Effect of sol-gel and solid-state synthesis techniques on structural,morphological and thermoelectric performance of Ca3Co4O9

Two methods of obtaining calcium cobalt oxide (Ca₃Co₄O₉) thermoelectric materials were studied: (I) solid state synthesis (SS) followed by high-energy ball-milling (HEBM) and (II) modified sol-gel method (SG). The obtained powders were heated at 900?°C for 12?h. They were subsequently compacted using spark plasma sintering (SPS). The crystal structure and morphology were characterized by X-ray diffraction and scanning electron microscopy. The calculated average crystallite sizes, before and after SPS, alter from 43 to 61?nm and from 38 to 41?nm, for the SS and SG powders, respectively. XRD studies of the obtained powders revealed presence of Co₃O₄ secondary phases, which turned into metallic cobalt after SPS. Thermoelectric measurements of sintered samples were performed in the range, 50–500?°C. The maximum Seebeck coefficient is 120?µV/K and 110?µV/K, for the SS and SG synthesized samples, respectively. The highest ZT was found to be 0.02 and 0.04 at 500?°C for the SS and SG samples, respectively.     

Low-temperature pressureless sintering of Al2O3-SiC-Ni nanocermets in air environment

This paper introduces a simplified method for low-temperature pressureless sintering of Al₂O₃-Ni-SiC nanocermets in air environment. In this method, a thin and continuous Ni shell was coated on the surface of Al₂O₃ particles using electroless deposition method. The composite powders were subsequently compressed to prepare bulk specimens. By preventing the ceramic particles from direct contact during the densification of green specimens, sintering temperature of cermet materials was reduced from that of Al₂O₃ (>?1400?°C) to the range of Ni solid-phase sintering temperature. Furthermore, dissolution of a low amount of phosphorus in the composition of Ni coatings caused the further decrease of the sintering temperature to 800?°C. At such low temperatures, pressureless sintering of the cermets in the air environment was successfully performed instead of the common hot pressing process in a reducing atmosphere. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD) characterizations indicated that the microstructure of such sintered samples consists of a continuous Ni network surrounding Al₂O₃ grains, without any structural defects or Ni oxidation. Furthermore, mechanical properties of the cermet materials were improved through reinforcement of the continuous Ni network by different amounts of SiC nanoparticles. The results showed that Al₂O₃-Ni-5?wt% SiC nanocermets sintered at 800?°C obtain the highest compressive strength of 242.5?MPa, hardness of 56.8 RA, and the lowest wear weight loss of 0.04?mg/m.     

High-temperature thermoelectric properties of Sm3+-doped Ca3Co4O9+δ fabricated by spark plasma sintering

Ca_3-xSm_xCo₄O_9+δ (0 ≤ x ≤ 0.3) samples were fabricated by the sol-gel method followed by spark plasma sintering in vacuum. The high-temperature thermoelectric properties of the Ca_3-xSm_xCo₄O_9+δ were also studied, with an emphasis placed on the partial substitution of Sm³⁺ for Ca²⁺. The sintered Ca_3-xSm_xCo₄O_9+δ formed a monoclinic Ca₃Co₄O₉ phase and exhibited fine lamellar grains and dense morphology. With increased Sm³⁺ content, the electrical and thermal conductivities decreased, whereas the Seebeck coefficient significantly increased. Of the prepared samples, Ca_2.7Sm_0.3Co₄O_9+δ had the largest dimensionless figure-of-merit (0.175) at 800 °C. The results showed that the partial substitution of Sm³⁺ for Ca²⁺ in Ca₃Co₄O_9+δ is effective for enhancing its thermoelectric properties.     

Enhanced thermoelectric properties induced by chemical pressure in Ca3Co4O9

We report the investigation of boron substitution on structural, electrical, thermal, and thermoelectric properties of Ca_3−xB_xCo₄O₉ (x=0, 0.5, 0.75, and 1) in the temperature range between 300 K and 5 K. X-ray diffraction studies show that the Ca₃Co₄O₉ phase is successfully preserved as the majority phase in the x=0.5 sample despite the small size of boron ions. Electrical transport measurements confirm that B³⁺ substitution for Ca²⁺ causes an increase in resistivity due to the decrease in carrier concentration. x=0.5 sample is found to have a Seebeck coefficient of 181 μV/K at room temperature which is ~1.5 times higher than that of the pure Ca₃Co₄O₉. Our results indicate that the chemical pressure due to the large ionic radii difference between B³⁺ (0.27 Å) and Ca²⁺ (1 Å) enhances the thermoelectric properties as long as the unique crystal structure of Ca₃Co₄O₉ is preserved.     

Preparation of CaCO3 coated corundum aggregates by dip-coating and heat treatment and its effects on the properties and microstructures of Al2O3–MgO castables

The CaCO₃ coated corundum aggregates were prepared by impregnating tabular corundum aggregates with sizes of 1–5 mm in calcium hydrogen citrate solution and heat treatment at 430 °C, which were also used in Al₂O₃–MgO castables. The effects of Ca²⁺/Cit^3? mole ratio in precursor solution on coating characteristics of CaCO₃ coated corundum aggregates as well as the effects of CaCO₃ coatings on properties and microstructure of castables were investigated. It is found that the thickness and continuity of CaCO₃ coating is increased and the size of CaCO₃ particles in coatings decreases first and then increases as Ca²⁺/Cit^3? mole ratio is decreased. High-temperature properties of castables are improved by in-situ formation of calcium hexaaluminate (CA₆) layer at aggregate/matrix interface after sintering at 1600 °C. The Al₂O₃–MgO castables exhibit the best thermal shock resistance when Ca²⁺/Cit^3? mole ratio is 1/3. It is contributed by deflections of cracks and consumptions of fracture energy in a continuous platelet CA₆ layer with thickness of 10 μm, which is in-situ formed through reaction between Al₂O₃ and CaO derived from CaCO₃ coatings. The present investigation provides a novel approach to enhance thermal shock resistance of the Al₂O₃–MgO castables.     

Thermoelectric properties in Ca3Co4−xMnxOy ceramics

Ca₃Co_4?xMn_xO_y polycrystalline thermoelectric ceramics with small amounts of Mn have been prepared by the classical solid state method. X-ray diffraction data have shown that Ca₃Co₄O₉ is the major phase, with small amounts of the Ca₃Co₂O₆ one. Moreover, they show that the Mn has been incorporated into these two phases. Electrical resistivity decreases, compared with the values for undoped samples, with Mn content until a minimum for the 0·03 doped ones, increasing for higher Mn substitution. Seebeck coefficient does not change in all the measured temperature range, independently of Mn content. The improvement in electrical resistivity leads ～30% higher power factor values for the 0·03 Mn doped samples than that obtained in the undoped ones. The maximum power factor at 800°C, ～0·28 mW K^?2 m^?1, is close to that obtained in much higher density samples, clearly indicating the good thermoelectric properties of these samples.     

Protective properties of SiO2 with SiO2 and Al2O3 nanoparticles sol-gel coatings deposited on FeCrAl alloys

Five layer SiO₂ coatings containing SiO₂ or Al₂O₃ nanopowders were deposited on FeCrAl alloy support by sol-gel method. Studies of protective properties of the coatings were carried out during high temperature cyclic oxidation. Changes in surface topography, structure and chemical composition of the surface layer of FeCrAl alloy were investigated. It has been shown that the type of nanofillers present in the SiO₂ coating (about 2.5?wt%) affects morphology of Al₂O₃ growing scale and determines the heat resistance of FeCrAl alloy. The lowest relative mass change (approx. 1.3%) after 10 oxidation cycles in air at 900?°C (one cycle = 12?h) was measured for the samples with coatings containing hydrophilic nanosilica (Aerosil 380) as filler. The protective efficiency of the coatings in the process of high-temperature oxidation is from 66% to 85%. The thickness of the formed scale and the value of the parabolic rate constant depend on the type of nanopowder in the coating.     

Directionally solidified Al2O3-ME3Al5O12 (ME: Y,Er and Yb) eutectic coatings for thermophotovoltaic systems

Selective emitters for thermophotovoltaic systems consisting of directionally solidified Al₂O₃-ME₃Al₅O₁₂ (ME: Y, Er and Yb) eutectic coatings on Al₂O₃ substrates were produced and characterizated. Coatings were deposited by dip-coating on cylindrical substrates. After sintering, a continuous-wave CO₂ laser was used to produce the surface resolidification. The optimization of the processing parameters yielded dense eutectic coatings with good adhesion to the substrate and with 90–200 µm in thickness. All coatings were free of voids and showed a eutectic microstructure consisting of a three dimensional interpenetrated network of Al₂O₃ and ME₃Al₅O₁₂. The mechanical properties of the coatings (hardness and fracture toughness) were evaluated by indentation techniques. Thermal emission was studied by heating the rods with a CO₂ laser at temperatures between 1000 and 1400 °C. Selective emission was observed in Er³⁺ and Yb³⁺ based coatings and attributed to the electronic transitions of the rare earth ions. Er³⁺-coatings showed the best emission properties as selective emitters for thermophotovoltaic converters.     

Microstructure and cytotoxicity of Al2O3-ZrO2 eutectic bioceramics with high mechanical properties prepared by laser floating zone melting

Developing new generation of strong, tough and stable bioceramics used in dental filed has been highly desired for attaining the clinical requirement of secure and reliable therapy. In this paper, a novel Al₂O₃-ZrO₂ eutectic bioceramics with nearly fully density and extremely aesthetic luster was in-situ prepared by innovative laser floating zone melting (LFZM) method. The influence of solidification rates on microstructure evolution, mechanical properties and cytotoxicity was investigated. The eutectic bioceramics displayed a special three dimensional interpenetrating microstructure evolving with increasing the solidification rate. The eutectic colony structure occurred when solidification rate overpassed 8?µm/s, and lamellar spacing was below 1?µm when solidification rate exceeded 30?µm/s. The eutectic bioceramics solidified at 100?µm/s exhibited optimal mechanical properties with an average hardness of 16.53?GPa, fracture toughness of 6.5?MPa?m^1/2 and flexural strength of 1.37?GPa. The cytotoxicity of Al₂O₃-ZrO₂ eutectic bioceramics was evaluated by MTT methods according to ISO 10993-5 standard. Non-cytotoxic behavior was detected for the eutectic bioceramics, indicating this eutectic bioceramic could be used as promising dental restoration material.     

Evolution of phase composition and microstructure of commercial Al2O3 gel in different heat treatment condition

Evolutions of phase composition and microstructure of commercial Al₂O₃ gel in different heat treatment conditions (temperature, atmosphere and additives) were investigated. There was almost no effect of atmosphere and carbon additive on phase evolution of Al₂O₃ gel during heating, γ-Al₂O₃ formed at 800?°C, γ-Al₂O₃ and minor θ-Al₂O₃ co-existed at 1000 °C, and single phase of ɑ-Al₂O₃ occurred at heating temperature ≥1200?°C. Atmosphere and carbon had great effects on morphology and crystal size of Al₂O₃ particles. Crystal size of spherical-shape Al₂O₃ particles was 10–20?nm after heating at 800–1000?°C in air, afterwards, they rapidly grew into micro or macro scale when temperature was above 1200?°C, and sintering phenomena of worm-like Al₂O₃ particles were observed. In the presence of carbon, spherical-shape Al₂O₃ particles grew slightly from 10 to 20?nm to 50–60?nm with the temperature increasing from 800?°C to 1500?°C in reducing atmosphere, carbon inclusions in Al₂O₃ grain boundaries triggered a steric hindrance of Al₂O₃ particles growth. Al₂O₃ gel had a high reactive ability and could react with microsilica to form nano mullite crystals at relatively lower temperature.     

Deposition behavior and characteristics of hydroxyapatite coatings on Al2O3, Ti,and Ti6Al4V formed by a chemical bath method

A simple chemical bath method was used to deposit hydroxyapatite (HA) coatings on Al₂O₃, Ti, and Ti6Al4V substrates at ambient pressure by heating to 65–95 °C in an aqueous solution prepared with Ca(NO₃)₂·4H₂O, KH₂PO₄, KOH, and EDTA. The deposition behavior, morphology, thickness, and phase of the coatings were investigated using scanning electron microscopy and X-ray diffractometry. The bonding strength of the coatings was measured using an epoxy resin method. The HA coatings deposited on the three kinds of substrates were fairly dense and uniform and exhibited good crystallinity without any additional heat treatment. A coating thickness of 1–1.8 μm was obtained for the samples coated once. By repeating the coating process three times, the thickness could be increased to 4.5 μm on the Al₂O₃ substrate. The bonding strength of these coatings was 18 MPa.     

Influence of heat treatment on alternant-layer structure and mechanical properties of Al2O3-TiO2-MgO coatings

Al₂O₃-TiO₂-MgO ceramic alternant layer coatings were prepared by atmospheric plasma spraying and heat treated at 600, 700, 800, 900, and 1000?°C. The influence of heat treatment on microhardness, fracture toughness, and the structural evolution of the coatings on steel were investigated. Heat treatment promoted alternant layer interdiffusion within ceramic coatings, which could result a transformation from a lamellar morphology to mutual pinning. The interfacial diffusion between the bond coating and substrate was clearly demonstrated after heat treatment at different temperatures. Heat treatment also significantly affected the evolution of the hardness and fracture toughness. Temperature strongly affected the microhardness of the specimens, and the hardness arrived to the highest value at 1000?°C. The formation of a new Mg₂Al₆Ti₇O₂₅ phase and alternant layer mutual pinning were beneficial to hardness improvement, and heat treatment also significantly improved fracture toughness.     

Fabrication and electrical properties of the fast response Mn1.2Co1.5Ni0.3O4 miniature NTC chip thermistors

Mn_1.2Co_1.5Ni_0.3O₄ ceramic chips with 90, 100, 110 and 120?µm thickness were developed using the tape casting method. The structure, electrical properties and thermal time constant have been investigated as a function of the thickness of the miniature NTC chip thermistors. The ρ₂₅ increased from 127 to 449.7?Ω?cm and the thermal constant B_25/50 increased from 52.8 to 180.5?K as the thickness increased. This could be a result of the increased grain boundary resistance and Mn³⁺/Mn⁴⁺ ratio (0.7–1.02) from the increased thickness. The thermal time constants were 0.83, 0.89, 1.05 and 1.10?s for chip thermistors with thicknesses from 90 to 120?µm, respectively. This reveals that the thickness of chips could affect the thermal time constant, a result of significantly different dissipation constants and heat capacities in MCN thermistors. The proposed sensors demonstrated a superior temporal response.     

The impact of Al2O3 amount on the synthesis of CASH samples and their influence on the early stage hydration of calcium aluminate cement

In this work the impact of Al₂O₃ amount on the synthesis (200?°C; 4–8?h) of calcium aluminium silicate hydrates (CSAH) samples and their influence on the early stage hydration of calcium aluminate cement (CAC) was examined. It was found that the amount of Al₂O₃ plays an important role in the formation of calcium aluminate hydrates (CAH) because in the mixtures with 2.7% Al₂O₃ only calcium silicate hydrates (CSH) intercalated with Al³⁺ ions were formed. While in the mixtures with a higher amount of Al₂O₃ (5.3–15.4%), calcium aluminate hydrate – C₃AH₆, is formed under all experimental conditions. It is worth noting that the largest quantity of mentioned compound was obtained after 4?h of hydrothermal treatment, in the mixtures with 15.4% of Al₂O₃. It was proved that synthesized C₃AH₆ remain stable up to 300?°C and at higher temperature (945?°C) recrystallized to mayenite (Ca₁₂Al₁₄O₃₃), which reacted with the rest part of CaO and amorphous structure compound, resulting in the formation of gehlenite (Ca₂Al₂SiO₇). Moreover, the synthesized C₃AH₆ addition induced the early stage of CAC hydration. Besides, in the samples with an addition, the induction period was effectively shortened: in a case of pure CAC (G70) paste, hydration takes about 6–6.5?h, while with addition – only 2–2.5?h. The synthesized and calcinated compounds was characterized by using XRD and STA analysis.     

Synthesis and electrochemical characteristics of Al2O3-coated LiNi1/3Co1/3Mn1/3O2 cathode materials for lithium ion batteries

LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials have been coated with Al₂O₃ nano-particles using sol-gel processing to improve its electrochemical properties. The X-ray diffraction (XRD) pattern of the as-prepared Al₂O₃ nano-particles was indexed to the cubic structure of the γ-Al₂O₃ phase and had an average size of ∼4 nm. The XRD showed that the structure of LiNi_1/3Co_1/3Mn_1/3O₂ was not affected by the Al₂O₃ coating. However, the Al₂O₃ coatings on LiNi_1/3Co_1/3Mn_1/3O₂ improved the cyclic life performance and rate capability without decreasing its initial discharge capacity. These electrochemical properties were also compared with those of LiAlO₂-coated LiNi_1/3Co_1/3Mn_1/3O₂ cathode material. The electrochemical impedance spectroscopy (EIS) was studied to understand the enhanced electrochemical properties of the Al₂O₃-coated LiNi_1/3Co_1/3Mn_1/3O₂ compared to uncoated LiNi_1/3Co_1/3Mn_1/3O₂.     

Preparation of a thick NiAl/Al2O3 coating by embedding method and its corrosion resistance under supercritical water environment

One of the critical issues in the application of supercritical water oxidation technology is to improve the corrosion resistance of reactor materials. Use of Al₂O₃ coating is one of the most promising methods to address this issue. In this study, thick NiAl/Al₂O₃ coatings on Inconel 625 substrates were prepared by a consecutive pack embedding and in-situ thermal oxidation process. The effect of aluminizing and oxidation temperature on phase structure and coating thickness is studied. Results show the diffusion of Al from the exterior to the interior of the alloy matrix to form intermetallic compounds between Al and metal elements in the matrix (Ni, Cr, Mo, etc.). Moreover, the coating thickness can reach above 300 μm at the aluminizing temperature of 950 °C. Increasing the aluminizing temperature above 950 °C will not increase the coating thickness further. After high temperature oxidation subsequently, only phases of NiAl and Al₂O₃ were detected. The formation of Al₂O₃ layer can be ascribed to the surface oxidization of Al. And the NiAl between the alloy substrate and Al₂O₃ coating provides an interfacial layer that can alleviate the crack or exfoliation of ceramic coating due to the mismatching of thermal expand coefficient. The thick NiAl/Al₂O₃ coatings prepared by aluminizing 950 °C and oxidizing at 1100 °C exhibit satisfied corrosion resistance after supercritical water test. This work would provide a significant method to develop advanced ceramics coating for the corrosion resistance of alloys.     

The role of sulphates in cement clinkering: Subsolidus phase relations in the system CaO-Aℓ2O3-SiO2-SO3

Experimental studies are reported on the CaO-rich portions of the system CaO-A?₂O₃-SiO₂-SO₃ in the temperature range 950°–1150°C. Six four-phase and 18 three-phase essemblages have been found. These define the equilibria between CaO, Ca₂SiO₄, Ca₃A?₂O₆, Ca₁₂A?₁₄O₃₃, CaA?₂O₄, calcium silicosulphate (Ca₅Si₂(SO₄)O₈) and aluminosulphate, Ca₄A?₆(SO₄)O₁₂. The equilibria above 1175°, where Ca₃SiO₅ becomes stable, are predicted. Possible departures from equilibrium during clinkering are discussed.     

Fabrication of Al2O3@BaFe12O19 core-shell powder by a modified heterogeneous precipitation method

Core-shell structural composites are promising to quench the desire for low-cost, lightweight and multifunctional magnetic materials, but the fabrication of highly pure shell nanoparticles with a desired morphology on a heterogeneous nuclear is still a big challenge. This study gives a successful illustration by in-situ synthesizing BaFe₁₂O₁₉ nanoparticles as a shell on Al₂O₃ core powders using BaCl₂ and FeCl₃ as precursors, instead of commonly used Ba(NO₃)₂ and Fe(NO₃)₃, by a modified heterogeneous precipitation process followed by calcination for crystallization. After calcination of the precipitants from the raw precursor mixture of BaCl₂:FeCl₃ for 1:10 at 800?°C, and for 1:8.5 at 900?°C, respectively, pure BaFe₁₂O₁₉ formed on pristine Al₂O₃, mostly with a hexagonal plane, about 100–200?nm in basal diameter and 20–100?nm in thickness. The highest saturation magnetization and coercivity of the coated Al₂O₃ @?BaFe₁₂O₁₉ powders was 23.3?emu/g and 5149?Oe, quite higher than that of the direct-mixed Al₂O₃-BaFe₁₂O₁₉ composite powder, and BaFe₁₂O₁₉-containing composites reported in literature, which could be attributed to the hexagonal shape of BaFe₁₂O₁₉ and uniform shell dispersion on Al₂O₃ core. The formation mechanism for the Al₂O₃@BaFe₁₂O₁₉ powders was discussed in detail.     

Preoxidation of bond coat in IN-738LC/NiCrAlY/LaMgAl11O19 thermal barrier coating system

The low thickness of thermally grown oxide (TGO) layer and presence of amorphous phase in the as-sprayed LaMgAl₁₁O₁₉ (LaMA) coating reduce the thermal cycling lifetime of thermal barrier coatings (TBCs). In the present study, the as-sprayed Ni-22Cr-10Al-1.0Y bond coat was preoxidized at 1060?°C to produce a continuous oxide scale prior to subsequent deposition of the ceramic top coat. The optimum time of peroxidation treatment and thickness of the continuous aluminum oxide layer were estimated 15?h and 2?µm respectively. The oxidized layer due to the preoxidation treatment of bond coating reduces the amorphous phase in as-sprayed LaMA coating and increases the microhardness of LaMA coating from approximately 600 to 900HV. Also, preoxidation of the NiCrAlY bond coating increases adhesion strength of the LaMA top coating, even slightly more than the adhesion strength of the as-spray 8YSZ coating. The LaMA coatings have a lower hardness in compared with the 8YSZ coating (~ 1010Hv), which results a better elastic behavior.     

Enhancement of Ca3Co4O9 thermoelectric properties by Cr for Co substitution

Ca₃Co_4−xCr_xO₉ polycristalline thermoelectric ceramics with small amounts of Cr have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Cr has been incorporated into the Ca₃Co₄O₉ structure and no Cr-containing secondary phases have been produced for Cr contents≤0.05. Apparent density measurements have shown that all samples are very similar, with densities around 75% of the theoretical one. Electrical resistivity decreases and Seebeck coefficient slightly raises when Cr content increases until 0.05 Cr addition. The improvement in both parameters leads to higher power factor values than the usually obtained by conventional solid state routes.