Crystallisation of amorphous spray-dried precursors in the Al2O3–SiO2 system

The crystallisation of amorphous precursors has been studied in the whole range of composition in the Al₂O₃–SiO₂ system. The amorphous precursors have been obtained by hydrolysing TEOS directly in a diluted aqueous solution of aluminium nitrate, spray drying the clear solution and heating the resulting powder. Up to 70 mol % Al₂O₃, only mullite crystallises around 980–1000 °C; between 70 and 80 mol % Al₂O₃ mullite and spinel crystallise together; and for more than 80 mol % Al₂O₃ only spinel is formed. In the 70–80 mol % Al₂O₃ range of composition, when both mullite and spinel crystallise, low heating favours the crystallisation of mullite and it is nearly possible to crystallise only mullite from a 75 mol % Al₂O₃ sample. By rapid heating it is also possible to crystallise only spinel from the same 75 mol % Al₂O₃ precursor. The enthalpy and the activation energy for crystallisation are maximum for 60–80 mol % Al₂O₃. Heating the samples up to 1700 °C for 1 h, the phase equilibrium is not reached, particularly when both mullite and spinel crystallise together, and θ-Al₂O₃ is still present.     

Effect of boron oxide on the microstructure of mullite-based glass-ceramic glazes for floor-tiles in the CaO–MgO–Al2O3–SiO2 system

The effect of increasing replacement of Al₂O₃ by B₂O₃ in a parent glass on the sintering and further crystallization of mullite was investigated. The composition of the parent glass was chosen in the mullite primary phase field of the CaO–MgO–Al₂O₃–SiO₂ quaternary system. Glass powder pellets were heated under standard (10 °C/min and 2 h of hold time) and fast heatings (25 °C/min and 5 min of hold time) at different temperatures from 700 to 1190 °C. Sintering of B₂O₃-containing glasses took place in the range between 850 and 1050 °C. X-ray diffraction results showed that mullite formed as unique crystalline phase for glasses containing amounts of B₂O₃ larger than 6 wt%. For lower amounts of boron oxide cordierite was formed as secondary crystalline phase. Quantitative determination of mullite by Rietveld analysis indicated that the higher amount of mullite present in the glass-ceramic fast heated at 1160 °C was 19.5 wt% for the glass containing 9 wt% of B₂O₃. The final microstructure of the glass-ceramic glazes showed the presence of well shaped, long acicular mullite crystals dispersed within the residual glassy phase. Results of glass-ceramic glazes when applied as slurry and under industrial heating conditions pointed out promising mechanical properties.     

Effects of additives on combustion synthesis of Al2O3–TiB2 ceramic composite

Al₂O₃, SiC and kaolin were employed as additives in combustion synthesizing Al₂O₃–TiB₂ ceramic composite. Effects of the additives on adiabatic temperature, combustion wave velocity, volume change and composite density were studied, and bending strength of the synthesized ceramics was evaluated. By theoretical calculation, the adiabatic temperature of Al–TiO₂–H₃BO₃ system is 2314.85 °C and decreases with increasing the additive addition. With Al₂O₃ addition, the phases presented in the ceramic composite are unchanged, and the phases of SiC and 3Al₂O₃·2SiO₂ emerges when SiC and kaolin are added. The addition of the additives results in a refined TiB₂ particulate size and reduces combustion wave velocity. The highest density is achieved with the addition of kaolin from 10 to 30 wt.% making the volume change from ?4.6 to ?1.2%. The bending strength of the TiB₂–Al₂O₃ composite is improved eight times with the addition of 30 wt.% kaolin.     

Recycling of waste fly ash for production of porous mullite ceramic membrane supports with increased porosity

Low-cost porous mullite ceramic membrane supports were fabricated from recycling coal fly ash with addition of natural bauxite. V₂O₅ and AlF₃ were used as additives to cause the growth of mullite crystals with various morphologies via an in situ reaction sintering. Dynamic sintering, microstructure and phase evolution of the membrane supports were characterized in detail and open porosity, pore size, gas permeation and mechanical properties were determined. It showed the membrane support with 3 wt.% V₂O₅ and 4 wt.% AlF₃ addition exhibits an open porosity of ∼50%, mechanical strength of 69.8 ± 7.2 MPa, an interlocking microstructure composed of anisotropically grown mullite whiskers with an aspect ratio of 18.2 ± 3.6 at 1300 °C. Addition of more V₂O₅ lowered the secondary mullitization temperature, resulting in more mullite formation at lower temperatures. The fabricated membrane supports feature high porosity without mechanical strength degradation, possible strengthening mechanism of the mullite whiskers was further discussed.     

Performance of V2O5/NPTiO2–Al2O3-nanoparticle- and V2O5/NTiO2–Al2O3-nanotube model catalysts in the SCR–NO with NH3

In this study, the NO reduction by NH₃ over V₂O₅/NPTiO₂–Al₂O₃ (nanoparticles) and V₂O₅/NTiO₂–Al₂O₃ (nanotubes) catalysts synthesized by the sol–gel method with 10 and 5 wt.% of Al₂O₃ and V₂O₅, respectively, is reported. The V₂O₅/NPTiO₂–Al₂O₃ and V₂O₅/NTiO₂–Al₂O₃ catalysts showed remarkable conversion, high acidity, structure stability, N₂ selectivity and a high resistance to deactivation in the presence of 10 vol.% of H₂O within the 200 to 500 °C temperature interval. The nanostructured catalysts developed in this work are an excellent alternative to improve the SCR–NH₃ process, both expanding its operation window and preventing deactivation by H₂O at high temperatures.     

Sintering behavior and microwave dielectric characteristics of BaO–Sm2O3–4TiO2 ceramics with B2O3 and BaB2O4 addition

We studied the low temperature sintering and the reaction in BaO–Sm₂O₃–4TiO₂ ceramics with boron-based additives for the application to microwave dielectric devices. The amount of the boride glasses of B₂O₃ and BaB₂O₄ was varied from 1 to 10 wt.% and the green compacts were sintered in the temperature range of 900–1200 °C for 2 h. When B₂O₃ was added, second phases of Sm₂Ti₂O₇, BaTi(BO₃)₂, Ba₂Ti₉O₂₀, and TiO₂ were formed, while BaB₂O₄ addition resulted in the formation of BaSm₂Ti₄O₁₂ single phase without second phases. On the basis of these results, it is regarded that the B₂O₃ is a reactive glass and the BaB₂O₄ is a non-reactive glass. The second-phase development, sintering behavior and microwave dielectric characteristics of BaO–Sm₂O₃–4TiO₂ ceramics were examined.     

Porous acicular mullite ceramics fabricated with in situ formed soot oxidation catalyst obtained from waste MoSi2

Porous acicular mullite (3Al₂O₃·2SiO₂) ceramics containing Cu₃Mo₂O₉ as a soot oxidation catalyst was fabricated by a novel approach using commercial powders of Al₂O₃ and CuO, and powder obtained by controlled oxidation of ground waste MoSi₂. The obtained material consisted of elongated mullite grains which are known to be effective in carbon soot removal from diesel engine exhaust. The presence of in situ created Cu₃Mo₂O₉ was found to catalyze the carbon burnout which is an extremely important feature when it comes to filter regeneration, i.e., the captured soot removal. The carbon burnout temperature in the sample containing 12 wt% CuO was by 90 °C lower than that in the sample without CuO. Effect of sintering temperature as well as the effect of amount of CuO additive on mullite properties were studied. It was found that the increase in amount of CuO in samples sintered at 1300 °C decreased porosity and increased compressive strength of the porous mullite ceramics. The addition of 12 wt% CuO increased the strength of the porous mullite ceramics up to 70 MPa, whereas the porosity was reduced from 62% in the mullite without CuO to 44% in the mullite ceramics containing 12 wt% CuO. Although affected by the amount of CuO, the microstructure still consisted of elongated mullite grains.     

Densification behaviors and high-temperature characteristics of Si3N4 sintered bodies using Al2O3–Yb2O3 additives

The densification behaviors (include α–β transformation) and high-temperature characteristics (especially oxidation resistance and high-temperature strength properties) of Si₃N₄ sintered bodies using Al₂O₃–Yb₂O₃ based sintering additive are investigated.Densification and α–β transformation behaviors were investigated by varying the compositions of Al₂O₃–Yb₂O₃ additives. In terms of the influence of the Y₂O₃/Al₂O₃ ratio on densification behavior, a greater Yb₂O₃/Al₂O₃ ratio tends to inhibit densification. The α–β transformation tended to be delayed in sintered bodies with a small additive amount of 3.4 mass%. Compared with the transformation behaviors of the sintered bodies using Al₂O₃–Y₂O₃ additives, those using Al₂O₃–Yb₂O₃ additives exhibited a narrower temperature zone for α–β transformation, which attributed to the finer structure for the sintered body using Al₂O₃–Yb₂O₃ additives. This is affected by the difference in solubility of Si₃N₄ in the two kinds of glass phase.High room temperature strength of 900–1000 MPa was obtained for sintered bodies with a 10.0 mass% addition of additives, and this is considered to be due to the finer micro-structure. Precipitation of a Yb₄Si₂N₂O₇ phase at the grain boundary glass phase, as induced by crystallization processing, enables the improvement of 1300 °C strength to about 650–720 MPa. Crystallization processing resulted in a 30% reduction in the amount of weight change during oxidation (from 3.42 to 2.46 mg/cm²), demonstrating the effectiveness in improving oxidation resistance.     

Phase diagram of the Al2O3–ZrO2–La2O3 system

The phase diagram of the Al₂O₃–ZrO₂–La₂O₃ system was constructed in the temperature range 1250–2800 °C. The liquidus surface of the phase diagram reflects the preferentially eutectic interaction in the system. Three new ternary and two new binary eutectics were found. The minimum melting temperature is 1665 °C and it corresponds to the ternary eutectic LaAlO₃ + T-ZrO₂ +  La₂O₃·11Al₂O₃. The solidus surface projection and the schematic of the alloy crystallization path confirm the preferentially congruent character of phase interaction in the ternary system. The polythermal sections present the complete phase diagram of the Al₂O₃–ZrO₂–La₂O₃ system. No ternary compounds or regions of remarkable solid solution were found in the components or binaries in this ternary system. The latter fact is the theoretical basis for creating new composite ceramics with favorable properties in the Al₂O₃–ZrO₂–La₂O₃ system.     

Effect of B2O3 on the sintering and microwave dielectric properties of M-phase LiNb0.6Ti0.5O3 ceramics

The effect of B₂O₃ addition on the sintering, microstructure and the microwave dielectric properties of LiNb_0.6Ti_0.5O₃ ceramics have been investigated. It is found that low-level doping of B₂O₃ (≤2 wt.%) can significantly improve the densification and dielectric properties of LiNb_0.6Ti_0.5O₃ ceramics. Due to the liquid phase effect of B₂O₃ addition, LiNb_0.6Ti_0.5O₃ ceramics could be sintered to a theoretical density higher than 95% even at 880 °C. No secondary phase was observed for the B₂O₃-doped ceramics. There is no obvious degradation in dielectric properties for the ceramics with B₂O₃ additions. In the case of 1 wt.% B₂O₃ addition, the ceramics sintered at 880 °C show good microwave dielectric properties of ɛ_r = 70, Q × f = 5400 GHz, τ_f = −6.39 ppm/°C. It represents that the ceramics could be promising for multilayer low-temperature co-fired ceramics (LTCC) applications.     

Selective oxidative dehydrogenation of ethane over MoO3/V2O5–Al2O3 catalysts: Heteropolymolybdate as a precursor for MoO3

Oxidative dehydrogenation of ethane to ethylene was investigated over a series of MoO₃ added V₂O₅–Al₂O₃ catalysts. The catalysts were characterized by BET, XRD, Laser-Raman and FT-IR spectroscopies and TPR technique. Catalytic tests were carried out in a fixed bed stainless steel reactor in the temperature range from 450 to 600 °C. Results revealed that the loading of molybdophosphoric acid (MPA) and the method of preparation had a clear influence on the catalytic performance. Among all, 10 wt.% MPA/V₂O₅–Al₂O₃ solid was found to possess superior activity and selectivity (X-C₂H₆ ~ 35% and S-C₂H₄ ~ 65%). Formation of Mo–V mixed oxide phases on Al₂O₃ appeared to be responsible for this improved performance. This best catalyst also exhibited good long-term stability over a period of ca. 36 h.     

Fabrication and characterization of highly transparent Er:Y2O3 ceramics with ZrO2 and La2O3 additives

In this study, we report highly transparent Er:Y₂O₃ ceramics (0–10 at% Er) fabricated by a vacuum sintering method using compound sintering additives of ZrO₂ and La₂O₃. The transmittance, microstructure, thermal conductivity and mechanical properties of the Er:Y₂O₃ ceramics were evaluated. The in-line transmittance of all of the Er:Y₂O₃ ceramics (1.2 mm thick) exceeds 83% at 1100 nm and 81% at 600 nm. With an increase in the Er doping concentration from 0 to 10 at%, the average grain size, microhardness and fracture toughness remain nearly unchanged, while the thermal conductivity decreases slightly from 5.55 to 4.89 W/m K. A nearly homogeneous doping level of the laser activator Er up to 10 at% in macro-and nanoscale was measured along the radial direction from the center to the edge of a disk specimen, which is the prominent advantage of polycrystalline over single-crystal materials. Based on the finding of excellent optical and mechanical properties, the compound sintering additives of ZrO₂ and La₂O₃ are demonstrated to be effective for the fabrication of transparent Y₂O₃ ceramics. These results may provide a guideline for the application of transparent Er:Y₂O₃ laser ceramics.     

Formation of high-temperature crystalline phases in ceramic from illite clay and dolomite

The formation of ceramic with the prevailing cordierite/mullite in the crystalline phase is greatly dependent on the raw materials used. In the present work the cordierite and mullite (or their solid—solutions accordingly with spinels and corundum) formation processes from compositions of natural raw materials (illite clay, dolomite) with synthetic additives (MgO, Al(OH)₃, K₂CO₃) are studied. It is shown that the illite clay acts as a low viscosity liquid medium as well as a precursor for cordierite and mullite crystallisation in ceramics.The phase development using XRD—analysis and the SEM is studied. Particles size distribution of staiting powder and some properties of ceramic obtained are also demonstrated. It is shown that formation of the above mentioned phases starts at temperatures 1050–1080 °C and remarkably increases at 1200 °C for mullite and at 1300 °C for cordierite.     

Fabrication and characterization of anorthite–mullite–corundum porous ceramics from construction waste

In this work, anorthite–mullite–corundum porous ceramics were prepared from construction waste and Al₂O₃ powders by adding AlF₃ and MoO₃ as mineralizer and crystallization catalyst, respectively. The effects of the sintering temperature and time on open porosity, mechanical properties, pore size distribution, microstructure, and phase composition were characterized in detail. The results showed that the formation of the mullite whiskers and the properties of the anorthite–mullite–corundum porous ceramics depended more on the sintering temperature than the holding time. By co-adding 12 wt% AlF₃ and 4 wt% MoO₃, mullite whiskers were successfully obtained at sintering temperatures upon 1350 °C for 1 h. Furthermore, the resultant specimens exhibited excellent properties, including open porosity of 66.1±0.7%, biaxial flexural strength of 23.8±0.9 MPa, and average pore size of 1.32 µm (the corresponding cumulative volume percent was 37.29%).     

Anti-reduction of Ti4+ in Ba4.2Sm9.2Ti18O54 ceramics by doping with MgO,Al2O3 and MnO2

The anti-reduction of Ti⁴⁺ ions in Ba_4.2Sm_9.2Ti₁₈O₅₄ (BST) ceramics at high sintering temperature over 1300 °C was investigated. MgO, Al₂O₃ and MnO₂ were added separately to suppress the reduction of Ti⁴⁺ ions so as to improve the microwave dielectric properties of BST ceramics. The microstructure of BST ceramics was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). X-ray photoelectron spectroscopy (XPS) was used to study the electroconductivity of BST ceramics and valency changes of Ti ions. The results showed that MgO or Al₂O₃, when acting as an acceptor, could effectively suppress the reduction of Ti⁴⁺ ions and significantly improve the Q × f values of BST ceramics at the cost of dielectric constant. Meanwhile, MnO₂ as an oxidant had also improved the Q × f values but with no decrease in dielectric constant. Excellent microwave dielectric properties were achieved in Ba_4.2Sm_9.2Ti₁₈O₅₄ ceramics doped with 0.2 wt.% Al₂O₃ sintered at 1340 °C for 3 h: ?_r = 76.9, Q × f = 10,120 GHz and τ_f  = ?22.7 ppm/°C.     

Oxidation behavior of Al2O3 reinforced MoSi2 composite coatings fabricated by vacuum plasma spraying

MoSi₂, MoSi₂–10 vol.% Al₂O₃, MoSi₂–30 vol.% Al₂O₃ (denoted as MA0, MA1, MA3, respectively) coatings were fabricated by vacuum plasma spraying (VPS), and their oxidation behavior was examined at low temperature (500 °C) and high temperature (1500 °C). The test at 500 °C showed that the addition of Al₂O₃ effectively restrained the pest oxidation of MoSi₂. The MA1 coating had satisfactory fluid surface and presented good oxidation resistance at 1500 °C. However, the MA3 coating showed worse oxidation resistant behavior compared with the MA0 coating because of mullite formation.     

Preparation,characterisation and catalytic evaluation of MgF2 supported V2O5 catalysts for ammoxidation of 3-picoline

Non-conventional MgF₂ supported V₂O₅ catalysts with different vanadia contents (2–15 wt.%) were prepared by impregnation (using NH₄VO₃), characterised and catalytically evaluated for selective ammoxidation of 3-picoline to nicotinonitrile. Oxygen and ammonia chemisorption uptakes increased continuously from 60 to over 600 μmol g^?1 and 275 to >750 μmol g^?1, respectively, with rise in V₂O₅ proportion indicating that the redox as well as acidic sites are increasing with increase in V₂O₅ content. Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) analysis revealed endothermic as well as exothermic thermal effects mainly due to liberation of water and ammonia, and also due to structural changes. XRD patterns showed the formation of crystalline V₂O₅ in the fresh solids having 8 wt.% V₂O₅ and above and NH₄VO₃ phase in the spent samples. The conversion of 3-picoline is observed to increase continuously with increase in V₂O₅ loading. However, the selectivity of nicotinonitrile is found to be independent on conversion of 3-picoline. The catalyst with the highest V₂O₅ loading (15.7 wt.%) displayed the best activity (X > 90%) and selectivity (S > 95%) compared to all other catalysts of this series. The 3-picoline conversion of 10% at 548 K is increased to almost 100% with rise in temperature to 663 K. Increase in 3-picoline feed rate and NH₃: 3-picoline ratio exhibited an inhibiting effect on the conversion, while an increase in air: 3-picoline ratio has no significant influence on the performance.     

Highly resistive SiC ceramics sintered with Al2O3-AlN-Y2O3 additions

A polycrystalline SiC ceramic prepared by pressureless sintering of α-SiC powders with 3 vol% Al₂O₃-AlN-Y₂O₃ additives in an argon atmosphere exhibited a high electrical resistivity of ~10¹³ Ω cm at room temperature. X-ray diffraction revealed that the SiC ceramics consisted mainly of 6H- and 4H-SiC polytypes. Scanning electron microscopy and high resolution transmission electron microscopy investigations showed that the SiC specimen contained micron-sized grains surrounded by an amorphous Al-Y-Si-O-C-N film with a thickness of ~4.85 nm. The thick boundary film between the grains contributed to the high resistivity of the SiC ceramic.     

Wetting,densification and phase transformation of La2O3/A2O3/B2O3-based glass–ceramics

A lead-free, non-alkali La₂O₃–Al₂O₃–B₂O₃ (LAB) glass with Al₂O₃ filler had been investigated for low temperature co-firing ceramic (LTCC) application. The glass forming window and several physical properties of the LAB systems were investigated by ICP, TMA, XRD, DSC, and SEM/EDS. The results show that the densification and crystallization temperatures of LAB/Al₂O₃ were between 700 °C and 950 °C and depended greatly on the formulation. Crystalline phase LaBO₃ (LB) and LaAl₂B₃O₉ (L2A3B) crystallized starting at 825 °C and 925 °C, respectively. High degree of densification and crystallization of one glass–Al₂O₃ composition (L30A) was observed with the microstructure composed of tabular L2A3B grains interlocking with submicron Al₂O₃ and LB grains.     

Solubility and microstructural development of TiO2-containing 3Al2O3·2SiO2 and 2Al2O3·SiO2 mullites obtained from single-phase gels

The interdependence of the titanium oxide amount and the anisotropic growth of mullites prepared from single-phase gels were investigated. Gels with stoichiometries 3(Al_2−xTi_xO₃)·2(SiO₂) and 2(Al_2−xTi_xO₃)·(SiO₂), with 0 ≤ x ≤ 0.15 were prepared by the semialkoxide method. Gels and specimens heated at temperatures between 1200 and 1600 °C were characterized by using infrared spectroscopy (IR), X-ray diffraction (XRD) and transmission and field emission scanning electron microscopies (TEM and FESEM). Al₂TiO₅ as minor impurity was detected in both series of mullites for gel precursor compositions x = 0.10 and x = 0.15, obtained at temperatures between 1200 and 1600 °C. Variations of lattice parameters of mullite, processed at temperatures from the range between 1400 and 1600 °C, with the starting nominal amount of titanium oxide indicated that the solubility limit of titanium oxide was in ranges 3.8–4.1 and 4.1–4.4 wt% TiO₂ for 3:2 and 2:1 mullites series, respectively. The anisotropic growth of titanium-doped mullite crystalline grains was significant only when the nominal amount of titanium oxide exceeded the limit of solubility into the mullite structure (for both mullite series). Stronger anisotropy occurred for the 3:2 series specimens, i.e. for the SiO₂-richer mullites. In both series of mullites, the anisotropic grain growth was observed for the process temperatures higher than 1400 °C; the crystalline grains of mullites processed at lower temperatures were equiaxials and of almost the same size.