New route to process uni-directional carbon fiber reinforced (SiC?+?ZrB2) matrix mini-composites

Unidirectional carbon fiber-reinforced (SiC + ZrB₂) matrix mini-composites were prepared by soft solution route. In this process, the matrix materials were prepared using water-soluble precursors of colloidal silica, sucrose, zirconium oxychloride, and boric acid as sources of silica, carbon, zirconia, and boron oxide respectively. The room temperature mechanical properties were investigated and the fracture features of the composites were examined. Tensile strength of 269 ± 36 MPa and fracture energy of 0.38 ± 0.05 MJ/m³ for the mini-composite, carbothermally reduced at 1,600 °C were attributed to the fiber pull out. In spite of a composite failure mode, the composite carbothermally reduced at 1,700 °C exhibited lower mechanical properties. It showed that carbon fibers reacted with ZrO₂ to form ZrC phase at 1,700 °C, formed chemical bonding, and led to a strong interface between fibers and matrix, which resulted in the degradation of mechanical properties of the mini-composites. The XRD and SEM investigations of the powders and the mini-composites revealed phase formation whereas cross-sectional microstructure indicated the uniform distribution of fibers within the matrix.     

Preparation of a hyperbranched polycarbosilane precursor to SiC ceramics following an efficient room-temperature cross-linking process

Room-temperature cross-linking of a hyperbranched polycarbosilane (HBPCS) with divinylbenzene (DVB) in the presence of the cyclohexanone peroxide–cobaltous naphthenate (CHP–CN) initiator system was studied. According to the Fourier transform infrared spectroscopy (FT-IR) and ¹H nuclear magnetic resonance (¹H NMR) results, the cross-linking reaction occurred via the vinyl polymerization. The GPC analysis confirmed the molecular weight of the cross-linked HBPCS significantly increased. Thermal behaviors of cross-linked HBPCS and original HBPCS were investigated by thermal gravimetric analysis-differential thermal analysis (TGA–DTA). The TGA results indicated that the ceramic yield of HBPCS remarkably increased by the cross-linking treatment. For the HBPCS/10 wt% DVB system, the maximum of reaction degree of HBPCS was obtained, which might be responsible for the highest ceramic yield of 70.1 wt% at 1000 °C. However, the ceramic yield of the non-crosslinked HBPCS was only 45 wt% at 1000 °C. The evolution of crystal structure of SiC as a function of pyrolysis temperature was traced by means of X-ray diffraction (XRD) and FT-IR. With the pyrolysis temperature increasing, the β-SiC peaks became sharper and the grain size also grew larger. As the DVB content increased, the intensity of β-SiC peaks significantly reduced, indicating smaller β-SiC grain size.     

Effect of impurities on characteristics of ZrO<Subscript>2</Subscript> and ZnO ceramic powders produced by spray pyrolysis

Previously it was observed that addition of impurities to a precursor solution may alter the size and morphology of the particles produced by spray pyrolysis. To investigate this further, the spray pyrolysis technique was used to prepare zirconia (ZrO₂) and zinc oxide (ZnO) ceramic powders, with addition of slight amounts of NaCl in various concentrations. The results show an increase in the percentage of nondisrupted particles which corresponds to an increase in the weight percentage of NaCl in the precursor in ZrO₂ powder produced at 400 °C. This effect is not repeated in ZnO powder produced at 400 °C, as the addition of NaCl to the precursor results in the disruption of individual particles into much smaller particles. As far as the morphology and strength of particles are concerned, it is concluded that the addition of NaCl to the precursor solution has a beneficiary effect on the morphology of ZrO₂ particles and an adverse effect on ZnO particles, both of which are negated at a higher reactor temperature of 600 °C.     

Sol–gel derived CaO–B2O3–SiO2 glass/CaSiO3 ceramic composites: processing and electrical properties

The CaO–B₂O₃–SiO₂ glass/CaSiO₃ ceramic (CBS/CS) composites were fabricated via sol–gel processing routes. Their densification behavior, structures and dielectric properties were investigated. The precursors of CBS glass and CS ceramic filler were firstly obtained via individual soft chemical route and then mixed together in various proportions. The results indicated that the structures of CBS/CS composites are characteristic of CS and CaB₂O₄ (CB) ceramic phases distributed in the matrix of glass phase at 800–950 °C. The CS ceramic phase not only acts as fillers, but nuclei for the crystallization of CBS glass as well such that the CS content exhibits an effect on the densification and dielectric properties of the composites. The CBS/CS composites with 10% CS sintered at 850 °C own dielectric properties of ε_r < 5 and tanδ = 6.4 × 10⁻⁴ at 1 MHz.     

ZrO<Subscript>2</Subscript> foams for porous radiant burners

In this work, Y₂O₃-stabilized ZrO₂ (YSZ) foams with low relative density were developed through the replication method, for application as porous radiant burners. The ceramic foams were produced by impregnation of open-cell polyurethane foams with aqueous suspensions and different fractions of raw materials: ZrO₂–8% Y₂O₃ (8YSZ) powder, and additives. The materials were milled for 10–40 min. The impregnated foams were dried and submitted to a heat treatment for polyurethane elimination at 1000 °C for 1 h, with subsequent sintering of the remaining ceramic structure at 1600 °C for 2 h, which resulted in YSZ foams with low relative density (0.07). The structural analysis revealed a cellular structure with an average mechanical strength of 95.6 kPa. The radiation efficiency (>19%) was obtained by tests with different air/fuel ratio. The ceramic matrixes exhibited high performance and structural integrity at high operation temperatures (1400 °C).     

The study of carbothermic reduction–sintering of ZrO<Subscript>2</Subscript>–ZrC–C composite microspheres prepared by internal gelation

ZrO₂–ZrC–C (ZrCO) ceramic microspheres were fabricated by a combination of internal gelation and carbothermic reduction. The effects of temperature on carbothermic reduction of ZrO₂ and the CO content in the heat treatment atmosphere on the phase composition, microstructure and properties of the microspheres were studied. The effects of different concentrations of CO in the reaction atmosphere on the formation of zirconium carbide were analyzed. The results indicate that addition of a small concentration of CO in the heat treatment atmosphere refined the microstructure, and high concentration of CO inhibited the formation of ZrC. Crack-free ZrCO ceramic microspheres with uniform microstructure, improved crush strength and density were successfully fabricated with C/Zr?=?3 in the broth and heat treated at 1550 °C for 4 h in argon containing 5%CO.     

Phase and structural evolution of sol–gel synthesized ZrO2/Si thin films under heat treatment

ZrO₂/Si thin films were fabricated using a sol–gel technique, and the chemical state change and structural evolution from sol to gel to Zr oxide by heat treatment were investigated. The precursor sol was synthesized using a Zr-acetylacetonate (Zr-acac) precursor, spin-coated, dried on Si(100) substrates, and then annealed at 300–700 °C in air. With increased annealing temperature of the sol–gel-derived layer, Zr-acac decomposed into Zr-acetate, an amorphous ZrO₂ phase formed, and crystallization into a tetragonal phase occurred. In addition, the ZrO₂ layer became denser and the interfacial layer between ZrO₂ and Si thickened, whereas the surface morphology was nearly unaffected and remained smooth with root-mean-square values < 4.5 ?.     

Silicone rubber ablative composites improved with zirconium carbide or zirconia

Silicone rubber ablative composites with zirconium carbide (ZrC) or zirconia (ZrO₂) were prepared and tested for thermal protection systems. The incorporation of ZrC or ZrO₂ powders increased the tensile strength but reduced the elongation at break of the composites. The thermal stabilities of the composites were enhanced by adding ZrC or ZrO₂. The ceramic layers containing ZrO₂, SiO₂ and SiC were formed on the ablated surface of the composites with ZrO₂ or ZrC, and acted as effective oxygen and heat barriers. The ablated surface of the composite with ZrO₂ was more uniform than that of the composite with ZrC in the experiment. The linear ablation rates of the composites were reduced by 40% and 72% by the incorporation of 40 phr ZrC and ZrO₂, respectively.     

Preparation of Y-TZP ceramic fibers by electrolysis-sol-gel method

Polycrystalline yttria stabilized tetragonal Zirconia (T-ZrO₂) fibers were obtained by pyrolysis of gel fibers using zirconium oxychloride octahydrate as raw material. The spinnable zirconia sol was prepared by electrolyzing the zirconium oxychloride octahydrate solution in the presence of acetic acid and sugar (sucrose, glucose or fructose), in which the molar ratio of CH₃COOH/ZrOCl₂ · 8H₂O and sugar/ZrOCl₂ · 8H₂O was in the range of 1.0–4.0 and 0.2–0.4, respectively. The relation of spinnability to the shape of colloidal particle was discussed. The as-prepared zirconia fibers sintered at different temperatures show smooth and crack-free surface with the diameter of 5–10 μm. Slow heating rate below 600 °C and then sintering at 1,400 °C for 30 min were necessary to obtain the dense tetragonal zirconia ceramic fibers, the particles composed the fibers had the size of ∼150 nm.     

Fabrication and oxidation behavior of a reaction derived graphite–ZrC composite for ultrahigh temperature applications

Graphite containing nominally 40 vol.% ZrC (graphite–ZrC) was prepared from commercially available ZrO₂ and graphite powders by hot pressing at 2000 °C in a vacuum. The oxidation behavior of the graphite–ZrC composite was carried out in dry stagnant air at the temperatures of 1200 and 2200 °C. Compared with the pure graphite, the graphite–ZrC composite exhibited good oxidation resistance because the mass loss of the composite powder was significantly lower than that of the pure graphite. The mass loss of graphite–ZrC at 2200 °C was lower than pure graphite at 1200 °C. Furthermore, the introduction of ZrC also improved the strength of the graphite–ZrC composite.     

Production of opaque frits with low ZrO<Subscript>2</Subscript> and ZnO contents and their industrial uses for fast single-fired wall tile glazes

The amounts of zirconium and zinc oxides, which raise the production costs of ceramic glazes, were decreased in fast single-fired wall tile frit compositions and an industrial frit production was conducted. Opacity of the fired frit-based glazes was accomplished by compositional modifications of frits with no other nucleating agent. It was determined that the ratios of Al₂O₃/ΣR₂O, Al₂O₃/ΣRO, and Al₂O₃/B₂O₃ have significant effects on decreasing ZrO₂ and ZnO levels in the frit composition. A reduction of 25% in both zirconia and zinc oxide contents of frit batches, with respect to the reference frit (R) containing 6–10% ZrO₂ and 6–10% ZnO for a glossy white opaque wall tile glaze, was achieved in the ZD glaze consisting of 4.5–7.5% zirconia and 4.5–7.5% ZnO in its frit composition. It was concluded that zircon was the main crystalline phase of the glaze contributing the opacity. The ZD frit-based glaze has a thermal expansion coefficient value of 61.13 ± 0.32 × 10⁻⁷ °C⁻¹ at 400 °C which well matches to that of the wall tile body. TS EN ISO 10545 standard tests were also applied to the final ZD glaze. It is confirmed that the production cost of a fast single-fired wall tile glaze can be decreased by 15–20% with the successful new frit developed.     

Processing and thermal stability of a Cfiber/SiCfiller/Si–C–Nmatrix composite: effect of oligomer vaporization and the surface oxide layer of SiC filler

The processing of carbon fiber-reinforced ceramic matrix composites (CMC) made by the precursor impregnation and pyrolysis (PIP) method was improved, and factors which deteriorate the thermal stability of the CMC were investigated. The processing time for cross-linking of a precursor polymer was substantially reduced by the application of a sealed metal container due to the suppression of the vaporization of oligomers. The strength of the as-fabricated CMC was 286 MPa and 77% of the original strength was retained after a heating at 1350 °C for 24 h in Ar. The reduction of the strength after the heating was due to the decomposition of SiO₂ which remained at the surface of the SiC filler particles. The decomposition reaction induced deterioration of carbon fibers and the matrix of the CMC at high temperature.     

Dielectric property and electrical conduction mechanism of ZrO2–TiO2 composite thin films

ZrO₂–TiO₂ composite films were fabricated by radio frequency magnetron sputtering and post annealing in O₂. It was found the films remained amorphous below the annealing temperature of 500 °C. The as-deposited ZrO₂–TiO₂ film has a high dielectric constant of 22, and which increases to 34 after annealing at 400 °C. At low electric field, the dominant conduction mechanisms are Schottky emission for both the as-deposited and the annealed thin films. At high electric field, the conduction mechanism changes to space-charge-limited current and then changes to Poole–Frenkel (PF) emission after annealing at 400 °C.     

Spark plasma sintering of ZrB<Subscript>2</Subscript>–ZrC powder mixtures synthesized by MA-SHS in air

Mechanical activation-assisted self-propagating high-temperature synthesis (MA-SHS) in air was successfully applied to the synthesis of the powder mixtures of ZrB₂ and ZrC as a precursor of the ZrB₂–ZrC composite. When the powder mixtures of Zr/B/C = 4/2/3–6/10/1 in molar ratio were mechanically activated (MA) by ball milling for 45–60 min and then exposed to air, they self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) was occurred to form ZrB₂ and ZrC. The ZrB₂–ZrC composites were produced from these MA-SHS powders by spark plasma sintering (SPS) at 1800 °C for 5–10 min and showed the fine and homogeneous microstructure composed of the <5 μm-sized grains. The mechanical properties of the composites evaluated by Vickers indentation method showed the values of Vickers hardness of 13.6–17.8 GPa and fracture toughness of 2.9–5.1 MPa·m^1/2, depending on the molar ratio of ZrB₂/ZrC. Thus, the better microstructure and mechanical properties of the ZrB₂–ZrC composites were obtained from the MA-SHS powder mixtures, compared with those obtained from the MA powder, the mixing powder and the commercial powder mixtures.     

A novel chemical route for the preparation of nanosized oxides, phosphates, vanadates, molybdates and tungstates using polymer precursors

A variety of nanosized (particle diameter ranging between 10–90 nm) ceramic oxide powders have been prepared from a versatile, efficient and technically simple polymer matrix based precursor solution. The precursor solution constituted of the metal nitrates mixed with the polymer-PVA/PAA/carboxylated starch in presence of mono-/disaccharides or, poly hydroxy compound. Thermolysis/flame pyrolysis of the precursor mass at external temperatures of around 300–500°C resulted in the oxide phase.     

Effects of Cf/C density on microstructure and properties of 3-D Cf/ZrC composites prepared by liquid metal infiltration

Three-dimensional braided carbon fiber-reinforced ZrC matrix composites, 3-D C_f/ZrC, were fabricated by Liquid metal infiltration process at 1200 °C. Porous carbon/carbon (C_f/C) composites with various densities were used as preforms, and the effects of C_f/C density on microstructure and properties of the 3-D C_f/ZrC composites were investigated. The results show that the composites are composed of carbon, ZrC and residual metal. Both microstructure and properties of the 3-D C_f/ZrC composites are apparently affected by C_f/C density. With increasing density of C_f/C preform, the density of 3-D C_f/ZrC composites decreases while the open porosity increases. The composites obtained from the C_f/C preform with a density of 1.12 g/cm³ have the best mechanical properties, with flexural strength of 286.2 ± 11.4 MPa, elastic modulus of 83.5 ± 6.8 GPa and fracture toughness of 9.2 ± 0.6 MPa m^1/2. The composites exhibit excellent ablation resistance, and the mass rate and the linear ablation rate under an oxyacetylene torch are as low as 5.1 ± 0.4 mg s⁻¹ and 1.1 ± 0.3 μm s⁻¹, respectively.     

Poly(furfuryl alcohol)-assisted pyrolysis synthesis of ceramic nanoparticles for solid oxide fuel cells

A pyrolysis synthesis method was developed to prepare ceramic nanoparticles for the fabrication of solid oxide fuel cells. Furfuryl alcohol was used as a polymerizable solvent to dissolve metal nitrates and then polymerized into poly(furfuryl alcohol) (PFA). During the pyrolysis at 600 °C, a mixture of nitrates/PFA was converted into ceramic nanoparticles/carbon networks nanocomposite, and the carbon networks act as a barrier to prevent the aggregation of newly formed nanoparticles during particle crystallization. Dispersible nanoparticles with particle sizes ranging from 40 nm to 200 nm were obtained after burning off carbon networks in air. As an example, Ce_0.8Sm_0.2O_1.9 nanoparticles were synthesized to prepare solid oxide fuel cells, and the fuel cells achieved maximum power densities of 444.5, 625.5 and 684 mW cm^?2 at 500 °C, 550 °C and 600 °C, respectively. Our study shows that the pyrolysis synthesis method described here is promising for the effective synthesis of high quality ceramic nanoparticles.     

Curing and pyrolysis of polysiloxane/divinylbenzene and its derived carbon fiber reinforced Si—O—C composites

The curing and pyrolysis of hydrogen-containing polysiloxane (PSO) and divinylbenzene (DVB) were investigated in this paper. It was found that H₂PtCl₆ was an effective catalyst for the curing of DVB/PSO. The mass ratio of DVB/PSO had great effect on ceramic yield. The cured DVB/PSO with a mass ratio of 0.5:1 had the highest ceramic yield (76%) at temperature up to 1000°C, and its pyrolysates consisted of 38.3 wt% silicon, 27.4 wt% oxygen, and 34.3 wt% carbon of which 26.3 wt% was free carbon. The composition and structure of pyrolysates of DVB/PSO were changed with increasing pyrolysis temperature. The pyrolysis behavior of DVB/PSO was characterized by thermal analysis. DVB/PSO-derived Si–O–C composites reinforced with carbon fiber cloth (C_f/Si–O–C) were fabricated. The results showed that the flexural strength of C_f/Si–O–C composites could be increased from 118.00 ± 5.00 MPa to 139.78 ± 7.68 MPa if the pyrolysis temperature was elevated from 1000 to 1400°C, which was ascribed to the weakened interfacial bonding.     

Effect of HNO<Subscript>3</Subscript> on crystalline phase evolution in lithium silicate powders prepared by sol–gel processes

An interesting observation is reported on the dramatic effect of HNO₃ on crystalline phase evolution in the 33.3 mol% Li₂O–SiO₂ glass–ceramic (stoichiometric composition of lithium disilicate Li₂Si₂O₅, LS₂) prepared by sol–gel processes from tetraethylorthosilicate (TEOS) and lithium ethoxide precursors. Nitric acid (65%), in molar ratio HNO₃/TEOS = 0.1, was added either to the precursor sol or to 95 °C dried gel. The product, which is amorphous at temperatures below 450 °C, transforms into crystalline lithium metasilicate (Li₂SiO₃, LS) at around 550 °C (starting temperature ∼450 °C), instead of forming crystalline LS₂. Phase separation in the glassy phase may be responsible for the formation of lithium metasilicate. XRD, ²⁹Si MAS, and ⁷Li static NMR were used to follow the crystallization evolution and network structures of the materials heat-treated at various temperatures.     

The Effect of Sintering Temperature Variation on the Superconducting Properties of ErBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">δ</Emphasis></Subscript> Superconductor Prepared via Coprecipitation Method

The effect of heat treatment on the superconducting properties of ErBa₂Cu₃O_7−δ (ErBCO) ceramic materials has been studied. The nano-metal oxalate precursor was prepared using coprecipitation (COP) method. The prepared materials were subjected to calcination process at 900 °C for 12 h and then sintered under oxygen environment for 15 h at 920 °C, 930 °C, 940 °C, and 950 °C, respectively. All samples showed a metallic behavior and single-step transition in the R–T curves. The best zero critical current, T _C(R=0)=91.4 K, was for the sample sintered at 920 °C. XRD data showed single phase of an orthorhombic structure. As the sintering temperature increases, the formation of nonsuperconducting phases (impurities) was observed when the samples sintered above 920 °C. The formation of nano-oxalate powders via COP method is a very efficient procedure to produce high-quality superconductors with less processing temperature required.