Effect of pressure on corrosion of Inconel 625 in supercritical water up to 100 MPa with acids or oxygen

The corrosion behavior of Inconel 625 in supercritical water was investigated under high pressure (30–100 MPa at 400 °C) with the addition of acids (5 mmol/kg CH₃COOH or 0.5 mmol/kg HNO₃) or O₂ (2.5–250 mmol/kg). Ni and Cr ions were the main dissolved metal ions in the effluent. The Ni ion concentration showed no clear dependence on the O₂ concentration or pressure in the experiment with O₂; the concentration was lower than 0.05 ppm. The concentration increased with pressure up to 0.09 ppm and 3 ppm (at 100 MPa) in the experiment with CH₃COOH and HNO₃, respectively. The Cr ion concentration increased with the O₂ concentration and pressure; the maximum value of the concentration was about 0.5 ppm. The Cr ion concentration was lower than 0.01 ppm in the experiment with CH₃COOH, while the concentration was considerably higher in the experiment with HNO₃: the concentration increased with pressure up to 0.87 ppm at 100 MPa. The effect of pressure on the corrosion behavior of Inconel 625 was discussed using potential-pH diagrams, metal oxide solubilities, pH, and equilibrium of ionic reactions. Pressure dependence of the metal ion concentrations was analyzed using a model with water density as a parameter; the log–log plots revealed a linear relationship.     

Dispersion of mixtures of submicrometer and nanometre sized titanias to obtain porous bodies

The stability and rheological behaviour of bimodal titania suspensions was studied. Bimodal mixtures were prepared by mixing nanosized TiO₂ powders with an average primary size of ～20–40 nm and surface area of ～50 m² g^?1 and/or a colloidal titania suspension of the same nanopowders dispersed in water with a submicrometer sized titania. The dispersing conditions were studied as a function of pH, type and content of dispersant, and sonication time for a constant solids content of 30 vol% (62 wt%). The mixtures were slip cast and presintered at low temperatures (800–1000 °C) in order to obtain porous materials with anatase as the major phase. The pore size distribution, microstructure and phase composition were characterised using MIP, SEM and XRD techniques, respectively.     

Dispersion and co-dispersion of ZrB2 and SiC nanopowders in ethanol

The dispersion and co-dispersion behavior of ZrB₂ and SiC nanopowders in ethanol solution was studied by sedimentation test, particle size measurement and transmission electron microscope (TEM) analysis. The dispersion behavior of ZrB₂ and SiC nanopowders in ethanol solution was strongly dependent on the pH values, types and amounts of dispersant. PEI was found to be effective for the dispersion of both ZrB₂ and SiC nanopowders in ethanol solution. With the addition of PEI, the isoelectric points of ZrB₂ and SiC nanopowders in ethanol solution were at pH 10 and pH 9.5, and shift to pH 13 and pH 12.3, respectively. The stability of ZrB₂ suspension increased with the dispersant content increasing until reached 0.7 wt% and well-dispersed SiC slurry can be obtained with more than 2.5 wt% PEI. The suitable pH value for the dispersion of ZrB₂ and SiC nanopowders should be lower than 12 and 10, respectively. The well co-dispersed ZrB₂ and SiC nanocomposite powders can be achieved by using 1 wt% or more PEI below pH 10.     

Selected sugar acids as highly effective deflocculants for concentrated nanoalumina suspensions

The possibility to fabricate nanoceramic materials by techniques based on colloidal processing requires the use of effectively working deflocculants (dispersing agents) which are able to stabilize and decrease the viscosity of aqueous nanopowders suspensions. Galacturonic and lactobionic acids have been used in the research as deffloculants for nanoalumina; they allowed to obtain ceramic suspensions of high solid loading (50 vol%) and relatively low viscosity (ca. 4 Pa s at a shear rate 10 s⁻¹). High solid loading could be achieved through combining three factors: the application of sugar acids as deflocculants, water leaching of the nanopowders from the surface impurities and precise defoaming of the slurries. Density of sintered bodies equalled 98% TD.     

Dispersion of Cu2O particles in aqueous suspensions containing 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt

Effect of 4,5-dihydroxy-1,3-benzenedisulfonic acid disodium salt's addition upon Cu₂O dispersion properties is studied to clarify dispersant's role in colloidal properties change and its underlying stabilization mechanism, and to quantify processing conditions for the oxide. Dispersing effectiveness was studied through adsorption, rheological and electrophoretic measurements, using as-received and surface charge modified Cu₂O particles. Maximum solid loading attained without dispersant was 73 wt.% (31 vol.%), with corresponding viscosity of 152.5 ± 7.3 Pa s. Addition of dispersant resulted in viscosity between 21.0 Pa s and 5.4 Pa s. No isoelectric point was found for as-received particles’ suspensions nor for dispersed suspensions, with particles presenting negative surface charge in all studied pH range, from pH 4 to 10. Adsorption of the organic molecule caused an absolute downshift of 8–25 mV of the electrophoresis curve. Dispersant/Cu₂O interaction was assessed through FTIR analysis. Attained results suggest that, at the natural suspensions pH, dispersant-modified Cu₂O suspensions are stabilized through inner-sphere complexation mechanism, resulting in high dispersion ability.     

Synthesis and characterization of Ba1−xSrxTiO3 nanopowders by citric acid gel method

Stoichiometric and monophasic Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders were successfully prepared by the citric acid gel method using barium nitrate, strontium nitrate and tetra-n-butyl titanate as Ba, Sr, Ti sources and citric acid as complexing reagent. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared (IR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the thermal decomposition behavior, the crystallization process and the particle size and morphology of the calcined powders. The results indicated that single-phase and well-crystallized Ba_1−xSr_xTiO₃ (x = 0.3) nanopowders with particle size around 80 nm could be obtained after calcining the dried gel at 950 °C for 2 h.     

Direct coagulation casting of yttria-stabilized zirconia using magnesium citrate and glycerol diacetate

Direct coagulation casting via controlled release of high valance counter ions (DCC-HVCI) has been reported in recent years. In this paper, concentrated yttria-stabilized zirconia (YSZ) suspensions were coagulated using DCC-HVCI method with magnesium citrate as coagulating agent assisted by pH shift in the presence of glycerol diacetate. The effect of ammonium polyacrylate (PAA-NH₄) on the dispersibility of YSZ powder was investigated. The influence of concentrations of glycerol diacetate and magnesium citrate on pH and viscosities of YSZ suspensions was studied. The results indicate that concentrated YSZ suspensions can be coagulated by adding 2 vol% glycerol diacetate and magnesium citrate above 0.5 wt% at room temperature for 2–5 h. The compressive strength of coagulated wet samples is above 2.0 MPa. YSZ ceramics sintered at 1450 °C show homogeneous microstructures with relative densities of 98.9–99.2%. Flexural strength of YSZ ceramics is 869±84 MPa.     

Alumina porous nanomaterials obtained by colloidal processing using d-fructose as dispersant and porosity promoter

Recently, great effort has been devoted to obtain porous materials with customized pore size distribution, high surface area and submicrometer sized microstructures or nanostructures. In this work, the viability of colloidal processing routes to obtain porous bulk ceramics using alumina nanopowders and d-fructose as a dispersant and a porosity former has been explored.The rheological behaviour of nanosuspensions was studied in order to assure their stability and to analyse the influence of different parameters (solids loading, fructose content, pH, sonication time). Mesoporous green bodies were obtained by slip casting with d-fructose in concentrations ranging from 5 to 50 wt%. The drying and burning-out conditions were determined by DTA-TG measurements and the sintering cycles were selected from the dynamic sintering curve. Sintered alumina materials with high porosity (>60%), open microstructures, submicrometer sized porosity (d_p = 140–210 nm) and grain size lower than 500 nm, were obtained for pieces sintered at temperatures of 1300 and 1400 °C. The influence of different processing parameters on the porosity and the microstructure of the sintered materials is discussed.     

The size and dispersion effect of modified graphene oxide sheets on the photocatalytic H2 generation activity of TiO2 nanorods

Nano graphene oxide (NGO) was produced by further refluxing graphene oxide (GO) sheets in HNO₃, and carboxylic acid functionalized graphene oxide (GO–COOH) was obtained by a simple etherification reaction between GO and chloroacetic acid. The GO, GO–COOH and NGO sheets are combined with TiO₂ nanorods by a two-phase assembling method, and confirmed by transmission electronic microscopy. The GO–TiO₂, GO–COOH–TiO₂ and NGO–TiO₂ composites are used in a comparative study of photocatalytic H₂ generation activity under UV light irradiation. The H₂ generation rate of TiO₂ nanorods was slightly increased from 15 to 30 mL h⁻¹ g⁻¹ by replacing oleic acid ligands with hydrophilic dopamine, and significantly increased to 105 mL h⁻¹ g⁻¹ after combining with GO sheets. The further comparative study shows that GO–COOH–TiO₂ composite has higher H₂ generation rate of 180 mL h⁻¹ g⁻¹ than that of GO–TiO₂ and NGO–TiO₂ composites.     

The influence of pH on particle packing in YSZ coatings electrophoretically deposited from a non-aqueous suspension

Yttria stabilized zirconia (YSZ) coatings were produced from a YSZ suspension in acetylacetone (ACAC) using electrophoretic deposition (EPD) and then sintered with substrate constraint at 1200 and 1300 °C. Before EPD, the operational pH of the suspension was adjusted by addition of acetic acid or triethanolamine (TEA) base. The effect of suspension pH on the deposition of EPD coatings was studied with respect to the suspension stability, coating density and microstructure. Results showed that the zeta potential had a high positive value on both sides of the iso-electric point (IEP). This probably resulted from the adsorption of TEA, detected by Fourier transform infrared spectroscopy. Three alkalies with different molecular structures were compared and the effect of their molecule length on the interparticle repulsion was discussed. Based on this, particle interactions were estimated for different pH suspensions. The reduced particle coagulation increased the packing density of the EPD coatings from 38% at pH 7.4 to 53% at pH 8.4. Therefore, subsequent sintering of coatings was promoted. The sinterability was evaluated by micro-hardness and microstructure. After sintering at 1200 °C, coatings made in pH 8.4 suspensions obtained a hardness of 786 MPa and had fewer big pores than coatings fabricated in pH 7.4 suspensions that had a hardness of 457 MPa.     

A novel route to obtain B4C nano powder via sol–gel method

Boron carbide nanopowders and nanowhiskers were synthesized using phenolic resin and boron alkoxide as precursors through sol–gel process in water–solvent–catalyst–dispersant system. The effects of soaking time on free carbon content and synthesized B₄C particles morphology were evaluated at 1270 °C. The synthesis process of B₄C nanopowders was completed at 1270 °C after 1 h while B₄C nanowhiskers were heterogeneously nucleated and grown from obtained nanopowders after 3 h.     

Dense nanostructured zirconia compacts obtained by colloidal filtration of binary mixtures

As starting materials two commercial nanosized zirconias doped with 3 mol% of Y₂O₃ were used: a powder of about 100 nm (TZ3YE, Tosoh, Japan) and a colloidal suspension of about 15 nm (Mel Chemicals, UK). Colloidal stability in water was studied for both zirconias in terms of zeta potential as a function of deflocculant concentration and pH. Concentrated suspensions were prepared by dispersing the powder in the colloidal suspension to solids loadings ranging from 5 to 30 vol.% using a sonication probe to achieve dispersion. The rheological behavior was optimized in terms of solids content, deflocculant content and sonication time. Optimized suspensions with up to 25 vol.% solids showed a nearly Newtonian behavior and extremely low viscosities and maintain stable for long times (days) which is an important drawback of conventional nanoparticle suspensions. Samples obtained by slip casting in plaster moulds were used for dynamic sintering studies and dense, nanostructured specimens were obtained at temperatures of 1300–1400 °C.     

High-pressure adsorption of methane on montmorillonite,kaolinite and illite

Methane (CH₄) adsorption of Ca^2 +-montmorillonite (Mt), kaolinite (Kaol) and illite (Il) at 60 °C and pressures up to 18.0 MPa was investigated, during which the adsorption capacity was evaluated by the Langmuir adsorption model. The influences of adsorbed water and the interlayer distance of the clay minerals on CH₄ adsorption were explored by using heated Mt products with different interlayer distances as the adsorbent. Mt, Kaol and Il showed high CH₄ adsorption capacities, and their maximum Langmuir adsorption capacities were Mt, 6.01 cm³/g; Kaol, 3.88 cm³/g; and Il, 2.22 cm³/g, respectively. CH₄ was adsorbed only on the external surface of Kaol and Il; however, adsorption also occurred in the interlayer space of Mt, which had a larger interlayer distance than the size of a CH₄ molecule (0.38 nm). CH₄ adsorption in the interlayer space of Mt was supported by the lower CH₄ adsorption capacity of heated Mt products (with the interlayer distance < 0.38 nm) than that of Mt at high pressures despite the higher external surface areas of the heated Mt samples. The entrance of CH₄ into the interlayer space of Mt occurred at low pressures, and more CH₄ molecules entered the interlayer space at high pressures. Moreover, the adsorbed water occupied the adsorption sites of the clay minerals and decreased the CH₄ adsorption capacity. These results indicate that clay minerals play a significant role in CH₄ adsorption of shale and indicate that the structure and surface properties of clay minerals are the important parameters for estimating the gas storage capacity of shale.     

Preparation of concentrated barium titanate suspensions incorporating nano-sized powders

In order to reduce agglomeration and overcome the low packing density issues of working with nano-sized powders, a colloidal processing route has been chosen in this study. Commercial BaTiO₃ (BT) powders with a particle size in the range of 50 nm have been dispersed in the aqueous media. Rheological properties have been analyzed on suspensions with different solids loading, dispersant concentration, and pH conditions. Optimum dispersing conditions were obtained for suspensions prepared at basic pH (pH 10) with 0.646 wt% ammonium poly (acrylic acid) (NH₄PAA) as a dispersant. Suspensions have been centrifugally cast to obtain the green body, and the sintering conditions have been investigated by examining the phase evolution, microstructures and electrical properties of the sintered samples through XRD, SEM and dielectric measurements, respectively. The results show that for a 45 vol% suspension sintered at 1325 °C, the density of bulk ceramic can reach 5.85 g/cm³, nearly 97.0% of the theoretical density.     

Ball milling assisted hydrothermal synthesis of ZrO2 nanopowders

The formation of ZrO₂ nanopowders under various hydrothermal conditions such as temperature, time, autoclave rotation speed, heating rate and particularly assistance of ball milling during reaction was investigated. Full ZrO₂ formation (with monoclinic phase) from zirconium solution was completed at shorter times with increasing temperature such as after 4 h at 150 °C, 2 h at 175 °C and less than 2 h at 200 °C. Crystallite size increased from 2.9 to 4 nm with increasing reaction temperature from 125 °C to 200 °C, respectively. Ball milling assisted hydrothermal runs were performed to understand the effect of mechanical force on phase formation, crystallinity and particle size distribution. Monoclinic ZrO₂ was formed in both milled and non-milled runs when zirconium solution was used. Mean particle size for the 2 M solution was measured to be 94 nm for the milled and 117 nm for the non-milled powders. However, when amorphous aqueous zirconia gels (precipitated at pH 5.8) were used, tetragonal phase was also formed in addition to monoclinic phase. Mean particle size was measured to be 0.7 μm (d₉₀≅1.3 μm) for the milled and 7.9 μm (d₉₀≅13 μm) for the non-milled powders. Ball milling during hydrothermal reactions of both zirconium solution and aqueous zirconium gel resulted in smaller crystallite size and mean particle size and, at the same time, effectively controlled particle size distribution (or agglomeration) of nanopowders.     

Electrophoretic deposition of CoFe2O4 films from aqueous suspensions

CoFe₂O₄ nano-particles with average size of ～40 nm were synthesized via the chemical coprecipitation method. PAMANH₄ was used as dispersant to improve the stability of aqueous suspensions. Zeta potential and sediment volumes were tested to study the effects of pH and dispersant amounts on the stability of suspensions. The most stable suspension was obtained when using 0.6 wt.% PAMANH₄ as dispersant at pH = 10. Conductivity results showed that thoroughly dispersed suspensions were formed after being ultrasonic agitated for 30 min. CoFe₂O₄ films on Al₂O₃/Pt substrates fabricated via EPD sintered at 1250 °C exhibited preferentially oriented structure. The XRD analyses showed (2 2 0) and (5 1 1) were the preferential orientations. Anisotropy was also observed in magnetic hysteresis loops. Stronger ferromagnetic effect was observed in the in-plane orientation, with saturation magnetization of ～290 emu/cm³.     

Effect of calcination temperature on structural,magnetic and optical properties of multiferroic YFeO3 nanopowders synthesized by a low temperature solid-state reaction

Nanosize multiferroic YFeO₃ powders have been synthesized via the low temperature solid-state reaction. Differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy all indicated that the phase-pure orthorhombic YFeO₃ powders were obtained at 800 °C with a size below 150 nm. X-ray photoelectron spectroscopy (XPS) showed the Fe³⁺ ions to be predominant. Magnetic hysteresis loops exhibited some ferromagnetic behaviour of the YFeO₃ nanopowders at ambient temperature. The maximum and remnant magnetizations of the powders were about 2.49 and 0.88 emu/g, respectively. Moreover, optical measurements demonstrated that the optical band gap of the nanopowders was around 2.4 eV, proving that they can strongly absorb visible light. So an easy and efficient way to synthesize YFeO₃ nanopowders with promising application in the magnetic and optical fields has been successfully developed.     

Preparation,structural characterization,and flocculation ability of amphoteric cellulose

A series of amphoteric celluloses (QACMCs) were prepared from sodium carboxymethyl cellulose (CMC) and 2,3-epoxypropyltrimethylammonium chloride (EPTMAC) with a cationic degree of substitution (DS_cat) of 0.24–1.06 and a carboxymethyl degree of substitution (DS_ani) of 0.60. The structures of the samples were characterized by FTIR and NMR spectroscopy, which revealed that DS_cat depended on the ratio of EPTMAC to CMC in the reaction mixture and that the substituent distribution of the cationic group at the C2, C3, and C6 positions of the QACMCs ranked as follows: C2 > C6 > C3. The QACMCs dissolved over a wide range of pH levels and exhibited flocculation ability against kaolin suspensions. In particular, the samples with high DS_cat exhibited excellent flocculation performance. In addition, the flocculation characteristics of the QACMCs depended on the pH of the kaolin suspension.     

Enhanced electrokinetic,dielectric and electrorheological properties of covalently bonded nanosphere-TiO2/polypyrrole nanocomposite

Nanosphere-TiO₂ was synthesized, surface functionalized with (3-aminopropyl) triethoxysilane (APTS) and then covalently bonded with polypyrrole (PPy) with bottom-up surface engineering strategy to obtain nanosphere-TiO₂/PPy core/shell hybrit nanocomposite. All the materials were subjected to full chemical and morphological characterizations by using various techniques. The presence of NaCl, AlCl₃, cetyltrimethyl ammonium bromide and sodium dodecylsulfate observed to cause high colloidal stabilities of the nanocomposite dispersions by reaching to zeta(ζ)-potential values of ζ > + 30 mV and ζ < − 30 mV. A series of suspensions were prepared by dispersing nanosphere-TiO₂ and nanosphere-TiO₂/PPy particles in insulating silicone oil (SO) and dielectric properties were determined using an LCR meter. Antisedimentation stabilities of these suspensions were determined against gravitational forces and 54% colloidal stability was achieved with the nanocomposite after 30 days. Polarizabilities of the suspended particles were observed using an optical microscope under externally applied electric field strength. Then the suspensions were subjected to electrorheological measurements by investigating the effects of shear rate, particle volume fraction, shear stress, and electric field strength. Non-Newtonian shear thinning behaviors were observed for the samples. Further, vibration damping characteristics of the materials were determined with shear stress and frequency oscillation measurements. Enhanced reversible viscoelastic deformations were observed for the dispersions from creep-recovery tests and 64% creep-recovery was obtained for nanosphere-TiO₂/PPy/SO system under E = 3.5 kV/mm.     

Reliable high strength alumina fabricated by DCC-HVCI using submicron calcium citrate complex

Ceramics with high strength and reliability are highly demanded in engineering applications. In this paper, a modified direct coagulation casting via high valence counter ions (DCC-HVCI) method for alumina using calcium citrate complex assisted by glycerol diacetate was investigated. Calcium citrate complex suspensions were prepared by mixing tri-ammonium citrate and calcium chloride in water. Effect of reaction time on the chelating properties of the prepared suspensions was investigated. Concentrated alumina suspensions with a solid loading of 50 vol% were prepared by mixing the calcium citrate complex suspensions and alumina powder at pH of 10.5. Then the suspensions were coagulated by adding 3–6 vol% glycerol diacetate at temperatures of 40–70 °C for 2–6 h. The compressive strength of the coagulated wet samples is in the range 1.1–2.4 MPa. Alumina ceramics sintered at 1550 °C shows homogeneous microstructures with flexural strength and Weibull modulus of 455±17 MPa and 30, respectively.