Densification of mullite–SiC nanocomposite sol–gel precursors by pressureless sintering

Mullite–SiC nanocomposite has been synthesised based on a nanoprecursor route and sintered to high density through pressureless sintering technique. The approach has been first to evolve a method to obtain high density, fine-grained mullite matrix phase through a sol–gel seeded route. Nanosize SiC particles (∼200 nm) were dispersed in a seeded mullite precursor sol to obtain mullite-coated SiC particles, which were further compacted and sintered to hybrid composites, resulting in distribution of SiC particles at the inter- and intra-mullite grain positions.     

Interfacial reactions in aluminum alloys/mullite–zirconia composites

Mullite and reaction-sintered mullite–zirconia bars were exposed to Mg- and Mg+Cu-containing molten aluminum alloyS, at 800°C for up to 2000 h, and afterwards characterized by X-ray diffraction (XRD), reflected light optical microscopy (RLOM) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). Mullite was completely attacked by Al penetrating through grain boundaries and reducing the mullite grains to alumina and silicon. Mullite–zirconia was not attacked, because the dense net of ZrO₂-grains prevented penetration of molten metal into the compact and, consequently, a mechanically and thermodynamically stable spinel layer (≈30 μm) was formed at the static interface, which protected the ceramic against further attack.     

Mechanical properties of 3D KD-I SiCf/SiC composites with engineered fibre-matrix interfaces

Three-dimensional (3D) silicon carbide (SiC) matrix composites reinforced with KD-I SiC fibres were fabricated by precursor impregnation and pyrolysis (PIP) process. The fibre-matrix interfaces were tailored by pre-coating the as-received KD-I SiC fibres with PyC layers of different thicknesses or a layer of SiC. Interfacial characteristics and their effects on the composite mechanical properties were evaluated. The results indicate that the composite reinforced with as-received fibre possessed an interfacial shear strength of 72.1 MPa while the composite reinforced with SiC layer coated fibres had a much higher interfacial shear strength of 135.2 MPa. However, both composites showed inferior flexural strength and fracture toughness. With optimised PyC coating thickness, the interface coating led to much improved mechanical properties, i.e. a flexural strength of 420.6 MPa was achieved when the interlayer thickness is 0.1 μm, and a fracture toughness of 23.1 MPa m^1/2 was obtained for the interlayer thickness of 0.53 μm. In addition, the composites prepared by the PIP process exhibited superior mechanical properties over the composites prepared by the chemical vapour infiltration and vapour silicon infiltration (CVI-VSI) process.     

Preparation,characterization and uses of mullite grain

The properties were compared of grain formed at low temperature from a homogeneous mullite precursor gel, and grain formed from the manufactured material produced at high temperature by fusion or by sintering. A homogeneous precursor gel with the oxide stoichiometry of mullite was prepared by treating technical ethyl silicate with aluminium chlorohydrate, with or without dibutyltin oxide as hydrolysis/gelation catalyst. The gel was dried and heated for 5 h at 700 ° C to remove organic residues then ground to 8 m nominal size to obtain a mullite precursor grain. Crystallites form even at this moderate temperature. Commercially produced fine mullite grain manufactured by fusion or by high temperature sintering was also ground to a give a grain of 8 m nominal size to provide a comparative standard with well known material. Each of the three grain materials, homogeneous precursor gel, fused mullite and sintered mullite, each ground to 8 m nominal size, representative of the fines fraction of a grain mix to be used in producing refractory shapes, was made into compacts which were sintered, and, when cold, tested for compressive and bend strength. Materials of this size were chosen for comparison because of their significance in applications where properties at temperature are important. The precursor gel compacts were 95% crystalline mullite and reached 86% of theoretical density. At 1500 ° C, heated in a 90 deg min^–1 schedule, they had strength comparable to sintered mullite; both materials were much stronger than fused mullite. The results show that sintering procedure has a profound effect on strength, and indicated that in the absence of binder, fused mullite is less reactive than sintered mullite. Some properties of refractory shapes and bricks made from the sintered or fused mullite grain are discussed and some uses in refractory shapes are considered.     

Densification and mechanical properties of mullite-SiC nanocomposites synthesized through sol-gel coated precursors

Mullite-SiC nanocomposites are synthesized by introducing surface modified sol-gel mullite coated SiC particles in the matrix and densification and associated microstructural features of such precursor are reported. Nanosize SiC (average size 180 nm) surface was first provided with a mullite precursor coating which was characterized by the X-ray analysis and TEM. An average coating thickness of 120 nm was obtained on the SiC particles. The green compacts obtained by cold isostatic pressing were sintered in the range 1500–1700°C under pressureless sintering in the N₂ atmosphere. The percentage of the theoretical sintered density decreases with increase in SiC content. A maximum sintered density of 97% was achieved for mullite-5 vol.% SiC. The fractograph of the sintered composite showed a highly dense, fine grained microstructure with the SiC particles uniformly distributed along the grains as well as at the grain boundaries inside the mullite. The Vicker’s microhardness of mullite-5 vol.% SiC composite was measured as 1320 kg/mm² under an applied indentation load of 500 g. This value gradually decreased with an increase in SiC content.     

Influence of grain size on wear behavior of SiC coating for carbon/carbon composites at elevated temperatures

To improve the wear performance of SiC coating for C/C composites at elevated temperatures, the grain was refined by adding small amounts of titanium, in the raw powders for preparing this coating. The related microstructure and mechanical characteristics were investigated by scanning electron microscopy, X-ray diffraction, energy dispersive spectroscopy and nano-indention. The results show that the grain size of SiC coating decreased from ∼30 μm to ∼5 μm due to the addition of grain refiner. TiC formed by reacting titanium with graphite, can act as perfect heterogeneous nucleus for the nucleation and growth of β-SiC. The wear resistance and fracture toughness of SiC coating was improved by grain refinement. However, the increasing interfaces increased the friction resistance and resulted in the high friction coefficient of fine-grained coating at room temperature. As the temperature rose, oxides layer formed on the surface of fine-grained coating, which can reduce the adhesive wear and decrease the friction coefficient. The fine-grained coating exhibited relative low friction coefficient of ∼0.41 owing to a compact silica film formed on the worn surface at 600 °C, and the wear was dominated by plastic deformation and shear of silica film. The wear of coarse-grained coating was controlled by the fracture of SiC at high temperature.     

Effect of size of reinforcement on thickness of anodized coatings on SiC/Al matrix composites

The effect of size of reinforcements on morphology and thickness of anodic coatings on 3.5 μm and 10 μm SiC particles reinforced 2024Al metal matrix composites (SiC_p/Al MMCs) formed in sulfuric acid was investigated with optical microscopy and scanning electron microscopy. The thickness of anodized coating on the MMCs is strongly dependent of size of SiC particles, and it is smaller for the MMC with smaller SiC particles because growth of more pores is affected when the concentration of SiC particles is fixed. The oxide/substrate interface became locally scalloped, and the anodized coatings formed on the MMCs were non-uniform in thickness, especially for the MMC reinforced by bigger particles.     

Dynamic deformation and fracture of mullite (3Al2O3·2SiO2) ceramics under hypervelocity impact

Shock-recovery experiments have been performed on mullite ceramics to clarify the effect of a phase transition on the microstructural change and deformation mechanism under shock loading. The recovered samples have been examined using the X-ray diffraction method and transmission electron microscope observation. In the samples shocked above the phase-transition pressure, an amorphization of mullite occurs. Mullite nano-crystals with grain sizes less than 10 nm are dispersed in the amorphous phase, indicating that the relatively large starting mullite crystals become nano-crystals accompanied with the amorphization. Mullite bumper-shield experiments have also been performed to examine the influence of shock-induced microstructural changes to ultimate fracture under hypervelocity impact. The results suggest that the phase transition of mullite has an effect on protection against high-velocity impact.     

Effects of particle size and distribution on the mechanical properties of SiC reinforced Al-Cu alloy composites

This paper studied the combined effects of particle size and distribution on the mechanical properties of the SiC particle reinforced Al-Cu alloy composites. It has been shown that small ratio between matrix/reinforcement particle sizes resulted in more uniform distribution of the SiC particles in the matrix. The SiC particles distributed more uniformly in the matrix with increasing in mixing time. It has also been shown that homogenous distribution of the SiC particles resulted in higher yield strength, ultimate tensile strength and elongation. Yield strength and ultimate tensile strength of the composite reinforced by 4.7 μm sized SiC particles are higher than those of composite reinforced by 77 μm sized SiC particles, while the elongation shows opposite trend with yield strength and ultimate tensile strength. Fracture surface observations showed that the dominant fracture mechanism of the composites with small SiC particle size (4.7 μm) is ductile fracture of the matrix, accompanied by the “pull-out” of the particles from the matrix, while the dominant fracture mechanism of the composites with large SiC particle size (77 μm) is ductile fracture of the matrix, accompanied by the SiC particle fracture.     

Microstructurally controlled mullite ceramics produced from monophasic and diphasic sol-derived pastes using extrusion

Mullite ceramics with controlled microstructure in terms of grain size/shape, pore and glassy phase content were produced from sol-derived pastes using extrusion. Particular attention has been given to the development of a continuous process which is suitable for the preparation of high-solids-loading mullite pastes from two different starting mullite precursors, namely, diphasic and molecular mixed mullite sols. A combined processing technique comprising vacuum filtering and pressure filtration was introduced in order to obtain extrudable mullite pastes from low solids-loading colloidal sols. It is shown that glassy phase free stoichiometric 3:2 mullite (3Al₂O₃·2SiO₂) with fine (0.94 m) equiaxed grain microstructure is achievable from monophasic precursors after pressureless sintering at 1400°C for 3 h using the developed technique which can control both the sol-derived paste microstructure and process parameters. It is also found that the room and high temperature (1300°C) flexural strength and toughness of extruded mullites are mainly controlled by the grain size, the presence and location of glassy phase, nano-inclusions and pores at the grain boundaries. Pressureless sintered mullite derived from the monophasic sol-derived pastes provides flexural strength values of 345 and 277 M Pa for room temperature and 1300°C, respectively.     

DAMAGE HEALING AND STRENGTHENING BEHAVIOUR IN INTELLIGENT MULLITE/SiC CERAMICS

Abstract Fifteen kinds of mullite/SiC samples with different microstructures were prepared in order to examine the effect of Sic volume% and Sic grain size on mullite morphology and mechanical properties. Special attention was paid to the effect of heat-treatment on fracture stress. It is shown that these materials have damage self-healing characteristics. The best mullite/SiC system, within the given test conditions, is 20% by volume of Sic, having a grain sue of 0.56 μm, and the best condition for damage healing is a 1 h heat treatment at 1300°C in an air atmosphere.     

Nanomechanical properties of functionally graded composite coatings: Electrodeposited nickel dispersions containing silicon micro- and nanoparticles

Functionally graded composite coatings constitute a class of materials which are mostly used for mechanical and tribological applications. Among these materials, nickel metal deposits with incorporation of SiC particles have excellent mechanical properties due to nickel metal and good tribological properties due to the SiC particles. In this work, nickel coatings containing different sizes of SiC particles, nanoparticles and microparticles (10 nm to 5 μm), were electrodeposited from an additive-free sulfate bath containing nickel ions and SiC particles. The material properties of the coatings were compared to nickel coatings containing microparticles (5 μm). The effect of current density, SiC content in the bath, and electroactive species concentration on the codeposition of SiC were studied. Afterwards, the effect of particle size and codeposition percentage of SiC particles on the nanomechanical properties on the morphology and structure of the electrodeposits were investigated. The coatings were analyzed with scanning electron microscopy (SEM), X-ray diffraction (XRD), nanoidentation and lateral force microscopy (LFM). The Ni–SiC electrocomposites, prepared at optimum conditions, exhibited improved nanomechanical properties in comparison to pure nickel electrodeposits. The improved properties of the composite coatings are associated to structural modifications of the nickel crystallites as well as the morphology of the electrodeposited layers. The improved nanomechanical properties of electrocomposites containing nanosized SiC particles, as compared to electrocomposites containing micron-sized SiC particles, is attributed to the increasing values of the density of embedded SiC particles with decreasing particle size and the mechanism of embedment of the SiC particles.     

High surface area neodymium phosphate nano particles by modified aqueous sol-gel method

Synthesis of nano rod shaped neodymium phosphate (NdPO₄) particles with specific surface area as high as 107 m²g⁻¹ and an average length of 50 nm with aspect ratio 5 was achieved using modified sol gel method. Crystallite size calculated from the X-ray diffraction data by applying Scherer equation was 5 nm for the precursor gel after calcination at 400 °C. NdPO₄ was first precipitated from neodymium nitrate solution using phosphoric acid followed by peptization using dilute nitric acid and further gelation in ammonia atmosphere. The calcined gel powders were further characterized by surface area (Brunauer-Emmet-Teller nitrogen adsorption analysis), Transmission electron microscopy, scanning electron microscopy, UV-vis and FT-IR analysis. Transmission electron microscopy confirms the formation of rod like morphology from the sol, gel and the calcined particles in nano size range. These particles could be compacted and sintered at as low as 1300 °C to a density of 98.5% (theoretical) with an average grain size of ∼1 μm.     

Chlorinated precursor study in low temperature chemical vapor deposition of 4H-SiC

Low temperature (1300 °C) chemical vapor deposition (CVD) of SiC has gained interest in the last years for being less demanding in terms of reaction chamber lifetime, but also for allowing higher p-type dopant incorporation. Chloride-based CVD at low temperatures has been studied using chloromethane with tetrachlorosilane or silane, respectively and with or without controlled HCl addition. In this study we explore the use of methyltrichlorosilane (MTS) at growth temperatures (1300 °C) significantly lower than what is commonly used for homoepitaxial growth of SiC (1600 °C). MTS is a molecule containing all the needed precursor atoms; its effects are compared to the standard CVD chemistry, consisting of silane, ethylene, and HCl.Very different chemistries between the two precursor systems are proposed; in the case of MTS, C/Si ratios higher than 1 were required, however using the standard chemistry ratios lower than 1 were needed to obtain a defect-free epitaxial layer. We also demonstrate the need of using Cl/Si ratios as high as 15 to achieve a growth rate of 13 μm/h for 8° off-axis 4H-SiC epitaxial layers at 1300 °C. Limitations due to the low growth temperature are discussed in light of the experimental evidence on the growth mechanism as determined by the morphology degradation and the limited growth rate. Finally a comparison between the epilayers morphology obtained on 4H-SiC substrates with different off-cuts are presented, confirming the importance of lower C/Si ratios for 4° off-axis material and the inevitable growth of the cubic SiC polytype on on-axis substrates.     

Fabrication of transparent mullite ceramic by spark plasma sintering from powders synthesized via sol–gel process combined with pulse current heating

Mullite nanopowders were synthesized by combining the advantages of the sol–gel process with the rapid synthesis provided by pulse current heating. The mullite ceramic with an infrared transmittance of 83–88% in the wavelength range from 2.5 to 4 μm with a fine grain size of 200 nm was obtained by spark plasma sintering at 1350 °C. Due to the high relative density and the small grain size, the hardness and toughness values of the sample reached 17.82 GPa and 3.6 MPa m^1/2, respectively. In contrast, when the mullite powders synthesized in a muffle furnace, an intermediate phase occurred so that the powder synthesis required high crystallization temperatures and resulted in agglomerated particles. Thus, the mullite ceramics required high temperatures for densification. As a result, the optical and mechanical properties of the ceramics were poor due to the low relative density and the elongated grain growth.     

Continuous synthesis of controlled size carbon-encapsulated iron nanoparticles

Carbon-encapsulated iron nanoparticles were continuously and selectively synthesised in a thermal plasma jet from ethanol (carbon source) and Fe powders with different grain sizes. The grain size of the Fe powder influenced the size distribution of the as-produced carbon encapsulates. The products obtained from large Fe particles (50-78 μm) were comprised of small encapsulates with diameters between 5 and 10 nm. Larger carbon encapsulates with a broad diameter distribution (10-100 nm) were synthesised from the finest Fe particles (18 μm). It was also found that Fe particle size was the most crucial parameter for determining the encapsulation yield. The encapsulation yield was also influenced by the carbon to iron ratio and the thermal conductivity of the plasma gas.     

Magnetite surface morphology during hematite reduction with CO/CO2 at 1073 K

The morphology of the magnetite phase that formed during the hematite/magnetite reduction process was studied. A very dense hematite ore of very high grade iron content (Fe = 69%) was used. Annealed ore specimens of about 2 g and 5 × 5 × 1 mm size were polished and reduced with 20%CO-80%CO₂ gaseous mixtures of total gas flow rate 1 L/min at 1073 K. The morphological observation of the surface of the specimens and its cross-section after reduction was detected by SEM. The initial formation of magnetite phase nuclei was detected as needles in shape. These needles of about 500-1000 nm diameter and length 10-15 μm come in a random distribution in the reduced surfaces. These needles are accumulated after forming a large centered area of a porous magnetite phase inside each surface grain.     

Multiphonon-based comparison of erbium-doped yttria in large, fine grain polycrystalline ceramics and precursor forms

We have studied the phonon-induced non-radiative decay in erbium doped yttria (Y₂O₃). The technique employed allows for the evaluation of potential ceramic and crystalline laser materials. The frequency of the dominant phonon that deactivates the fluorescing levels and an approximate prediction of 0 K lifetime can be determined. Results show no significant quantitative difference between very large grain polycrystalline (with grain size ∼200-500 μm) ceramic, fine grain polycrystalline (with grain size ∼0.3 μm) ceramic and the precursor powder (with ∼30 nm particle size) of Er³⁺ doped Y₂O₃, when it comes to the dominant phonon frequency and the phonon occupancy number.The results show that a correct evaluation of the final product can be made in the precursor stage of the process eliminating the need to proceed to crystalline or fully sintered ceramic form to evaluate the spectroscopic properties of the material. It should be noted that the powders must be carefully prepared and handled. Adsorbed species such as water can drastically change the effective lifetimes observed in powder samples.     

Microstructure and mechanical behavior of friction stir processed ultrafine grained Al-Mg-Sc alloy

Twin-roll cast (TRC) Al-Mg-Sc alloy was friction stir processed (FSP) to obtain ultrafine grained (UFG) microstructure. Average grain size of TRC alloy in as-received (AR) condition was 19.0 ± 27.2 μm. The grain size reduced to 0.73 ± 0.44 μm after FSP. About 80% of the grains were smaller than 1 μm in FSP condition. FSP resulted into 80% of the grain boundaries to have high angle grain boundary (HAGBs) character. Uniaxial tensile testing of UFG alloy showed an increase in yield strength (YS) and ultimate tensile strength (UTS) (by ∼100 MPa each) of the alloy with a very marginal decrease in total and uniform elongation (total - 27% in AR and 24% in UFG and uniform - 19% in AR and 14% in UFG). A theoretical model predicted that the grain refinement cannot take place via discontinuous dynamic recrystallization. Zener pinning model correctly predicted the grain size distribution for UFG alloy. From work hardening behaviors in both the conditions, it was concluded that grain boundary spacing is more important than the character of grain boundaries for influencing extent of uniform deformation of an alloy.     

An investigation of X-ray luminosity versus crystalline powder granularity

At the High-Throughput Discovery of Scintillator Materials Facility at Lawrence Berkeley National Laboratory, scintillators are synthesized by solid-state reaction or melt mixing, forming crystalline powders. These powders are formed in various granularity and the crystal grain size affects the apparent luminosity of the scintillator. To accurately predict a “full-size” scintillator's crystal luminosity, the crystal luminosity as a function of crystal granularity size has to be known. In this study, we examine Bi₄Ge₃O₁₂ (BGO), Lu₂SiO₅:Ce³⁺ (LSO), YAlO₃:Ce³⁺(YAP:Ce), and CsBa₂I₅:Eu²⁺ (CBI) luminosities as a function of crystalline grain size. The highest luminosities were measured for 600- to 1000-μm crystal grain sizes for BGO and LSO, for 310- to 600-μm crystal grain sizes for CBI, and for crystal grains larger than 165 μm for YAP:Ce. Crystal grains that were larger than 1 mm had a lower packing fraction, and smaller grains were affected by internal scattering. We measured a 34% decrease in luminosity for BGO when decreasing from the 600- to 1000-μm crystal grain size range down to the 20- to 36-μm range. The corresponding luminosity decrease for LSO was 44% for the same grain size decrease. YAP:Ce exhibited a luminosity decrease of 47% when the grain size decreased from the 165- to 310-μm crystal grains to the 20- to 36-μm range, and CBI exhibited a luminosity decrease of 98% when the grain size decreased from the 310- to 600-μm crystal grain range to the 36- to 50-μm range. We were able to very accurately estimate full-size crystal luminosities from crystalline grains that are larger than 90 μm.