mainly occurred at low relative pressures, multimolecular layer adsorption occurred with the increasing of relative pressure, and the filling and emptying of the mesopores by capillary condensation occurred at high relative pressures. But in PAN‐ACHF which went through the activation at 900°C, a mass of mesopores resulted in the large pore filling by capillary condensation. The dominant pore sizes of mesopores in PAN‐ACHF are from 2 nm to 5 nm. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3778–3783, 2006     

Surface area and pore structure of thorium dioxide

Water vapour isotherms at 35° were measured on two samples of thorium dioxide, one crystalline and the other an amorphous gel. Specific surface areas and pore size distributions were estimated and compared with those obtained from nitrogen adsorption at ?196°. The effect of thermal treatment on both samples was also investigated at 35–500°. It was shown that the adsorption of water vapour on thoria gel is purely physical in nature, and that the gel surface is characterised by the existence of pores of very particular sizes, namely of hydraulic radii 4–6 ǎ. On the other hand, water seems to interact chemically with the surface of the crystalline sample. In this case the specific surface area could not be evaluated from the adsorption branch of the isotherm, but was successfully evaluated through the application of the ‘corrected modelless’ method to the desorption branch of the water isotherms. This gave the surface area of the total pore system which was found to be in good agreement with the B.E.T.-nitrogen area.     

Processes and kinetics of mass gain in archeological brick following drying and reheating

The mass gain behavior of archeological bricks was examined following drying (130°C)/reheating (500°C) and aging at a range of temperatures (25°C, 35°C, 45°C). For drying or reheating, samples exhibit a two‐stage mass gain behavior, the second stage, Stage 2, continuing indefinitely and better described by a t^1/n model (1/n=1/6‐1/2); a correlation between the 1/n value and the specific surface area/pore volume demonstrates diffusion mechanisms with some pore geometry/morphology dependence. Stage 2 is shown to have an Arrhenius temperature dependence with activation energies of similar orders of magnitude following both drying and reheating. Supported by thermogravimetric‐mass spectrometry (TG‐MS), Stage 2 is demonstrated as likely due to the recombination of chemisorbed water, previously removed, whereas following reheating due to two components, a chemisorbed component associated with drying and a component associated with rehydroxyls removed at higher temperatures during reheating. Differences between activation energies of chemisorption and rehydroxylation components support this. Evidence for a fundamental compositional relationship between these processes is presented by a strong linear relationship between the drying and reheating mass gain rates. Stage 1, following drying or reheating, is shown to be likely associated with physisorption processes alone.     

Thermal decomposition,structural changes and surface properties of chromium oxide gel

Structural and phase changes of a prepared sample of chromium oxide gel were explored by differential thermal analysis and X-ray diffraction analysis. Dehydration products were obtained at various temperatures up to 740 °C, by heating in air for 3 hours. Samples obtained by heating below 320 °C were amorphous to X-rays whereas those obtained at 320 °C, and between 320 and 510°C proved to be a mixture of α- and γ-Cr₂O₃. However, at temperatures above 510 °C, only one form, the stable α-Cr₂O₃ exists. The glow phenomenon is attributed to the presence of excess oxygen, resulting from the reduction of Cr(VI) to Cr(III), just prior to the conversion of the amorphous gel to the crystalline γ-Cr₂O₃, which is accompanied by a release of energy. Surface area measurements were carried out by cyclohexane, methanol and water adsorption at 35 °C, and the areas were compared with those measured by nitrogen at liquid nitrogen temperature. Pore structure analysis for both micro- and mesopores were carried out. The low temperature samples are characterised by the presence of both types of pores, with the micropore fraction predominating acquiring thus the property of molecular sieves; the pores being accessible to water and nitrogen but not to cyclohexane or methanol. At temperatures above 320 °C, the microporosity collapses with the production of a heterogeneous assembly of mesopores. In the mean time the surface changes to be hydrophobic in nature; accordingly it cannot be measured from water adsorption.     

Thermal decomposition and pore structure of pure and doped stannic oxide gel

The surface properties of a stannic oxide gel and its thermal dehydration products obtained both in vacuo and in the presence of air in the temperature range 100–600°C have been examined by N₂ adsorption. Phase and structural changes have been followed by differential thermal analysis and X-ray diffractometry. Complete pore structure analysis showed that samples dehydrated at or below 250°C were microporous. Above 250°C the pores were found to widen with increase of temperature, the widening occurring concurrently with the crystallisation process. Doping with cations of lower valency (Li⁺ and Al³⁺) than the host cation (Sn⁴⁺) had little effect on the pore structure and specific surface area for the low temperature samples (≥250°), whereas at higher temperatures, e.g. 600°C, it increased the specific area remarkably. The dope ions produce oxygen vacancies and hinder or retard sintering in SnO₂.     

Surface properties of cement hydration products. I. Pore structure of calcium silicate hydrates prepared in a suspension form

Adsorption–desorption isotherms of water vapour at 30° were determined on preparations of calcium silicate hydrates of different molar ratios of lime/silica before and after free lime and free silica extractions. From these isotherms specific surface areas were measured and total pore structure analysis was evaluated. A surface layer thickness (t) curve for the adsorption of water vapour on calcium silicate hydrates was constructed, based on experimental curves previously published for the number of layers adsorbed on a variety of non-porous solids with the same heat of adsorption as the calcium silicate hydrates. The existence of both wide pores and micropores was detected in the calcium silicate hydrates. When the samples were free from uncombined lime and silica the micropores were estimated to have an average hydraulic radius of 4–5 Å, whereas the wide pores had an average size of 10 Å. Wide pores up to 200 Å also exist. The pores were predominantly parallel plates.     

Hydrogen sulphide oxidation over teflon treated activated alumina and titanium dioxide catalysts

Commercial activated alumina and titanium dioxide catalysts were treated with Teflon to reduce the negative effects of water vapour on the oxidation of hydrogen sulphide by sulphur dioxide (Claus Reaction) near the sulphur dew point. The tests were conducted at 200°C and 130°C (108 kPa), with and without 30% water vapour in the feed gas. An alumina/Teflon composite catalyst was found to be superior to both untreated commercial catalysts at 130°C. This improvement was probably due to an increase in macroporosity rather than to a wetproofing effect. At 200°C, the performance of the composite was similar to that of the untreated titanium dioxide which in turn was superior to the untreated activated alumina.     

Synthesis of hierarchically porous mullite ceramics with improved thermal insulation via foam-gelcasting combined with pore former addition

ABSTRACT

To further improve the thermal insulation performance of porous mullite ceramics used in important industrial sectors, a combined foam-gelcasting and pore-former addition approach was investigated in this work, by which hierarchical porous mullite ceramics with excellent properties, in particular, thermal insulation property, were prepared. Both mesopores (2–50?nm) and macropores (117.8–202.7?μm) were formed in porous mullite ceramics resultant from 2?h firing at 1300°C with various amounts of submicron-sized CaCO₃ pore former. The former mainly arose from the decomposition of CaCO₃, and the latter from the foam-gelcasting process. The porous samples prepared with CaCO₃ addition had low linear shrinkage of 2.35–4.83%, high porosity of 72.98–79.07% and high compressive strength of 5.52–14.82?MPa. Most importantly, they also exhibited a very low thermal-conductivity, e.g. 0.114?W?m^?1?K^?1 at 200°C, which was much lower than in the cases of their counterparts prepared via the conventional foam-gelcasting route.     

Thermal degradation of poly(3‐hydroxybutyrate) and poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) in drying treatment

This study examines the isothermal treatment of poly(3‐hydroxybutyrate) (PHB) and poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) powders and films. The PHB and PHBV crystallinities were determined using X‐ray diffractometry, and shown to increase with temperature (130–150°C) and then decreased from 55% to 45% at 180°C. The crystal morphology of crystal planes (101) and (111) became sharp at a high temperature. The weight average molecular weight (M_w) of PHB decreased from 1,028,000 to 41,800 g/mol when heated at 180°C for 30 min. The molecular weight of PHB decreased more rapidly than that of PHBV with time. No peak signal was observed in gel permeation chromatography after heating at 150°C because the solubility of PHB changed with crystallinity. The thermal behaviors of PHB and PHBV were analyzed by differential scanning calorimetry and thermogravimetric analysis. The roughness, contact angle, and surface morphology of PHB and PHBV films were also measured to determine the surface properties. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3659–3667, 2013     

Reactivity of lime and related oxides. XVIII. Production of plasters

Conditions for production of plasters are surveyed in relation to variations in phase composition, surface area and crystallite size. The commercially known α- and β-hemihydrate plasters prepared by autoclaving and dry calcination are not two distinct phases—they differ in crystal habit, surface area, crystallite size and lattice perfection. The terminal β-hemihydrate and γ-CaSO₄ produced in vacuo have greater surface areas than samples produced in air. The most active γ-CaSO₄ (Class C plaster) samples are finely porous and rapidly rehydrate in atmospheric water vapour with considerable loss of surface area and porosity. ‘Wet’ rehydration (liquid water) or γ-CaSO₄ causes more rapid development of the 6.01 Å (10.10) spacing, in accord with the c-direction elongation of the ‘α’-hemihydrate crystals prepared by autoclaving or precipitation from 50% HNO₃. The porosity of the γ-CaSO₄ diminishes considerably between 200° and 300°, but remains to a smaller extent up to 500° when the γ-CaSO₄ has changed to β-CaSO₄, anhydrite (Class D plaster). Anhydrite sintered extensively between 500° and 900° before decomposing to lime at temperatures up to 1400° (Estrich Gips plaster). Removal of the SO₂ and O₂ is facilitated by streams of nitrogen or carbon dioxide, which increases the decomposition rate. Calcination at temperatures below 1200° gives anhydrite and lime crystals of more comparable sizes, and avoids complete ‘dead-burning’ of the lime.     

Characterisation of steam-treated nanoporous carbide-derived carbon of TiC origin: structure and enhanced electrochemical performance

Modifications of pore size distribution and structural order of nanoporous carbide-derived carbon (CDC) materials with variety of surface areas and pore sizes were investigated using physical activation by etching with water vapour. Variable etching duration was used to explore the activation impact on the pore size distribution and the adsorption behaviour of TiC-derived carbon. A distribution of micro- and mesopores, modified during physical activation, was studied using N₂ and CO₂ adsorption. Notable impact of preceding carbon structure on the activation product was revealed by the results of scanning electron microscopy, powder X-ray diffraction and Raman spectroscopy. An infrared spectroscopy, energy dispersive spectroscopy and X-ray photoelectron spectroscopy confirmed that water-induced etching of CDC followed by high-temperature treatment in inert gas atmosphere does not change notably the total amount of surface oxygen, however, leads to the changes in a composition of oxygen containing functional groups in post-activated carbon. The electrochemical evaluation was performed in triethylmethylammonium tetrafluoroborate/acetonitrile electrolyte to elaborate the structure-electrochemical properties relationships on post-activated nanoporous CDC materials. It was observed that the degree of improvement in double-layer capacitance achievable with a steam-treatment significantly depends on the preceding properties of CDC prior treatment, whereby the highest capacitance, ~?160 Fg^?1, was reached for the steam-treated TiC-derived CDC made at 800 °C, which clearly is a very promising material for the electrical double-layer capacitor.     

Condensation enthalpies of <Emphasis Type="Italic">n</Emphasis>-hexane in micelle-templated mesoporous silicas

The isosteric enthalpies of adsorption of n-hexane on ordered mesoporous silica of pore diameter between 3 and 10 nm have been measured. The heat of capillary condensation increases when mesopores are smaller. Capillary condensation of n-hexane in 3 nm mesopores is 20% more exothermic than the condensation on a flat liquid surface. The results are in good agreement with a model which takes into account the energetic contribution of the interface between the adsorbed layer and the vapour phase.     

Hydrothermal reaction between C6S6H and kaolinite for the formation of tobermorite at 180°C

11 Å tobermorites were made from CSH (Ca/Si = 1.14) and kaolinite with Ca/(Si+Al) = 0.8 and Al/(Si+Al)= 0.15 at 180°C. The CSH was prepared from colloidal silica and lime at 130°C and 180°C for 2 h. Reaction gives in succession CSH, poorly crystalline Al-substituted tobermorite, and highly crystalline Al-substituted tobermorite. The addition of kaolinite markedly accelerates the formation of tobermorite within 4 h, more effectively with CSH prepared at 130°C than with that prepared at 180°C. X-ray fluorescence diffractometry shows that the Al coordination number is a mixture of 4 and 6 in the initial products and shifts to 4 with an increase in processing time. This agrees with the results for the degree of reaction of the kaolinite.     

Thermal decomposition and pore structure of pure and doped stannic oxide gel

The surface properties of a stannic oxide gel and its thermal dehydration products obtained both in vacuo and in the presence of air in the temperature range 100–600°C have been examined by N₂ adsorption. Phase and structural changes have been followed by differential thermal analysis and X-ray diffractometry. Complete pore structure analysis showed that samples dehydrated at or below 250°C were microporous. Above 250°C the pores were found to widen with increase of temperature, the widening occurring concurrently with the crystallisation process. Doping with cations of lower valency (Li⁺ and Al³⁺) than the host cation (Sn⁴⁺) had little effect on the pore structure and specific surface area for the low temperature samples (≤250°), whereas at higher temperatures, e.g. 600°C, it increased the specific area remarkably. The dope ions produce oxygen vacancies and hinder or retard sintering in SnO₂.     

Enhanced Hydrothermal Resistance of Y‐TZP Ceramics Through Colloidal Processing

Two commercial zirconia powders with 3 mol% of yttria (TZ3YE and TZ3YS, labeled as ZE and ZS, respectively) supplied by Tosoh (Japan) were used for this study. Maximum colloidal stability for ZE was achieved by dispersing the powders in a mixture of water/ethanol of 90:10 (wt/wt) using a sonication probe. The rheological behavior of the suspensions was optimized in terms of solids content ranging from 20 to 33 vol% and sonication time (0–6 min), the best results being obtained after 2 min. ZS samples were prepared to a solids loading of 30 vol% in water dispersing with 2 min‐sonication. Samples obtained by slip casting in plaster molds were used for dynamic sintering studies, and fully dense and nanostructured specimens were obtained at temperatures of 1300°C–1350°C (ZE samples) and 1400°C per 2 h (ZS samples). The Hardness (H) and Young's Modulus (E) properties of the specimens were studied by nanoindentation technique giving 17 and 250 GPa mean values for H and E, respectively. The specimens were then forced to a low‐temperature degradation (LTD) treatment at 130°C for 240 h in steps of 60 h. Raman spectroscopy and nanoindentation results of hydrothermally treated samples showed the absence of transformation from tetragonal to monoclinic phase until 180 h whereas the mechanical properties maintained constant even at the sample surface. After 240 h of LTD, the monoclinic phase was detected on all specimens by Raman peaks centered at 180, 191, and 383 cm^?1. The nanoindentation study revealed an important loss of mechanical features reaching 10 and 175 GPa for H and E, respectively. In the case of the ZS specimens, no monoclinic phase is detected after 240 h of LTD treatment and no decay of E or H is detected. The free defect microstructure reached for the ZS specimen revealed a higher hydrothermal resistance so that it is concluded that the excellent behavior against thermal degradation is possible due to the large uniformity obtained by colloidal processing rather than the particle size of the starting powders.     

Thermal decomposition of strontium oxalate — effect of various atmospheres

Decomposition products have been prepared from strontium oxalate monohydrate by heating for 2 h at 410, 470 and 510 °C in presence of air, water vapour at various pressures, nitrogen, hydrogen or carbon dioxide atmosphere. Structural, textural and morphological changes have been studied by X-ray diffraction and electron microscopy; the influence of various atmospheres is discussed. Differential thermal analysis and thermogravimetry reveal that dehydration of the starting material proceeds in two steps: a main dehydration process takes place at 180 °C, followed by the release of a small amount of water (～5%) at 270 °C. Decomposition of oxalate into carbonate covers the range 400 – 480 °C; a very small endotherm is observed at 450 °C, masked by a strong exotherm (530 °C) due to the release of energy probably accompanying the conversion of the amorphous decomposition product of the oxalate into a crystalline form.