Rapid synthesis,electrical, and mechanical properties of polycrystalline Fe2AlB2 bulk from elemental powders

Polycrystalline Fe₂AlB₂ bulk including minor Al₂O₃ is synthesized by reactive hot pressing from Fe, Al, and B powders at 1200°C and 30 MPa for 30 minutes, with a relative density of 96%. The present approach enables a markedly reduced holding time compared with previous studies. The derived Fe₂AlB₂ shows an electrical resistivity of 2.27±0.01 μΩ·m, Vickers hardness of 10.2±0.2 GPa, flexural strength of 232±25 MPa, compressive strength of 2101±202 MPa, fracture toughness of 5.4±0.2 MPa·m^1/2 and work of fracture of 117±12 J/m². No dominant indentation cracks are observed, indicating that Fe₂AlB₂ may be quite damage tolerant. Interestingly, a noncatastrophic failure is present in the SENB test, with a high work of fracture. The energy‐absorbing mechanisms in inhibiting crack formation are delamination and pullout of Fe₂AlB₂ grains.     

Bioinspired nacre-like 2024Al/B4C composites with high damage tolerance

The bio-inspired 2024Al/B₄C composites with a laminate-reticular hierarchical architecture were constructed by squeeze casting of 2024Al into loose freeze-cast ceramic scaffolds. This pressurized infiltration process provided a clean and well-bonded interface without physical gaps. By regulating the initial suspension concentration (20, 25, 30 and 35 vol%), the effects of different ceramic content on the microstructure, damage-tolerance behavior and toughening mechanisms of the composites parallel and perpendicular to the ice-growth direction were investigated. The strength and toughness in the longitudinal direction were greater than that in the transverse direction. The 2024Al/20 vol% B₄C composite in the longitudinal direction yielded the highest flexural strength of 658 MPa, crack-initiation toughness (K_Ic) of 18.4 MPa m^1/2 and crack-growth toughness (K_Jc) of 27.5 MPa m^1/2. The unique damage-tolerant properties were attributed to multiple toughening mechanisms, including crack deflection, branching and blunting, ductile-ligament bridging and multiple-crack propagation, as evidenced by the stable crack growth and rising R-curve behavior during fracture. The significantly decreased damage tolerance in the transverse direction was mainly due to inadequate toughening tools. On the other hand, both the flexural strength and fracture toughness reduced remarkably as the ceramic content increased. The 2024Al/35 vol% B₄C composite fractured in a single-crack mode and the crack growth path was almost straight, showing a relatively low ?exural strength (502 MPa) and crack-initiation toughness (9.1 MPa m^1/2). The toughening mechanism was discussed in terms of the relationship between structural characteristics and cracking mode.     

Effect of texturing on thermal,electric and elastic properties of MoAlB,Fe2AlB2, and Mn2AlB2

Herein, MoAlB, Fe₂AlB₂, and Mn₂AlB₂ with textured microstructures were sinter-forged from pre-reacted preforms in a hot press at 1400, 1200, and 1050 °C, respectively. X-ray diffraction and energy dispersive X-ray spectroscopy confirmed the textured nature and high phase purities. The effect of texturing on the thermoelectric properties was evaluated by measuring thermal diffusivities, electrical resistivities, and Seebeck coefficients on the sinter-forged samples and those of less textured, reactively hot-pressed samples. While the thermoelectric properties were essentially isotropic for sinter-forged Fe₂AlB₂ and Mn₂AlB₂, differences included electron-dominated thermal transport in Fe₂AlB₂ and phonon-dominated transport in Mn₂AlB₂ over most of the 323–873 K range. Magnetic phase transitions at 281 K in Fe₂AlB₂ and 308 K in Mn₂AlB₂ were identified in electrical resistivity measurements. MoAlB was found to have anisotropic thermoelectric properties, with notably small relative Seebeck coefficients ranging from ? 2 μV/K at 323 K and increasing to 5 μV/K at 873 K.     

Residual stress versus microstructural effects on the strength and toughness of phase-separated PbO–B2O3–Al2O3 glasses

A few authors have reasonably proposed that liquid–liquid phase-separated (LLPS) glasses could show improved fracture strength, S_f, and toughness, K_Ic, as the second phase could provide a barrier to crack propagation via deflection, bowing, trapping, or bridging. Due to the associated tensile or compressive residual stresses, the second phase could also act as a toughening or a weakening mechanism. In this work, we investigated five glasses of the PbO–B₂O₃–Al₂O₃ system spanning across the miscibility gap: Four of them undergo LLPS—three are binodal (two B₂O₃-rich and one PbO-rich) and one is spinodal—and one does not show LLPS (composition outside the miscibility gap). Their compositions were designed in such a way that the amorphous particles are under compressive residual stresses in some and under tensile residual stresses in others. The following mechanical properties were determined: the Vickers hardness, ball on three balls (B3B) strength, and toughness, K_Ic-SEVNB (single-edge V-notch beam [SEVNB]). The microstructures and compositions were analyzed using scanning electron microscopy with energy-dispersive X-ray spectrometry. The spinodal glass showed, by far, the best mechanical properties. Its K_Ic-SEVNB = 1.6 ± 0.1 MPa m^1/2, which embodies an increase of almost 50% over the B₂O₃-rich binodal composition, and 90% considering the PbO-rich binodal composition. Moreover, its fracture strength, S_f = 166 ± 7 MPa, is one of the highest ones ever reported for an LLPS glass. Fracture analyses evidenced that the spinodal composition exhibited the lowest net stress at the fracture point. Moreover, calculations indicate that the internal residual stress level is the lowest in the spinodal glass. The overall results indicate that the microstructural effect of the spinodal glass is the most significant factor for its superior mechanical properties. This work corroborates the idea that LLPS provides a feasible and stimulating solution to improve the mechanical properties of glasses.     

Preparation and mechanical properties of laminated B4C–TiB2/BN ceramics

Laminated B₄C–TiB₂ ceramics with h-BN interface layers were successfully prepared by roll forming and tape casting, and samples with different numbers of stacked layers were obtained. Scanning electron microscopy and X-ray diffraction were used to analyze the microstructure and interlayer crystal phases of the composites, and the bending strength, fracture toughness, and work of fracture were measured. As the number of h-BN layers increased, the fracture toughness increased from 7.38 ± 0.5 MPa m^1/2 to 9.01 ± 0.61 MPa m^1/2, which is 2–3 times higher than that of monolithic B₄C ceramics. As the fracture toughness increased, the hardness remained at a high level (31.67 GPa). Bending tests showed that cracks deflected when they encountered the h-BN interfacial layers. The toughening mechanisms included the deflection and branching of cracks and generation of new microcracks, which increased the length of the propagation path and work of fracture.     

Rapid synthesis of Ti3AlC2/TiB2 composites by the spark plasma sintering (SPS) technique

Dense Ti₃AlC₂/TiB₂ composites were successfully fabricated from B₄C/TiC/Ti/Al powders by spark plasma sintering (SPS). The microstructure, flexural strength and fracture toughness of the composites were investigated. The experimental results indicate that the Vickers hardness increased with the increase in TiB₂ content. The maximum flexural strength (700 ± 10 MPa) and fracture toughness (7.0 ± 0.2 MPa m^1/2) were achieved through addition of 10 vol.% TiB₂, however, a slight decrease in the other mechanical properties was observed with TiB₂ addition higher than 10 vol.%, which is believed to be due to TiB₂ agglomeration.     

Fabrication and characterization of SiC-ZTA ceramic composites by hot pressing

Al₂O₃/ZrO₂/SiC ceramic composites with different SiC contens have been prepared by hot pressuring. The effect of SiC content on the microstructure and mechanical properties of composites have been studied. The results show that SiC has obvious grain refinement effect on ZTA ceramics and change the fracture mode of the matrix from intergranular fracture to transgranular fracture. Simultaneously, it has been found that the mechanical properties of the material are significantly enhanced in comparison with ZTA matrix. The highest strength is acquired at 10% SiC content, the flexural strength and toughness are obtained when the SiC content is 15 vol%, and the values are 18.86 GPa, 1262 MPa and 6.13 MPa m^1/2, respectively. The mechanisms of hardening, strengthening and toughening have been discussed.     

Application of Nitramines Coated with Nitrocellulose in Minimum Signature Isocyanate‐Cured Propellants

Cyclotrimethylenetrinitramine (RDX) coated with nitrocellulose (NC‐RDX) is prepared by an internal solution method and applied in a minimum signature isocyanate‐cured propellant. It was found that RDX was coated or bonded by NC to form NC‐RDX particles; the median particle diameter (d₅₀) and specific surface area of NC‐RDX are in the range from 150 to 240 μm and 0.03 to 0.04 m²⋅g⁻¹, respectively. The NC‐RDX particles could swell in nitrate ester plasticizers with relatively low swelling rate compared with NC added directly in the plasticizers. Different types of ballistic modifiers can be effectively added to NC‐RDX. It was experimentally shown that NC‐RDX can increase the content of NC in the propellant with viscosities in the range from 371 to 394 Pa s and improve the mechanical characteristics of the propellant with maximum tensile strength (σ_m) between 0.48 MPa<σ_m<1.92 MPa, elongation at maximum tensile strength (ε_m) between 28.0%<ε_m<37.3%, and elastic modulus between 3.18 MPa<E<8.68 MPa in the temperature range from −40 to +50 °C.     

Phase composition,crystal structure,and microwave dielectric properties of LiIn1-xAlxO2 ceramics

A series of LiIn_1-xAl_xO₂ (x =0.05, 0.10, 0.15, 0.20, 0.25) microwave dielectric ceramics with low permittivity were synthesized via a solid-state reaction method. XRD, Raman spectra, and SEM analysis reveal that a single LiInO₂ tetragonal structure phase could be obtained at the x < 0.10, and with the x increased further to 0.15–0.25, the diffraction peaks of the secondary phase LiAlO₂ were detected. In the LiIn_1-xAl_xO₂ ceramics, the τ_f was closely related to the ε_r, and the relative density, microstructure, and microwave dielectric properties were effectively improved by the Al³⁺ substitution for In³⁺. Bond valence theory analysis demonstrates that the Al³⁺ entered the In³⁺ site exhibits a strength rattling effect, which is beneficial to the increase of ε_r. While Al³⁺ substitution for In³⁺ simultaneously lowers the average ionic polarizability, resulting in a decrease in ε_r. A near-zero τ_f (0.74 ppm/°C) combined with ε_r approximately 12.83, Q × f = 58 200 GHz, was obtained in LiIn0_.85Al0_.15O₂ ceramic sintered at 970°C.     

Microstructural,electrical, and mechanical properties of conductive SiC ceramics fabricated by spark plasma sintering

Nitrogen (N)-doped conductive silicon carbide (SiC) of various electrical resistivity grades can satisfy diverse requirements in engineering applications. To understand the mechanisms that determine the electrical resistivity of N-doped conductive SiC ceramics during the fast spark plasma sintering (SPS) process, SiC ceramics were synthesized using SPS in an N₂ atmosphere with SiC powder and traditional Al₂O₃–Y₂O₃ additive as raw materials at a sintering temperature of 1850–2000°C for 1–10 min. The electrical resistivity was successfully varied over a wide range of 10⁻³–10¹ Ω cm by modifying the sintering conditions. The SPS-SiC ceramics consisted of mainly Y–Al–Si–O–C–N glass phase and N-doped SiC. The Y–Al–Si–O–C–N glass phase decomposed to an Si-rich phase and N-doped Y_xSi_yC_z at 2000°C. The Vickers hardness, elastic modulus, and fracture toughness of the SPS-SiC ceramics varied within the ranges of 14.35–25.12 GPa, 310.97–400.12 GPa, and 2.46–5.39 MPa m^1/2, respectively. The electrical resistivity of the obtained SPS-SiC ceramics was primarily determined by their carrier mobility.     

Experimental and DFT insights into elastic,magnetic, electrical,and thermodynamic properties of MAB-phase Fe2AlB2

The elastic, magnetic, electrical, and thermodynamic properties of bulk polycrystalline Fe₂AlB₂ are reported, with further theoretical insights provided through first-principles calculations. DFT simulations, along with an appropriate empirical method to treat magnetic contributions, reproduce well the experimental heat capacity, phonon thermal conductivity, and thermal expansion. The shear, Young's and bulk moduli all decrease linearly from 107.7, 264.5, and 162.4 GPa at 298 K to 91.4, 227.7, and 149.6 GPa at 1273 K, respectively, while the Poisson's ratio shows a weak dependence on temperature, hovering around 0.23-0.26. The mechanical damping, Q⁻¹, is found to be a weak function of temperature up to 973 K, but above this temperature increases sharply, peaking at a specific temperature that becomes higher with increasing resonant frequency. The phonon contribution plays a dominant role in the measured heat capacity from 4.2 to 1000 K, while the magnetic contribution becomes significant around the Curie temperature (301 K). The electronic contribution increases with temperature above 100 K. Furthermore, the standard enthalpy, entropy, and Gibbs free energy are calculated as 16.60 kJ/mol, 92.92 J/(mol·K) and −11.09 kJ/mol, respectively. The thermal conductivity increases linearly with increasing temperature from RT to 1173 K, up to around 13.0 W/(m·K) at 1323 K after hovering at around 7.8 W/(m·K) in the low-temperature range. The electrons play a dominant role in heat conduction, while the phonons contribute only a little to the thermal conductivity. The dilatometric coefficient of thermal expansion of Fe₂AlB₂ is measured as 13.5 × 10⁻⁶ K⁻¹ in the temperature range RT-1348 K.     

Effects of zirconium source and content on zirconia crystal form,microstructure and mechanical properties of ZTM ceramics

ZTM ceramics were successfully derived from coal gangue. The effect of zirconium source (ZrO₂ and ZrOCl₂?8H₂O) and content on properties of the ZTM ceramics has been studied. The phase composition, density, and microscopic morphology were characterized with X-ray diffraction (XRD), Archimedes method and scanning electron microscopy (SEM). The influence of zirconium source, sintering temperature, zirconia content on flexural properties and fracture toughness was studied. The sample, with ZrOCl₂?8H₂O added 12% zirconia (Z2TM12) sintered at 1475 °C for 3 h has the highest density of 2.83 g/cm³. Partially stable t-ZrO₂ was present in samples with zirconium oxychloride (ZrOCl₂?8H₂O) as the zirconium source, due to the constraints of mullite crystals. Therefore, Z2TM12 had both microcrack toughening and stress phase transformation toughening mechanisms. The flexural strength was increased from 162.40 MPa to 285.04 MPa, while the fracture toughness was improved to 3.55 MPa m^1/2 from 2.38 MPa m^1/2. Our achievement can be used as a reference to fabricate ZTM ceramics from coal gangue with high-value additions.     

High strength and hardness BNNSs/Al2O3 composite ceramics prepared by hot-press sintering of in-situ composite BNNSs/Al2O3 powder

In this paper, BNNSs/Al₂O₃ composite powder was prepared by in-situ reaction using borate nitridation method and BNNSs/Al₂O₃ composite ceramics were prepared by hot-pressing sintering. This method achieves uniform mixing of BNNSs and Al₂O₃ ceramic matrix and reduces the introduction of impurities in the processing process. The BNNSs/Al₂O₃ composite ceramics have excellent bending strength (549.4 MPa), fracture toughness (5.18 MPa m^1/2) and hardness (21.3 GPa). The high hardness of composite ceramics is attributed to high grain boundary strength and density. The reinforcing mechanisms of ceramics include BNNSs pull-out, BNNSs bridging, crack deflection as well as the transgranular fracture and intergranular fracture of Al₂O₃ matrix.     

Design of crack deflection induced high toughness laminated Si3N4 ceramics based on hollow oriented one-dimensional microstructure

A laminated silicon nitride (Si₃N₄) ceramic material with a hollow, oriented, one-dimensional microstructure was successfully prepared based on the tape casting and sacrificial template method. The results show that hollow, oriented, one-dimensional microstructures can effectively induce crack deflection. Different arrangements of the structural design layer and dense layer will have different effects on the material. In particular, bulks with a single-layer orthogonal arrangement of the structural design layer possess high toughness and obvious crack deflection during the fracture process. A kind of multiscale crack deflection mode was realized. Compared with the fracture toughness of the monolithic Si₃N₄ ceramic bulk (5.55 MPa m^1/2), the fracture toughness can reach 8.73 MPa m^1/2, and the flexural strength can still reach 391.47 MPa with only a slight decrease.     

Preceramic paper-derived Ti3Al(Si)C2-based composites obtained by spark plasma sintering

The paper describes the structure and properties of preceramic paper-derived Ti₃Al(Si)C₂-based composites fabricated by spark plasma sintering. The effect of sintering temperature and pressure on microstructure and mechanical properties of the composites was studied. The microstructure and phase composition were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. It was found that at 1150 °C the sintering of materials with the MAX-phase content above 84 vol% leads to nearly dense composites. The partial decomposition of the Ti₃Al(Si)C₂ phase becomes stronger with the temperature increase from 1150 to 1350 °C. In this case, composite materials with more than 20 vol% of TiC were obtained. The paper-derived Ti₃Al(Si)C₂-based composites with the flexural strength > 900 MPa and fracture toughness of >5 MPa m^1/2 were sintered at 1150 °C. The high values of flexural strength were attributed to fine microstructure and strengthening effect by secondary TiC and Al₂O₃ phases. The flexural strength and fracture toughness decrease with increase of the sintering temperature that is caused by phase composition and porosity of the composites. The hardness of composites increases from ~9.7 GPa (at 1150 °C) to ~11.2 GPa (at 1350 °C) due to higher content of TiC and Al₂O₃ phases.     

Enhancement mechanical properties of in-situ preparated B4C-based composites with small amount of (Ti3SiC2+Si)

B₄C-based composites were synthesized by spark plasma sintering using B₄C、Ti₃SiC₂、Si as starting materials. The effects of sintering temperature and second phase content on mechanical performance and microstructure of composites were studied. Full dense B₄C-based composites were obtained at a low sintering temperature of 1800 °C. The B₄C-based composite with 10 wt% (TiB₂+SiC) shows excellent mechanical properties: the Vickers hardness, fracture toughness, and flexural strength are 33 GPa, 8 MPa m^1/2, 569 MPa, respectively. High hardness and flexural strength were attributed to the high relative density and grain refinement, the high fracture toughness was owing to the crack deflection and uniform distribution of the second phase.     

Effect of Ti5Si3 on wear properties of Ti3Si(Al)C2

Near-fully dense Ti₃Si(Al)C₂/Ti₅Si₃ composites were synthesized by in situ hot pressing/solid–liquid reaction process under a pressure of 30 MPa in a flowing Ar atmosphere at 1580 °C for 60 min. Compared to monolithic Ti₃Si(Al)C₂, Ti₃Si(Al)C₂/Ti₅Si₃ composites exhibit higher hardness and improved wear resistance, but a slight loss in flexural strength (about 26% lower than Ti₃Si(Al)C₂ matrix). In addition, Ti₃Si(Al)C₂/Ti₅Si₃ composites maintain a high fracture toughness (K_IC = 5.69–6.79 MPa m^1/2). The Ti₃Si(Al)C₂/30 vol.%Ti₅Si₃ composite shows the highest Vickers hardness (68% higher than that of Ti₃Si(Al)C₂) and best wear resistance (the wear resistance increases by 2 orders of magnitude). The improved properties are mainly ascribed to the contribution of hard Ti₅Si₃ particles, and the strength degradation is mainly due to the lower Young's modulus and strength of Ti₅Si₃.     

B4C–TiB2 composites fabricated by hot pressing TiC–B mixtures: The effect of B excess

Previous research have reported that B₄C–TiB₂ composites could be prepared by the reactive sintering of TiC–B powder mixtures. However, due to spontaneous oxidation of raw powders, using TiC–B powder mixtures with a B/TiC molar ratio of 6: 1 introduced an intermediate phase of C during the sintering process, which deteriorated the hardness of the composites. In this report, the effects of B excess on the phase composition, microstructure, and mechanical properties of B₄C–TiB₂ composites fabricated by reactive hot pressing TiC–B powder mixtures were investigated. XRD and Raman spectra confirmed that lattice expansion occurred in B-rich boron carbide and B_xC–TiB₂ (x > 4) composites were obtained. The increasing B content improved the hardness and fracture toughness but decreased the flexural strength of B_xC–TiB₂ (x > 4) composites. When the molar ratio of B/TiC increased from 6.6:1 to 7.8:1, the Vickers hardness and the fracture toughness of the composites were enhanced from 26.7 GPa and 4.53 MPa m^1/2 to 30.4 GPa and 5.78 MPa m^1/2, respectively. The improved hardness was attributed to the microstructural improvement, while the toughening mechanism was crack deflection, crack bridging and crack branching.     

Microstructure,mechanical properties and thermal conductivity of (Ti0.5Nb0.5)C–SiC composites

A series of (Ti_0.5Nb_0.5)C-x wt.% SiC (x = 0, 5, 10, 20) composites were prepared by spark plasma sintering. Dense microstructures with well‐dispersed SiC particles were obtained for all composites. With the increment of SiC content, the Vickers hardness, Young's modulus and fracture toughness increase monotonically. An optimized flexural strength of 706 MPa was achieved in (Ti_0.5Nb_0.5)C-5 wt.%SiC composite. (Ti_0.5Nb_0.5)C-20 wt%SiC composite exhibits the highest fracture toughness of 6.8 MPa m^1/2. The crack deflections and the suppression of grain growth were the main strengthening and toughening mechanisms. Besides, (Ti_0.5Nb_0.5)C-20 wt%SiC composite exhibit the highest thermal conductivity of 45 W/m·K at 800 °C.     

Effect of calcination temperature on optical,magnetic and dielectric properties of Sol-Gel synthesized Ni0.2Mg0.8-xZnxFe2O4 (x = 0.0–0.8)

The Ni_0.2Mg_0.8-xZn_xFe₂O₄ (x = 0.0, 0.2, 0.4, 0.6 & 0.8) nanomaterials were prepared via sol-gel technique. These samples were calcined at three different temperatures (T) such as 400, 450 and 500 °C/5 h. Furthermore, the X-ray diffraction (XRD) patterns of all the calcined samples revealed the single phase cubic spinel structure. The lattice constants (a = b = c) were noticed to be increasing with increase of ‘x’. The grain shape, size and distribution of x = 0.0–0.8 contents were analyzed using field emission electron microscope (FESEM). The x = 0.2 content provided higher optical band gap energy (E_g) value than the remaining contents. Furthermore, the magnetization versus magnetic field (M − H) curves indicated the superparamagnetic nature of x = 0.0–0.8 contents. The high saturation magnetization (M_s) was noticed for x = 0.4 and 0.6 contents. In addition, the distribution of cations like Ni⁺², Mg⁺², Zn⁺², Fe⁺³ and Fe⁺² was performed between the tetrahedral (A) and octahedral (B) sites. The frequency dependence of dielectric constant (ε′), dielectric loss (ε") and ac-electrical conductivity (σ_ac) was investigated as a function of composition. Moreover, the temperature variation of ε′ showed the decreasing trend of dielectric transition temperature (T_e) with increase of ‘x’. The high ε′ of 163.1 (at 1 MHz) was noticed at x = 0.2 content calcined at 500 °C. Using the power law fit applied to the log σ_ac-log ω plots, the dc-electrical conductivity (σ_dc) and exponent (n) parameters were calculated.