Effect of Al2O3 particle size on preparation and properties of ZTA ceramics formed by gelcasting

Zirconia-toughened alumina (ZTA) ceramics were prepared using three different kinds of Al₂O₃ powders (marked PW-A average particle size: 7.53 μm, marked PW-B average particle size: 1.76 μm, marked PW-C average particle size: 0.61 μm) by gelcasting. Effect of Al₂O₃ particle size on zeta potential, dispersant dosage and solid volume fractions of ZTA suspensions as well as the mechanical properties of ZTA green bodies and ceramics were investigated. The optimum dosages of dispersant for ZTA suspensions prepared by PW-A, PW-B and PW-C are 0.4 wt%, 0.5 wt% and 0.7 wt%, respectively. The highest solid volume fractions of ZTA suspensions can reach 62 vol% (SP-A), 60 vol% (SP-B) and 52 vol% (SP-C), respectively. The green bodies show a bending strength as high as 20 MPa, which can meet the requirement of machining. The Al₂O₃ powder with fine particle size is beneficial to the improvement of mechanical properties. The ZTA ceramics prepared by PW-B Al₂O₃ powder show the highest bending strength (680 MPa) and toughness (7.49 MPa m^1/2).     

Direct synthesis of TiO2 nanoparticles by using the solid-state precursor TiH2 powder in a thermal plasma reactor

We attempt the direct synthesis of TiO₂ by using the solid state precursor TiH₂ powder with oxygen in a thermal plasma reactor. Nanocrystalline titanium dioxide powder has been synthesized by using thermal plasma synthesis in a non-transferred arc thermal plasma reactor. The thermal plasma-synthesized powder product consists of nano-sized particles of anatase and rutile phases of titanium dioxide. Particle compositions were observed on collecting powder from different positions of the reactor and varying the amount of flow rate of reactive gases (O₂). The characteristics of the powder such as particle size, size distribution and phases were analyzed using various techniques such as XRD, SEM, TEM, XPS, EDS and particle size analyzer. UV–visible reflection spectroscopy of the plasma-synthesized TiO₂ powders showed the absorbance in the visible region leading to effective photocatalytic activity, which is clearly confirmed from the XPS analysis. XPS analysis reveals the presence of –OH bonds on the surface of nanoparticles, which is the significant evidence of better quality of powders in comparison to other methods. Also, we have investigated the phase transformation phenomenon of anatase to rutile. At 1000 °C, complete transformation of the anatase to rutile occurs. Powders prepared in this procedure are white in colour and their diameter varies from 10 nm to 150 nm. Average particle size distributes in the range of 20–50 nm. The unique property about the plasma-synthesized powders is high resistance to heat treatment, with enhanced photocatalytic activity.     

Low pressure plasma-sprayed Al2O3 and Al2O3/SiC nanocomposite coatings from different feedstock powders

This paper describes a preliminary investigation of a nanocomposite ceramic coating system, based on Al₂O₃/SiC. Feedstock Al₂O₃/SiC nanocomposite powder has been manufactured using sol-gel and conventional freeze-drying processing techniques and then low pressure plasma sprayed onto stainless steel substrates using a CoNiCrAlY bond coat. Coatings of a commercial Al₂O₃ powder have also been manufactured as a reference for phase transformations and microstructure. The different powder morphology and size distribution resulting from the different processing techniques and their effect on coating microstructure has been investigated. Phase analysis of the feedstock powders and of the as-sprayed coatings by X-ray diffractometry (XRD) and nuclear magnetic resonance (NMR) showed that the nano-scale SiC particles were retained in the composite coatings and that equilibrium α-Al₂O₃ transformed to metastable γ- and δ-Al₂O₃ phases during plasma spraying. Other minority phases in the sol-gel Al₂O₃/SiC nanocomposite powder such as silica and aluminosilicate were removed by the plasma-spraying process. Microstructure characterisation by scanning electron microscopy (SEM) of the as-sprayed surface, polished cross-section, and fracture surface of the coatings showed evidence of partially molten and unmolten particles incorporated into the predominantly lamella microstructure of the coating. The extent of feedstock particle melting and consequently the character of the coating microstructure were different in each coating because of the effects of particle morphology and particle size distribution on particle melting in the plasma.     

Fabrication and characterization of ceramic coatings with alumina–silica sol-incorporated α-alumina powder coated on woven quartz fiber fabrics

The formation and properties of dense silica/α-alumina coatings derived from alumina–silica sol-incorporated coarse alumina powder (median particle size d₅₀=0.509 μm) using Al(NO₃)₃·9H₂O and tetraethyl orthosilicate as precursors have been investigated. Phase compositions and thermal evolution analysis of the coatings as well as interface microstructure between the coatings and the matrix were characterized by means of XRD, XRF, DSC-TG, TEM, SEM supplemented with EDS, and tensile strength tester. Silica/α-alumina coatings combined with quartz fiber matrix are homogeneously integrated with distinct inter-diffusion near the interface. The heat conduction mechanism of the quartz fiber matrix was influenced by the dense silica/α-alumina ceramic coatings, and the prepared coatings provide practical thermodynamic stability and desired mechanical strength for the woven quartz fiber fabrics.     

Co-precipitated ZnAl2O4 spinel precursor as potential sintering aid for pure alumina system

Ultra-fine ZnAl₂O₄ spinel hydrogel precursor synthesized from mixed salt solutions of Zn²⁺ and Al³⁺ ions using ammonium hydroxide–hexamethylenetetramine as basic media for co-precipitation was used as bonding material and sintering aid for pure alumina system. The hydrogel powder exhibited some well-defined ZnAl₂O₄ spinel phases at 800 °C. Alumina compacts were fabricated by incorporating small proportions of the precursor in alumina powder and firing at different temperatures (1350–1500 °C). The degree of densification was studied by measurement of fired shrinkage, apparent porosity, bulk density and cold crushing strength. Phase compositions and microstructural features of sintered samples were evaluated by XRD and SEM respectively. Addition of 0.2% hydrogel powder to alumina exhibited remarkable influence on development of high mechanical strength. The in situ formed ZnAl₂O₄ spinel dopant acted as a grain growth inhibitor in the alumina system.     

Electrodeposition and electrochemical characterisation of thick and thin coatings of Sb and Sb/Sb2O3 particles for Li-ion battery anodes

The possibilities to electrodeposit thick coatings composed of nanoparticles of Sb and Sb₂O₃ for use as high-capacity anode materials in Li-ion batteries have been investigated. It is demonstrated that the stability of the coatings depends on their Sb₂O₃ concentrations as well as microstructure. The electrodeposition reactions in electrolytes with different pH and buffer capacities were studied using chronopotentiometry and electrochemical quartz crystal microbalance measurements. The obtained deposits, which were characterised with XRD and SEM, were also tested as anode materials in Li-ion batteries. The influence of the pH and buffer capacity of the deposition solution on the composition and particle size of the deposits were studied and it is concluded that depositions from a poorly buffered solution of antimony-tartrate give rise to good anode materials due to the inclusion of precipitated Sb₂O₃ nanoparticles in the Sb coatings. Depositions under conditions yielding pure Sb coatings give rise to deposits composed of large crystalline particles with poor anode stabilities. The presence of a plateau at about 0.8 V versus Li⁺/Li due to SEI forming reactions and the origin of another plateau at about 0.4 V versus Li⁺/Li seen during the lithiation of thin Sb coatings are also discussed. It is demonstrated that the 0.4 V plateau is present for Sb coatings for which the (0 1 2) peak is the main peak in the XRD diffractogram.     

The effect of argon shielding gas at plasma spray process on the structure and properties of MoSi2 coating

In this work, MoSi₂ powder was agglomerated for using it in atmospheric plasma spray (APS) process. MoSi₂ coatings were manufactured by APS and argon-shrouded plasma spray (ASPS) processes. Phase composition and structural properties of coatings were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Also, the mechanical properties of coatings (such as microhardness and adhesion strength) were evaluated. Using the ASPS method, the coatings were found to have a low porosity and highly homogeneous structure. In addition, the argon protection gas was used to prevent the oxidation of the powder. Finally, the ASPS method revealed better microhardness and bending strength results for the corresponding tests.     

Preliminary study on suspension plasma sprayed ZrO2 + 8 wt.% Y2O3 coatings

ZrO₂ + 8 wt.% Y₂O₃ powder of a mean diameter d_VS = 38 μm was milled to obtain fine particles having mean size of d_VS = 1 μm. The fine powder was used to formulate a suspension with water, ethanol and their mixtures. The zeta potential of obtained suspensions was measured and found out to be in the range from −22 to −2 mV depending on suspension formulation. The suspension was injected through a nozzle into plasma jet and sprayed onto stainless steel substrates. The plasma spray experimental parameters included two variables: (i) spray distance varying from 40 to 60 mm and (ii) torch linear speed varying from 300 to 500 mm/s. The microstructure of obtained coatings was characterized with scanning electron microscope (SEM) and X-ray diffraction (XRD). The coatings had porosity in the range from 10% to 17% and the main crystal phase was tetragonal zirconium oxide. The scratch test enabled to find the critical load in the range of 9-11 N. Finally, thermal diffusivity of the samples at room temperature, determined by thermographic method, was in the range from 2.95 × 10⁻⁷ to 3.79 × 10⁻⁷ m²/s what corresponds to thermal conductivities of 0.69 W/(mK) and 0.97 W/(mK) respectively.     

Characterization of high tap density Li[Ni1/3Co1/3Mn1/3]O2 cathode material synthesized via hydroxide co-precipitation

Li[Ni_1/3Co_1/3Mn_1/3]O₂ cathode material for lithium ion batteries was prepared by mixing metal hydroxide, (Ni_1/3Co_1/3Mn_1/3)(OH)₂, with 6% excess LiOH followed by calcinations. The (Ni_1/3Co_1/3Mn_1/3)(OH)₂ with secondary particle of about 12 μm was prepared by hydroxide co-precipitation. The tap density of the obtained Li[Ni_1/3Co_1/3Mn_1/3]O₂ powder was 2.56 ± 0.21 g cm⁻³. The powder was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), particle size distribution (PSD) and galvanostatic charge-discharge cycling. The XRD pattern of Li[Ni_1/3Co_1/3Mn_1/3]O₂ revealed a well ordered hexagonal layered structure with low cation mixing. Secondary particles with size of 13-14 μm and primary particles with size of about 1 μm can be identified from the SEM observations. In the voltage range of 2.8-4.3 V, the initial discharge capacity of the Li[Ni_1/3Co_1/3Mn_1/3]O₂ electrode was 166.6 mAh g⁻¹, and 96.5% of the initial capacity was retained after 50 charge-discharge cycling.     

Dried particle plasma spray in-flight synthesis of spinel coatings

Powder particle diameters currently used for spraying are generally between 5 and 100 μm with a preferred size range around 40–60 μm. Future trends in plasma spraying involve the use of fine or ultrafine powders and the reduction of the number of steps between raw material preparation and coating. The use of non-sintered spray dried ceramic aggregates as feedstock material for plasma spraying has accordingly been investigated. Al₂O₃ based coatings were prepared by this route of dried particle plasma spray (DPPS). The microstructure and crystallographic phases of these deposits were characterised using scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). Given the intimate mixing of the starting oxides, reactions occur during spraying leading to the formation of spinel (MgAl₂O₄ and/or ZnAl₂O₄) and zinc aluminum oxide (Zn₄Al₂₂O₃₇). The layered structure of the deposit is characteristic of conventional plasma-sprayed coatings but the features are smaller in size. Depending on the operating conditions (plasma characteristics and powder injection), two different melting modes of the particles were identified; the first leads to individual well-melted droplets that splash regularly even if generating some fingers and the second leads to aggregates that are well-melted on their outer parts and strengthened in their cores.     

Surface Modification of Graphite Particles Coated by Atomic Layer Deposition and Advances in Ceramic Composites

Graphite particles have been coated with Al₂O₃ via atomic layer deposition. Alumina content was measured via inductively coupled plasma spectrometry (ICP), LECO combustion analysis, and thermogravimetric analysis (TGA). While alumina was present, adherence was limited, and nonconformal films were deposited on the graphite particles. Coatings produced changes in particle interactions and dispersability. These changes were observed via sedimentation rates of particle suspensions in water, Zeta potential values, and particle size distributions. Alumina‐Graphite composites were sintered using coated and uncoated particles. Differences in bulk thermal properties are ascribed to enhanced dispersability of the coated particles in presintered powder mixtures. EDS mapping of the sintered composites confirms the enhanced dispersion of the coated graphite particles. Particle coating through atomic layer deposition provides a means to improve particle dispersion with low material loadings. It has been shown that changes in particle interaction characteristics can be achieved even without uniform and conformal coatings. These particle interaction changes can result in sintered composites with enhanced thermal properties.     

Reaction mechanism and size control of CaB6 micron powder synthesized by the boroncarbide method

Reaction synthesis mechanism of Calcium hexaboride (CaB₆) powder was investigated by using CaCO₃-B₄C-C system. Micron-scale CaCO₃ and B₄C powders were used as main raw materials. The synthesized powder was determined by X-ray diffraction, showing no left reactants if enough CaCO₃ was added to compensate the evaporation of calcium atoms at high temperature. The powder morphology was observed through SEM. The synthesized CaB₆ powder formed hard agglomerates which consisted of cubic CaB₆ crystallites when the reaction completely finished. Reaction process was illustrated indicating it was a solid-state reaction occurred from B₄C surface to the centre. The dry high-energy ball milling was used to investigate the influence of ball-milling time on the shape and size of powder particles. The particle granularity was measured by laser size analysis method. It is obvious that the particles were refined greatly after ball milling for 8 h. However, the CaB₆ powder could not been refined markedly after 16 h. Finally, optimized parameters for size controlling were given in this paper.     

Densification ability of combustion-derived Al2O3 powders

Nanocrystalline Al₂O₃ powders containing different amounts of MgO (0.1–5.0 mol%) or added boehmite (AlOOH) have been synthesized by combustion synthesis from aluminium nitrate and magnesium nitrate, using urea or sucrose as fuels. The as synthesized alumina powders were deagglomerated, compacted by dry pressing and sintered at 1625 °C for 2 h. For comparison purposes, a commercial high purity α-Al₂O₃ powder (ACC) was also processed following the same route. The sintered materials were characterized for bulk density (BD), apparent porosity (AP), and water absorption (WA) capacity, microstructure using SEM, and XRD phase composition. In comparison to boehmite, the MgO had a considerable effect on the densification behaviour of combustion-synthesized powder.     

Synthesis of mono-dispersed spherical Nd:Y2O3 powder for transparent ceramics

Transparent Nd:Y₂O₃ ceramic was obtained by sintering mono-sized spherical powder. The powder was prepared by homogeneous precipitation method in aqueous media using urea to regulate the pH. The structure and morphology of the powder were investigated by TG-DTA, XRD, SEM and IR spectrum. The effect of aging temperature, time, and the concentration of urea, [Y³⁺], and [Nd³⁺] were investigated. Results showed that the obtained precursor was R₂(OH)CO₃·H₂O (R = Y, Nd), and the least size of mono-sized spherical yttria particles was 72 nm by a microwave oven method after calcinations at 850 °C for 4 h. After dry press and CIP, the particles accumulated closely, and no defects can be detected in the green body.     

Composite Ni/Al2O3 coatings and their corrosion resistance

Composite coatings Ni/Al₂O₃ were electrochemically deposited from a Watts bath. Al₂O₃ powder with particle diameter below 1 μm was codeposited with the metal. The obtained Ni/Al₂O₃ coatings contained 5-6% by weight of corundum. The structure of the coatings was examined by scanning electron microscopy (SEM). It has been found that the codeposition of Al₂O₃ particles with nickel disturbs the nickel coating's regular surface structure, increasing its microcrystallinity and surface roughness. DC and AC electrochemical tests were carried out on such coatings in a 0.5 M solution of Na₂SO₄ in order to evaluate their corrosion resistance. The potentiodynamic tests showed that the corrosion resistance of composite coating Ni/Al₂O₃ is better than that of the standard nickel coating. After 14 days of exposure the nickel coating corrodes three times faster than the Ni/Al₂O₃ coating. The electrochemical behaviour of the coatings in the corrosive solution was investigated by electrochemical impedance spectroscopy (EIS). An equivalent circuit diagram consisting of two RC electric circuits: one for electrode, nickel corrosion processes and the other for processes causing coating surface blockage, were adopted for the analysis of the impedance spectra. The changes in the charge transfer resistance determined from the impedance measurements are comparable with the changes in corrosion resistance determined from potentiodynamic measurements.     

A comparison of preparation method on the electrochemical performance of cathode material Li[Li0.2Mn0.54Ni0.13Co0.13]O2 for lithium ion battery

Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ as a cathode material for Li-ion battery has been successfully prepared by co-precipitation (CP), sol–gel (SG) and sucrose combustion (SC) methods. The prepared materials were characterized by XRD, SEM, BET and electrochemical measurements. The XRD result shows that the Li[Li_0.2Mn_0.54Ni_0.13Co_0.13]O₂ materials prepared by different methods all form a pure phase with good crystallinity. SEM images and BET data present that the SC-material exhibited the smallest particle size (ca. 0.1 μm) and the highest surface area (7.4635 m² g⁻¹). The tap density of SC-material is lower than that of CP- and SG-materials. The result of rate performance tests indicates that the SC-material showed the best rate capability with the highest discharge capacity of 178 mAh g⁻¹ at 5.0 C, followed by SG-material and then CP-material. However, the cycling stability of SC-material tested at 0.1 and 0.5 C is relatively poor as compared to that of SG-material and CP-material. The result of EIS measurements reveals that large surface area and small particle size of the SC-electrode result in more SEI layer formation because of the increased side reactions with the electrolyte during cycling, which deteriorates the electrode/electrolyte interface and thus leads to the faster capacity fading of the SC-material.     

A novel micro-spherical CoSn2/Sn alloy composite as high capacity anode materials for Li-ion rechargeable batteries

Micro-scaled spherical CoSn₂/Sn alloy powders synthesized from oxides of Sn and Co via carbothermal reduction at 800 °C were examined for use as anode materials in Li-ion battery. The phase composition and particle morphology of the CoSn₂/Sn alloy composite powders were investigated by XRD, SEM and TEM. The prepared CoSn₂/Sn alloy composite electrode exhibits a low initial irreversible capacity of ca. 140 mAh g⁻¹, a high specific capacity of ca. 600 mAh g⁻¹ at constant current density of 50 mA g⁻¹, and a good rate capability. The stable discharge capacities of 500-515 mAh g⁻¹ and the columbic efficiencies of 95.8-98.1% were obtained at current density of 500 mA g⁻¹. The relatively large particle size of CoSn₂/Sn alloy composite powder is apparently favorable for the lowering of initial capacity loss of electrode, while the loose particle structural characteristic and the Co addition in Sn matrix should be responsible for the improvement of cycling stability of CoSn₂/Sn electrode.     

One-stage synthesis of (Y0,5Bi0,5)3(Fe0,5Ga0,5)5O12 garnet using the organometallic gel auto-combustion approach

Nanosized powder of (Y_0.5Bi_0.5)₃(Fe_0.5Ga_0.5)₅O₁₂ ferrite garnet with a narrow particle size distribution was prepared via the gel auto-combustion approach based on polyvinyl alcohol (PVA) and metal nitrates. The peculiarities of garnet phase formation were studied by infrared spectroscopy (IR spectroscopy), simultaneous thermal analysis (TGA/DSC), C, H and N-elemental analysis, and powder X-ray diffraction (XRD). The final powdery product was characterized using powder XRD, IR spectroscopy, X-ray fluorescence (XRF) and scanning electron microscopy (SEM). We established that the use of PVA provides one-stage crystallization of a single-phase ferrite-garnet due to the formation of a thermally stable PVA-salt complex. Crystallization of the (Y_0.5Bi_0.5)₃(Fe_0.5Ga_0.5)₅O₁₂ powder occurred at 700?°C, without intermediate and impurity phases.     

Large scale synthesis of non-transformable tetragonal Sc2O3, Y2O3 doped ZrO2 nanopowders via the citric acid based gel method to obtain plasma sprayed coating

Non-transformable tetragonal scandia, yttria doped zirconia (SYDZ) nanopowders were prepared in large scale by the citric acid (CA) based gel method. The effect of ethylene glycol monobutyl ether (EGM):CA ratios and pH on the structure, morphology and SYDZ particle size was investigated. The microstructure of SYDZ was characterized by XRD, Raman scattering, TG–DTA, SEM, TEM, and FTIR analyses. The SYDZ nanopowders, synthesized with 1Zr⁴⁺:4EGM:4CA mole ratio in acidic medium (pH ∼1) at 700 °C, had an average diameter of 15±2 nm. Finally, air plasma spray (APS) coatings were produced from nanostructured SYDZ agglomerated powders.     

Fabrication and characterization of spray dried Al2O3-ZrO2-Y2O3 powders treated by calcining and plasma

In this paper, the Al₂O₃-20 wt.%ZrO₂ (8 wt.%Y₂O₃) feedstocks were fabricated and treated by spray drying, calcination, and plasma treatment technology. The morphology of feedstocks was characterized by scanning electron microscope (SEM). The phase structure and grain size were analyzed by X-Ray diffraction (XRD). The flowability and density were measured by Hall Flowmeter and density instrument, respectively. The sphericity and flowability of feedstocks treated by plasma flame increased greatly compared with that of the feedstocks without plasma treatment, and the particle surfaces were very smooth. The optimum flowability was obtained when the critical plasma spray parameter (CPSP) was 363. The compactness also increased greatly with the increment of CPSP, and the maximum value of compactness was got with CPSP of 325. Calcination can make the grain grow and plasma treatment can lead to the decrement of grain size. The phase structure of Al₂O₃ did not change, which was α-Al₂O₃ in the composites. The phase structure of ZrO₂ (8 wt.%Y₂O₃) changed from t-phase to c-phase which was affected greatly by plasma treatment.