Preparation and growth mechanism of the flower-like whiskers of γ-, θ-, and α-Al2O3 phases by homogeneous precipitation/calcination method

The different alumina whiskers phases are extensively used to reinforce high-performance composite materials. The understanding of the mutual relationship of the different phases is key for simulating Al₂O₃ whiskers containing systems and is of general interest for the metal oxide and ceramic communities. In this paper, we used homogeneous precipitation to synthesized flower-like boehmite whisker precursors from aluminum sulfate and urea. The synthesized precursors were calcined at 600, 900, and 1200 °C to obtain flower-like whiskers of γ-, θ-, and α-Al₂O₃ phases, respectively. The precursor boehmite crystals prepared in closed, semi-closed, and open reactor systems were qualitatively analyzed using SEM. The results showed that the usea hydrolysis, which serves as a source of OH^? ions, is slow under closed-reactor conditions, which favors the growth of the boehmite crystal nuclei into whiskers. We adopted the L₁₆ (4³) orthogonal design of experiment to analyze the influence of reaction temperature, time, and the amount of urea (relative to the amount of Al₂(SO₄)₃) in the closed system. Synthesis temperature of 160 °C and the processing time of 8 h yielded optimal boehmite flower-like whiskers. Physicochemical properties of the precursor and series of alumina phases were characterized using thermogravimetric/differential thermal analysis (TG/DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) methods. Finally, the obtained results and the thorough analysis of the growth mechanism suggested the step-growth model of boehmite flower-like whiskers.     

Determination of the shape,size and porosity of fine α-Al2O3 powders prepared by emulsion evaporation

Stable water in oil emulsions were prepared from Al(NO₃)₃ solutions of different molarities. Dark brown precursors were obtained from these emulsions which were evaporated by adding them dropwise into hot mineral oil. The precursors containing diaspore as raw alumina were calcined at 1000 °C and fine α-Al₂O₃ powders were obtained. The photographs of the powders were taken by the use of an electron microscope and their particle size distributions were determined. The shapes and sizes of the powders were discussed. Their mesopore size distributions were obtained from the data of the desorption of nitrogen at 77 K. The relation between the obtained results and the concentrations of the Al(NO₃)₃ solutions were discussed.     

The processing and properties of (Zr,Hf)B2–SiC nanostructured composites

(Zr, Hf)B₂–SiC nanostructured composites were fabricated by high energy ball milling and reactive spark plasma sintering (RSPS) of HfB₂, ZrSi₂, B₄C and C. Highly dense composites with homogeneously intermixed ultra-fine (Zr, Hf)B₂ and SiC grains (100–300 nm) were obtained after RSPS at 1600 °C for 10 min. The densification was promoted by high energy ball milling and ZrSi₂ additive. The additives were almost completely transformed into ZrB₂ and SiC during densification. The improvement of flexural strength and fracture toughness (641 MPa and 5.36 MPa m^1/2, respectively) was achieved. The relationships between the ultra-fine microstructure and mechanical properties were discussed.     

Isothermal crystallization of PLA: Nucleation density and growth rates of α and α' phases

From a technological point of view, poly(lactic acid) (PLA) is one of the most important polymers produced from renewable sources, due to its versatility, relatively acceptable processability, and low cost. However, a significant limitation exists in its slow crystallization kinetics, which results in amorphous products having low mechanical properties and thermal resistance. For this reason, quantitative knowledge of the phenomenon of crystallization kinetics is fundamental. In this work, the crystallization kinetics in quiescent conditions of a commercial grade of PLA was analyzed in terms of nucleation and growth rates by direct morphological observations at different crystallization temperatures (T_c) and by calorimetric analysis in isothermal and non-isothermal conditions. The optical analysis showed a spherulitic morphology with radial growth of the lamellae. The analysis of the growth rate evidenced the α/α'-crystals polymorphism with a transition temperature of ~120°C. Based on experimental observation, the crystallization kinetics of the two crystalline phases were assessed.     

Comparative investigations of the catalytic properties of an anodic Al2O3-coated catalyst and of α- and γ-Al2O3 bulk catalysts

The catalytic properties of an Al₂O₃-coated catalyst formed by anodic oxidation of aluminium were compared with those of α- and of γ-Al₂O₃- bulk catalysts in the dehydration of 2-propanol and in the isomerization of n-butenes. In the dehydration reaction both the Al₂O₃-coated catalyst and the γ-Al₂O₃ bulk catalyst show approx. the same activities and activation energies. However, the reaction rate based on the unit of the surface area for the Al₂O₃-coated catalyst is approx. 30 times larger than that for the γ-A1₂O₃ bulk catalyst. In the isomerization of the individual isomeric n-butenes the Al₂O₃-coated catalyst was more active than the γ-Al₂O₃, bulk catalyst and under otherwise equal conditions equilibrium was reached at approx. 80 K lower temperature.     

Role of immiscible and miscible second phases on the sintering kinetics and microstructural development of nano-crystalline α-Al2O3-based materials

An ultra-fine alumina powder was doped with yttrium or zirconium chloride to produce Al₂O₃-5 vol.%ZrO₂ (AZ-5) and Al₂O₃-5 vol.%YAG (AY-5) composite powders. Composite samples and pure alumina, used as a reference, were submitted to dilatometric analyses up to 1500-1550 °C at 2, 5 and 10 °C/min for supplying the data required for the modeling of their sintering behaviour. The best fit for the three samples was obtained by applying an Avrami-Erofeev nucleation and growth model (An) and a subsequent power law reaction (AnFn). The evolution of the activation energy with densification is given for the three samples, as well as the prediction of their best sintering conditions. Finally, densification mechanism for the immiscible (AZ-5) and miscible (AY-5) systems are hypothesized and correlated with their final microstructures.     

Extraction kinetics and mechanism of Zr and Hf by DIBK−TOPO system using Lewis Cell

Kinetics studies on the extraction of Zr and Hf using the mixture of diisobutyl ketone (DIBK) and trioctylphosphine oxide (TOPO) as extractant were performed in a Lewis Cell. The results showed that the extraction rate of Zr was governed by chemical reaction in the bulk phase. The extraction rate of Hf was controlled by chemical reaction and mass diffusion at the interface. It testified that the overall extraction reaction was an exothermic and non-spontaneous process, which occurred via an S_N2 mechanism. Under the given conditions, the overall extraction rate equation and rate-limiting chemical reaction step of Zr and Hf were proposed.     

MAX phases Hf2(SexS1−x)C (x = 0–1) and their thermal expansion behaviors

Herein, a series of chalcogen-containing MAX phases, Hf₂(Se_xS_1?x)C, were successfully synthesized, whose lattice parameter change follows the Vegard’s law. The average coefficient of thermal expansion (CTE) can be continuously tuned from 7.59 μK^?1 to 9.93 μK^?1 when the occupancy rate x of Se changes from 0 to 1. The substitution of Se for S effectively soften the crystal structures that is reflected by long average M-A bond in Se-alloying Hf₂(Se_xS_1?x)C. However, the CTEs along a and c axes in all Hf₂(Se_xS_1?x)C MAX phases are almost same which may be find application in thermal barrier coating (TBC) that isotropic volume change is highly demanding.     

Role of B2O3 in formation of mullite from kaolinite and α-Al2O3 mixtures

Abstract

The microstructure, phase constitution, and physical properties of mullite bodies prepared from α-Al₂O₃- kaolin mixtures with added B₂O₃ were investigated. Densification was found to be enhanced with small additions of B₂O₃. The results indicate that 0.5 wt-% B₂O₃ increases the content and growth rate of the mullite. It was found to be the optimum addition with respect to densification and resulting properties.     

Zirconium and hafnium separation with molten salt extraction in Sn–Cu–Zr–Hf and Cu–Zr–Hf alloy systems

Chemical-grade zirconium contains about 1–3 wt% hafnium, which is harmful for nuclear applications due to its high neutron-capture cross section. In the present paper, Zr-Hf separation in Sn-Cu-Zr-Hf and Cu-Zr-Hf alloy systems using molten salt containing CuCl₂ or CuF₂ was thermodynamically evaluated and lab-scale experiments on zirconium and hafnium separation in different molten salt systems were conducted. The best single-step Hf removal efficiency and Zr-Hf separation factor of about 95% and 9.0, respectively, were obtained with a NaCl-CaCl₂-CuCl₂ (3 wt%) molten salt system at 850°C and the CuCl₂/Hf stoichiometric ratio of 1.5.     

Molten salt synthesis and formation mechanisms of ternary V-based MAX phases by V–Al alloy strategy

V₂AlC and V₄AlC₃ are obtained in much milder conditions by a new facile approach using low-cost V–Al alloys and carbon black with the assistance of molten salts. Density functional theory (DFT) calculations show that our strategy is feasible and thermodynamically superior to the traditional method, with the following X-ray diffraction and Scanning Electron Microscopy fully revealing the phase and morphology evolutions of both reaction processes. Unreported formation mechanisms are revealed for the first time. Al₈V₅ transforms directly into V₂AlC with both lattices perfectly overlapped due to the existence of multiple parallel planes between V₂AlC and Al₈V₅ with consistent interplanar spacings, which leads to the overlapped phenomenon of the diffraction spots as well. For V₄AlC₃, an embedded structure of V₂AlC–VC is first formed through an aluminum removing behavior in V₂AlC layers, causing the increase of the VC content, and V₄AlC₃ is transformed by the embedded V₂AlC–VC structure through diffusion and interface migration with a possible climb behavior of Frank partial dislocation.     

Pixelated sintering of α-Al2O3

The sintering of α-alumina by a brand new and innovative technique, called pixelated sintering (PS), is here studied. Densification and grain growth by PS of perfectly controlled granular compacts are analysed and compared to results obtained using Spark Plasma Sintering (SPS) and Pressure-Less SPS (PL-SPS). Materials are exposed to the same temperature profiles whatever the sintering technique used in order to assess the potential of PS in terms of microstructure control. It is shown that PS can be used as an alternative technique to SPS for fast sintering with the advantages of a much simpler and cost-effective set-up, as well as a better control of the localised heat input. PS also appears to be a very modular technology in the way it controls the temperature gradients allowing its implementation for multi-step sintering approaches, as well as for the fabrication of large and complex parts.     

Effect of carbon and α-Al2O3 on controlled synthesis of AlON powder

The α-Al₂O₃/C mixtures were prepared by ball milling, and then AlON powders were synthesized by carbothermal reduction and nitridation (CRN). The effects of α-Al₂O₃ particle size, carbon powders morphology and particle size on the morphology and particle size of the synthesized AlON powders were studied. The results showed that as the particle size of α-Al₂O₃ increases, the particle size of the synthesized AlON powders will also increase, but the surface morphology will not be affected. The increase of the carbon particle size will increase the particle size of the synthesized AlON powders. The pore size and number of pores of the synthetic AlON powders were very similar to the morphological characteristics of the carbon powders. In addition, the mechanism of controlling the synthesis of AlON powder with α-Al₂O₃ and carbon as raw materials was also discussed.     

Coarsening of Mesoporous α-Al2 3 Ceramics

Until recently, the processing of high surface area alumina ceramics has been restricted to transitional phases such as , , and . In this study, a nanocrystalline -Al_{2 3} powder (100 m²/g) was processed into mesoporous -Al_{2 3} ceramics with surface areas in the range 20 to 80 m²/g. Hence, the opportunity exists to study the effect of thermal treatment on the pore structure of a high surface area -Al_{2 3} ceramic in the temperature range 600°C to 1000°C. The reduction in surface area was characterized as pure coarsening by surface diffusion. Examination of the pore structure showed both intraparticle pores and interparticle pores in the temperature range 600°C to 800°C and only interparticle pores above 800°C. The powder particles were dense and polycrystalline. However, much of their internal structure is lost in heat treatments above 1000°C.     

Phase equilibria and stability of intermediate phases in the Sm2O3–BaO–Fe2O3 system

Phase equilibria in the ½ Sm₂O₃–BaO–½ Fe₂O₃ system were systematically studied at 1100°C in air. Two individual compounds: Sm_1.875Ba_3.125Fe₅O_15-δ and Ba₃SmFe₂O_7.5+δ have been detected in the studied system at 1100°C in air. One more complex oxide with double perovskite structure SmBaFe₂O_5+w has been obtained under reduced oxygen partial pressure. Thermodynamic stability of SmBaFe₂O_5+w and its thermal stability in metastable state in air were determined. The subsolidus phase diagram for the ½ Sm₂O₃ – BaO – ½ Fe₂O₃ system at 1100°C in air has been constructed.     

Determination of interdiffusion coefficients of Si and Al in Ti3SiC2–Ti3AlC2 diffusion couple at 1373-1673 K

In the diffusion couple of Ti₃SiC₂ and Ti₃AlC₂, only interdiffusion of Si and Al occurred during diffusion treatment process. Based on the concentration profiles of Si and Al measured by electron probe microanalysis (EPMA), the interdiffusion coefficients of Si and Al at 1373-1673 K in Ti₃SiC₂–Ti₃AlC₂ diffusion couple were determined by both the Boltzmann-Matano (B-M) method and the Saucer-Freise (S-F) method. At the position of Matano plane with the composition of Ti₃Al_0.5Si_0.5C₂, the interdiffusion coefficient could be expressed as D_int (m²/s) = 5.6 × 10⁻⁴⋅exp [−246 ± 14 (kJ/mol)/RT]. Based on the two methods, the calculated interdiffusion coefficients increased with increasing temperature, and the magnitudes of their absolute values were on the order of 10^–13-10^–11 m²/s at 1373-1673 K. At 1373-1573 K, the calculated interdiffusion coefficients decreased monotonously with the increase of Si concentration, that is, x_Si/(x_Al + x_Si). But at 1673 K, the variation trend of interdiffusion coefficients with x_Si/(x_Al + x_Si) was no longer monotonous, probably due to the presence of Ti₅Si₃ phase and voids on Ti₃AlC₂ side.     

Ultrafine α-Al2O3. Explosive Method of Synthesis and Properties

An explosive method for producing ultrafine -Al₂O₃ is developed and optimal synthesis parameters are determined. Particles of ultrafine -Al₂O₃ have a spherical shape and are separated from one another. The size distribution is log-normal (number-averaged size 70 nm and variance 1.9). Special features of phase transitions in ultrafine aluminum oxide under shock-wave action are studied. Results of x-ray phase analysis suggest stabilization of the new high-pressure phase -Al₂O₃ with a face-centered cubic lattice with a parameter a = 8.53 Key words: metastability, corundum, shock-wave synthesis, surface, modification.     

Stability and excitation of potassium promoter in iron catalysts – the role of KFeO2 and KAlO2 phases

The kinetics of the NO SCR with propane has been studied on a low‐exchanged Cu‐ZSM‐5 catalyst. The study of the kinetics of individual reaction stages (2‐nitrosopropane isomerization to acetone oxime and reaction of adsorbed acetone oxime with gaseous NO) has shown that the NO reaction with acetone oxime is the rate‐determining stage in the whole chain of transformations leading to the formation of molecular nitrogen in the low‐temperature region below 300 °C. The kinetic analysis of the reaction has revealed that at the temperatures above 300 °C propane plays a more important role. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mechanochemical synthesis of α-Al2O3-Cr3+ (Ruby) and χ-Al2O3

In this work we present a unique method to synthesize χ-Al₂O₃ and α-Al₂O₃ doped with Cr³⁺ (ruby). The ruby is synthesized by mechanical milling of pseudoboehmite that is doped in-situ with chromium. The doping is carried by adding chromium sulfate hydrate to an aqueous solution rich in aluminum sulfate hydrate. The pH in the solution is controlled to be between 9 and 10 by using ammonia, which induces the pseudoboehmite precipitation. The Cr³⁺ is added for its remarkable effects on the detectability of ruby emitting luminescent R₁ and R₂ bands that are traceable in Raman spectroscopy. The formation of ruby is detected at milling times as short as 5 hours and increased with the milling time. Ruby phase is further confirmed by means of true atomic resolution Transmission Electron Microscopy (TEM).     

Synthesis and Characterization of α- Fe2O3 NPs/P-Si Heterojunction for High Sensitive Photodetector

Silicon - In this work, Fe2O3 nanoparticles (NPs) were synthesized successfully through simple chemical precipitation method. The absorption spectra of the synthesized Fe2O3 NPs was studied by...