Structure study of Bi4Ti3O12 produced via mechanochemically assisted synthesis

Nanosized bismuth titanate was prepared via high-energy ball milling process through mechanically assisted synthesis directly from their oxide mixture of Bi₂O₃ and TiO₂. Only Bi₄Ti₃O₁₂ phase was formed after 3 h of milling time. The excess of 3 wt% Bi₂O₃ added in the initial mixture before milling does not improve significantly the formation of Bi₄Ti₃O₁₂ phase comparing to stoichiometric mixture. The formed phase was amorphized independently of the milling time. The Rietveld analysis was adopted to determine the crystal structure symmetry, amount of amorphous phase, crystallite size and microstrains. With increasing the milling time from 3 to 12 h, the particle size of formed Bi₄Ti₃O₁₂ did not reduced significantly. That was confirmed by SEM and TEM analysis. The particle size was less than 20 nm and show strong tendency to agglomeration. The electron diffraction pattern indicates that Bi₄Ti₃O₁₂ crystalline powder is embedded in an amorphous phase of bismuth titanate. Phase composition and atom ratio in BIT ceramics were determined by X-ray diffraction and EDS analysis.     

Fast amorphization and crystallization in Al-Fe binary system by high-energy ball milling

Large amount of amorphous phase of Al-Fe binary system was obtained by MA of elemental powders using a high-energy ball mill at milling intensity of 150G (G is the gravitational acceleration). XRD, HRTEM and DSC were used to analyze the process of amorphization and crystallization. The time required achieving almost complete amorphous state is only 4.2 ks for Al-25 at.%Fe system and 3 ks for Al-30 at.%Fe system, respectively. The time of amorphous formation is very shorter than that of previous reports on Al-Fe binary system. Further milling causes rapid crystallization of the amorphous phase. By analysis of S(Q), the presence of a strong Al-Fe chemical short-range order in the amorphous matrix is suggested. Moreover, the superstructure of these Al-Fe clusters in the amorphous matrix is similar to the solid structure of Al₅Fe₂, and the clusters transform into the nucleus of Al₅Fe₂ intermetallic compound under the action of milling energy.     

Mechanical alloying and sintering of nanostructured tungsten carbide-reinforced copper composite and its characterization

Elemental powders of copper (Cu), tungsten (W) and graphite (C) were mechanically alloyed in a planetary ball mill with different milling durations (0–60 h), compacted and sintered in order to precipitate hard tungsten carbide particles into a copper matrix. Both powder and sintered composite were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and assessed for hardness and electrical conductivity to investigate the effects of milling time on formation of nanostructured Cu–WC composite and its properties. No carbide peak was detected in the powder mixtures after milling. Carbide WC and W₂C phases were precipitated only in the sintered composite. The formation of WC began with longer milling times, after W₂C formation. Prolonged milling time decreased the crystallite size as well as the internal strain of Cu. Hardness of the composite was enhanced but electrical conductivity reduced with increasing milling time.     

Hyperfine interactions and structural features of Fe–44Co–6Mo (wt.%) nanostructured powders

Nanocrystalline Fe–44Co–6Mo (wt.%) powders have been prepared by high-energy ball milling from elemental Fe, Co and Mo pure powders in a P7 planetary ball mill. The obtained powders were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Mössbauer spectrometry techniques. The influence of milling process and Mo substitution for Co in equiatomic FeCo have been examined in order to study structural evolution and formation mechanism of nanostructured Fe(CoMo) solid solution. XRD results show the formation of a BCC Fe(CoMo) solid solution (a = 0.2874 nm) where unmixed nanocrystalline Mo with a BCC structure is embedded. Disordered Fe(CoMo) solid solution is characterized by a broad hyperfine magnetic field distribution with two regions centered at B₁ = 35.0 T and B₂ = 30.7 T, respectively, attributed to disordered Fe(Co) solid solution and CoMo enriched environments. Prolonged milling and Mo addition cause the decrease of average hyperfine magnetic field while the average isomer shift remains nearly constant.     

Effect of Ni addition on formation of amorphous and nanocrystalline phase during mechanical alloying of Al-25 at.%Fe-(5,10) at.%Ni powders

Al-Fe-Ni ternary powder mixtures containing 25 at.%Fe-5 at.%Ni and 25 at.%Fe-10 at.%Ni were mechanically alloyed by a high-energy planetary ball mill. Structural evolution of these powders during milling was investigated by X-ray diffraction technique and transmission electron microscopy. Almost complete amorphous phase in Al₇₀Fe₂₅Ni₅ system is observed at the early milling stage. The amorphous phase transforms into metallic compound Al₅(Fe,Ni)₂ and then the compound changes to ordered Al(Fe,Ni) phase. The last milling products in Al₇₀Fe₂₅Ni₅ system are amorphous phase plus nanocrystalline of the disordered Al(Fe,Ni) phase changed from the ordered Al(Fe,Ni) phase. During milling of Al₆₅Fe₂₅Ni₁₀ system, α-Al and α-Fe solid solutions formed at the early stage change to the ordered Al(Fe,Ni) compound and at last the ordered phase changes to the disordered Al(Fe,Ni) phase. Ten percent of Ni addition promotes retardation of the formation of the amorphous phase.     

Mechanochemical synthesis of NaNbO3

The mechanochemical synthesis of NaNbO₃ is studied. It is shown that NaNbO₃ can be prepared by milling the constituents, i.e. Na₂CO₃ and Nb₂O₅ in the planetary mill. After 40 h of mechanochemical treatment NaNbO₃ nanoparticles in the range of 10-20 nm are obtained. Furthermore, the high-energy milling leads to the mechanochemically-triggered carbonate decomposition, which has been observed for a few cases in the open literature.     

Effects of milling on the thermal stability of synthetic hydromagnesite

Hydromagnesite is a basic magnesium carbonate that undergoes an endothermic decomposition with water and carbon dioxide release in the temperature range of 200-550 °C. Due to this thermal behaviour it has been studied as flame retardant filler for polymers in cable applications. For this purpose the particle size distribution should be optimized, as it is in most cases responsible for decrease in final composite mechanical properties. This work describes the variations found in the thermal behaviour of hydromagnesite associated with the process of particle size reduction. Air jet micronization was compared with mechanical milling. Thermogravimetry and differential scanning calorimetry were used to study thermal decomposition. FTIR spectroscopy and XRD analysis of the solid residue after heating were used to follow structural changes. Decomposition behaviour of synthetic hydromagnesite was shown to be dependent of the applied particle size reduction process. A remarkable increase in the decomposition rate was observed for the milled sample, which was attributed to the introduction of defects in the crystalline structure during the mechanical milling. Therefore, it was concluded that the mechanical milling process may affect the thermal decomposition of hydromagnesite and therefore its characteristics as flame retardant.     

Microstructural characterization of amorphous and nanocrystalline boron nitride prepared by high-energy ball milling

Microstructural parameters like crystallite size, lattice strain, stacking faults and dislocation density were evaluated from the X-ray diffraction data of boron nitride (BN) powder milled in a high-energy vibrational ball mill for different length of time (2-120 h), using different model based approaches like Scherrer analysis, integral breadth method, Williamson-Hall technique and modified Rietveld technique. From diffraction line-broadening analysis of the successive patterns of BN with varying milling time, it was observed that overall line broadening was an operative cause for crystallite size reduction at lower milling time (∼5 h), whereas lattice strains were the prominent cause of line broadening at higher milling times (>19 h). For intermediate milling time (7-19 h), both crystallite size and lattice strain influence the profile broadening although their relative contribution vary with milling time. Microstructural information showed that after long time milling (>19 h) BN becomes mixture of nanocrystalline and amorphous BN. The accumulations of defects cause this crystalline to amorphous transition. It has been found that twin fault (β′) and deformation fault (α) significantly contributed to BN powder as synthesized by a high-energy ball-milling technique. Present study consider only three ball-milled (0, 2 and 3 h) BN powder for faults calculation because fault effected reflections (1 0 1, 1 0 2, 1 0 3) disappear with milling time (>3 h). The morphology and particle size of the BN powders before and after ball milling were also observed in a field emission scanning electron microscope (FESEM).     

Structural and spectroscopic studies of mechanochemically activated nanosized apatite from Syria

Phosphorite sample from Syria is subjected to mechanochemical treatment in planetary mill equipped with 20 mm steel milling bodies. A complex of chemical and physical methods was applied, namely chemical analysis, powder XRD, IR spectroscopy, and SEM in order to prove changes in the structure of the minerals in the modified samples. These changes are manifested mainly in transformations of a considerable part of apatite into a nanosized phase, breakage of the coordination complex CaO₆F, and localization of OH⁻ groups in the position of F⁻ and CO₃²⁻ in the position of PO₄³⁻ of the apatite structure. As a result of these transformations the assimilable content of in the activated samples is increased and this gives opportunity to suppose that the activated phosphorite could be used as a phosphorous fertilizer.     

Structural characterization of a mechanically milled carbon nanotube/aluminum mixture

The structural evolution of carbon nanotubes (CNTs) during mechanical milling was investigated using SEM, TEM, XRD, XPS and Raman spectroscopy. The study showed that milling of the CNTs alone introduces defects but preserves the tubular structure. When milling the CNTs with aluminum (Al) powder in order to produce a composite, Raman spectroscopy has shown that most of the nanotubes are destroyed. During sintering of the CNT/Al milled mixture, the carbon atoms available from the destruction of the nanotubes react with the Al to form aluminum carbide (Al₄C₃). The effect of milling on the Al matrix was also studied.     

Lead zinc niobate (PZN)-barium titanate (BT) ceramics from mechanochemically synthesized powders

Nanosized PZN-BT powders were synthesized directly from their constituent component oxide mixture via a high-energy ball milling process. XRD patterns showed that perovskite phase could be formed after milling for 5 h, while single phase perovskite was achieved when the milling was prolonged for 12 h. Further increase in milling time (20 h) led to the formation of pyrochlore phase. PZN-BT ceramics were obtained by sintering the milled powders at temperatures from 1000 to 1100°C for 1 h. The 1100°C-sintered PZN-BT samples derived from the 12 h milled powders have a density of ∼99% of the theoretical density with an average grain size of about 4 μm, a dielectric constant of 2300 and a dielectric loss of 0.03, being in good agreement with the reported results for PZN-BT prepared by the conventional solid-state reaction process.     

Solid-state reactions to synthesize nanostructured lead selenide semiconductor powders by high-energy milling

Both solid-solid and gas-solid reactions have been traced during high-energy milling of Se and PbO powders under vial (P, T) conditions in order to synthesize the PbSe phase. Chemical and thermodynamic arguments are postulated to discern the high-energy milling mechanism to transform PbO-Se micropowders onto PbSe-nanocrystals. A set of reactions were evaluated at around room temperature. Therefore an experimental campaign was designed to test the nature of reactions in the PbO-Se system during high-energy milling.     

Microstructure and martensitic transformation in Si-coated TiNi powders prepared by ball milling

Si was coated on the surface of Ti–49Ni (at%) alloy powders by ball milling in order to improve the electrochemical properties of the Si electrodes of secondary Li ion batteries and then the microstructure and martensitic transformation behavior were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Ti–Ni powders coated with Si were fabricated successfully by ball milling. As-milled powders consisted of highly deformed Ti–Ni powders with the B2 phase and amorphous Si layers. The thickness of the Si layer coated on the surface of the Ti–Ni powders increased from 3–5 μm to 10–15 μm by extending the milling time from 3 h to 48 h. However, severe contamination from the grinding media, ZrO₂ occurred when the ball milling time was as long as 48 h. By heating as-milled powders to various temperatures in the range of 673–873 K, the highly deformed Ti–Ni powders were recovered and Ti₄Ni₄Si₇ was formed. Two-stage B2–R–B19′ transformation occurred when as-milled Si-coated Ti–49Ni alloy powders were heated to temperatures below 873 K, above this temperature one-stage B2–B19′ transformation occurred.     

Fabrication of Sr- and Co-doped lanthanum chromite interconnectors for SOFC

Many studies have been performed dealing with the processing conditions of electrodes and electrolytes in solid oxide fuel cells (SOFCs). However, the processing of the interconnector material has received less attention. Lanthanum chromite (LaCrO₃) is probably the most studied material as SOFCs interconnector. This paper deals with the rheology and casting behaviour of lanthanum chromite based materials to produce interconnectors for SOFCs. A powder with the composition La_0.80Sr_0.20Cr_0.92Co_0.08O₃ was obtained by combustion synthesis. Aqueous suspensions were prepared to solids loading ranging from 8 to 17.5 vol.%, using ammonium polyacrylate (PAA) as dispersant and tetramethylammonium hydroxide (TMAH) to assure a basic pH and providing stabilization. The influence of the additives concentrations and suspension ball milling time were studied. Suspensions prepared with 24 h ball milling, with 3 wt.% and 1 wt.% of PAA and TMAH, respectively, yielded the best conditions for successful slip casting. Sintering of the green discs was performed in air at 1600 °C for 4 h leading to relatively dense materials.     

Occurrence and propagation of delamination during the machining of carbon fibre reinforced plastics (CFRPs) - An experimental study

The machining of carbon fibre reinforced plastics (CFRPs) is often accompanied by delamination of the top layers of the machined edges. Such damage necessitates time-consuming and costly post-machining and in some cases leads to rejection of components. The work described in this paper systematically investigates the occurrence of delamination of the top layers during the machining of CFRP tape, with the focus being on the process of contour milling. The occurrence and propagation of delamination were studied by milling slots in unidirectional CFRP specimens having different fibre orientations and mainly analysing the slot tip. This allowed the key mechanisms to be clarified. The results show that delamination is highly dependent on the fibre orientation and the tool sharpness. The experiments allow derivation of a novel system for describing the occurrence and propagation of delamination during milling. Furthermore, the principles also apply for drilling. The results allow customisation of the machining procedure to reduce and in some cases totally avoid delamination, leading to a significant increase in the quality of components.     

Thermal and mechanically activated decomposition of LiAlH4

Thermal stability of as-received LiAlH₄ and milled LiAlH₄ has been investigated. The thermal decomposition mechanism of as-received LiAlH₄ depends on the temperature-time history. Apparent activation energies and enthalpies of the reactions have been obtained. During milling treatment, the high temperature and pressures locally induced by shocks lead to LiAlH₄ mechanically decomposition. The decomposition temperatures of LiAlH₄ and Li₃AlH₆ are both reduced by ∼60 °C due to particle size reduction produced by mechanical milling. Besides, the activation energy of the decomposition reaction of LiAlH₄ decreases as compared to as-received LiAlH₄. Moreover, a layer of oxide (∼5 nm) at the surface of the milled alanate Li₃AlH₆ is observed. This layer could have a drastic influence on decomposition H-kinetics.     

SbSI nanocrystal formation in As–Sb–S–I glass under laser beam

As–Sb–S–I glasses are obtained by co-melting of As₂S₃ and SbSI in a broad compositional interval. Their structure and composition are confirmed by the studies of scanning electron microscopy, energy dispersive X-ray spectroscopy, and micro-Raman scattering. Laser-induced crystallization of SbSI crystallites from the glass matrix is observed in the course of the micro-Raman measurement as a result of local laser beam heating.     

Formation of TiB2 by volume combustion and mechanochemical process

Titanium diboride was produced both by volume combustion synthesis (VCS) and by mechanochemical synthesis (MCP) through the reaction of TiO₂, B₂O₃ and Mg. VCS products, expected to be composed of TiB₂ and MgO, were found to contain also side products such as Mg₂TiO₄, Mg₃B₂O₆, MgB₂ and TiN. HCl leaching was applied to the reaction products with the objective of removing MgO and the side products. Formation of TiN could be prevented by conducting the VCS under an argon atmosphere. Mg₂TiO₄ did not form when 40% excess Mg was used. Wet ball milling of the products before leaching was found to be effective in removal of Mg₃B₂O₆ during leaching in 1 M HCl. When stoichiometric starting mixtures were used, all of the side products could be removed after wet ball milling in ethanol and leaching in 5 M HCl when pure TiB₂ was obtained with a molar yield of 30%. Pure TiB₂ could also be obtained at a molar yield of 45.6% by hot leaching of VCS products at 75 °C in 5 M HCl, omitting the wet ball milling step. By MCP, products containing only TiB₂ and MgO were obtained after 15 h of ball milling. Leaching in 0.5 M HCl for 3 min was found to be sufficient for elimination of MgO. Molar yield of TiB₂ was 89.6%, much higher than that of VCS. According to scanning electron microscope analyses, the TiB₂ produced had average grain size of 0.27 ± 0.08 μm.     

Neutron diffraction study of the oxychloride layered perovskite, (CuCl)LaNb2O7

The structure of the oxychloride layered perovskite, (CuCl)LaNb₂O₇, has been examined by neutron diffraction. Rietveld refinement of room temperature neutron TOF data, while consistent with the previous X-ray study, allows for an improved modeling of the structure. The structure consists of double perovskite layers (LaNb₂O₇) separated by copper chloride layers. The copper is octahedrally coordinated, bridging between apical oxygens from the perovskite layer and surrounded by four chlorines in the CuCl plane; this gives rise to edge-sharing CuO₂Cl₄ octahedra. The chlorines within the CuCl plane were found to move off the ideal (0, 0, 1/2) position to a more general position, (x, 0, 1/2). This disorder leads to a combination of four short and two long distances, common to the Jahn-Teller distorted environments for d⁹ copper. Structural details are discussed with respect to their influence on the chlorine disorder.     

Effect of mechanical activation on the structure and ferroelectric property of Na0.5K0.5NbO3

Mechanical activation synthesis of Na_0.5K_0.5NbO₃ (NKN) was studied in order to explore the effect of mechanochemical interaction on the crystal structure and microstructure of NKN powder and ceramic. A single phase, nanocrystalline perovskite NKN powder has been derived from a mixture of oxide/carbonates via a mechanical activation route with heating at an elevated temperature. With the increase in milling time, the distortion of orthorhombic structure for NKN was weakened and the cell volume of NKN powder slightly decreased. The relative density and remnant polarization of NKN ceramics were improved, and the grain became uniform and smaller for prolonged milling NKN. The developed method is well suited for the production of NKN nanocrystallite powders and refined grain NKN ceramics.