Optical properties of magnesium aluminate spinel crystals implanted with helium ions

Measurements of the optical properties of helium-implanted spinel crystals have been used to obtain information on the formation of point defects and their evolution during thermal annealing. Optical absorption spectra showed that the main defects in helium-implanted spinel were the anion vacancies which were removed by annealing to 1000 K. Additional defects in implanted and annealed specimens were observed and are related to cation vacancies and anti-site defects. The increased cationic disorder in ion-implanted spinel crystals was demonstrated by measuring the radio-luminescence in these crystals. Optical micrographs show that, at low fluences, the heterogeneous nucleation of bubbles take place, but at a fluence of 1×10²¹ ion/m² the homogeneous formation of small bubbles of higher density was obtained. Finally, at the highest fluence of 3×10²¹ ion/m² the bubbles begin to grow by coalescence and become large enough to cause flaking of the implanted surface.     

Effect of magnesium aluminum isopropoxide hydrolysis conditions on the properties of magnesium aluminate spinel powders

We have studied the influence of the conditions of hydrolysis of the magnesium aluminum double alkoxide MgAl₂(OPr ⁱ )₈ and the heating rate and calcination temperature of aluminum magnesium hydroxides on the particle size and morphology of magnesium aluminate spinel powders. Conditions of the two-step process have been identified that make it possible to reproducibly prepare MgAl₂O₄ nanopowders with controlled properties. We have obtained MgAl₂O₄ powders ranging in specific surface area from 2 to 160 m²/g.     

Rate effects on the mechanical response of magnesium aluminate spinel

The uniaxial compressive, biaxial flexural strength and fracture toughness of a polycrystalline transparent MgAl₂O₄ spinel were characterized over a wide range of loading rates. The flexural tests were carried out by means of ring-on-ring equibiaxial bending, while the fracture toughness was determined by four-point bending on samples with Chevron notch (CN) configuration. The surface crack (SC) method was also attempted in determining the fracture toughness. Quasi-static experiments were conducted on a servohydraulic testing machine, while the high-rate experiments were performed on a modified Kolsky bar. Results showed that both the failure strength and fracture toughness of the spinel were rate sensitive. Edge beveling in sample preparation did not affect the ring-on-ring flexural strength significantly, and the failure initiation sites were found to be inside the loading ring area regardless of edge conditions. Fracture toughness tests following ASTM standard were largely affected by the inherent coarse microstructure of this material.     

Effects of argon-ion bombardment on the properties of the surface layer in spinel crystals of different composition

Ion-induced photon emission (IPE) during bombardment of magnesium aluminate spinel crystals MgO·nAl₂O₃ by 20 keV Ar⁺ ions was studied. The dependence of the yield of particles in specific excited states on the fluence of incident ions in the range of (0.1–1.6)×10¹⁷ ions/cm² was measured. It was shown that yield of magnesium and aluminum atoms and ions in most excited states do not depend (or slightly depend) on the fluence of ion bombardment. An exception was found for yields of Mg⁺ ions in the 4s ²S excited state and Al atoms in the 5p ²P⁰ excited state leading to emission lines at 292.8 and 669.6 nm, respectively. The yield of particles in these excited states drastically decreases at the start of ion bombardment. Analysis of these results and published data on the bombardment-induced surface modification of spinel crystals allows to elucidate the role of crystal structure and chemical bonding in the formation of some excited states. The dependence of excited state yield (except of Mg and Al indicated above) from spinel crystals of different composition MgO·nAl₂O₃ (n=1.0, 1.5, 2.0, and 2.5) does not reflect quantitatively the variation of the calculated bulk concentration of constituent atoms in these targets.     

Controlled synthesis of high-quality W-Y_2O_3 composite powder precursor by ascertaining the synthesis mechanism behind the wet chemical method

As an emerging preparation technology,wet chemical method has been employed widely to produce lots of alloy materials such as W and Mo based alloys,owing to its unique technical advantages.Ascertaining the synthesis mechanism behind wet chemical method is indispensable for controlled synthesis of highquality W-Y2 O3 composite powder precursor.The co-deposition mechanism of yttrium and tungsten component behind the wet chemical method of preparing yttrium-doped tungsten composite nanopowder was investigated systematically in this work.A series of co-deposited composite powders fabricated under different acidity conditions were used as research targets for investigating the effect of surface composition and structure on co-deposition efficiency.It was found that white tungstic acid has more W-OH bonds and much higher co-deposition efficiency with Y^3+ions than yellow tungstic acid.It is illustrated that the coordination reaction between W-OH bonds on tungstic acid particles and Y^3+ions brings the co-deposition of yttrium and tungsten component into being.Through displacing H^+ions in W-OH bonds,Y^3+ions can be adsorbed on the surface of or incorporated into tungstic acid particles in form of ligand.Consequently,to control and regulate Y2 O3 content in powder precursor accurately,H^+ion concentration in wet chemical reaction should be in range of 0.55-2.82 mol L^-1 to obtain white tungstic acid.Besides,H^+ion concentration also has prominent effect on the grain size and morphology of reduced powder precursor.The optimal value should be around 1.58 mol L^-1,which can lead to minimum W grain size(about 17 nm) without bimodal structure.The chemical mechanism proposed in this work could produce great sense to preparation of high-quality precursor for sintering high-performance Y2 O3 dispersion strengthened W based alloys.Our work may also shed light on the approach to exploit analogous synthesis mechanism in other alloy systems.     

Synthesis of nanocrystalline MgAl2O4 spinel powders by a novel chemical method

A novel chemical method for the preparation of nanocrystalline MgAl₂O₄ spinel powders has been developed in this paper. The mixed magnesium-aluminum hydroxide precipitates were initially formed in a three-dimensional space network microarea. After being calcined at above 700 °C, the nanocrystalline MgAl₂O₄ spinel powders were obtained. The precursor and as-calcined powders were characterized using TGA-DTA, XRD and TEM. The MgAl₂O₄ spinel powders calcined at 850 °C for 2 h are of narrow distribution, little agglomeration and small particle size of ∼ 24 nm. The reason for the synthesis of high-quality powders was explained.     

Diffusion and formation energies of adatoms and vacancies on magnesium surfaces

This paper reports classical molecular statics calculations of magnesium {0 0 0 1}, , , and surfaces, specifically formation energies of defects (adatoms and surface vacancies) and flat surfaces and diffusion energy barriers of the defects. The formation energies show that the surface is thermodynamically more favorable than , and surfaces; in contrast, literature reports have often ignored the surface. The diffusion energy barriers of both adatoms and surface vacancies show strong diffusion anisotropy on , , and surfaces. Based on this anisotropy, the ratio of diffusion distances (of either adatoms or surface vacancies) along two orthogonal directions on is 37–55 at room temperature. Using the results of formation energies and diffusion energy barriers we develop a more complete understanding of surface orientations in Mg nanoblades synthesized by physical vapor deposition [F. Tang, T. Parker, H.-F. Li, G.-C. Wang, T.-M. Lu, J. Nanosci. Nanotechnol. 7 (2007) 3239]. In contrast to previous reports, we postulate that the side surfaces of Mg nanoblades are because (a) they have the second lowest surface formation energy and (b) the ratio of diffusion distances on them agrees with the experimental value of approximately 50.     

The influence of trace magnesium content on the phase composition of silicon-stabilized calcium phosphate powders

A novel silicon-stabilized calcium phosphate phase mixture possesses a characteristic phase composition of ∼ 75 wt.% silicon-stabilized α-tricalcium phosphate (Si-α-TCP) with the balance being calcium hydroxyapatite (HA) and traces of β-tricalcium phosphate (β-TCP). Variability in the phase composition has been shown to be caused by trace magnesium (Mg) contained in the calcium nitrate tetrahydrate used to prepare the sol gel. Mg contents between 250 and 300 ppm are sufficient to form significant quantities of β-TCP at the expense of the Si-α-TCP phase.     

不同锰源对尖晶石型锰酸锂性能影响的研究

分别以自制锰源和工业用电解二氧化锰为原料,采用固相合成法,将锰源和碳酸锂的混合物合成尖晶石型锰酸锂.通过扫描电子显微镜、X射线衍射的方法研究了不同锰源合成的尖晶石型锰酸锂之间形貌和结构的区别,通过电性能测试研究了不同锰酸锂对锂离子电池电性能的影响.结果表明：合成的锰酸锂都有良好的尖晶石型立方结构,以碳酸锰为原料的自制三...     

The effects of delamination deficiencies on compressive properties of composite grid-stiffened structures

This article provides a systematic approach to accurately predict the effects of delamination deficiencies on compressive properties of composite grid-stiffened structures. The deformation of the stiffener and the skin is investigated during a mechanical test. Cohesive elements in the finite element method were employed to calculate the ultimate load-carrying capacity. The delamination deficiencies in the interface between the skin and the stiffener have a significant effect upon its ultimate load-carrying capability compared with the delamination deficiencies in the skin and in the stiffener.     

Clamping effects on the dynamic characteristics of composite machine tool structures

Substituting composite structures for conventional metallic structures has many advantages for structural dynamic characteristics because composite materials have the higher specific stiffness and damping characteristics than conventional materials. However, the dynamic characteristics such as the fundamental natural frequency and damping of composite structures are influenced much by their joints. In this work, the effects of clamping conditions on the dynamic characteristics of cantilever type composite machine tool structures with clamped joint were investigated to increase the natural frequency and damping of structures. In order to improve the shear property of the clamping part of composite machine tool bar, a new method for the clamping part was developed with metal core or sleeve inserted in the composite body at the clamping part. From the finite element and experimental results, suitable clamping conditions for the maximum dynamic stiffness were obtained for the composite structures with clamped joint.     

Effects of volume ratio on the microstructure and mechanical properties of particle reinforced magnesium matrix composite

In the present study, the AZ91 alloy reinforced by (submicron + micron) SiCp with four kind volume ratio was fabricated by the semisolid stirring casting technology. The influence of volume ratio between submicron and micron SiCp on the microstructure and mechanical properties of Mg matrix was investigated. Results show that the submicron SiCp is more conducive to grain refinement as compared with micron SiCp. With the increase of volume ratio, the submicron particle dense regions increase and the average grain size decreases. The yield strength of bimodal size SiCp/AZ91 composite is higher than monolithic micron SiCp/AZ91composite. Both Δσ_Hall–Petch and Δσ_CTE increase as the volume ratio changes from 0:10, 0.5:9.5, 1:9 to 1.5:8.5. Among the composite with different volume ratio, the S-1.5 + 10-8.5 composite has the best mechanical properties. The interface debonding is found at the interface of micron SiCp-Mg. As the increase of volume ratio, the phenomenon of interface debonding weakens and the amount of dimples increases.     

An efficient strategy for the calculation of end effects on composite pipes: The thermoelastic case

Through the study of a linear problem, this paper introduces some of the ingredients that are needed in order to conduct efficient nonlinear calculations on composite pipes under thermomechanical loading in the presence of defects. Based on the observation that the main degradation scenarios take place at the extremity of a pipe, a refined model is used to describe this end zone while the rest of the pipe is described using a beam model. In order to avoid creating spurious effects in connecting the beam theory and the 3D end model, the Saint-Venant solution is used as the boundary condition on the latter. This solution is achieved through what is called the exact beam theory [Ladevèze and Simmonds, New concepts for linear beam theory with arbitary geometry and loading, Eur. J. Mech. Solids 1998;17(3):377]. In order to obtain the solution efficiently, the 3D end problem is uncoupled through a dedicated Fourier series expansion into a series of 2D problems. This proposed representation is dealt with efficiently thanks to the use of fast Fourier transforms.     

聚合物前驱体法合成铌酸锶钡铁电薄膜

采用聚合物前驱体法制备铌酸锶钡(Srx Ba1-x Nb2O6,SBN)铁电薄膜.以柠檬酸为配位剂与金属离子配合,水作为溶剂,用乙二醇加速溶液聚合形成稳定的前驱体溶胶.采用旋转法在Si/SiO2/Ti/Pt基片上镀膜.研究了柠檬酸(CA)/乙二醇(EG)(质量比)、pH值对前驱体溶胶稳定性的影响.结果表明,当Sr-Ba-Nb前驱体溶胶的pH值为7,CA/EG=40:60时,可以获得稳定性好的溶胶.薄膜经750℃热处理,可以获得单相、表面均匀、无裂纹的铌酸锶钡铁电薄膜.     

Scale effects on the response of composite structures under impact loading

For several years, composite materials have taken a significant part in the realization of structures designed for transport (aeronautical, nautical, automotive…). In order to qualify the behavior of such structures, preliminary validation tests have to be done. These specific tests are often very expensive and difficult to set up, especially when the structure dimensions are large (fuselages of aircraft, ship hulls…). An alternative way is then to employ small-scale models.The use of these reduced scale structures requires the identification of similitude models allowing the extrapolation of the small-scale model behavior to the real structure. Although largely developed in the case of homogeneous materials, such similitude techniques are not clearly identified for composite materials taking into account the damage evolution during an impact.The purpose of this article is firstly to present existing similitude techniques making it possible to predict the composite structure behaviour from the knowledge of small-scale model response. Secondly, experiments were done on two scale of samples carried out by stratification of unidirectional carbon/epoxy plies. These results were finally compared with the analytical predictions of similitude laws currently used.The aim of this paper is to contribute to similitude laws development applied to composite structures. These laws permit to extrapolate the small-scale model behavior to the real scale one. Existing approaches have been established following two different methods. They are summarized in this paper and applied to impact loadings on two laminated plate scales. In order to complete data collected by “conventional” instrumentation (force transducer, displacement sensor, accelerometer…), optical device such as an high-velocity CCD camera, associated with optical techniques for the monitoring of markers, were used. These techniques make possible to compare displacement lines corresponding to each scale. It is shown that existing similitude laws, used for elastic materials, do not allow to simulate the behavior of the real scale when this one is damaged.     

The effect of rotatory inertia on the dynamic response of laminated composite plate

The strip element method (SEM) is extended to include rotatory inertia for analysing the dynamic response of laminated composite plates. The transient responses of rectangular symmetric laminated plates are computed for various loading using the newly developed SEM program. The effect of the rotatory inertia is investigated for plates of different thickness. It is found that the rotatory inertia has less effect on thin plates whose thickness–length ratio is less than 1/20, but significant effect on thicker plates. The effects of other parameters such as elastic constants, material density and fiber orientation on the responses of plate are also studied and discussed in detail.     

The effect of PVD coatings on the corrosion behaviour of AZ91 magnesium alloy

In this study, multilayered AlN (AlN + AlN + AlN) and AlN + TiN were coated on AZ91 magnesium alloy using physical vapour deposition (PVD) technique of DC magnetron sputtering, and the influence of the coatings on the corrosion behaviour of the AZ91 alloy was examined. A PVD system for coating processes, a potentiostat for electrochemical corrosion tests, X-ray difractometer for compositional analysis of the coatings, and scanning electron microscopy for surface examinations were used. It was determined that PVD coatings deposited on AZ91 magnesium alloy increased the corrosion resistance of the alloy, and AlN + AlN + AlN coating increased the corrosion resistance much more than AlN + TiN coating. However, it was observed that, in the coating layers, small structural defects e.g., pores, pinholes, cracks that could arise from the coating process or substrate and get the ability of protection from corrosion worsened were present.     

The effect of the diffusive, composite chromium nitride layers produced by a hybrid surface treatment on the corrosion behavior of AZ91D magnesium alloy

The layers based on chromium nitride were produced on the AZ91D magnesium alloy using a hybrid surface treatment. The treatment consists of nitriding of the chromium-precoated magnesium alloy. The proposed treatment yields diffusive, composite gradient-type chromium nitride layers which are diffusion-bonded to the substrate. The effect of these layers on the corrosion behavior of the AZ91D magnesium alloy was investigated by the potentiodynamic and impedance spectrometry methods. It was found that the formation of the diffusive, composite chromium nitride layers on the AZ91D alloy may result in a significant decrease of the corrosion activity measured by the corrosion potential. This effect is correlated with the layer thickness and becomes significant with relatively thin layers (less than 1 μm thick). Moreover, with the thin layers the impedance modulus is the highest and phase angle has a beneficial character.     

Buckling and free vibration finite strip analysis of composite plates with cutout based on two different modeling approaches

A Reddy type, third order shear deformation theory of plates is applied to the development of two versions of finite strip method (FSM), namely semi-analytical and spline methods, to predict the behavior of the moderately thick plates containing cutouts. The internal cutouts are modeled based on two different modeling approaches, and the effects of cutouts on the buckling critical stresses as well as natural frequencies are investigated.     

Assessment of a composite beam finite element based on the proper generalized decomposition

In this paper, a finite element based on the Proper Generalized Decomposition (PGD) is presented for the analysis of bi-dimensional laminated beams. The displacement field is approximated as a sum of separated functions of x (axial coordinate) and z (transverse coordinate). This choice yields to an iterative process that consists of computing a product of two one-dimensional functions at each iteration. The capability and the behavior of the PGD approach are shown on isotropic beam with different slenderness ratios. A second and fourth-order expansion with respect to the thickness are considered. Mechanical tests for thin/thick laminated and sandwich beams are presented in order to evaluate the two approaches. They are compared with elasticity and 2D finite element reference solutions.