The kinetics and mechanism of combusted Zr–B–Si mixtures and the structural features of ceramics based on zirconium boride and silicide

The study focuses on investigation of the combustion kinetics and mechanisms, as well as the phase- and structure formation processes, during elemental synthesis of ceramics based on zirconium diboride and silicide doped with aluminum. The effect of the degree of dilution with an inert component and initial temperature T₀ on the combustion kinetics of the Zr–Si–Al–B mixture is studied. An increase in T₀ in the range of 298–700 K causes a directly proportional rise in the combustion temperature T_c and rate U_c, which demonstrates that staging of the reactions of formation of zirconium boride and silicide remains invariant. The effective activation energy E_eff of the combustion process is 225 kJ/mol, suggesting that the liquid-phase processes have a decisive effect on the reaction kinetics. The interaction of zirconium with boron and silicon runs through the Zr–Si–Al–B melt that is formed in the combustion zone. Staging of chemical transformations during phase and structure formation of SHS products is studied. The primary ZrB₂ grains crystallize from the melt in the combustion zone; the ZrSi silicide phase is formed with a delay of no longer than 0.5 s. Compact ceramics with composition ZrB₂–ZrSi–ZrSi₂–ZrSiAl₂ synthesized by forced SHS- pressing showing a great potential for high-temperature applications both as a construction material and as a precursor for ion-plasma deposition of coatings.     

Conditions for fabricating single-phase (Ta,Zr)C carbide by SHS from mechanically activated reaction mixtures

The process parameters for the fabrication of binary carbide (Ta, Zr)C using MA–SHS were optimized. After MA processing in air, the SHS product was the virtually single-phase binary carbide (Ta, Zr)C with a ZrO₂ content less than 3%. Activation of the exothermic mixture in argon or under vacuum did not give rise to the single-phase product because the TaC and ZrC phases were present in addition to (Ta, Zr)C.Introduction of excess carbon, an increase in the combustion and post-reaction temperatures, and an increase in the duration of thermal relaxation using a “chemical oven” yielded the single-phase (Ta, Zr)C solid solution with lattice parameter a=0.4493 nm, which corresponds to 17.5 at% of dissolved ZrC. Dense samples were fabricated from submicron-sized carbide powder via hot pressing and spark plasma sintering; their properties were studied.     

Low-temperature synthesis of tantalum carbide by facile one-pot reaction

Tantalum carbide (TaC) was synthesized by polycondensation and carbothermal reduction reactions from an inorganic hybrid. Tantalum pentachloride (TaCl₅) and phenolic resin were used as the sources of tantalum and carbon, respectively. FTIR of as-synthesized dried complexes revealed formation of Ta-O. Pyrolysis of the complexes at 800 °C/1 h under argon resulted in tantalum oxide which after heat treatment at 1000–1200 °C transformed to tantalum carbide. The mean crystallite size of the precursor-derived TaC ceramics was less than 40 nm and Ta and C elements were homogeneously distributed in the ceramic samples. Mechanism for formation of TaC ceramic was analyzed.     

Densification,microstructure, elastic and mechanical properties of reactive hot-pressed ZrB2–ZrC–Zr cermets

In this study, cermets composed of zirconium diboride and zirconium carbide with intergranular zirconium were sintered by reactive hot-pressing. Relative density exceeding 97% was obtained for the sintered cermets having four distinct compositions varying in concentration of excess Zr. Their densification behaviour was examined by monitoring displacement during sintering. The microstructure was characterized by scanning electron microscopy and X-ray diffraction, and the elastic and mechanical properties were evaluated at room temperature. The effects of Zr concentration on the densification and mechanical properties were assessed. The ZrB₂ and ZrC micron-grains coarsened with increasing amount of Zr starting material. In addition, the cermets exhibited high flexural strength (546–890 MPa) and fracture toughness (6.63–10.24 MPa m^1/2), which simultaneously increased with increasing Zr concentration. However, the elastic moduli and hardness (11–18 GPa) decreased with increasing Zr. The shear modulus and Young's modulus were in the range of 150–190 GPa and 360–440 GPa, respectively.     

The ZrC–C eutectic structure and melting behaviour: A high-temperature radiance spectroscopy study

A fast spectro-pyrometer has been employed for radiance measurements of zirconium carbide samples laser-heated to very high temperature, for compositions 0.7 ≤ C/Zr ≤ 2.61 and in a spectral range 0.550 μm ≤ λ ≤ 0.900 μm. The ZrC–C eutectic temperature has been taken as the radiance reference. The measured normal spectral emissivity (NSE) ?_λ of solid zirconium carbide is close to 0.6 at 0.650 μm, in agreement with previous literature. Its high-temperature behaviour, value in the liquid, carbon-content and wavelength dependences in the visible-near infrared range have been determined here for the first time. Liquid zirconium carbide seems to interact with electromagnetic radiation in a more metallic way than the solid. A considerable NSE increase has been observed at increasing carbon content, which can be interpreted on the basis of preferential growth along the “c” plane of the carbon lamellae in the eutectic structure.     

Preparation and characterization of ultrafine ZrB2–SiC composite powders by a combined sol–gel and microwave boro/carbothermal reduction method

A combined sol–gel and microwave boro/carbothermal reduction technique was investigated and used to synthesize ultrafine ZrB₂–SiC composite powders from raw starting materials of zirconium oxychloride, boric acid, tetraethoxysilane and glucose. The effects of reaction temperature, molar ratios of n(B)/n(Zr) and n(C)/n(Zr+Si) on the synthesis of ultrafine ZrB₂–SiC composite powders were studied. The results showed that the optimum molar ratios of n(B)/n(Zr) and n(C)/n(Zr+Si) for the preparation of phase pure ultrafine ZrB₂–SiC composite powders were 2.5 and 8.0, respectively, and the firing temperature required was 1300 °C. This temperature was 200 °C lower than that require by using the conventional boro/carbothermal reduction method. Microstructures and phase morphologies of as-prepared ultrafine ZrB₂–SiC composite powders were examined by field emission-scanning electron microscopy (FE-SEM) and transmission electron microscope (TEM), showing that SiC grains were formed evenly among the ZrB₂ grains, and the grain sizes of ZrB₂ in the samples prepared at 1300 °C for 3 h were about 1–2 μm. The average crystalline sizes of these two phases in the as-prepared samples were calculated by using the Scherrer equation as about 58 and 27 nm, respectively.     

SHS of Ti3SiC2: ignition temperature depression by mechanical activation

The influence of high-energy mechanical alloying on the self-propagating high-temperature synthesis (SHS) of titanium silicon carbide (Ti₃SiC₂) was investigated. A depression of the SHS ignition temperature as a function of milling time was observed. After 107 min of milling, a spontaneous combustion reaction (MASHS) occurred within the milling vial at 67 °C, which corresponded with an 25 °C rise in the vial temperature. Observed changes in the microstructure of the milled powders gave considerable insight into the process, which provides a means of controlling a previously difficult synthesis procedure.     

Microstructure evolution and tribological properties of TiB2/Ni–Ta cermets

Cermets containing TiB₂ and single or mixed metals were produced by conventional hot-pressing technique at 2100 °C for 1 h. Nickel, tantalum and their mixtures were used as alloying substances to enhance the density of TiB₂ composites. The influence of metal addition on the microstructure and tribological properties were investigated. The addition of Ta powder greatly refined the microstructure of sintered samples. Similarly, the mixture of Ni and Ta metals hindered the grain growth of TiB₂ particles during the hot-pressing while the samples were sintered up to 98% of theoretical density. The wear behaviour of the composites was assessed by ball on disk tests. The wear rate against alumina counterbody varied in the range of (5.9–21.2) × 10^?6 mm³/Nm. The friction coefficient was not affected significantly by the alloying substances and only slightly increased from 0.58 for pure TiB₂ to 0.67 for samples with Ta addition.     

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.     

Enhanced piezoelectric properties of (Na0.53K0.47)(Nb1−xTax)O3 ceramics by Ta substitution

The effects of Ta substitution for B-site Nb in (Na_0.53K_0.47)(Nb_1?xTa_x)O₃ (NKNT) ceramics were investigated in the range of x = 0–0.6. It was found that polymorphic phase transitions (PPT) were significantly influenced by Ta substitution. Transitions among orthorhombic, tetragonal, and cubic phases in sequence with temperature, T_O-T and T_C, respectively, decreased linearly with x. At x = 0.45, T_O-T was reduced to room temperature from 182 °C at x = 0, and subsequently piezoelectric coefficient (d₃₃) at room temperature was enhanced up to 284 pC/N from 120 pC/N at x = 0 due to the coexistence of ferroelectric orthorhombic and tetragonal NKNT phases. With x further increasing beyond x = 0.45, d₃₃ decreased due to there being no orthorhombic but only a tetragonal NKNT phase at room temperature with T_O-T below room temperature.     

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.     

Synthesis of nanosized zirconium carbide by a sol–gel route

Nanosized zirconium carbide was synthesized by a new simple sol–gel method using zirconium n-propoxide, acetic acid as chemical modifier, and saccharose as carbon source. When heat-treated at 900 °C under flowing argon, gels transformed into intimately mixed amorphous carbon and nanosized tetragonal ZrO₂. Further heat treatments above 1200 °C led to the formation of zirconium carbide with some dissolved oxygen in the lattice. Oxygen content could be reduced by increasing the heat treatment temperature from 1400 to 1600 °C, which unfortunately also induced a mean crystallites size increase from 90 to 150 nm. Short heat treatments above 1600 °C were carried out to further purify the samples and to limit the particles growth. A compromise between purity and average crystallite's size could then be found. Powders were assessed using X-ray diffraction, thermal analysis and scanning electron microscopy.     

Straightforward synthesis and structural characterization of the first alkoxy-zircono-silsesquioxanes — Potential models for zirconia–silica epoxidation catalysts: Molecular hybrid materials mimicking solution exchange in MOFs

The first representatives of alkoxy-zircono-silsesquioxane compounds, dinuclear [Cy₇Si₇O₁₂]Zr(ROH)(μ-OR)₂Zr(ROH)[O₁₂Si₇Cy₇], where R = ⁿPr, ⁿBu, ^tBu; Cy = c-C₆H₁₁, and [Cy*₇Si₇O₁₂]Zr(μ-ROH)(μ-OR)₂Zr[O₁₂Si₇Cy*₇], R = ^tBu; Cy* = c-C₅H₉, have been prepared with quantitative yield by interaction of the corresponding zirconium alkoxides with the cycloalkyl-substituted cage silsesquioxanes in hexane. The X-ray single crystal study of the n-butoxide derivative revealed that zirconium atoms are hexacoordinated with the 3 oxygen atoms of the cage ligand, 2 — from the bridging alkoxide groups and one — from the solvating alcohol molecule in the coordination sphere. The molecule is additionally supported by a hydrogen bond between the solvating alcohol and an oxygen atom belonging to the cage ligand coordinated by the other zirconium atom. The structures of produced silsesquioxanes display behavior typical for metal–organic frameworks as they crystallize initially as hexane clathrates, but lose the solvent on storage in inert atmosphere. This results in formation of empty channels situated along the c-axis.     

Formation of chromium borides by combustion synthesis involving borothermic and aluminothermic reduction of Cr2O3

Chromium borides of various phases were fabricated through combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) by adopting the powder compacts of Cr₂O₃ + xB (with x = 4–9) and Cr₂O₃ + 2Al + yB (with y = 1–8). Because aluminothermic reduction of Cr₂O₃ is more exothermic than borothermic reduction, the reaction temperature and combustion front velocity of the Al-added samples are much higher than those of the Cr₂O₃–B samples. In agreement with the composition dependence of reaction exothermicity, the fastest combustion wave was observed in the compact with x = 6 for the Cr₂O₃–B sample and y = 4 for the Cr₂O₃–Al–B sample. According to the XRD analysis, Cr₅B₃, CrB, and CrB₂ were produced in the monolithic form respectively from the Cr₂O₃–B samples of x = 4, 5, and 9, or in the composite form from samples of other stoichiometries. On the other hand, five different borides were identified in Al₂O₃-added products. Among them, Cr₂B, CrB, and CrB₂ were yielded as the sole boride compound from the Al-added samples of y = 1.2, 3, and 7 or 8, respectively. Cr₅B₃ and Cr₃B₄ were produced along with CrB as the secondary phase. Based upon experimental evidence, it was found that an excess amount of boron in the reactant mixture was required to facilitate the formation of chromium borides.     

Microstructure and properties of ZrB2–SiC composites prepared by spark plasma sintering using TaSi2 as sintering additive

ZrB₂–SiC composites were fabricated by spark plasma sintering (SPS) using TaSi₂ as sintering additive. The volume content of SiC was in a range of 10–30% and that of TaSi₂ was 10–20% in the initial compositions. The composites could be densified at 1600 °C and the core–shell structure with the core being ZrB₂ and the shell containing both Ta and Zr as (Zr,Ta)B₂ appeared in the samples. When the sintering temperature was increased up to 1800 °C, only (Zr,Ta)B₂ and SiC phases could be detected in the samples and the core–shell structure disappeared. Generally, the composites with core–shell structure and fine-grained microstructure showed the higher electrical conductivity and Vickers hardness. The completely solid soluted composites with coarse-grained microstructure had the higher thermal conductivity and Young's modulus.     

Hydrothermal and electrochemical growth of complex oxide thin films for electronic devices

Thin film growth of complex oxides including BaTiO₃, SrTiO₃, BaZrO₃, SrZrO₃, KTaO₃, and KNbO₃ were studied by the hydrothermal and the hydrothermal–electrochemical methods. Hydrothermal–electrochemical growth of ATiO₃ (A = Ba, Sr) thin films was investigated at temperatures from 100 to 200 °C using a three-electrode cell. Current efficiency for the film growth was in the range from ca. 0.6 to 3.0%. Tracer experiments revealed that the ATiO₃ film grows at the film/substrate interface. Thin films of AZrO₃ (A = Ba, Sr) were also prepared on Zr metal substrates by the hydrothermal–electrochemical method. By applying a potential above ca. +2 V versus Ag/AgCl to the Zr substrates, AZrO₃ thin films were formed uniformly. Thin films of KTaO₃ and KNbO₃ were prepared on Ta metal substrates by the hydrothermal method. Perovskite-type KTaO₃ thin films were formed in 2.0 M KOH at 300 °C. Pyrochlore-type K₂Ta₂O₆ thin films were formed at lower temperatures and lower KOH concentrations.     

Synthesis,structure and catalytic properties of a novel zirconium guanidinato complex [Zr{ArNC(NMe2)N(SiMe3)}(μ2-Cl)Cl2]2[Ar = 2,6-iPr2-C6H3]

A novel non-symmetric zirconium guanidinato complex, [{Zr{ArNC(NMe₂)N(SiMe₃)}(μ₂-Cl)Cl₂}₂] (Ar = 2,6-ⁱPr₂-C₆H₃), was synthesized and structurally characterized. Catalytic studies showed that the zirconium complex was active for ethylene polymerization with the activity of 4.98 × 10⁵ g PE/mol Zr h. The influences of cocatalysts, Al/Zr molar ratios and ethylene pressures on the activities were investigated.     

Improving electrical conductivity and wear resistance of hafnium nitride films via tantalum incorporation

Transition metal nitrides are being widely applied, as durable sensors, semiconductor and superconductor devices, their electrical conductivity and wear resistance having a significant influence on these applications. However, there are few reports about how to improve above properties. In this paper, tantalum was incorporated into hafnium nitride films through Hf_1-xTa_xN_y [x=Ta/(Hf+Ta), y=N/(Hf+Ta)] solid solution. The electrical conductivity and wear resistance of the films were significantly improved, due to the increase of the electron concentration (tantalum has one more valence electron than hafnium) and the increase in H/E and H³/E² ratios caused by the effect of solid solution hardening, respectively. The highest electrical conductivity of Hf_1-xTa_xN_y films is 8.3×10⁵ S m⁻¹, which is 1.7 times and 5.2 times of that of hafnium nitride and tantalum nitride films, respectively. In addition, the lowest wear rate of films is 1.2×10⁻⁶ mm³/N m, which is only 10% and 48% of that of hafnium nitride and tantalum nitride films, respectively. These results indicate that alloying with another transition metal is an effective method to improve electrical conductivity and wear resistance of transition metal nitrides.     

Combustion of Al–Al2O3 mixtures in air

An experimental study on the aluminum oxynitride and aluminum nitride formation by combustion of mixtures of micron-sized aluminum powder (average particle diameter a_s ∼9.0 μm) and alumina nanopowder (a_s ∼0.05 μm) of the fixed mass (∼10 g) and different mass ratios (Al/γ-Al₂O₃ = 0.1–19.0) in air is reported. The formation of aluminum oxynitride (Al₃O₃N) and aluminum nitride during the combustion of powdery aluminum-based mixtures in air is discussed in this study. The combustion synthesis of Al₃O₃N and AlN was carried out in self-sustaining way. XPS-FESEM, XRD and chemical analysis were executed on final products of synthesis. The combustion process was also recorded by a video-camera. It was found that powdery mixtures, ignited by local heating, burned in one- or two-stage self-propagating regime. The combustion regime is different for different initial mass ratios Al/γ-Al₂O₃ and mainly depends on the content of fuel (aluminum powder) in mixture.     

Large E-field induced strain and polar evolution in lead-free Zr-doped 92.5%(Bi0.5Na0.5)TiO3–7.5%BaTiO3 ceramics

Structure, dielectric permittivity, strain, electric (E) polarization, and piezoelectric responses of (Bi_1/2Na_1/2)_0.925Ba_0.075(Ti_1−xZr_x)O₃ (BNT7.5BT-100xZr; x = 0–0.04) ceramics were investigated as functions of poling E field and temperature. The BNT7.5BT ceramic reveals a phase transition from P4bm nanodomains to long-range-ordered P4mm domains. The Zr-doped BNT7.5BT ceramic reveals a reversible change of unit cell with dynamically fluctuating polar nanoregions, which are responsible for the large strain. The poled BNT7.5BT ceramic displays a depolarization temperature of T_d = 90 °C, which correspond to a phase transition from ferroelectric to relaxor states. The Zr-doped BNT7.5BT ceramics have Burns temperatures (T_B) in the region of 400–435 °C, below which polar nanoregions begin to develop. The Zr-doped BNT7.5BT ceramics display wide diffuse phase transitions, suggesting a transition from R + T to T phases. BNT7.5BT-2Zr ceramic shows a temperature dependent linear large strain of 0.482% at 150 °C and can be a potential candidate for lead-free actuator.