Preparation of magnesium aluminate spinel by self-propagating high-temperature synthesis metallurgy methods

Experimental data are presented on the high-temperature synthesis of cast inorganic materials in the MgO–Al₂O₃ system. Experiments were carried out in multipurpose self-propagating high-temperature synthesis (SHS) reactors at an argon pressure p = 5 MPa. The starting mixtures used in the experiments consisted of molybdenum(VI) and magnesium(II) oxides and aluminum. It has been shown that, varying synthesis parameters, one can control the phase composition and microstructure of the final oxide products. We have optimized synthesis conditions for the preparation of single-phase magnesium aluminate spinel, MgAl₂O₄. The SHS products have been characterized by X-ray diffraction and local microstructural analysis.     

Evaluation of the magnesium and silicon diffusion rates in forsterite synthesis using various magnesium compounds

In forsterite synthesis from highly disperse SiO₂ (white soot or silica sol) and magnesium chloride, nitrate, sulfate, acetate, or citrate through sol-gel processing, with polyvinyl alcohol as a gel former, the reaction products are forsterite, magnesium oxide, silica, and clinoenstatite. From analysis of the synthesis products, we assess the relationship between the diffusion rates of the magnesium and silicon cations. Phase-pure forsterite can be obtained by reacting white soot and magnesium acetate.     

Mechanochemical synthesis of amorphous and crystalline magnesium diboride

We have studied the phase composition of materials obtained by mechanochemical processing and subsequent heat treatment of mixtures of magnesium and boron powders in the atomic ratio 1: 2. Differential dissolution, differential scanning calorimetry, and X-ray diffraction data indicate that, during mechanical processing, some of the magnesium reacts with boron to form amorphous magnesium diboride. During annealing of the activated powder mixture, X-ray amorphous magnesium diboride forms at 340°C and crystallizes at 480°C. As shown by high-resolution transmission electron microscopy, the unreacted crystalline magnesium is covered with an amorphous layer consisting of magnesium diboride and boron. The amorphous material obtained by milling contains nuclei of MgB₂ crystallites 3–5 nm in size. During subsequent heating of the activated mixture, magnesium and boron react further to form amorphous magnesium diboride and the amorphous phase crystallizes. Heating of mechanically activated mixtures to just below the crystallization temperature allow MgB₂ nanoparticles to be produced. The formation of nanocrystalline magnesium diboride nuclei along with the amorphous phase during mechanochemical processing facilitates mechanochemical synthesis compared to thermal synthesis.     

Synthesis of magnesium borate nanorods

Single-crystalline magnesium borate Mg₂B₂O₅ nanorods have been synthesized via a simple route based on the calcinations of mixed powders containing Mg(OH)₂ and H₃BO₃. The nanorods have the typical diameters in the range of 70-120 nm and the lengths up to a few micrometers. An optimal synthesis temperature for Mg₂B₂O₅ nanorods was obtained, and the possible growth mechanism was also presented.     

Double tetrametaphosphates Mn2−x Mg x P4O12

The tetrametaphosphates of the type Mn_2−x Mg _x P₄O₁₂, wherex=0 to 2, have been synthesized as new binary compounds, and their existence has been proved. The synthesis is based on a thermal procedure making use of the reversible transformation of tetrametaphosphates to higher linear phosphates. Temperatures and heats of formation of these products have been determined (i.e. formation by thermal recrystallization from higher linear phosphates) along with the yields of this procedure which increase with increasing magnesium content,x. The structure of the binary tetrametaphosphates belongs to the monoclinic system (over the whole range ofx); the structural parameters determined slowly decrease with increasing magnesium content. With respect to the proposed application of these products as special inorganic pigments, the following properties have also been determined experimentally: density, thermal stability, colour. With increasing magnesium content the density of these products slowly decreases and the melting temperatures increase; the intensity of pink hue is also changed with changing magnesium or manganese content.     

Novel in-situ synthesis and characterization of nanostructured magnesium substituted β-tricalcium phosphate (β-TCMP)

Tricalcium phosphate (TCP, Ca₃(PO₄)₂) in its pure form cannot be synthesized under physiological conditions in normal aqueous solutions due to phase instability, resulting in its transformation to hydroxyapatite (HA, Ca₁₀(PO₄)₆(OH)₂) in the presence of water. However, substituting magnesium in lieu of calcium is known to stabilize TCP, preventing its conversion to hydroxyapatite. There are several methods known for synthesizing magnesium substituted tricalcium phosphate (TCMP). In the present study, a novel in-situ method has been developed to synthesize β-TCMP using magnesium substituted brushite as a precursor.Substitution of 50% of calcium by magnesium results in the formation of semi-spherical nanocrystalline particles (～ 100 nm) of brushite. Boiling the nanocrystalline brushite powder in aqueous condition for only 30 min results in the generation of rosette shaped nanocrystals (～ 80 nm) of β-TCMP that emerge from the original brushite spheres. The β-TCMP particles exhibit a specific surface area of ～ 200 m²/g. Details about the synthesis procedure and the possible mechanisms involved in the formation of β-TCMP from Mg-substituted brushite is further discussed.     

Role of magnesium in cast aluminium alloy matrix composites

Wetting between the dispersoid and the matrix alloy is the foremost requirement during the preparation of metal matrix composites (MMC) especially with the casting/liquid metal processing technique. The basic principles involved in improving wetting fall under three categories: (i) increasing the surface energies of the solids, (ii) decreasing the surface tension of the liquid matrix alloy, and (iii) decreasing the solid/liquid interfacial energy at the dispersoid matrix interface. The presence of magnesium, a powerful surfactant as well as a reactive element, in the aluminium alloy matrix seems to fulfil all the above three requirements. The role played by magnesium during the synthesis of aluminium alloy matrix composites with dispersoids such as zircon (ZrSiO₄), zirconia (ZrO₂), titania (TiO₂), silica (SiO₂), graphite, aluminium oxide (Al₂O₃) and silicon carbide (SiC), has been analysed. The important role played by the magnesium during the composite synthesis is the scavenging of the oxygen from the dispersoid surface, thus thinning the gas layer and improving wetting and reaction-aided wetting with the surface of the dispersoid. The combinations of magnesium and aluminium seem to have some synergistic effect on wetting.     

Comparison between mechanochemical effect on die cast and extruded magnesium alloys

Abstract

The mechanochemical effect (MCE) of several magnesium based alloys, obtained by both die casting and extrusion methods, was studied by potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) techniques. The mechanochemical behaviour of each alloy was evaluated as a function of die cast parameter, environment, and alloy composition. Electrochemical tests were performed in a buffer solution of sodium tetraborate, (Na₂B₄O₇) with and without magnesium hydroxide (Mg(OH)₂). The MCE was correlated with the microstructure of the Mg alloys.     

Design and microstructural analysis of magnesium alloys for dynamical applications

A microanalytical characterization of cast magnesium alloys of eutectic origin based on the Mg–Al–Ca ternary matrix system has been carried out in order to investigate the influence of alloying elements on their microstructure as well as microchemistry-processing-microstructural relations using structure-sensitive techniques of electron microscopy, mechanical spectroscopy (internal friction), X-ray diffractometry, and advanced microanalytical methods including electron probe compositional analysis. Following the data obtained here there is direct correlation of microstructure with creep properties of the new experimental magnesium alloys. The creep and heat-induced properties of the multicomponent magnesium alloys containing low range of inexpensive additions of titanium (0.07–0.2%) or strontium (of about 1.8%) are defined by resulting structure dynamically formed during creep strain (up to 200 h). It is noteworthy that Ti as novel alloying element competes for creep resistance and cost with Sr and attracts as-cast desirable properties minimizing solute effects at ambient temperatures because of the pinning of slowly moving dislocations with the binding energy no more then 0.3 eV as well as because of stress-induced strengthening. The Ti and Sr solute atmosphere dragging is believed to be the rate-controlling mechanism responsible for radical improvement of creep resistance and long-term strength in the newly developed magnesium alloys at elevated temperatures. The new experimental alloys are superior to commercial alloys AZ91D, AE42, and AS21 following their creep resistance, long-term strength, heat resistance, and castability because of their novel microstructure having desirable engineering properties for structural applications (creep strain ε_c less than 0.3–0.4% at 423 K and 70 MPa for 200 h; έ_c ~ 10⁻⁹ s⁻¹). The newly developed magnesium alloys with improved castability could be used in die-casting technology and automobile (powertrain) industry for manufacturing of components and parts which are difficult to cast with more desirable microstructure.     

镁基储氢材料研究进展

从气固反应的角度对近几年镁基储氢材料研究中的新技术和新方法进行了综述。主要包括球磨法、晶态及非晶态改性、烧结法和添加添加剂等方法。并简要对各种方法的工艺条件及所制备产物的吸放氢性能进行了讨论。     

Superplastische Eigenschaften einer konventionellen AM20‐Magnesiumlegierung

Superplastic Characteristics of a Conventional AM20 Magnesium Alloy Despite the increasing interest of the industry in lightweight materials during the last years, an intensive industrial use of magnesium based alloys due to the their restricted cold‐workability caused by the hexagonal lattice is still very limited. Considering this limitation a solution is provided by the superplastic forming of magnesium based alloys which, in contrast to other types of materials, is neither metallurgically developed nor process optimized. A very promising step is the extrusion of the conventional AM20 magnesium alloy followed by controlled cooling of the extruded material in order to reduce the grain growth caused by secondary recrystallisation are suitable means to produce a fine grained microstructure. After a short presentation of the theoretical background and methods to determine the superplastic characteristics, the strongly improved material properties of the AM20 magnesium alloy are revealed by increased m‐values and higher elongation‐to‐fracture‐values (ε_max = 550%) determined by tensile tests at constant strain rates.     

Optimizing the methods of synthesis for barium hexagonal ferrite—An experimental and theoretical study

M-type barium hexaferrite (BaM) is a hard ferrite, crystallizing in space group P6₃/mmc possessing a hexagonal magneto-plumbite structure, which consists of alternate hexagonal and spinel blocks. The structure of BaM is thus related to those of garnet and spinel ferrite. However the material has proved difficult to synthesize. By taking into account the presence of the spinel block in barium hexagonal ferrite, highly efficient new synthetic methods were devised with routes significantly different from existing ones. These successful variations in synthetic methods have been derived by taking into account a detailed investigation of the structural features of barium hexagonal ferrite and the least change principle whereby configuration changes are kept to a minimum. Thus considering the relevant mechanisms has helped to improve the synthesis efficiencies for both hydrothermal and co-precipitation methods by choosing conditions that invoke the formation of the cubic block or the less stable Fe₃O₄. The role played by BaFe₂O₄ in the synthesis is also discussed. The distribution of iron from reactants or intermediates among different sites was also successfully explained. The proposed mechanisms are based on the principle that the cubic block must be self-assembled to form the final product. Thus, it is believed that these formulated mechanisms should be helpful in designing experiments to obtain a deeper understanding of the synthesis process and to investigate the substitution of magnetic ions with doping ions.     

Application of a Bioactive Coating on Resorbable,Neodymium Containing Magnesium Alloys,and Analyses of their Effects on the In Vitro Degradation Behavior in a Simulated Body Fluid

Whilst bioactive coatings are commonly used as layers for non‐resorbable implant materials, such as titanium or steel to improve cell adhesion, this study investigates the application of bioactive SiO₂–CaO–P₂O₅ on resorbable magnesium alloys (Nd2 and LANd442). The bioactive coating was applied to the magnesium alloys by a dip‐coating process, where a parameter set of 20 immersions and a 10 s drying time between each immersion generated a reproducible layer with regard to its thickness and homogeneity. In vitro mass loss, strength loss, and pH value measurements were used to determine the coating's effects on the degradation behavior in a simulated body fluid. Here, it could be observed that bioactive layers on magnesium alloys lead to an increased degradation in comparison to specimens in the uncoated states. In addition to this, pitting corrosion was determined for bioactive coated magnesium samples during comparatively early periods of the investigation. Due to the decreased corrosion resistance and induced pitting corrosion of bioactive coated magnesium alloys, it is suggested that one carefully tests if the enhanced cell adhesion, which occurs with bioactive coatings, warrants the increased degradation of magnesium based implant materials.     

Preparation and characterization of lamellar-like Mg(OH)2 nanostructures via natural oxidation of Mg metal in formamide/water mixture

Facile, scale-up, and controllable fabrication of lamellar-like Mg(OH)₂ on a magnesium substrate through natural oxidation of Mg metal in formamide/water mixtures has been demonstrated. The one-step, wet-chemical approach has displayed well-controlled growth of densely packed magnesium hydroxide with large-area homogeneity and uniform morphology. The chemical-liquid-deposition process, an analogue to the widely used chemical-vapor-deposition technique, has been well developed for production of specific morphological Mg(OH)₂ through continuous supply, transport, and thermal decomposition of magnesium complexes in a liquid phase. The formation mechanism of lamellar-like Mg(OH)₂ was also discussed based on experimental phenomena.     

Effect of the structure of precursors on the formation of nanotubular magnesium hydrosilicate

We have studied the effect of the structure of precursors on the formation of Mg₃Si₂O₅(OH)₄ nanotubes under hydrothermal conditions. The results demonstrate that a chemical match between the components of a starting mixture has an advantageous effect on the synthesis of nanotubular magnesium hydrosilicate and the morphology of the hydrothermal treatment products.     

磷酸钾镁水泥水化产物六水磷酸钾镁(K-Struvite)定量分析

磷酸钾镁水泥的各项性能与其中水化产物六水磷酸钾镁(K-Struvite)的含量息息相关。使用基于X射线衍射的绝热法、Rietveld法分析了不同配比磷酸钾镁水泥中K-Struvite和MgO的相对含量,并提出了将相对含量转化为绝对含量的公式。之后使用热重分析法确定了K-Struvite的脱水温度和绝对含量,并和绝热法、Rietveld法所得结果进行了比较,发现三种方法所得结果较为一致。绝热法和Rietveld法在分析K-Struvite相对含量时简便快速,但换算为绝对含量时比热重分析法需要的相关信息要多,可操作性弱于热重分析法。     

Synthesis and mechanism of a new environment-friendly flame retardant (anhydrous magnesium carbonate) by hydrothermal method

In this study, the synthesis and formation mechanism of anhydrous magnesium carbonate (AMC) using hydrothermal method is proposed from the perspective of ionic reaction. Validation experiments were conducted, and the effect of the pH of the reaction solution and the form of carbon source on AMC preparation was investigated. It is found that HCO₃^? can accelerate crystal growth and reduce crystal size, and the preparation conditions of AMC can be quantified. The AMC particles were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The AMC prepared using urea as carbon source has a small particle size in the presence of ion pairs of Na⁺/Cl^? or K⁺/SO₄^2?. AMC prepared using magnesium chloride as magnesium source has smaller composition units than those prepared using magnesium sulfate as magnesium source. The effects of ions types, reaction temperature, reaction time and filling degree on the phase transition and crystal growth of AMC are also discussed.     

Synthesis of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> nanopowders

A procedure has been developed for the synthesis of MgAl₂O₄ nanopowders with a characteristic particle size of 10–40 nm. Translucent hydrous xerogels have been synthesized as precursors to MgAl₂O₄. The synthesized magnesium aluminum spinel nanopowders are promising for the fabrication of optical ceramics.     

Improving the wear resistance of AZ91D magnesium alloys by laser cladding with Al-Si powders

To improve the wear resistance of AZ91D magnesium alloy, laser surface cladding with Al and Si powders was investigated using a Nd:YAG pulsed laser. With appropriate processing parameters and the suitable weight ratio of Al to Si in powders, a modified surface layer free of cracks and pores was formed by reaction synthesis of Mg with Al and Si. X-ray diffractometry (XRD) confirmed the main phases in the layer to be Mg₂Si and Mg₁₇Al₁₂. The surface hardness increased from 35 HV for as-received magnesium alloy to more than 170 HV for laser treated sample. Accompanying the increase in hardness, the wear resistance of the clad layer increased more than 4 times that of the substrate.