Interface properties of Ti3SiC2/Al2O3 ceramics: Combined experiments and first-principles calculations

The synthesis, characterization, and first-principles calculations of Ti₃SiC₂/Al₂O₃ ceramics were reported. X-ray diffraction measurements showed that the composite ceramics were highly pure. Scanning electron microscopy and transmission electron microscopy were used to characterize the interface information for Ti₃SiC₂ and Al₂O₃ crystals. Surface energies and interface properties were calculated using the first-principles method. The results suggested that Ti₃SiC₂ with Ti terminations and Al₂O₃ with O terminations are more stable than other terminations crystals. Thus powerful attraction between the coordinatively unsaturated Ti and O atoms on the Ti₃SiC₂∥Al₂O₃ interface would result in higher work of adhesion (Wad) and shorter boundary distance, demonstrating the intercrystalline strengthening of Ti₃SiC₂/Al₂O₃ composite ceramics.     

Pressureless-sintered boron nitride nanosheets/glass composite ceramics for excellent mechanical,dielectric and thermo-conductive performances

Because they have a high application potential in the thermal management of insulation environments, high-quality hexagonal boron nitride (h-BN)-based multiphase ceramics have been highly desired. However, so far, their synthesis is still full of challenges. Here, a kind of boron nitride nanosheets (BNNSs)/glass (GS) composite ceramics was prepared by a pressureless sintering method at a lower temperature of 900 °C. Due to a tightly bonded interaction between BNNSs and GS, the formed BNNSs/GS ceramics exhibit excellent multifunction performance. They have an outstanding compressive strength in the range of 19 ∼ 64 MPa and Vickers hardness ranging from 50 to 179 HV. For the BNNSs/GS ceramics with BNNS’s filling fraction of 90 wt%, their maximum side-surface TC values are 12.01 ± 0.18 W m⁻¹ K⁻¹ at 25 °C and 13.64 ± 0.37 W m⁻¹ K⁻¹ at 300 °C, respectively. In the ultra-high frequency range of 26.5 ∼ 40 GHz, the dielectric constant values of the BNNSs/GS ceramics are primarily between 2 and 3, and the corresponding loss tangent values are < 0.3. In addition, based on the remarkable integrity of their structure, these BNNSs/GS ceramics exhibit outstanding thermal-shock stability and prominent thermal management capacity during lots of heating/cooling-testing cycles. Therefore, we believe this kind of BNNSs/GS ceramic system will have great application potential in the new-generation thermal management and/or insulation packaging fields.     

Oxygen-ion conductivity and mechanical properties of Lu2O3-doped ZrO2 as a solid electrolyte

The characteristics of Lu₂O₃-doped ZrO₂ as a solid electrolyte material were investigated in terms of its oxygen ion conductivity and flexural strength to realize its electrolytic function at intermediate and high temperatures. The effect of doping Lu³⁺, which has a high nuclear charge electric field strength, was examined through impedance spectroscopy, open-circuit potential measurements, and bending tests. The results with Lu₂O₃ dopant were compared with those obtained with a widely used dopant, Y³⁺, having a similar ionic radius with Lu³⁺, as well as a dopant that provides high ionic conduction, Sc³⁺, having a smaller ionic radius with Zr⁴⁺. The results revealed that, at the same dopant concentration, both the ionic conductivity and the flexural strength of Lu₂O₃-doped ZrO₂ are higher than those of the widely used Y₂O₃-doped ZrO₂. The conductivity of 8 mol% Lu₂O₃-doped ZrO₂ surpassed that of 8 mol% Sc₂O₃-doped ZrO₂ in the range of 800–950 °C (0.153 S/cm vs. 0.121 S/cm at 900 °C). These results indicate the potential of Lu³⁺ as a dopant for enhancing the performance of ZrO₂ solid electrolytes.     

Non-isothermal nitridation kinetics of Ti6Al4V in Al2O3-based refractories

To establish a kinetic model of nitridation of Ti6Al4V in Al₂O₃-based refractories, the non-isothermal nitridation of Ti6Al4V–Al₂O₃ composite refractories at various heating rates was investigated using a thermogravimetric (TG) analyzer for large samples. The activation energy (E) and kinetic model (G(α)) for the nitridation of Ti6Al4V were determined using the isoconversional and master plots methods, respectively. The nucleation and growth of nitriding products of the TiN solid solution was the controlling step in the nitridation of Ti6Al4V in Al₂O₃-based refractories. The Avrami-Erofeev kinetic model, depicted by the G(α) = [-ln (1-α)]⁴ equation, is the most rational kinetic model. The values of E and A for the nitridation of Ti6Al4V were calculated to be 214.99 kJ/mol and 1.46 × 10⁷ (S^?1), respectively.     

Tuning the electronic and thermal transport behaviors of metal-dispersed Ti2O3 composites derived by metal–insulator transition

Thermal management requires an understanding of the relations among the thermal energy transfer, electronic properties, and structures of thermoconductive materials. Here, we enhanced the metal–insulator transition (MIT)-induced effect on the thermal conductivities of microstructure-controlled Ti₂O₃ composites containing W as a thermal conductive filler at approximately 450 K. To change the electronic and thermal transport properties, we varied the particle radii of the conductive phases in the raw material. The change in the calculated electronic thermal conductivity relative to the electrical conductivity of the W_x(Ti₂O₃)_1?x composite was enhanced by compounding the material. When x was reduced from 50 vol% to 20 vol% and the W particle diameter was reduced from 150 μm to 5 μm, the variation in the estimated electronic thermal conductivity of the W_x(Ti₂O₃)_1?x composite was increased by a factor of 2.01. The total thermal conductivity was also changed by the MIT. At x = 50 vol% and a W particle diameter of 5 μm, the maximum thermal conductivity change was 6.34 times larger than that of pure Ti₂O₃. The detailed relation between the MIT-induced changes in thermal transport and the microstructure were elucidated in classical effective medium approximations.     

Evaluation of mechanical properties of YSZ TBCs doped by different ratios of Eu3+ ions after isothermal oxidation

Thermal barrier coatings (TBCs) are essential to improve the thermal insulation performance of high-temperature components. Rare earth element (Eu³⁺) doped yttrium stabilized zirconia (YSZ) TBCs have been proved to be an ideal solution for non-destructive testing of internal damages. Based on this theory, two types of coatings deposited by air plasma spray (APS) on Hastelloy-X were investigated: (1) Eu³⁺ doped YSZ (dopant ratios 1 mol%, 2 mol%, 4 mol%, respectively), (2) traditional undoped 8YSZ. Isothermal oxidation treatment at 1100 °C, in increments of 10h until the failure of the coatings are conducted to evaluate the mechanical properties of different coatings. The microscopic morphology and phase of the coatings were analyzed by scanning electron microscope (SEM) and X-ray diffraction (XRD) patterns, respectively. The indentation testing methods were used to study the apparent interfacial fracture toughness and the hardness of the ceramic top coat. Results show that the Vickers hardness of the top coat increases with the decrease of porosity in the early stage and then decreases with the heat treatment time increasing in the long-term stage. Simultaneously, compared with the undoped 8YSZ coating, the fracture toughness increased with the dopant of Eu³⁺ ions increasing, from 1 mol% to 2 mol%, nevertheless, that of 4 mol% Eu³⁺ doped YSZ decreased compared with in the undoped 8 YSZ. For all types of specimens, the interfacial fracture toughness decreases with the increase of isothermal oxidation time. Results also indicate that the content of Eu³⁺ doping does not affect the microstructure and interfacial morphology of the YSZ coating as well as the growth law of thermally grown oxides (TGO). Furthermore, EDS detection found that the Eu³⁺ ions almost do not diffuse inside the TBCs system after isothermal oxidation treatment.     

Fe3O4-intercalated reduced graphene oxide nanocomposites with enhanced microwave absorption properties

Fe₃O₄-intercalated reduced graphene oxide (Fe₃O₄-rGO) nanocomposites were synthesized by an in situ reduction process. The results of XRD and XPS analyses suggested the successful formation of a Fe₃O₄ crystal phase within the rGO sheets. The SEM and TEM images demonstrated that Fe₃O₄ was flaky and was inserted stably within the rGO layers to form a typical sandwich-like structure. The hysteresis loops revealed the superparamagnetic behavior of the Fe₃O₄-rGO nanocomposites at room temperature. The electromagnetic parameters revealed that Fe₃O₄-rGO nanocomposites exhibited multiple dielectric relaxation and magnetic resonance. The reflection loss revealed that the maximum loss was −49.53 dB at 6.32 GHz for a thickness of 3.4 mm while the highest effective absorption bandwidth was 2.96 GHz.     

A comprehensive exploration on the development of nano Y2O3 dispersed in AA 7017 by mechanical alloying and hot-pressing technique

The main objective of the present study is to develop AA 7017 alloy matrix reinforced with yttrium oxide (Y₂O₃, rare earth element) nanocomposites by mechanical alloying (MA) and hot pressing (HP) techniques for armor applications. AA 7017+10 vol % Y₂O₃ nanocomposites were synthesized in a high-energy ball mill with different milling times (0, 5, 10, and 20 h) to explore the structural refinement effect. The phase analysis and homogeneous dispersion of Y₂O₃ in AA 7017 nanocrystallite matrix were investigated by X-ray diffraction (XRD), various electron microscopes (HRSEM, and HRTEM), Particle Size Analyzer (PSA), and Differential Thermal Analysis (DTA). The nanostructured powders were hot-pressed at 500 MPa pressure with a temperature of 673k for 1hr. The consolidated sample results revealed significant grain refinement and the enhanced mechanical properties with the function of milling time in which the 20h sample exhibited improvement in the hardness (142 VHN - 260 VHN) and ultimate compressive strength (514 MPa–906.45 MPa) due to effective dispersion of Y₂O₃. The various strengthening mechanisms namely, grain boundary (27.02–32.69 MPa), solid solution (57.21 MPa), precipitate (189.79–374.62 MPa), Orowan (135.68–206.92 MPa), and dislocation strengthening (84.99–149.82 MPa) were determined and correlated to the total strength.     

Phase-gradient atomic layer deposition of TiO2 thin films by plasma-induced local crystallization

Atomic layer deposition (ALD) is a thin-film fabrication method that can be used to deposit films with precise thickness controllability and uniformity. The low deposition temperature of ALD, however, often interrupts the facile crystallization of films, resulting in inferior optical and electrical properties. In this study, the extremely localized crystallization of TiO₂ thin films was demonstrated by per-cycle plasma treatment during the plasma-enhanced ALD process. By layering crystalline and amorphous films, a phase-gradient TiO₂ film with precisely modulated optical and electrical properties was fabricated. Moreover, the ratio between the amorphous and crystalline layer thicknesses for a high dielectric constant and low leakage current density was optimized.     

Fabrication of Al2O3/AlN composite ceramics with enhanced performance via a heterogeneous precipitation coating process

To improve the thermal and mechanical properties of Al₂O₃/AlN composite ceramics, a novel heterogeneous precipitation coating (HPC) approach was introduced into the fabrication of Al₂O₃/AlN ceramics. For this approach, Al₂O₃ and AlN powders were coated with a layer of amorphous Y₂O₃, with the coated Al₂O₃ and AlN powders found to favor the formation of an interconnected YAG second phase along the grain boundaries. The interconnected YAG phase was designed to act as a diffusion barrier layer to minimize the detrimental interdiffusion between Al₂O₃ and AlN particles. Compared with samples prepared by a conventional ball-milling method, the HPC Al₂O₃/AlN composites exhibited less AlON formation, a higher relative density, a smaller grain size and a more homogeneous microstructure. The thermal conductivity, bending strength, fracture toughness and Weibull modulus of the HPC Al₂O₃/AlN composite ceramics were found to reach 34.21 ± 0.34 W m⁻¹ K⁻¹, 475.61 ± 21.56 MPa, 5.53 ± 0.29 MPa m^1/2 and 25.61, respectively, which are much higher than those for the Al₂O₃ and Al₂O₃/AlN samples prepared by the conventional ball-milling method. These results suggest that HPC is a more effective technique for preparing Al₂O₃/AlN composites with enhanced thermal and mechanical properties, and is probably applicable to other composite material systems as well.     

Optimized pore structure and high permeability of metakaolin/fly-ash-based geopolymer foams from Al– and H2O2–sodium oleate foaming systems

Geopolymer foams have been widely studied as adsorbents owing to their high specific surface areas, high heavy metal immobilization efficiencies, low cost, environmental friendliness, and resource-recycling benefits. In this study, geopolymer foams with different pore structures were prepared from Al– and H₂O₂–sodium oleate foaming systems, and their chemical properties, pore structures, and permeabilities were characterized. The effects of the foaming agent type and surfactant content on the crystal structure and chemical bonding of the materials were analyzed by X-ray diffraction analysis and Fourier-transform infrared spectroscopy, and the pore morphology and structural characteristics were characterized by morphological observations, three-dimensional (3D) reconstruction, and compression tests. Numerical simulations were also carried out to study the structural characteristics of the 3D-reconstructed pores. Furthermore, variations in the permeability coefficient and flow characteristics were tested and analyzed by experiments and simulations. The pores in the geopolymer from the H₂O₂–sodium oleate foaming system tended to be more connected, whereas those in the Al–sodium oleate geopolymers were more complete and closed. The highly connected pore structure facilitates the even diffusion of the solution and effectively increases the amount of adsorption sites. These properties are significant for adjusting the adsorption capacity of geopolymer foams as adsorption monoliths.     

Direct ink writing of Pd-Decorated Al2O3 ceramic based catalytic reduction continuous flow reactor

Continuous flow reactor, due to the characteristics of safety and stability, faster reaction, less solvent demand, smaller occupied area and less energy requirements, is a satisfactory selection for heterogeneous catalysis compared with the traditional intermittent and non-intermittent reactors. Herein, the emerging 3D printing is employed to build the all-Al₂O₃ ceramic based continuous flow reactor (Pd/Al₂O₃ CFR) with the direct ink writing technique, where the outer wall is the dense Al₂O₃ ceramic and the inner is the Pd immobilized porous Al₂O₃ ceramic carrier for catalytic hydrogenation reaction. Polymethylmethacrylate (PMMA) microspheres are used to realize the inside micro-/nanometer porous structures during calcination, which offer plentiful sites for Pd anchoring via the vacuum self-assembly. The Pd/Al₂O₃ CFR therefore exhibits outstanding catalytic performance in the reduction of 4-nitrophenol (4-NP). Importantly, the continuous flow reactor is programmable in diameter, length and shape, and reusable as well. A 3 cm length Pd/Al₂O₃ CFR can be reused for a period of 4 cycles consecutively and still achieve 93.87% catalytic efficiency at 90 s for the reduction of 4-NP with no any post-treatment but washing. Combining the freeform fabrication 3D printing and the porous Al₂O₃ ceramic, the present direct ink writing continuous flow reactor is promising in practical application due to the stability, reusability, and designability etc.     

Mechanical properties and microstructure evolution of MgO–Al–C slide plate refractories in presence of Al powder-modified magnesia aggregates

MgO–Al–C slide plate refractories were fabricated using sintered magnesia and modified sintered magnesia as aggregates, fused magnesia aggregates and fines, Al powder and carbon black (N220) as fines, and thermosetting phenolic resin as the binder. Al powder-modified magnesia aggregates were prepared and characterized and were introduced into the MgO–Al–C slide plate refractories. The effects of the modified aggregates on the properties, phase composition, and microstructure were investigated. 1) The Al powder-modified magnesia aggregates exhibited considerably high bonding strengths and low Al powder shedding ratios, thus meeting the preparation requirements of MgO–Al–C slide plate refractories. 2) At high temperatures, more needle-like and fibrous Al₄C₃, AlN and octahedral MgAl₂O₄ were generated on the surface of the modified magnesia aggregates, which enhanced the bond between the matrix and the aggregates and increased the hot modulus of rupture of the material. 3) Non-oxide Al₄C₃ and AlN phases were formed in situ and had high thermal conductivity and low coefficient of expansion; this could relieve the internal thermal stress of the material and create a toughening effect, improving the thermal shock resistance of the material.     

Sintering characteristics and microwave dielectric properties of ultralow-loss SrY2O4 ceramics

A SrY₂O₄ microwave dielectric ceramic suitable for 5G systems is synthesised via a solid-state reaction in a sintering temperature range of 1425–1525 °C. X-ray diffraction patterns and Rietveld refinement analysis show that the ceramic has an antispinel orthorhombic crystal structure belonging to the Pnma space group. Scanning electron microscopy images show that the ceramic particles are closely connected, the grain boundaries are clear, and the particles are uniform at the optimal sintering temperature of 1475 °C. The optimal microwave dielectric performances are ε_r = 14.78, Q × f = 84090 GHz, τ_? = ?14.98 ppm/°C. The relatively low dielectric constant, high Q × f value, low τ_? value, and easily available raw materials indicate that it is a good choice for 5G equipment.     

Study on the microstructure and mechanical properties of microwave sintered NbC–Ni cermets reinforced by multilayer graphene

In recent years, NbC–Ni cermets has been proposed as a potential substitute for WC-Co cemented carbide in machining and other fields because of its economy and good performance, which has attracted extensive attention of scholars. Research on improving its mechanical properties will help to explore its application potential. Graphene-reinforced NbC–Ni cermets were prepared using a microwave sintering technique, and the effects of multilayer graphene (MLG) on its mechanical properties and microstructure were investigated. The experimental results show that the addition of a certain content of graphene contributes to the densification of the material and inhibits the grain growth. The Vickers hardness, toughness, and bending strength increased and then decreased with an increase in the MLG content. When 0.75 wt% MLG was added, the comprehensive mechanical properties of NbC–Ni cermets were optimal, with a Vickers hardness, fracture toughness, and bending strength of 1297.5 kg/mm², 18.23 MPa m^1/2, and 1464.5 MPa, respectively, which were 12.01%, 38.95%, and 18.97% higher than those without MLG. At low MLG content, the graphene sheet layers were well dispersed in the matrix grain boundaries, whereas graphene agglomerates and pores appeared in cermets with 1 wt% MLG, which degraded their mechanical properties. The strengthening and toughening mechanisms of MLG include grain refinement, large-angle deflection of cracks, crack bridging, and pullout of graphene sheet layers.     

Shrinkage behavior and mechanical properties of alkali activated mortar incorporating nanomaterials and polypropylene fiber

Blended ground granulated blast slag (GGBS), low-calcium fly ash (FA), nano silica (NS), and nano alumina (NA) with/without polypropylene fiber (PPF) had a significant effect on the development length of alkali activated mortar (AAM) cured at various humidity levels and curing ages. This paper presents the behavior of alkali activated mortar with 50% FA and 50% GGBFS binder materials and the alkali ratio of sodium silicate to sodium hydroxide (SS/SH) is 2.5. The molar concentration of sodium hydroxide (NaOH) used was 12 M. Feasibility Comparisons between the different humidity 60%–98% of at 23 ± 3 C° were examined. The strength behavior of alkali-activated mortar with different curing ages was also evaluated. Scanning electron microscopy (SEM) analysis and X-ray diffractions (XRD) were also conducted to clarify the effect of nanomaterials and PPF on the microstructures of AAM. It was found that the shrinkage values of AAM were decreased with the addition of polypropylene fiber and nanomaterials. The combined use of both nanomaterials had better performance than the use of nano SiO₂ and/or nano Al₂O₃ alone. The combined use of PPF with nanomaterials had a superior reduction in shrinkage and expansion and increment on strength values; the minimum shrinkage, expansion values, and maximum strength were found at AAM mix incorporating 2%NS-1%NA-0.5%PPF. The SEM analysis and XRD evaluation indicates the significant effect of nanomaterials on the microstructures and bond strength of AAM. The microstructure of the mixes incorporating both nanomaterials and PPF was denser than other mixes without nanomaterials and/or PPF and showed lower micro cracks.     

Mechanical and dielectric properties of porous magnesium aluminate (MgAl2O4) spinel ceramics fabricated by direct foaming-gelcasting

Porous ceramic materials have been broadly applied in various fields due to their multifunctional properties. Optimization of their microstructural characteristics, such as pore morphology, total porosity, and pore size distribution, which determine various properties of the final products, is crucial to improve their performances and thus extend their applications. In this study, single-phase porous MgAl₂O₄ materials were fabricated by direct foaming–gelcasting. With an increase in the foam volume from 260 to 350 mL, the total porosity and pore size of the porous ceramic increased, and its microstructure varied from mostly closed cells to open cells containing interconnected large pores (40–155 μm) and small circular windows (10–40 μm) in the ceramic skeleton. The total porosity could be tailored from 84.91% to 76.08% by modulating the sintering temperature and foam volume and the corresponding compressive strengths were in the range of 2.8–15.0 MPa. The compressive strength exhibited a power-law relationship with the relative density with indices of approximately 3.409 and 3.439, respectively. Porous MgAl₂O₄ ceramics exhibited low dielectric constants in the range of 1.618–1.910 at room temperature, which are well matched with theoretical calculations on account of a modified Bruggeman model. The porous MgAl₂O₄ ceramics with good mechanical and dielectric properties controlled easily by various sintering temperatures and foam volumes are promising for practical applications.     

Tuning specific loss power of CoFe2O4 nanoparticles by changing surfactant concentration in a combined co-precipitation and thermal decomposition method

New synthetic approaches of nanoparticles (NPs) can be used for magnetic hyperthermia, destroying malignant cells without damaging healthy tissues. Here, a combination of co-precipitation and thermal decomposition techniques was employed to synthesize monodisperse CoFe₂O₄ NPs. A mixture of oleylamine and oleic acid with different concentrations was utilized as a surfactant, significantly changing magnetic, morphological and structural properties of the NPs. Increasing the surfactant concentration from 1 to 7.5 mmol resulted in maximum and minimum coercivity and saturation magnetization of 420.0 Oe 73.6 emu/g, and 67.2 Oe and 48.3 emu/g, respectively, arising from the prevention of agglomeration and reduction in crystallite size. The first-order reversal curve analysis was employed to clarify the role of the surfactant in magnetic distributions and detailed characteristics. The specific loss power of the NPs was found to be tuned for the different surfactant concentrations, achieving a maximum of 268.5 W/g at 7.5 mmol for CoFe₂O₄ NPs with enhanced superparamagnetic contribution in Néel and Brownian mechanisms. MTT assay of the NPs was also carried out, indicating their low cytotoxicity.     

In-situ growth flower-like binder-free 3D CuO/Cu2O-CTAB with tunable interlayer spacing for high performance lithium storage

Copper-based oxides are attractive anode materials for lithium-ion batteries (LIBs) due to their abundant resources, low cost, non-toxic and high capacity. However, copper-based oxides will produce a huge volume change during lithiation/delithiation, and the structural strain caused by periodic volume changes may cause the exfoliation of active materials. Herein, a flower-like binder-free three-dimensional (3D) CuO/Cu₂O-CTAB was prepared by introducing CTAB, which homogeneously grew in situ on a copper mesh framework. The binder-free 3D sample guarantees direct contact between the active material and the copper mesh, maintaining the structure stability. The flower-like CuO/Cu₂O-CTAB with a small size reveals larger active interfaces and provides more active sites. The introduction of CTAB enlarges the interlayer spacing of CuO/Cu₂O, increases the active sites for lithium storage, and adapts to the volume change of the material during lithiation/delithiation. In addition, the expanded interlayer structure helps decrease the ion diffusion energy barrier for accelerating electrochemical reaction kinetics. Therefore, CuO/Cu₂O-CTAB exhibits better lithium storage performance (2.9 mAh cm^?2 at 0.5 mA cm^?2) than bare CuO/Cu₂O (1.8 mAh cm^?2 at 0.5 mA cm^?2).