Novel class of precursor-derived Zr–La–B–C(O) based ceramics containing nano-crystalline ultra-high temperature phases stable beyond 1600 °C

In this work, a novel ultra-high temperature resistant precursor-derived ceramic containing Zr, La, B, and C was synthesized through precursor modification of phenol formaldehyde resin. The thermal stability and resistance to crystallization of the ceramic at a temperature of 1600 °C was investigated and was found to be profoundly influenced by the boron content in the starting precursors. The ceramics remained amorphous at 1600 °C for 2 h in argon and upon sustained heat-treatment for up to 16 h resulted in nano-crystalline ultra-high temperature phases such as ZrB₂, ZrC, LaB₆ and La₂Zr₂O₇. Thermodynamic equilibrium phase calculations show that even longer durations of heat treatment may be required to achieve thermodynamic equilibrium. High-resolution transmission electron microscopy revealed encapsulation of nanocrystals (<5 nm) in an amorphous matrix surrounded by turbostratic layers of carbon inhibiting its growth. Spectrochemical techniques confirmed the presence of boron substituted carbon in the amorphous matrix of the ceramic. The unique nature of the amorphous matrix lends the ceramic resistance to crystallization and chemical degradation that can surpass the likes of classical silicon-based precursor-derived ceramics.     

Effect of carbon content on electrical,thermal, and mechanical properties of porous SiC ceramics with B4C and C additives

The electrical, thermal, and mechanical properties of porous SiC ceramics with B₄C-C additives were investigated as functions of C content and sintering temperature. The electrical resistivity of porous SiC ceramics decreased with increases in C content and sintering temperature. A minimal electrical resistivity of 4.6 × 10^?2 Ω·cm was obtained in porous SiC ceramics with 1 wt% B₄C and 10 wt% C. The thermal conductivity and flexural strength increased with increasing sintering temperature and showed maxima at 4 wt% C addition when sintered at 2000 °C and 2100 °C. The thermal conductivity and flexural strength of porous SiC ceramics can be tuned independently from the porosity by controlling C content and sintering temperature. Typical electrical resistivity, thermal conductivity, and flexural strength of porous SiC ceramics with 1 wt% B₄C-4 wt% C sintered at 2100 °C were 1.3 × 10^?1 Ω·cm, 76.0 W/(m·K), and 110.3 MPa, respectively.     

Preparation of mullite ceramics with equiaxial grains from powders synthesized by the sol-gel method

Four different alumina content of mullite ceramics were fabricated by powders synthesized using the sol-gel method. The synthesis process of powders, microstructure evolution, mechanical and optical properties of the mullite ceramics were studied. The XRD results showed that the precursors transformed into aluminosilicate spinel phase at 1000 °C and mullite phase at 1200 °C. Equiaxial grains were easy to form in the alumina-rich mullite ceramics while elongated grains were easy to form in the alumina-poor mullite ceramics. With the increase of alumina content, the grain size of the samples firstly increased and then decreased, the number of elongated grains decreased while equiaxed grains increased. The flexural strength, compression strength, fracture toughness, and Vickers hardness all decreased firstly and then increased. While the infrared transmittance increased firstly and then decreased. The transmittance at 4 μm (thickness of 0.75 mm) of the ceramics containing 66mol% Al₂O₃ reached the highest (72%) when sintered at 1780 °C because of the equiaxial grains.     

Transparent polycrystalline MgO ceramic fabricated by low-temperature vacuum sintering with MgF2 additive

This study introduces transparent MgO ceramics produced via simply vacuum sintering at 1200–1500 °C by optimal incorporation of MgF₂ as a sintering additive. The effect of MgF₂ content and sintering temperature on the densification process, optical, and thermal properties of MgO ceramics is presented with emphasis on its function as a sintering aid and adverse effect of MgF₂ evaporation in the condition of high MgF₂ content or high sintering temperature. MgO ceramic with 1.0 mol% MgF₂ sintered at 1300 °C exhibits the highest relative density of 99.95% with average grain size of 17.46 μm. The in-line transmittance attains 60% at 1000 nm and >80% in the infrared range (3.8–6.8 μm), without absorption bands originated from the carbon contamination. The corresponding room-temperature thermal conductivity reaches 47.25 W/(m∙K). These results demonstrate that MgF₂ is an outstanding sintering additive for the preparation transparent MgO ceramics.     

Carbothermal reduction synthesis of high porosity and low thermal conductivity ZrC-SiC ceramics via an one-step sintering technique

In this work, porous ZrC-SiC ceramics with high porosity and low thermal conductivity were successfully prepared using zircon (ZrSiO₄) and carbon black as material precursors via a facile one-step sintering approach combining in-situ carbothermal reduction reaction (at 1600 °C for 2 h) and partial hot-pressing sintering technique (at 1900 °C for 1 h). Carbon black not only served as a reducing agent, but also performed as a pore-foaming agent for synthesizing porous ZrC-SiC ceramics. The prepared porous ZrC-SiC ceramics with homogeneous microstructure (with grain size in the 50–1000 nm range and pore size in the 0.2–4 µm range) possessed high porosity of 61.37–70.78%, relatively high compressive strength of 1.31–7.48 MPa, and low room temperature thermal conductivity of 1.48–4.90 W·m^?1K^?1. The fabricated porous ZrC-SiC ceramics with higher strength and lower thermal conductivity can be used as a promising light-weight thermal insulation material.     

Preparation and properties of porous mullite-based ceramics fabricated by solid state reaction

In this study, the effect of the alumina particle size on the formation of mullite using a silica gel powder and micro- and nano-scale Al₂O₃ powders as raw materials was investigated. The optimized Al₂O₃ source was then reacted with the silica gel to prepare porous mullite-based ceramics. The results revealed that the highly reactive nano-Al₂O₃ powder could form mullite at a relatively low firing temperature. Therefore, the nano-Al₂O₃ powder was used to prepare porous mullite-based ceramics by firing at 1600 °C, 1650 °C and 1700 °C. The pore size of the prepared porous mullite-based ceramics ranges from tens to hundreds of micrometres, with the apparent porosity being 42.8–58.0%. Further, the mullite content in the samples increased with increasing firing temperature, and a higher firing temperature promoted sintering, resulting in improved strength of the sample. After calcination at 1700 °C, the mullite content in the sample reached 81.8%, and the sample showed excellent thermal shock resistance. The strengths of the samples before and after thermal shock were found to be 23.6 and 15.58 MPa, with the residual strength ratio being 66%.     

Microstructural evolution and microwave transmission/absorption transition in polymer-derived SiOC ceramics

Herein, we present the structural evolution of polymer-derived SiOC ceramics with the pyrolysis temperature and the corresponding change in their microwave dielectric properties. The structure of the SiOC ceramics pyrolyzed at a temperature lower than 1200 °C is amorphous, and the corresponding microwave complex permittivity is pretty low; thus, the ceramics exhibit wave transmission properties. The Structural arrangement of free carbon in the SiOC ceramics mainly happens in the temperature range of 1200 °C-1300 °C due to the separation from the Si–O–C network and graphitization, while the structural arrangement of the Si-based matrix mainly occurs in the range of 1300 °C-1400 °C owing to the separation of SiC₄ from the Si–O–C network to form nanocrystalline SiC. In pyrolysis temperature range of 1200 °C-1400 °C, the microwave permittivity of SiOC shows negligible change. At a pyrolysis temperature exceeding 1400 °C, the carbothermal reaction of free carbon and the Si–O backbone becomes significant, leading to the formation of crystalline SiC. The as-formed SiC and residual defective carbon improve the polarization loss of SiOC ceramics. In this case, the SiOC ceramics show significantly increased complex permittivity, exhibiting electromagnetic absorption characteristics. These characteristics promote the application of polymer-derived SiOC ceramics to high-temperature electromagnetic absorption materials.     

Polymer-derived SiOC ceramics: A potential catalyst support controlled by the sintering temperature and carbon content

A series of silicon oxycarbide ceramics with varying carbon content from ca. 10 wt% to ca. 40 wt% were prepared by thermal pyrolysis of four commercially available polysiloxanes and subsequent spark plasma sintering (SPS) at 1200 °C, 1400 °C, and 1600 °C. The results showed that the high carbon content led to a porous microstructure, and for SiOC with ca. 40 wt% carbon content, its porosity and specific surface area at 1600 °C reached 34% and 262 m²/g, respectively. The electrochemical behavior of materials was evaluated. It was shown that SiOC has a certain degree of electrocatalytic activity, and the sample with 10 wt% carbon content obtained at 1200 °C exhibited an overpotential of 450 mV vs. RHE at 10 mA·cm⁻² in acid medium. Finally, it was analyzed that the electrochemical behavior of SiOC is closely related to the phase composition and microstructure of the resulting ceramics.     

Processing of CaLa2S4 infrared transparent ceramics: A comparative study of HP and FAST/SPS techniques

The densification of CaLa₂S₄ (CLS) powders prepared by combustion method was investigated by the use of Field-Assisted Sintering Technique (FAST) and Hot Pressing (HP). CLS powders were sintered using FAST at 1000°C at different pressures and heating rates and sintered by HP under 120 MPa from 800°C to 1100°C for 6 hours with a heating rate of 10°C/min. Comparison of both techniques was further realized by use of the same conditions of pressure, dwell time, and heating rate. Complementary techniques (XRD, SEM-EDS, density measurements, FTIR spectroscopy) were employed to correlate the sintering processes/parameters to the microstructural/compositional developments and optical transmission of the ceramics. Both sintering techniques produce ceramics with submicrometer grain size and relative density of about 99%. Nevertheless, HP is more suitable to densify CLS ceramics without fragmentation and also reach higher transmission than FAST. Transmission of 40%–45% was measured out of a possible maximum of 69% based on the Fresnel losses in the 8-14 μm window when HP is applied at 1000°C for 6 hours under 120 MPa. In both techniques, ceramics undergo reduction issues that originate from graphitic sintering atmosphere.     

Synthesis of Fe/Fe3C nanoparticles encapsulated in nitrogen-doped carbon with single-source molecular precursor for the oxygen reduction reaction

Ammonium ferric citrate (AFC) was used as a single-source molecular precursor to prepare Fe/Fe₃C nanoparticles encapsulated in nitrogen-doped carbon by pyrolysis in Ar atmosphere followed by acid-leaching. Comparative studies, using citric acid and ferric citrate as the precursors, indicated that the ammonia and ferric ion in AFC and the pyrolysis temperature affected the composition of iron species and the properties of carbon in AFC-derived materials. Above the pyrolysis temperature of 600 °C, the iron species were Fe/Fe₃C, and the carbon had a hollow graphitic nanoshell structure in AFC-derived materials. The specific surface area and content of nitrogen element decreased with increasing pyrolysis temperature. The AFC-derived material pyrolyzed at 600 °C had the optimal graphitization degree, specific surface area (489 m² g⁻¹) and content of nitrogen (1.8 wt.%), thus resulted in the greatest activity for oxygen reduction reaction among the AFC-derived materials pyrolyzed at different temperatures. The AFC-derived material pyrolyzed at 600 °C exhibited improved methanol-resistance ability compared with Pt/C catalyst.     

Preparation and high-temperature performance of HfC-based nanocomposites derived from precursor with Hf-(O,N) bonds

A novel precursor was synthesized by reacting hafnium chloride with dicyandiamide and dimethylformamide. The precursor was characterized via FT-IR and NMR, as well as TG. Subsequently, the precursor was annealed in Ar over a range of temperatures from 1000 °C to 2000 °C, and the microstructural evolution of the ceramics was investigated by XRD, XPS, and TEM. The results show that the carbothermal reduction of the precursor starts at 1150 °C and the ceramic yields at 1500 °C reach 44.6 wt%. The obtained powders exhibit a uniform distribution and are composed of N-doped HfC and graphite. The N-doped structure postponed the oxidation of the HfC(N) ceramics. The HfC(N) ceramics were first oxidized to yield HfO₂, carbon, and nitrogen, and then the carbon was oxidized with the evolution of CO₂. The presented synthesis method is believed to be applicable to the preparation of other high-performance ceramics.     

Low temperature sintering and microwave dielectric properties of TiO2 ceramics

The TiO₂ ceramics were prepared by a solid-state reaction in the temperature range of 920–1100 °C for 2 h and 5 h using TiO₂ nano-particles (Degussa-P25 TiO₂) as the starting materials. The sinterability and microwave properties of the TiO₂ ceramics as a function of the sintering temperature were studied. It was demonstrated that the rutile phase TiO₂ ceramics with good compactness could be readily synthesized from the Degussa-P25 TiO₂ powder in the temperature range of 920–1100 °C without the addition of any glasses. Moreover, the TiO₂ ceramics sintered at 1100 °C/2 h and 920 °C/5 h demonstrated excellent microwave dielectric properties, such as permittivity (Ɛ_r) value >100, Q × f  > 23,000 GHz and τ_f ≅ 200 ppm/°C.     

Synthesis and pyrolysis of non-oxide precursors for ZrC/SiC and HfC/SiC composite ceramics

Multiphase ceramics like ZrC/SiC are promising candidates as ultra-high temperature ceramics for applications in extreme environments. In this work, non-oxide precursors for ZrC/SiC and HfC/SiC composite ceramics were synthesized by a one-pot reaction of three components – metal source, silicon source, and activating reagent. Molecular structures of the precursors were identified by ¹H NMR and FTIR. Transformation process of the precursors to the ZrC/SiC ceramics was investigated via XRD and SEM. After heat-treatment at 1600 °C under argon, the obtained ZrC/SiC and HfC/SiC ceramics features a particle size of 100–200 nm and high metal content without excess carbon. The elemental composition of pyrolyzed ceramics can be tuned by varying the ratio of the reagents in the synthesis of precursors. This strategy also inspires a facile fabrication of composite ceramics with other elemental compositions.     

Synthesis of rare earth containing single-phase multicomponent metal carbides via liquid polymer precursor route

Novel high-entropy carbide ceramics (HEC) containing rare earth metals, namely (Ti, Zr, Hf, Ta, La, Y)C, (Ti, Zr, Hf, Ta, Nb, La, Y)C, and (Ti, Zr, Hf, Ta, Nb, Mo, W, La)C were prepared with single-phase structure by polymer precursor method. Controlled co-hydrolysis and polycondensation of equiatomic metal-containing monomers were conducted successively, followed by blending allyl-functional novolac resin as carbon source, and the polymer precursors were obtained as clear viscous liquid solutions. The single-phase formation possibility was theoretically analyzed from the aspects of size-effect parameter δ of the designed compositions. All as-obtained ceramics possessed single face-centered-cubic structure of metal carbides and high-compositional uniformity from nanoscale to microscale. The (Ti, Zr, Hf, Ta, Nb, Mo, W, La)C ceramic powder pyrolyzed at 1800°C exhibited low-oxygen impurity content of 1.2 wt%. Thus, multicomponent high-entropy carbide nanoceramics with over five metal elements containing even rare earth element were firstly synthesized and characterized.     

Phase evolution and microstructure characteristics during the carbothermal reduction of Hf(C,N,O) ceramics derived from a precursor

In this study, nanosized Hf(C,N,O) ceramics were successfully prepared from a novel precursor synthesised by combining HfCl₄ with ethylenediamine and dimethylformamide. Subsequently, the carbothermal reduction of these Hf(C,N,O) ceramics into hafnium carbide was investigated. The Hf(C,N,O) ceramics comprised Hf₂ON₂ and HfO₂ nanocrystals and amorphous carbon. Upon carbothermal reduction, conversion began at 1300 °C, when HfC first appeared, and continued to completion at 1500 °C, resulting in irregularly shaped crystallites measuring 50–150 nm. Upon increasing the dwelling time, the oxides were completely converted into carbides at 1400 °C. Furthermore, nitrogen was introduced into the reaction to catalyse the conversion of oxides into carbides considering the beneficial gas–solid reaction between CO and Hf₂ON₂. We expect that the ceramics prepared in this study will be suitable for the fabrication of high-performance composite ceramics, with properties superior to those of current materials.     

Enhanced microwave absorption properties of Fe-doped SiOC ceramics by the magnetic-dielectric loss properties

The combination of multiple loss characteristics is an effective approach to achieve broadband microwave wave absorption performance. The Fe-doped SiOC ceramics were synthesized by polymer derived ceramics (PDCs) method at 1500 °C, and their dielectric and magnetic properties were investigated at 2–18 GHz. The results showed that adding Fe content effectively controlled the composition and content of multiphase products (such as Fe₃Si, SiC, SiO₂ and turbostratic carbon). Meanwhile, the Fe promoted the change of the grain size. The Fe₃Si enhanced the magnetic loss, and the SiC and turbostratic carbon generated by PDCs process significantly increased the polarization and conductance loss. Besides, the magnetic particles Fe₃Si and dielectric particles SiO₂ improved the impedance matching, which was beneficial to EM wave absorption properties. Impressively, the Fe-doped SiOC ceramics (with Fe addition of 3 wt %) presented the minimum reflection coefficient (RC_min) of ?20.5 dB at 10.8 GHz with 2.8 mm. The effective absorption bandwidth (EAB, RC < ?10 dB) covered a wide frequency range from 5 GHz to 18 GHz (covered the C, X and Ku-band) when the absorbent thickness increased from 2 mm to 5 mm. Therefore, this research opens up another strategy for exploring novel SiOC ceramics to design the good EM wave-absorbing materials with broad absorption bandwidth and thin thickness.     

Polymer-derived Ta4HfC5 nanoscale ultrahigh-temperature ceramics: Synthesis,microstructure and properties

Nanoscale Ta₄HfC₅ ceramics were synthesized from curing and pyrolysis of novel polymer precursors which were synthesized by cohydrolysis and polycondensation of acetyl acetone coordinated tantalum alkoxide and hafnium alkoxide followed by blending with phenolic resin as carbon source. Pyrolysis of the polymer precursor at 1600 °C in vacuum produced Ta₄HfC₅ nanocrystallites with an average grain size of 21 nm and well-distributed elements, encapsulated by an amorphous carbon shell. Near full dense Ta₄HfC₅ monoliths can be prepared by spark plasma sintering (SPS) at 1600 °C with 10 vol% MoSi₂ as an additive, of which the Vicker micro-hardness and flexural strength achieved 17.58 GPa and 466 MPa, respectively. The polymer precursor method shed light on the fabrication of ceramic matrix composites. Besides, the high electrical conductivity of 1.5 × 10⁴ S/m entitled the ceramics to a prospective of utilization in microwave absorbing/shielding fields under harsh conditions.     

Fabrication and laser operation of Yb:Lu2O3 transparent ceramics from co-precipitated nano-powders

In this article, 5 at.% Yb:Lu₂O₃ transparent ceramics were fabricated by vacuum sintering combined with hot isostatic pressing (HIP) posttreatment using co-precipitated nano-powders. The influence of precipitant molar ratio, ammonium hydrogen carbonate, to metal ions (AHC/M³⁺, R value) on the properties of Yb:Lu₂O₃ precursors and calcined powders was investigated systematically. It was found that the powders with different R value calcined at 1100°C for 4 hours were pure cubic Lu₂O₃ but the morphologies of precursors and powders behaved differently. The opaque samples pre-sintered at 1500°C for 2 hours grew into transparent ceramics after HIP posttreatment at 1750°C for 1 hour. The final ceramic with R = 4.8 showed the best optical quality with the in-line transmittance of 79.7% at 1100 nm. The quasi-CW laser operation was performed at 1034 nm and 1080 nm with a maximum output power up to 8.15 W as well as a corresponding slope efficiency of 58.4%.     

Synthesis of Fe:ZnSe nanopowders via the co-precipitation method for processing transparent ceramics

Fe:ZnSe nanopowders were synthesized via the co-precipitation method for fabricating transparent ceramics. Fe_xZn_1−xSe (0.00 ≤ x ≤ 0.06) powders that were calcined at 400°C yielded a single-phased cubic ZnSe, but when the calcination temperature was raised to 500-600°C, ZnO phase was created. Introduction of pressure could avoid appearance of ZnO. XRD Scherrer analysis revealed a monotonic increase in lattice parameter with increasing Fe²⁺ content. The average powder particle size increased with calcination temperature from several nanometers at 80°C to hundreds of nanometers at 600°C. Attempts to pressurelessly sinter ZnSe powders resulted in the partial decomposition of ZnSe, thus spark plasma sintering was employed to sinter Fe_0.01Zn_0.99Se transparent ceramics with pure ZnSe phase composition, which could be well sintered at 950°C for 30 minutes under an applied pressure of 60 MPa. SEM observations of the polished and thermally etched microstructure of the ceramic revealed a dense microstructure with average grain size of approximately 35 μm, and a few micropores were observed at the grain boundaries. The transparent ceramic exhibited good transmittance in the mid-far infrared range, with the highest transmittance 57% at 12 μm. This paper confirmed the scheme of synthesis of Fe:ZnSe nanopowders by liquid-phase co-precipitation method for sintering transparent ceramics.     

Si/B/C/N/Al precursor-derived ceramics: Synthesis,high temperature behaviour and oxidation resistance

The Si/B/C/N/H polymer T2(1), [B(C₂H₄Si(CH₃)NH)₃]_n, was reacted with different amounts of H₃Al·NMe₃ to produce three organometallic precursors for Si/B/C/N/Al ceramics. These precursors were transformed into ceramic materials by thermolysis at 1400 °C. The ceramic yield varied from 63% for the Al-poor polymer (3.6 wt.% Al) to 71% for the Al-rich precursor (9.2 wt.% Al). The as-thermolysed ceramics contained nano-sized SiC crystals. Heat treatment at 1800 °C led to the formation of a microstructure composed of crystalline SiC, Si₃N₄, AlN(+SiC) and a BNC_x phase. At 2000 °C, nitrogen-containing phases (partly) decomposed in a nitrogen or argon atmosphere. The high temperature stability was not clearly related to the aluminium concentration within the samples. The oxidation behaviour was analysed at 1100, 1300, and 1500 °C. The addition of aluminium significantly improved the oxide scale quality with respect to adhesion, cracking and bubble formation compared to Al-free Si(/B)/C/N ceramics. Scale growth rates on Si/B/C/N/Al ceramics at 1500 °C were comparable with CVD–SiC and CVD–Si₃N₄, which makes these materials promising candidates for high-temperature applications in oxidizing environments.