Al2O3 Modification Research Articles

Formation and Evolution of Tribo-Layer on Al<sub>2</sub>O<sub>3</sub> and Al<sub>2</sub>O<sub>3</sub>-Based Ceramics in Sliding against Si<sub>3</sub>N<sub>4</sub> under Dry Sliding

Alumina (Al2O3) and Al2O3-based ceramics are candidate wear-resistant materials for many applications. In the past decades, the friction and wear behaviors of Al2O3 and Al2O3-based ceramics in dry sliding and lubricated conditions have been extensively investigated. In dry sliding, surface modification of Al2O3 and Al2O3-based ceramics at tribo-interface can be observed. In other words, the original surfaces of Al2O3 and Al2O3-based ceramics can be modified by friction and friction-induced material interaction (fracture, mechanical mixing, tribo-sintering) at the tribo-interface, and hence give birth to a tribo-layer. The formation and evolution of the tribo-layer on the worn surfaces of Al2O3 and Al2O3-based ceramics are the keys to understand the friction and wear behaviors of Al2O3 and Al2O3-based ceramics. This is conducted by scanning electron microscopy (SEM) observation and classification of the morphology and composition of the worn surfaces of Al2O3 and Al2O3-based ceramics at different sliding stages. The typical characteristics of the tribo-layers on the worn surfaces of Al2O3 and Al2O3-based ceramics in sliding against Si3N4 in a reciprocating motion are smooth surface with many interlinked cracks in morphology, mechanical mixing in chemical composition. The condition allowing the formation of the tribo-layer is briefly discussed.

X-ray diffraction and calorimetric studies of powder nanocrystalline systems based on ZrO2(Y) and Al2O3 with second insoluble component

Four nanocrystalline ZrO2(Y)- and Al2O3-based powders synthesized by plasma spray pyrolysis have been studied. It has been shown that the ZrO2(3Y)-based system with second component Al2O3 forms a nonequilibrium solid solution ZrO2(3Y, Al) with the tetragonal structure. It has been found that the existence of an component (ZrO2(Y)) insoluble in the coarse-grained state in Al2O3-based systems causes the delay of the γ → α transformation and decreases the size of the coherently scattering domains of formed nanosized modifications of Al2O3.

Glycine-nitrate combustion synthesis of finely dispersed alumina

The possibility of synthesizing different crystalline modifications of alumina Al2O3 by combustion reactions of a mixture of aluminum nitrate with glycine has been analyzed. Aggregated powders of γ-Al2O3 that do not transform into the corundum structure after annealing at a temperature of 1000°C have been prepared. The influence of the synthesis conditions on the specific surface area, bulk density, porosity, and morphology of alumina particles has been investigated.

Synthesis, photophysics of two new perylene bisimides and their photovoltaic performances in quasi solid state dye sensitized solar cells

Two new symmetrical compounds A and P based on perylene-anthracene and perylene-pyrene, respectively, were synthesized and characterized by FT-IR, 1H NMR, TGA and TMA. These compounds contained tert-butyl groups which enhanced their solubility, decomposed above 400 °C and gave char yields of 46–65% at 800 °C in N2. Compound A showed significantly higher glass transition temperature (124 °C) than P (75 °C). Their absorption spectra were broad with longer wavelength absorption at 467–525 nm and optical band gap of 2.05 eV. The solutions of the compounds emitted green-yellow light with maximum at 555 nm, while their films were not photoluminescent. The compound A shows better photovoltaic response than compound P. Quasi solid state dye sensitized solar cells (DSSCs) have been fabricated employing compound A as sensitizer and polymer sol gel as electrolyte and characterized through the current–voltage characteristics in dark as well as under illumination and electrochemical impedance spectra. We found that the Al2O3 modification of TiO2 layer significantly improves the dye absorption resulting in enhancement of power conversion efficiency (PCE) (from 1.15 to 2.13%) which is attributed to the increase in electron lifetime and reduction in back transfer of electrons. Finally, the TiO2 has been incorporated into the polymer electrolyte gel to improve the power conversion efficiency (3.42%) of the quasi solid state DSSC. The faster electron diffusion in the device, the high ionic conductivity and the low activation energy of the polymer electrolyte are also responsible for enhanced PCE, when TiO2 nano-particles are incorporated in the polymer electrolyte.

Simultaneous removal of soot and NO over thermal stable Cu–Ce–Al mixed oxides

Cu–Ce–Al mixed oxides were synthesized by depositing Al2O3 powders in Cu–Ce gel and then were treated at 800 °C in air for 10 h. The modification of Al2O3 obviously increases the textural stability of the catalyst and improves the dispersion of CuO and CeO2, which induces a lower soot oxidation temperature and a higher simultaneous NO reduction. In addition, the adsorption behavior of NOx on the Cu–Ce–Al surface is quite different from that for Cu–Ce, which may be also an important factor for the improved activity.

Encapsulated Titanium Oxide with Al<sub>2</sub>O<sub>3</sub> and its Ultraviolet Shielding Properties

In this work, we encapsulated the titanium oxide particles with a layer of Al2O3 by a sol-gel process and investigated the UV-shielding properties of the obtained Al2O3-TiO2 composites. In this method, Al(NO3)3 aqueous solution mixed with raw titanium oxide powder was hydrolyzed by adding NH3·H2O. The hydrated materials were calcined at 500°C and then the composite of Al2O3-TiO2 was prepared. Different techniques including ICP-AES, XRD, SEM-EDS, and TEM were used to characterize the raw TiO2 and encapsulated TiO2 particles. Chemical analysis showed that TiO2 mass content in the raw titanium oxide particles is 98.25%, and Al2O3 is just 0.75%. After modification, the percentage Al2O3 was increased to 4.48% due to the encapsulation of Al2O3. Trace elements analysis displayed that the concentration of the trace elements were much lower than their original content in the raw titanium oxide, which prove that proved that there was no impurities incorporated into in the sol-gel process. XRD analysis indicated that the modified titanium oxide powders are mainly in phase of rutile, ca. 99.1%, and the profiles exhibit no peaks of crystalline Al2O3 suggesting the layer of Al2O3 exists in amorphous phase. SEM image suggests that the size is in 80~600nm of the raw TiO2 granula and became rougher on the surface, the packing of the agglomerations got looser after modification with Al2O3. HRTEM image shows that the TiO2 particles was evenly coated with a packed layer ~8 nm in thickness. The UV-Vis absorbency experiments showed that the range of wavelength shielded was narrowed from 208-316 nm in the raw titanium oxide to 225-285 nm in the case of the Al2O3-TiO2. The average absorbency of UV was improved from 1.07 to 1.98 at the same content 1.0 μg/mL in the suspended solution. The results reflected that Al2O3 modification process improve the titanium oxide particles UV-shielding properties in selectivity and performance.

Electronic Excitations and Defects in Nanostructural Al[sub 2]O[sub 3

A time-resolved cathodo-and photoluminescence study of nanostructural modifications of Al2O3 (powders and ceramics) excited by heavy-current electron beams, as well as by pulsed synchrotron radiation, is reported. It was found that Al2O3 nanopowders probed before and after Fe+ ion irradiation have the same phase composition (the γ-phase/δ-phase ratio is equal to 1), an average grain size equal to ∼17 nm, and practically the same set of broad cathodoluminescence (CL) bands peaking at 2.4, 3.2, and 3.8 eV. It was established that Al2O3 nanopowders exhibit fast photoluminescence (PL) (a band at 3.2 eV), whose decay kinetics is described by two exponential stages (τ1 = 0.5 ns, τ2 = 5.5 ns). Three bands, at 5.24, 6.13, and 7.44 eV, were isolated in the excitation spectrum of the fast PL. Two alternate models of PL centers were considered, according to which the 3.2-eV luminescence either originates from radiative relaxation of the P− centers (anion-cation vacancy pairs) or is due to the formation of surface analogs of the F+ center (FS+-type centers). In addition to the fast luminescence, nano-Al2O3 was found to produce slow luminescence in the form of a broad band peaking at 3.5 eV. The excitation spectrum of the 3.5-eV luminescence obtained at T = 13 K exhibits two doublet bands with maxima at 7.8 and 8.3 eV. An analysis of the luminescent properties of nanostructural and single-crystal Al2O3 suggests that the slow luminescence of nanopowders at 3.5 eV is due to radiative annihilation of excitons localized near structural defects.

A Nano-Effect in the Generation of Alumomanganese Catalysts for the Deep Oxidation of Methane

Alumomanganese catalysts for the deep oxidation of methane have been investigated by X-ray phase analysis and temperature programmed reduction. Application of the active component and stabilizing additives (La, Ba, Sr) under non-equilibrium conditions facilitates retention of nano-particles of the carrier and stabilization of low-temperature modifications of Al2O3 (γ and χ phases), which determines the high activity and thermal stability of the catalysts. A phase dimension effect was observed during high temperature treatment of the catalysts (900 °C, 5 h), which includes a decrease in the phase transitions of aluminum oxide with a decrease in the size of its particles and the formation of the metastable θ-phase in the more dispersed γ-Al2O3 carrier (L = 4 nm).

Doped CVD Al2O3 coatings for high performance cutting tools

Today, high performance applications of cemented carbide cutting tool inserts demand coatings with enhanced wear resistance. Chemical vapour deposited coatings based on Ti(C,N) and Al2O3 are the systems of choice for these applications. The influence of experimental variables on process stability and Al2O3 modification and structure was investigated. Deposition characteristics of Al2O3 as a function of doping combinations, generated by additional gases (TiCl4, BCl3), and deposition temperature are reported. The Al2O3 coatings were deposited in the temperature range of 930–1030 °C at atmospheric pressure. Doping element concentrations and distribution were investigated by inductively coupled plasma mass spectrometry, energy-dispersive X-ray spectra and scanning transmission electron microscopy. Microstrain and substructure were evaluated by high temperature X-ray diffraction analysis and discussed in consideration of doping element type and deposition temperature and phase stability in the application temperature range of 800–1100 °C. The investigations have shown that the deposition rates can be enhanced by doping with Titanium. The temperature range of κ-Al2O3 phase stability during deposition is extended by Boron. Ti-and B-doped κ-Al2O3 exhibited lower full width half maximum values at temperatures above 1000 °C. It was concluded that the kinetic of the κ→α-Al2O3 transformation is affected thereby and results in enhanced wear resistance for high temperature applications of κ-Al2O3 coated cutting tool inserts.

NOX removal in excess oxygen by plasma-enhanced selective catalytic reduction

In the off-gases of internal combustion engines running with oxygen excess, non-thermal plasmas (NTPs) have an oxidative potential, which results in an effective conversion of NO to NO2. In combination with appropriate catalysts and ammonia (NH3-SCR) or hydrocarbons (HC-SCR) as a reducing agent, this can be utilized to reduce nitric oxides (NO and NO2) synergistically to molecular nitrogen.The combination of SCR and cold plasma enhanced the overall reaction rate and allowed an effective removal of NOX at low temperatures. Using NH3 as a reducing agent, NOX was converted to N2 on zeolites or NH3-SCR catalysts like V2O5–WO3/TiO2 at temperatures as low as 100–200°C. Significant synergetic effects of plasma and catalyst treatment were observed both for NH3 stored by ion exchange on the zeolite and for continuous NH3 supply.Certain modifications of Al2O3 and ZrO2 have been found to be effective as catalysts in the plasma-assisted HC-SCR in oxygen excess. With an energy supply of about 30eV/NO-molecule, 500ppm NO was reduced by more than half at a temperature of 300°C and a space velocity of 20000h−1 at the catalyst. The synergistic combinations of NTP and both NH3- and HC-SCR have been verified under real diesel engine exhaust conditions.

The characteristics of alumina scales formed on HVOF-sprayed MCrAlY coatings

Turbine blades are protected against high-temperature oxidation by thermal barrier coating (TBC) systems, which consist of a ceramic top coating (ZrO2/Y2O3) and a metal bond coating (MCrAlY, M=Ni, Co). At high temperatures and under oxidative conditions between the bond coating and the ceramic top coating an oxide scale is formed, which protects the metal against further oxidation. The oxidation behavior of thermally sprayed MCrAlY coatings is influenced by the coating process and the composition of the metal alloy. This work is concerned with the isothermal oxidation behavior of high-velocity oxygen fuel (HVOF) MCrAlY (M=Ni, Co) coatings. During thermal spraying two elements, yttrium and aluminum, oxidize. As a consequence the HVOF MCrAlY coatings exhibit a microstructure with fine dispersed Al2O3 and aluminum yttrium oxides. Free-standing bodies of HVOF coatings were isothermally oxidized in synthetic air between 850 and 1050°C for different periods. Oxidation experiments show that the oxidation rate of HVOF coatings is two times slower than the oxidation rate of corresponding VPS MCrAlY coatings. The oxidation mechanism is mainly changed in the transient stage (no metastable modification of Al2O3 was formed). It is supposed that the oxide dispersion favors the formation of α-Al2O3 scales. The presence of the fine oxide dispersion also influences the adherence of the oxide scale. The microstructure of the transient oxide scales were examined by X-ray diffraction, scanning electron microscopy and transmission electron microscopy.

Modification of a filler's surface by the molecular layering method

Modification of a filler's surface by the molecular layering (ML) method is carried out by successive, repeated and alternate processing of the solid surfaces by excess of appropriate reagents. It is followed by the removal of those reagents that have not taken part in the reaction and of gaseous products formed in each stage of the treatment involved. In every cycle of ML reactions, no more than one monolayer of the new structural units joins the surface; thus, the monoatomic assembly of the solid substance is accomplished on the surface of a chosen matrix. The present paper presents the experimental results on the modification of SiO2' Al2O3 and some other fillers (glass and phenolformaldehyde microspheres for polymeric compositions) with volatile chlorides of V, Ti, P, Cr and vapours of H2O.

Surface modifications of crystalline SiO2and Al2O3induced by energetic heavy ions

(1989). Surface modifications of crystalline SiO2 and Al2O3 induced by energetic heavy ions. Radiation Effects and Defects in Solids: Vol. 110, No. 1-2, pp. 185-188.

New possibilities of NMR spectroscopy in studies of adsorption and catalysis☆

By using modern Fourier spectrometers operating at high (about 7 T) magnetic field and having special devices for magic angle spinning (MAS), one can obtain within reasonable periods of time, the NMR spectra for nuclei of various elements present in heterogeneous catalysts, including nuclei with small magnetic moments and low natural abundance, as well as the high resolution spectra of these catalysts and of molecules adsorbed on their surfaces. The following examples of such studies are presented: A comparison of 51V-, 17O- and 23NA-NMR spectra of vanadium catalysts, for SO2 oxidation to SO3 with those of various reference compounds made it possible to identify some chemical compounds present in the active component of these catalysts, as well as to elucidate the interaction between the active component and the support. Coordination of vanadium by pyrosulphate has been studied in the melt of the active component under typical conditions (500°C) at which the catalysts usually operate. By using 27Al-MAS NMR, it is possible to resolve the lines from aluminium ions in tetrahedral and octahedral positions for various crystalline modifications of Al2O3. For zeolites using 27Al NMR, some of the aluminium was found on certain treatments to escape from tetrahedral positions in the lattice and to acquire octahedral coordination in the presence of water. The 29Si-MAS spectra support this conclusion, indicating a simultaneous decrease in the fraction of silicon atoms of the lattice that have four and three aluminium atoms in the second coordination sphere, and than increase in the fraction of silicon atoms that have two and one aluminium atoms in the second coordination sphere. In 1H-MAS spectra of zeolites, lines from various hydroxyl groups can be resolved. The higher sensitivity of modern spectrometers, together with the use of MAS techniques, open novel possibilities for the study of adsorbed species. As an example, results obtained for the adsorption of hydrogen, methane, carbon monoxide and propylene on heterogenous catalysts are presented.

Superacid alumina catalysts II

The acidity and catalytic activity of alumina doped with AlCl3, CH3AlCl2, PCl5, PCl3, I2, Cl2 and H2PO3F were studied. Vapors of AlCl3 and CH3AlCl2 induce superacid sites in alumina, whereas PCl3 and PCl5 produce less acidic systems, which nevertheless catalyze the skeletal isomerization of alkenes. The modification of Al2O3 with other specified compounds affects only slightly the acidity and catalytic activity of alumina.

Sintering behavior of materials based on alumina and titanium and zirconium nitrides

A study was made of the sintering kinetics of TiN-Al2O3 and ZrN-Al2O3 materials in various protective atmospheres, ft was established that during sintering in nitrogen ZrN-Al2O3 composites act as reactive substances. ZrO2 and AlN are formed, the ZrN lattice constant increases, the microhardness of the materials decreases, and the hexagonalα- Al2O3 phase becomes transformed into the cubicη- Al2O3 modification witha =7.92 A. The densification kinetics of specimens of the composite materials and the variation of their electrical resistance were investigated. It is shown that the sintering process occurs in several stages as a result of changes in the mechanism controlling mass transport.

The dissolution of aluminum oxide in cryolite melts

Several modifications of Al2O3 were prepared. Al2O3 samples pressed to tablets were dissolved in cryolite melts at temperatures between 975 and 1090°C. The dissolution rates obtained are consistent with the consecutive occurrence of two rate laws. The dissolution rate changes at an Al2O3 content of 5 to 6 pct in the cryolite melts. The results are discussed with consideration of other experimental and theoretical work. It is concluded that the alteration of the rate law may be explained by structural changes in the melts.

Polymorphism and catalytic properties of Al2O3

1. Together withγ- anda-Al2O3, other polymorphic modifications of Al2O3 described in the literature (χ, κ, Θ, and δ phases) were also prepared and it was shown that their existence is not caused by differences in the residual water content of the aluminum oxide and their total and specific activities in the catalytic dehydration of iso- C3H7OH were determined for the first time. 2. The total catalytic activity of the phases changed symbatically with a change in the specific surface. According to the specific activity, found for preparations with the same residual water content per m2 of surface, the phases form the series Θ>χ>κ> γ( a. It was shown that Aspec of some phases obtained at high temperatures was higher than that of those obtained at low temperatures. 3. It was confirmed that there are three series of phase conversions, depending on the nature of the original aluminum hydroxides. 4. It was established by x-ray methods that the phases retained their structural characteristics during catalytic experiments.