Microstructure and ablation behavior of carbon/carbon composites infiltrated with Zr–Ti

Carbon/carbon(C/C) composites infiltrated with Zr–Ti were prepared by chemical vapor infiltration and reactive melt infiltration. Their microstructure and ablation behavior at different temperatures and time were investigated. The results show that C/C composites infiltrated with Zr–Ti have good interface cohesion between carbon fibers, pyrocarbon and carbide. Compared with C/C composites and C/C–ZrC composites, the synthesized sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 exhibits better ablation resistance at 2500 °C due to the newly formed protective layer composed of ZrTiO₂ pinned by ZrO₂ grains after ablation. The ablation resistance of the sample with Zr_0.57Ti_0.43C_1.01 increased gradually with the decrease of temperature from 3000 °C to 2000 °C, whereas the ablation resistance of the sample with Zr_0.83Ti_0.17C_0.92 and Ti_0.82Zr_0.18C_0.92 first increased obviously and then decreased slightly. In addition, the work indicates that surplus particles or liquid phases of oxides cannot protect the matrix, and that the liquid oxides may even cause severe ablation. Furthermore, a protective layer of oxides tends to be formed with the increase of ablation time.     

Ablation behavior and mechanism of C/C–ZrC–SiC composites under an oxyacetylene torch at 3000 °C

C/C–ZrC–SiC composites with continuous ZrC–SiC ceramic matrix were prepared by a multistep technique of precursor infiltration and pyrolysis process. Ablation properties of the composites were tested under an oxyacetylene flame at 3000 °C for 120 s. The results show that the linear ablation rate of the composites was about an order lower than that of pure C/C and C/C–SiC composites as comparisons, and the mass of the C/C–ZrC–SiC composites increased after ablation. Three concentric ring regions with different coatings appeared on the surface of the ablated C/C–ZrC–SiC composites: (i) brim ablation region covered by a coating with layered structure including SiO₂ outer layer and ZrO₂–SiO₂ inner layer; (ii) transition ablation region, and (iii) center ablation region with molten ZrO₂ coating. Presence of these coatings which acted as an effective oxygen and heat barrier is the reason for the great ablation resistance of the composites.     

Preparation,ablation behavior and mechanism of C/C-ZrC-SiC and C/C-SiC composites

ZrC precursor was synthesized by a solution approach using ZrOCl₂·8H₂O, acetylacetonate, glycerol and boron-modified phenolic resin. A ZrC yield of ~ 40.56 wt% was obtained at 1500 °C in the C/Zr molar ratio of 1:1. C/C-ZrC-SiC composites were fabricated by a combined processes of chemical vapor infiltration (CVI) and precursor infiltration and pyrolysis (PIP) using the synthesized ZrC precursor. For comparison, C/C-SiC composites were prepared by CVI. Thermogravimetric analysis showed that C/C-ZrC-SiC composites exhibited better oxidation resistance than C/C-SiC composites. After oxyacetylene torch ablation, the mass ablation rate of C/C-ZrC-SiC composites was 9.23% lower than that of C/C-SiC composites. The porous ZrO₂ skeleton in the ablation center was prone to be peeled off by the flame flow, resulting in the higher linear ablation rate of C/C-ZrC-SiC composites. The oxide layers of ZrO₂ and SiO₂ were formed on the transition and brim region of C/C-ZrC-SiC composites and acted as effective heat and oxygen barriers. For C/C-SiC composites, the C-SiC matrix was severely depleted in the ablation center and the formed SiO₂ layer in the brim region could protect the matrix against further ablation.     

Effect of Cu on the ablation properties of Cf/ZrC composites fabricated by infiltrating Cf/C preforms with Zr-Cu alloys

C_f/ZrC composites were fabricated by reactive melt infiltration at 1200 °C, Low melting Zr₇Cu₁₀, ZrCu and Zr₂Cu alloys were used as infiltrators and the effect of Cu on ablation properties of the composites was investigated. The results show that the C_f/ZrC composites exhibit excellent anti-ablative properties affected apparently by the Cu contents. With the increase of Cu in infiltrators, the linear recession rates decrease from 0.0019±0.0006 to −0.0006±0.0002 mm s⁻¹, whereas the mass loss rates increase from 0.0006±0.0003 to 0.0047±0.0009 g s⁻¹. The formation of a dense ZrO₂ protective layer and the evaporation of residual Cu are in favor of their ablation resistance.     

Microstructure and ablation behavior of SiC coated C/C–SiC–ZrC composites prepared by a hybrid infiltration process

In this study, C/C–SiC–ZrC composites coated with SiC were prepared by precursor infiltration pyrolysis combined with reactive melt infiltration. The pyrolysis behavior of the hybrid precursor was investigated using thermal gravimetric analysis-differential scanning calorimetry, X-ray diffraction, and scanning electron microscopy techniques. The microstructure and ablation behavior of the composites were also investigated. The results indicate that the composites exhibit an interesting structure, wherein a ceramic coating composed of SiC and a small quantity of ZrC covers the exterior of the composites, and the SiC–ZrC hybrid ceramics are partially embedded in the matrix pores and distributed around the carbon fibers as well. The composites exhibit good ablation resistance with a surface temperature of over 2300 °C during ablation. After ablation for 120 s, the mass and linear ablation rates of the composites are 0.0026 g/s and 0.0037 mm/s, respectively. The great ablation resistance of the composites is attributed to the formation of a continuous phase of molten SiO₂ containing SiC and ZrO₂, which seals the pores of the composites during ablation.     

Effects of porous C/C density on the densification behavior and ablation property of C/C–ZrC–SiC composites

C/C–ZrC–SiC composites were prepared by precursor infiltration and pyrolysis process using a mixture solution of organic zirconium-containing polymer and polycarbosilane as precursors. Porous carbon/carbon (C/C) composites with density of 0.92, 1.21 and 1.40 g/cm³ were used as preforms, and the effects of porous C/C density on the densification behavior and ablation resistance of C/C–ZrC–SiC composites were investigated. The results show that the C/C preforms with a lower density have a faster weight gain, and the obtained C/C–ZrC–SiC composites own higher bulk density and open porosity. The composites fabricated from the C/C preforms with a density of 1.21 g/cm³ exhibit better ablation resistance with a surface temperature of over 2400 °C during ablation. After ablation for 120 s, the linear and mass ablation rates of the composites are as low as 1.02 × 10⁻³ mm/s and −4.01 × 10⁻⁴ g/s, respectively, and the formation of a dense and continuous coating of molten ZrO₂ solid solution is the reason for their great ablation resistance.     

Fabrication and microstructure of 3D Cf /ZrC-SiC composites: through RMI method with ZrO2 powders as pore-making agent

3D C_f/ZrC–SiC composites were prepared by a combination process of slurry infiltration and reactive melt infiltration. ZrO₂ powders and ZrSi₂ alloy, both of which reacted with amorphous carbon, were used as pore-making agent and infiltrator, respectively. After carbothermal reduction at 1650 °C, X-ray diffraction analysis revealed that ZrO₂ powders were completely converted into ZrC by reacting with amorphous carbon, and an in-situ formed submicron porous configuration was observed at the areas containing ZrO₂. Results showed that the matrix in composites mainly consisted of SiC, ZrC and a small quantity of residual metal. SEM and TEM images revealed the formation of ZrC or SiC intergranular particles in the matrix and the characteristic around the residual resin carbon. The composites had a bending strength of 94.89±16.7 MPa, fracture toughness of 11.0±0.98 MPa m^1/2, bulk density of 3.36±0.01 g/cm³, and open porosity of 4.64±0.40%. The formation mechanisms of ZrC–SiC dual matrix and intrabundles׳ structure were discussed in the article.     

Hot corrosion behavior of TiO2 doped,Yb2O3 stabilized zirconia exposed to V2O5 + Na2SO4 molten salt at 700–1000 °C

3.5 mol% Yb₂O₃ stabilized zirconia (YbSZ) doped with 10 mol% TiO₂ (Ti-YbSZ) was produced, and its hot corrosion behavior exposed to Na₂SO₄ + V₂O₅ molten salt was investigated. The as-fabricated ceramic mainly consists of metastable tetragonal (t′) phase. When exposed to the molten salt at 700 °C, 800 °C, 900 °C and 1000 °C for 2 h and 10 h, YbVO₄ and m-ZrO₂ formed as corrosion products due to chemical reactions between the ceramics and the salt. Ti⁴⁺ in Ti-YbSZ solid solution keeps stable during the hot corrosion tests, which acts as a stabilizer for ZrO₂, preventing total decomposition of the t′ phase. After the hot corrosion tests, Ti-YbSZ has an apparently lower m phase content than Y₂O₃ doped Zirconia and YbSZ, indicative of better corrosion resistance. The hot corrosion mechanism of Ti-YbSZ is proposed based on Lewis acid-base rule, phase diagrams and thermodynamics.     

C/C–ZrC composite prepared by chemical vapor infiltration combined with alloyed reactive melt infiltration

A high performance and low cost C/C–ZrC composite was prepared by chemical vapor infiltration combined with zirconium–silicon (Zr: 91.2 at.%; Si: 8.8 at.%) alloyed reactive melt infiltration. The density of the as-received composite is 2.46 g/cm³ and the open porosity is 5%. Due to the reaction between the pyrolytic carbon and Zr–Si alloy in the composite, ZrC and Zr₂Si phases were formed, the formation and distribution of which were investigated by thermodynamics and phase diagram. The as-received C/C–ZrC composite, with the flexural strength of 239.5 MPa, displayed a pseudo-ductile fracture behavior. Ablation properties of the C/C–ZrC composite were tested by a pulse laser. The linear ablation rate was 0.028 mm/s. A ZrO₂ barrier layer was formed on the ablation surface and the composite presented excellent ablation resistance.     

Microstructure and mechanical properties of ZrC-TaCx composite fabricated by displacive compensation of porosity at 1300 °C

ZrC-TaC_x composites synthesized via Displacive Compensation of Porosity method 1300 °C using porous TaC preforms and Zr-Cu binary melts (Zr₂Cu, ZrCu and Zr₁₄Cu₅₁) are presented in this work. Effect of Zr content in the infiltration on microstructure is investigated by XRD, FE-SEM and TEM analysis. The results indicate that ZrC and platelet-like Ta₂C formed during the reactive infiltration of Zr-Cu melts into TaC preforms. The nano precipitations in TaC_x/ZrC grains and Ta atoms dissolved in ZrC suggests a probable dissolution-precipitation mechanism of the reaction between Zr-Cu melts and TaC. The ZrC-TaC_x composite exhibits a flexural strength of 142 MPa and a fracture toughness of 5.34 MPa m^1/2. The platelet-like Ta₂C phase has toughening effect by crack deflection and bridging.     

Reaction and phases from monoclinic zirconia and calcium aluminate cement at high temperatures

Solid state reaction using m-ZrO₂ and high alumina cement as starting materials was studied. Various compositions containing different proportions of calcium aluminate cement (5–50 mol% CaO in ZrO₂) were reaction sintered at 1300–1500 °C. Crystalline phase formation and densification of Ca stabilized ZrO₂ composites was investigated by X-ray diffraction analysis, density and shrinkage measurements. Scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy was used to examine the microstructure. The main crystalline phases formed are related to the expected with the equilibrium phase diagram of the ZrO₂–CaO–Al₂O₃ system. Stabilized c-ZrO₂ is formed with the composition of Ca_0.15Zr_0.85O_1.85. The sintering of the mixtures leads to porous composites materials. Textural properties were analyzed considering the initial composition and the present crystalline phases.     

Synthesis and pyrolysis behavior of a soluble polymer precursor for ultra-fine zirconium carbide powders

A soluble polymer precursor for ultra-fine zirconium carbide (ZrC) was successfully synthesized using phenol and zirconium tetrachloride as carbon and zirconium sources, respectively. The pyrolysis behavior and structural evolution of the precursor were studied by Fourier transform infrared spectra (FTIR), differential scanning calorimetry, and thermal gravimetric analysis (DSC–TG). The microstructure and composition of the pyrolysis products were characterized by X-ray diffraction (XRD), laser Raman spectroscopy, scanning electron microscope (SEM) and element analysis. The results indicate that the obtained precursor for the ultra-fine ZrC could be a Zr–O–C chain polymer with phenol and acetylacetone as ligands. The pyrolysis products of the precursor mainly consist of intimately mixed amorphous carbon and tetragonal ZrO₂ (t-ZrO₂) in the temperature range of 300–1200 °C. When the pyrolysis temperature rises up to 1300 °C, the precursor starts to transform gradually into ZrC, accompanied by the formation of monoclinic ZrO₂ (m-ZrO₂). The carbothermal reduction reaction between ZrO₂ and carbon has been substantially completed at a relatively low temperature (1500 °C). The obtained ultra-fine ZrC powders exhibit as well-distributed near-spherical grains with sizes ranging from 50 to 100 nm. The amount of oxygen in the ZrC powders could be further reduced by increasing the pyrolysis temperature from 1500 to 1600 °C but unfortunately the obvious agglomeration of the ZrC grains will be induced.     

Hot corrosion behavior of V2O5-coated Gd2Zr2O7 ceramic in air at 700–850 °C

Gd₂Zr₂O₇ ceramic was prepared by solid state reaction at 1650 °C for 10 h in air, and exhibited a defect fluorite-type structure. Reaction between molten V₂O₅ and Gd₂Zr₂O₇ ceramic was investigated at temperatures ranging from 700 to 850 °C using an X-ray diffractometer (XRD) and scanning electron microscopy (SEM). Molten V₂O₅ reacted with Gd₂Zr₂O₇ to form ZrV₂O₇ and GdVO₄ at 700 °C; however, in a temperature range of 750–850 °C, molten V₂O₅ reacted with Gd₂Zr₂O₇ to form GdVO₄ and m-ZrO₂. Two different reactions observed at 700 °C and 750–850 °C could be explained based on the thermal instability of ZrV₂O₇.     

Microstructure and mechanical properties of Cf/ZrC composites fabricated by reactive melt infiltration at relatively low temperature

Carbon fiber-reinforced zirconium carbide matrix composites (C_f/ZrC) were prepared by vacuum infiltrating porous carbon/carbon preforms with molten Zr₂Cu alloy at 1200 °C. X-ray diffraction, scanning electron microcopy and transmission electron microscopy analysis were used to characterize the composition and microstructure of the final composites. It was found that the matrix of the composites were composed of the Cu–Zr–C amorphous phase dispersed with either single- or polycrystalline ZrC. Based on the microstructural analysis, the formation mechanism of the matrix was proposed to be a solution-precipitation and grain coalescence process. The influence of the heat treatment at 1800 °C was also investigated. Results indicated that at very high temperature the volatilization of residual metal somewhat deteriorated the flexural strength and the elastic modulus, but the fracture toughness of the composites was improved due to the sintering of ZrC grains.     

Ultra-high temperature ceramic coating for carbon/carbon composites against ablation above 2000 K

To improve the ablation resistance of carbon/carbon composites at the temperature above 2000 K, a ZrB₂-SiC-ZrC ultra-high temperature ceramic coating was prepared by combination of supersonic atmosphere plasma spray (SAPS) and reaction melt infiltration. The micro-holes in ZrB₂-Si-ZrC coating prepared by SAPS were effectively filled and the compactness and interface compatibility between the coating and C/C composites was improved through the reaction melt infiltration process. The ultra-high temperature ceramic coating exhibited good ablation resistance under oxyacetylene torch ablation above 2000 K. After ablation for 120 s, the mass and linear ablation rates of the ZrB₂-SiC-ZrC coated C/C samples were only ?0.016 × 10^?3 g/s and 1.30 µm/s, respectively. Good ablation resistance of the ultra-high temperature ceramic coating is mainly attributed to the dense coating structure and the improvement of interface compatibility between the coating and C/C composites.     

Effect of PyC interphase thickness on mechanical and ablation properties of 3D Cf/ZrC–SiC composite

Three-dimensional (3D) C_f/ZrC–SiC composites were successfully prepared by the polymer infiltration and pyrolysis (PIP) process using polycarbosilane (PCS) and a novel ZrC precursor. The effects of PyC interphase of different thicknesses on the mechanical and ablation properties were evaluated. The results indicate that the C_f/ZrC–SiC composites without and with a thin PyC interlayer of 0.15 µm possess much poor flexural strength and fracture toughness. The flexural strength grows with the increase of PyC layer thickness from 0.3 to 1.2 µm. However, the strength starts to decrease with the further increase of the PyC coating thickness to 2.2 µm. The highest flexural strength of 272.3±29.0 MPa and fracture toughness of 10.4±0.7 MPa m^1/2 were achieved for the composites with a 1.2 µm thick PyC coating. Moreover, the use of thicker PyC layer deteriorates the ablation properties of the C_f/ZrC–SiC composites slightly and the ZrO₂ scale acts as an anti-ablation component during the testing.     

Study of structure and conductivity of Li3/8Sr7/16-3x/2LaxZr1/4Nb3/4O3 solid electrolytes

Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0, 0.05, 0.10, 0.15, 0.20) were synthesized using the conventional solid-state reaction method. In order to increase the vacancy concentration, La³⁺ was doped on the Sr²⁺ site. Crystal structures of doped samples were characterized by X-ray diffraction. Except, perovskite-type Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0, 0.05, 0.10, 0.15) samples were fabricated by heat treatment at 1250 °C, 1275 °C, 1275 °C and 1275 °C, respectively, for 15 h. Lattice sizes decreased with the increase of doping amounts because of the smaller ion radius of La³⁺ compared to that of Sr²⁺. Ionic conductivities of the samples were measured by AC impedance spectroscopy. The results showed that the ionic conductivity increases at first and then decreases with raising doping amounts and sintering temperatures. So the optimized composition Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0.05) sintered at 1275 °C was selected with the highest total conductivity of 3.33 × 10^?5 S cm^?1at 30 °C and an activation energy of 0.27 eV. Additionally, potentiostatic polarization test was used to evaluate the electronic conductivity. The optimal composition Li_3/8Sr_7/16-3x/2La_xZr_1/4Nb_3/4O₃ (x = 0.05) as a possible Li-ion conducting solid electrolyte has an electronic conductivity of only 8.39 × 10^?9 S cm^?1.     

Molten salt synthesis of La2Zr2O7 ultrafine powders

Ultrafine powders of pyrochlore-type La₂Zr₂O₇ were synthesized via a simple molten salt mediated process using zirconium oxide and lanthanum oxide as raw materials, and sodium chloride, potassium chloride and sodium fluoride to form a reaction medium. The effects of reaction temperature, salt/reactant ratio and salt type on the La₂Zr₂O₇ formation were investigated. Among the three attempted salt assemblies (KCl–LiCl, Na₂CO₃–K₂CO₃, and NaCl–KCl–NaF), NaCl–KCl–NaF showed the best accelerating effect on the La₂Zr₂O₇ formation. At a given temperature, the La₂Zr₂O₇ content in the final products increased with the increase in the salt amount. Phase pure submicron sized La₂Zr₂O₇ ultrafine powders were obtained after 3 h firing at 1100 °C with the salt/reactant weight ratio of 5:1 or at 1200 °C with salt/reactant weight ratio of 3:1. The synthesis temperature (1100 °C) was much lower than that required by the conventional solid-state mixing method or a wet chemical method. The “dissolution–precipitation” mechanism had dominated the synthesis process.     

Improving the Li-ion conductivity and air stability of cubic Li7La3Zr2O12 by the co-doping of Nb,Y on the Zr site

The elements Nb and Y were simultaneously substituted to the Zr sites of an Li₇La₃Zr₂O₁₂ (LLZO) electrolyte to improve its Li-ion conductivity and air stability. Samples of Li₇La₃Zr_2-2xNb_xY_xO₁₂ were fabricated using a solid-state reaction method. The results show that the introduction of Nb and Y can stabilise cubic-phase LLZO. The total conductivity of Li₇La₃ZrNb_0.5Y_0.5O₁₂ electrolyte can reach 8.29 × 10^?4 S cm^?1 at 30 °C when sintered at 1230 °C for only 15 h. Surprisingly, the conductivity of Li₇La₃ZrNb_0.5Y_0.5O₁₂ can be maintained at 6.91 × 10^?4 S cm^?1 after exposure to air for 1.5 months, indicating excellent air stability. Furthermore, a LiFePO₄/Li₇La₃ZrNb_0.5Y_0.5O₁₂/Li cell displayed stable charge/discharge and cycling performance at ambient temperature, suggesting there is potential to use Li₇La₃ZrNb_0.5Y_0.5O₁₂ electrolyte in Li-ion batteries. Additionally, the effects of varying the co-doping amount and dwelling time on the Li-ion conductivity of Li₇La₃Zr_2-2xNb_xY_xO₁₂ were investigated.     

Effects of ZrO2-La2O3 co-addition on the microstructural and optical properties of transparent Y2O3 ceramics

Commercial Y₂O₃ powder was used to fabricate Y₂O₃ ceramics sintered at 1600 °C and 1800 °C with concurrent addition of ZrO₂ and La₂O₃ as sintering aids. One group with different contents of La₂O₃ (0–10 mol%) with a fixed amount of 1 mol% ZrO₂ and another group with various contents of ZrO₂ (0–7 mol%) with a fixed amount of 10 mol% La₂O₃ were compared to investigate the effects of co-doping on the microstructural and optical properties of Y₂O₃ ceramics. At low sintering temperature of 1600 °C, the sample single doped with 10 mol% La₂O₃ exhibits much denser microstructure with a few small intragranular pores while the samples with ZrO₂ and La₂O₃ co-doping features a lot of large intergranular pores leading to lower density. When the sintering temperature increases to 1800 °C, samples using composite sintering aids exhibit finer microstructures and better optical properties than those of both ZrO₂ and La₂O₃ single-doped samples. It was proved that the grain growth suppression caused by ZrO₂ overwhelms the acceleration by La₂O₃. Meanwhile, 1 mol% ZrO₂ acts as a very important inflection point with regard to the influence of additive concentration on the transmittance, pore structure and grain size. The highest in-line transmittance of Y₂O₃ ceramic (1.2 mm in thickness) with 3 mol% of ZrO₂ and 10 mol% of La₂O₃ sintered at 1800 °C for 16 h is 81.9% at a wavelength of 1100 nm, with an average grain size of 11.2 µm.