Effect of milling speed on mechanical activation of Al/ZrO2/H3BO3 system to prepare Al2O3–ZrB2 composite powder

A powder mixture of Al/ZrO₂/H₃BO₃ system was mechanically milled under argon in a high-energy planetary mill at different speeds. The XRD and DSC analyses of the as-synthesized samples show that milling operation affects the mechanism, efficiency, and ignition temperature of combustion behavior of the system. XRD analysis of the as-milled samples shows that low-speed milling does not change the phases presented in the sample, while high-speed milling (350, 450 rpm) has led to the formation of new intermetallic phases. This confirms that some reactions between reactants have occurred during high-speed milling.     

Characteristics of CuAl<Subscript>2</Subscript>-Cu<Subscript>9</Subscript>Al<Subscript>4</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposites synthesized by mechanical treatment

Summary Reactive milling of Cu-hydroxycarbonate - powder aluminium mixture brings many complex chemical reactions such as decomposition, aluminothermic reduction and mechanical alloying resulting in the formation of nanometer size composites that contain intermetallic phases, -Cu₉Al₄ and -CuAl₂, with aluminium oxide.     

Comments on: “Toward improved heat transfer performance of annular heat exchangers with water/ethylene glycol based nanofluids containing graphene nanoplatelets,Journal of Thermal Analysis and Calorimetry 126.3 (2016): 1427–1436”

The present study has investigated the complex mechanisms in the aluminum–titanium system with different percentages of titanium through a combination of thermal and X-ray analyses. Thermogravimetry, derivative thermogravimetric, X-ray diffraction, scanning electron microscope and transition electron microscope were used for characterization of the samples. Initially, different Al–Ti powder mixtures were produced by high-energy ball milling and after 30 h of milling the phases generated at different percentages of Ti were analyzed. The XRD results revealed that the intermetallic Al₃Ti powder is obtained after a certain duration of milling. In addition, L1₂ to D0₂₃ phase transformation is possible with increase of the Ti percent. Analyses of the powder annealed at different temperatures yielded interesting results, including the effect of stearic acid as the surface control agent on phase transformations of the aluminum–titanium system and also the formation of unexpected phases such as Al₄C₃ and TiC. Moreover, ductile to brittle transition during phase transformations of the intermetallic Al₃Ti powder was quite conspicuous, which could result in more homogeneity of the powders and the occurrence of more reactions in the system. For example, formation of D0₂₃-Al₃Ti powder which is more brittle compared to L1₂ resulted in the exit of Al from among its layers, leading to the increase of the chances for Al reaction with the system impurities.     

Effect of mechanical activation of reagents’ mixture on the high-temperature synthesis of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–TiB<Subscript>2</Subscript> composite powder

A powder mixture of Al/TiO₂/H₃BO₃ = 10/3/6 in molar ratio was used in this study to form the Al₂O₃–TiB₂ ceramic composite via thermite reactions (combustion synthesis). As no combustion synthesis occurred for an unmilled sample in a furnace, the mixture was milled in a planetary ball-mill for various milling times, and the as-milled samples were in situ synthesized in the furnace at a heating rate of 10 °C/min. The differential scanning calorimetry (DSC) measurements were performed with the same heating rate on the unmilled and the as-milled samples to evaluate the influences of the milling on the mechanisms and efficiencies of reactions. Although no combustion synthesis occurred for the unmilled sample in the furnace, two exothermic peaks were detected in its DSC curve after the melting of the Al. For the as-milled samples, significant changes revealed in the DSC curves, suggest that the milling process before the combustion synthesis changed the mechanisms and efficiencies of reactions. In addition, the intensity and the temperature of the exothermic peaks in the DSC curves changed by increasing the milling time. According to the XRD analyses, by enhancing the milling time, the purity of the final products would increase, confirming that the efficiency of the reactions increased. Finally, the microstructures of the as-milled and as-synthesized samples were examined by a SEM, and it was shown that the morphology of the reactant powders was altered by increasing the milling time.     

Hydrogenation of CO2 conjugated with dehydrogenation of cyclohexane over an intermetallic catalyst

Transformations of carbon dioxide catalyzed by the hydride form of [TiFe_0.95Zr_0.03Mo_0.02]H_x, by the industrial Pt/Al₂O₃ catalyst, and by a mixture of the above materials were studied. Study of the thermal desorption of H₂ showed the presence of two forms of absorbed hydrogen, namely, the weakly bound hydrogen, which is evolved from the intermetallic structure on heating to 430 °C under Ar, and the strongly bound hydrogen (SBH), which remains in the intermetallic compound at higher temperatures (up to 700 °C). In a carbon dioxide medium, the SBH enters into selective CO₂ reduction to give CO at 350—430 °C and 10—12 atm. The selectivity of the formation of CO reaches 80—99% for conversion of CO₂ between 50—70%, the SBH being consumed almost entirely for the reduction of CO₂. In the presence of the mixed catalyst, conjugate reactions proceed efficiently; dehydrogenation of cyclohexane yields hydrogen, which is consumed for CO₂ hydrogenation.     

Crystal structure and thermal stability of Ni151.5Pb24S92. Search for selenium- and tellurium-containing analogs

The crystal structure of the subvalent nickel—lead sulfide, which has been described previously as Ni₆₀Pb₉S₃₁, was established and the composition of this sulfide was refined based on powder X-ray diffraction data. The true Ni_151.5Pb₂₄S₉₂ composition of this compound was confirmed by the EDX techniques. The temperature range of stability of this compound (490—578 °C) was refined by differential thermal analysis. In the search for analogs, the triangulation of the Ni—Pb—Se and Ni—Pb—Te systems at 540 °C was carried out for the first time. No new ternary phases were detected.     

Influence of Homogenizing Methodology on Mechanical and Tribological Performance of Powder Metallurgy Processed Titanium Composites Reinforced by Graphene Nanoplatelets

In the present work, 0.25 wt%GNP-Ti composites were prepared through powder metallurgy route by adopting three types of mixing modes to investigate the extent of mixing on the mechanical and tribological properties. Dry ball milling, wet ball milling, and rotator mixing were independently employed to homogenize the composite constituents. Three types of composite powders obtained were subsequently sintered into composite pellets by cold compaction followed by vacuum sintering. Morphological investigation of composite powders performed by SEM revealed better homogenization of GNPs in Ti matrix for dry ball milled composite powder, whereas wet ball milled and rotator mixed composite powders showed aggregation and bundling of GNPs. Micro Vickers hardness of composites produced via dry ball milling is 4.56% and 15.7% higher than wet ball milled and rotator mixed samples, respectively. Wear test performed by pin-on-disk tribometer showed higher wear loss for wet ball milled and rotator mixed composites in comparison to dry ball milled.     

Thermodynamic analysis of (Ni,Fe)3Al formation by mechanical alloying

(Ni, Fe)₃Al intermetallic compound was synthesized by mechanical alloying (MA) of Ni, Fe and Al elemental powder mixtures of composition Ni₅₀Fe₂₅Al₂₅. Phase transformation and microstructure characteristics of the alloy powders were investigated by X-ray diffraction (XRD). The results show that mechanical alloying resulted in a Ni (Al, Fe) solid solution. By continued milling, this structure transformed to the disordered (Ni, Fe)₃Al intermetallic compound. A thermodynamic model developed on the basis of extended theory of Miedema is used to calculate the Gibbs free-energy changes. Final product of MA is a phase having minimal Gibbs free energy compared with other competing phases in Ni–Fe–Al system. However in Ni–Fe–Al system, the most stable phase at all compositions is intermetallic compound (not amorphous phase or solid solution). The results of MA were compared with thermodynamic analysis and revealed the leading role of thermodynamic on the formation of MA product prediction.     

AES investigations of plasma sprayed Al2O3 coatings on Ni

Summary The system of plasma sprayed Al₂O₃ on Ni substrates is investigated by means of AES/depth profiling. The influence of two process parameters — preoxidation procedure and spraying temperature — is examined. Rupture between substrate and ceramic layer occurs between a residual — or, in the case of excessive preoxidation, a superfluous — NiO layer on Ni, the thickness of the former depending on preoxidation conditions and the Al₂O₃ layer, the back side of which being partially covered with NiO. The thickness of this NiO layer increases up to about 1 m with the thickness of the initial NiO layer on the substrate, until this layer is about 1.3 m thick, and remains constant thereafter. The same dependence is observed for the width (0.1–1 m range) of the mixed oxide interface between the sprayed Al₂O₃ layer and the NiO layer below. These results represent the chemical contribution to adherence. Contrary to excessive preoxidation, an increase of the spraying temperature from 300°C to 500°C effects broader interfaces.This poster was awarded the First Prize in Poster Section C by the Deutscher Arbeitskreis für Spektroskopie (DASp)     

Contribution to the analytical characterization of coatings produced by thermal spraying

This paper reports on the use of Auger electron spectroscopy (AES)/ depth profile analysis for the investigation of plasma-sprayed coatings. Prior to spraying the St 37 substrates are heated to 300 °C or 500 °C for ceramic or metallic layers, respectively. Studies of the starting materials and of the interfaces are important if the adhesion mechanism is to be understood. Therefore the initial components—the unheated and heated substrates and the powder particles NiCrAl, Al₂O₃ and ZrO₂-7.25Y₂O₃—are analyzed. Depth profiles obtained from two coatings St 37/NiCrAl and St 37/Al₂O₃ show the influence of plasmaspraying on substrate surfaces and sprayed particles. Plasma-spraying mainly causes a decrease of superficial carbon contamination for both coating layers. In the case of St 37/NiCrAl incorporation of carbon in the sprayed layer is observed. The whole layer is almost completely oxidized except for some areas where substrate and particle material are present. It is assumed that these areas are identical with so-called adherence zones.Dedicated to Professor Günther Tölg on the occasion of his 60th birthday     

Structural and thermal investigation of Ta–25 mass% Cu alloy prepared by mechanosynthesis route

A mixture of Ta and 25 mass% Cu elemental powders was subjected to mechanical alloying in a high-energy ball mill up to 60 h. The results are composite particles formed by nanocrystalline Cu and amorphous Ta phases. Thermal stability of amorphous was investigated by DSC. The XRD, FTIR and EDX analyses of Ta–25 mass% Cu powder milled for 60 h performed after DSC at 800 and 900 °C have revealed large amounts of Ta nitride and Ta oxides even though the milling process was done in Ar atmosphere. This is due to high reactivity of Ta fine particles with oxygen and nitrogen from air. During manipulations of the powder (taking samples from vials and its investigation), the adsorption phenomena on its surface occur, and both surface-adsorbed N₂ and O₂ are processed with powder and embedded in it. While heating of Ta–25% Cu milled powder in DSC, nitrogen and oxygen diffusion into tantalum is activated, and Ta₂N and TaO₂/Ta₂O₅ compound forms.

     

Effect of alumina concentration on thermal and structural properties of mas glass and glass-ceramics

Magnesium aluminum silicate (MAS) glass samples with different concentrations of alumina (7.58 to 14.71 mol%) were prepared by melt and quench-technique. Total Mg content in the form of MgF₂+MgO was kept constant at 25 mol%. MAS glass was converted into glass-ceramics by controlled heat treatment at around 950°C. Crystalline phases present in different samples were identified by powder X-ray diffraction technique. Dilatometry technique was used to measure the thermal expansion coefficient and glass transition temperature. Scanning electron microscopy (SEM) was employed to study the microstructure of the glass-ceramic sample. It is seen from X-ray diffraction studies that at low Al₂O₃ concentrations (up to 10.5 mol%) both MgSiO₃ and fluorophlogopite phases are present and at higher Al₂O₃ concentrations of 12.3 and 14.7 mol%, fluorophlogopite and magnesium silicate (Mg₂SiO₄), respectively are found as major crystalline phases. The average thermal expansion co-efficient (_avg) of the glass samples decreases systematically from 9.8 to 5.5·10^–6 °C^–1 and the glass transition temperature (T g) increases from 610.1 to 675°C with increase in alumina content. However, in glass-ceramic samples the _avg varies in somewhat complex manner from 6.8 to 7.9·10^–6 °C^–1 with variation of Al₂O₃ content. This was thought to be due to the presence of different crystalline phases, their relative concentration and microstructure.Authors wish to thank Dr V. C. Sahni, Director Physics Group and Dr J. V. Yakhmi, Head TPPED, BARC for encouragement and support to the work. They would like to thank Dr S. K. Kulshreshtha for many useful discussions. Technical assistance from Shri B. B. Sawant, Mrs Shobha Manikandan, Mr Rakesh Kumar and Shri P. A. Wagh is gratefully acknowledged. One of authors (BIS) would like to thank BRNS-DAE for awarding him KSKRA fellowship.     

Nanocomposites based on opal matrices and iron subgroup metal nanoparticles

Three-dimensional nanocomposites based on ordered opal matrices (OMs) and metal nanoparticles were prepared by the reduction of salts and oxides of iron subgroup metals (M = Ni, Co, and Fe) and their binary and ternary mixtures with isopropanol in a supercritical state. The effect of the composition of the initial salts (nitrates or chlorides) on the phase composition of OM/M composites was determined. For a binary system of Ni and Co nitrates (1 : 1), the particles of a NiCo solid solution in a cubic modification were formed in an opal matrix after treatment in supercritical isopropanol. For the Ni-Fe and Co-Fe systems, the nanoparticles of solid solutions based on nickel or ??-, ??-cobalt metal and also oxides or an MFe₂O₄ phase with the spinel structure were formed in opal matrices with the use of iron trichloride. The nanoparticles of iron metal and Ni₃Fe, NiFe, and CoFe intermetallic compounds with regular distributions of metal atoms were detected for the first time in addition to spinel phases upon the reduction of composites with Fe, Ni-Fe, and Co-Fe nitrates with supercritical isopropanol. The reduction of composites obtained by the thermal treatment of a ternary mixture of nickel and cobalt nitrates and iron chloride in supercritical isopropanol led to the formation of solid solution nanoparticles based on Ni, Co, and Fe with an fcc structure and an oxide phase with the spinel structure in the voids of opal matrices. In the composite based on an opal matrix and a ternary system of Ni-Co-Fe nitrates (1 : 1 : 1), the complete reduction of spinel phases to the intermetallic phases of Ni₃Fe, NiFe, and CoFe was noted.     

Investigations into the formation and characterization of microemulsions. I. Phase diagrams of the ternary system water—sodium alkyl benzene sulfonate—hexanol and the quaternary system water—xylene—sodium alkyl benzene sulfonate—hexanol

The phase diagrams of the ternary system water—sodium alkylbenzene sulfonate (NaDBS)-hexanol and the quaternary system water—xylene—NADBS—hexanol have been established at three different temperatures, namely 25, 37, and 50°C. The different phases formed have been qualitatively examined using optical (phase contrast and polarizing) microscopy. The textures of the various liquid crystalline phases in the ternary system have been identified, by comparison with previous studies in the literature. Some of the liquid crystalline phases have been quantitatively assessed using low angle X-ray diffraction. The latter measurements were also used to determine the unit cell dimensions in the various phases studied. With the quaternary system, particular attention was paid to the transparent region which consisted of an L₂ (inverse micellar) phase extending into another transparent region which has a blue “tinge” in some cases, namely the microemulsion (M) region. The amount of water solubilized in the L₂ (reverse micelle) or M + L₂ phase was calculated from the phase diagrams. With the ternary system the results showed a maximum in moles of water solubilized per mole total surfactant (NaDBS + hexanol) at a concentration of 0.3 mole surfactant, at an optimum molar ratio of n-hexanol to NaDBS of 4.5:1. This maximum was about twice with the quaternary system, when compared with that of the ternary system, indicating the importance of the role of xylene in solubilization of water by the surfactants. The present investigation has also shown that the extent of the microemulsion region is significantly reduced by increases of temperature when the NaDBS is lower than 15 wt%.     

Phase relationships in the Ce-Pd-In system

The interaction of cerium with palladium and indium was studied, and the 773°C isothermal section of the Ce-Pd-In phase diagram was plotted using physicochemical methods: X-ray powder diffraction, single-crystal X-ray diffraction, and electron probe microanalysis. The existence of 12 ternary intermetallic phases in the title system was confirmed, and three new phases were discovered. Crystal structure was determined for seven intermetallic compounds. A single-crystal X-ray diffraction study of CePdIn was carried out for the first time. A high-temperature phase CePdIn₄ was found, and its crystal structure was solved.     

Studies on the effect of milling on reduction of niobium pentaoxide with aluminium

The effect of milling on the aluminothermic reduction of niobium pentaoxide was investigated. Charges of Nb₂O₅ and Al (containing 5 % excess Al) were milled for different time (2, 5 and 10 h). XRD profile of milled samples indicated no phase formations during milling; only peak broadening were seen. Milled and unmilled charges were heated in a thermal analyser up to 1,400 °C. Products of milled charges showed formation of Nb, NbO and Al₂O₃; whereas unmilled hand mixed charges showed formations of Nb₃Al along with Nb and Al₂O₃. The tendency of milled charges towards Nb formations without the presence of aluminides was explained from the increase in surface area of charges caused by particle reduction.     

Analytical scheme for tracing sources of contamination during processing of Si3N4 in clean rooms

This work investigates the uptake of impurities during processing of Si₃N₄ and describes an analytical scheme for detecting sources of contamination. For this purpose a process as simple and short as possible was chosen, using commercial starting materials with a high standard of purity and reproducibility. The uptake of non-metallic and metallic contaminants was investigated by choosing elements which were specific for individual processing steps. This was difficult in the determination of metallic impurities in a powder consisting of Si₃N₄ with Y₂O₃/Al₂O₃ additives, because the powder mixture and the sources of contamination (milling balls, attritor disk, wall materials) were similar in composition and the available analytical methods were not precise enough to detect the small increase in concentration that occurred. Therefore pure Si₃N₄ powders were milled in order to get an indication of the kind and concentration of impurity introduced by the individual milling materials and steps. These elements can then be used as monitor elements to trace sources of contamination and to optimize processing parameters. Experience with the processing of Si₃N₄ with Y₂O₃/Al₂O₃ additives by cyclic milling, spray drying, burn-out and isopressing are reported. Contamination by carbon is unavoidable. Its concentration during the process is relatively high, as it is added in the form of processing aids (deflocculants, binders), but temporary, as it can be completely burned out. Oxygen is predominantly taken up during milling. Good deflocculation reduces the milling time and thus limits the uptake of oxygen. As a consequence of these findings the processing parameters could be optimized. Thus the uptake of metallic impurities, e.g. Fe could be limited to 10 g/g and the uptake of oxygen was found to be less than 0.2 wt%.     

Exploration of the lithium–aluminum–silver system. Crystal and electronic structure analysis of new phases Li6.98Al4.15Ag0.87 and LiAlAg2

The lithium–aluminum–silver ternary system has been investigated and two new phases Li_6.98Al_4.15Ag_0.87 and LiAlAg₂ were characterized using both powder and single crystal X-ray diffraction techniques. These phases crystallize in the cubic system, space group (a=6.344(1), 6.3124(5) Å), they, respectively, display the Pearson's cF12- and cF16-type structural arrangements. Compared to Li₂AlAg, the only phase in the system reported so far, the structure of Li_6.98Al_4.15Ag_0.87 is disordered owing to atomic substitutions and contains a full site vacancy. Band structures and densities of states of Li_6.98Al_4.15Ag_0.87, LiAlAg₂ and Li₂AlAg have been calculated by a first principle pseudopotential method using the plane-wave basis CASTEP package.     

Interfacial analyses for elucidation of the intergranular disintegration upon oxidation of intermetallic phases

Summary In a range of intermediate temperatures from 600 to 1000°C and in the presence of oxygen, numerous intermetallic phases show the phenomenon pest, an intergranular disintegration into small pieces. Thermo-gravimetric investigations and annealings in quartz ampoules have been performed, for annealing the oxygen pressures were established in the range 10^–30 to 10^–10 bar O₂ using metal/oxide mixtures. The specimens after annealing were fractured in UHV and the intergranular fracture faces were analyzed by AES. The Auger peaks of Al are markedly different for the intermetallic phase and for Al₂O₃, therefore it can be distinguished if oxygen has diffused into grain boundaries and not yet reacted or if Al₂O₃ was formed. The fracture face of NbAl₃ shows oxide precipitates near the surface and oxygen which had penetrated into the interior. Also in NiAl, Al₂O₃ was detected as well as oxygen penetrated into the grain boundaries. The pest obviously is a complex interplay of the processes: 1) penetration of oxygen through the outer oxide layer on the surface into grain boundaries of the intermetallic phase; 2) inward diffusion of oxygen along the grain boundaries into the interior of the intermetallic phase; 3) precipitation of Al₂O₃, beginning near the surface or (at low oxygen pressure) in the whole cross section, and cracking of the materials by the growth of Al₂O₃ into grain boundaries and cracks. Depending on the range of oxygen pressure different steps can be rate determining.     

Preparation and optical characterization of hydroxyapatite and ceramic systems with titanium and zirconium formed by dry high-energy mechanical alloying

Calcium phosphate-based bioceramics, mainly in the form of hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂—HAP, is the main mineral constituent of teeth and bones with excellent biocompatibility with hard and muscle tissues. These materials exhibit several problems of handling and fabrication, which can be overcome by mixing them with a suitable binder. The dry milling process of fabrication of HAP presents the advantage that melting is not necessary and the powder obtained is nanocrystalline. The high efficiency of the dry milling process opens a new way to produce commercial amount of nanocrystalline HAP and others bioceramic. In this work dry mechanical alloying has been used to produce nanocrystalline powders of HAP using three different experimental procedures (HAPA: Ca(H₂PO₄)₂ + Ca(OH)₂; HAPB: Ca(H₂PO₄)₂ + CaCO₃; and HAPC: CaHPO₄ + CaCO₃). HAP was obtained after 5, 10 and 15 h of milling in the reactions HAPA and HAPB, but it is necessary 15 h of milling in the reaction HAPC to obtain HAP. In order to improve the mechanical properties of HAP calcium phosphate ceramics, with titanium (CaP-Ti) and zirconium (CaP-Zr), have been prepared by dry ball milling using two different experimental procedures: CaP-Ti1: Ca(H₂PO₄)₂ + TiO₂; CaP-Ti2: CaHPO₄ + TiO₂; and CaP-Zr1: Ca(H₂PO₄)₂ + ZrO₂, CaP-Zr2: CaHPO₄ + ZrO₂. The calcium titanium phosphate phase, CaTi₄P₆O₂₄, was obtained in the reaction CaP-Ti1. In the reactions CaP-Ti2, CaP-Zr1 and CaP-Zr2, it was not observed the formation of any calcium phosphate phase even after 15 h of dry mechanical alloying. The milled HAP and the ceramics systems obtained were characterized by X-ray powder diffraction, infrared and Raman scattering spectroscopy.