Functionally graded structure of a nitride-strengthened Mg_(2)Si-based hybrid composite

The ex-situ incorporation of the secondary SiC reinforcement,along with the in-situ incorporation of the tertiary and quaternary Mg_(3)N_(2) and Si_(3)N_(4) phases,in the primary matrix of Mg_(2)Si is employed in order to provide ultimate wear resistance based on the laser-irradiation-induced inclusion of N_(2) gas during laser powder bed fusion.This is substantialized based on both the thermal diffusion-and chemical reactionbased metallurgy of the Mg_(2)Si–SiC/nitride hybrid composite.This study also proposes a functional platform for systematically modulating a functionally graded structure and modeling build-direction-dependent architectonics during additive manufacturing.This strategy enables the development of a compositional gradient from the center to the edge of each melt pool of the Mg_(2)Si–SiC/nitride hybrid composite.Consequently,the coefficient of friction of the hybrid composite exhibits a 309.3%decrease to–1.67 compared to–0.54 for the conventional nonreinforced Mg_(2)Si structure,while the tensile strength exhibits a 171.3%increase to 831.5 MPa compared to 485.3 MPa for the conventional structure.This outstanding mechanical behavior is due to the(1)the complementary and synergistic reinforcement effects of the SiC and nitride compounds,each of which possesses an intrinsically high hardness,and(2)the strong adhesion of these compounds to the Mg_(2)Si matrix despite their small sizes and low concentrations.

Effect of ZrO_2-nitrides Composite Powder Addition on Oxidation Resistance of Al_2O_3-C Refractories

Oxidation of carbon is the main problem or Al2O3 - C refractories. ZrO2 - nitrides composite powder was synthesized through carbothermal reduction and nitridation （CRN） of zircon. The effect of ZrO2 - nitrides composite powder addition on oxidation resistance of the Al2O3 - C refractories was investigated by measuring the thickness of oxidation layer. Phase compositions of the Al2O3 - C refractories before and after oxidation were investigated by X-ray diffraction （ XRD ）. Results show that the oxidation resistance of the Al2O3 - C refractories can be obviously improved by adding the synthesized ZrO2 - nitrides composite powder. The formation of mullite and zircon in the oxidation layer results in the densification of oxidation layer, which prevents oxygen diffusion and bnproves the oxidation resistance of the Al2O3 - C refractories.

TiC Particle Reinforced Silicon Nitride Composite Joined With Y_2O_3-Al_2O_3-SiO_2 Mixture

Silicon nitride composite is joined to itself by heating interlayer of Y2 O3 -AL2O3 -SiO2 mixtures above their liquidus temperatures in flowing nitrogen. The joined specimens are tested in four point flexure from room temperature to 1373 K. The interface microstruclure and fractured surfaces after testing are observed and analyzed by SEM, EPMA and XRD respectively. The results show that F2 O3 -A12 O3 -SiO2 glass reacts with Si3 N4 at interface, forming the Si3 N4/Si2 N2 O( Y-AlrSi-O-N glass/ Y-Al- Si-O glass gradient interface. With the increase of bonding temperature and holding time, the joint strength first increases, reaching a peak, and then decreases . According to interfacial analyses , the bonding strength depends on joint thickness .

Mechanical Properties and Electrical Conductivity of TiN-Al_2O_3 Composites

TiN- Al2O3 composite powders with different TiN contents （0, 10 vol%, 20 vol%, 30 vol% and 40 vol% ）were prepared with micrometer TiN and α-Al2O3 powder （ their purities were 99% ） as starting materials by wet ball milling for 5 h. TiN-Al2O3 composite were then prepared by pressing the above composite powders, drying at 200 ℃ for 12 h and firing at 1 800 ℃ for 3 h in nitrogen atmosphere in hot-pressing furnace. The influences of TiN content on mechanical properties and electrical conductivity of TiN -Al2O3 composites were studied. The results showed that the mechanical properties of the composite increased with TiN content increasing, while the resistivity of composites decreased. A composite with 40% TiN had 498 MPa ben- ding strength, 4. 285 MPa· m1/2 fracture toughness, 1.34×10^-3 Ω· cm resistivity. The SEM analysis showed that the fine TiN crystals distributed among the crystal boundary of Al2O3 matrix. They bonded together forming a net-like structure which played a role of restraining Al2O3 grains from growing up, toughening and strengthening, so the mechanical properties of TiN -Al2O3 composite were enhanced.

Phase Composition Analysis of TiN-AI_2O_3Synthesized from Aluminum-containing Dross and Rutile by Aluminothermic Reductionnitridation Method

TiN- Al2O3 composite powder was prepared by aluminothermic reduction- nitridation method with starting materials of aluminum-containing dross and rutile,and metallic aluminum in the aluminum-containing dross as reducer. The influences of synthesis temperature（600-1 400 ℃） and aluminum-containing dross addition（20% lower than theoretical value,theoretical value,20% higher than theoretical value,and 50% higher than theoretical value） on phase compositions and microstructure of the composites were investigated,and the reaction mechanism was analyzed. The results show that（1） TiN- Al2O3 composite powder can be synthesized under the experimental conditions; the main phases are TiN,α-Al2O3,a little bytownite,and MgAl2O4;（2）enhancing synthesis temperature or increasing aluminumcontaining dross addition favors the reaction of aluminothermic reduction- nitridation;（3） in the synthesized products,α-Al2O3 is platy or columnar; TiN is sub-micron granular,which reinforces and toughens the composite.

Synthesis of ZrN-Si_3N_4 Composite by Carbothermal Reduction and Nitridation of Zircon

Zircon （ ≤44 μm） and carbon black （≤30μm） were used as starting materials and mixed for 24 h using anhydrous ethanol as medium with the mass ratio of 100：40, dried fully at 60 ℃ and then dry mixed for 10 h. Specimens with size of Ф20 mm × 5 mm were pressed under 60 MPa, then dried fully at 120 ℃ , put into a furnace with 1. 0 L ·min^-1 nitrogen gas and fired at 1 400, 1 450, 1 480 and 1 500℃ for 6, 9 and 12 h, respectively. The phase composition and microstructure of the specimens were studied by XRD and SEM, and the carbothermal reduction and nitridation reaction process was thermodynamically analyzed. The results show that using zircon and carbon black as starting materials, ZrN - Si3N4 composite is synthesized by carbothermal reduction and nitridation reaction in nitrogen atmosphere. The composites with different compositions are obtained by controlling the firing temperature and partial pressure of CO gas. The proper firing temperature and holding time to synthesize ZrN - Si3N4 composite are 1 500 ℃ for 12 h.

TiN/Al_2O_3 Composite Material Synthesized by Aluminothermic Reduction Process in Coke Bed

TiN/Al2O3 composite material was prepared by aluminothermic reduction of TiO2 in coke bed. The phase composition, lattice parameter and microstructure of products treated at 1 300 ℃ , 1 400 ℃, 1 500℃ for 3 h respectively were analyzed by XRD, SEM and TEM. The results showed that TiN/Al2O3 composite material can be prepared in coke bed via aluminothermic reduction method. However, the treatment temperatures affect the evolution of aluminothermic reaction and morphology of the materials obviously. The content, grain size and lattice parameter of TiN in materials in coke bed increased with the increase of treatment temperature. Thermodynamic calculation confirmed that metal Al reacted with O2 in coke bed while it was involved in aluminothermic reduction reaction, causing the lack of Al and surplus rutile in the products.

Electrostabilized homogeneous dispersion of boron nitride nanotubes in wide-range of solvents achieved by surface polarity modulation through pyridine attachment

Boron nitride nanotubes(BNNTs)show exceptional physical properties including high mechanical strength and thermal conductivity;however,their applications have been restricted due to limited dispersibility in processing solvents.Here,a novel BNNT dispersion method with exceptional dispersibility in a wide range of solvents has been demonstrated by surtace polarity modulation through short-molecule pyridine attachment.Nitrogen atoms in pyridine are selectively bonded to electron-deficient boron atoms of the BNNT surface through Lewis acid-base reaction,which changes the surface polarity of BNNTs from neutral to negative.Re-dispersing pyridine-attached BNNTs(Py-BNNTs)create a thick and stable electronic double layer(EDL),resulting in uniform dispersion of BNNTs in solvents with an exceptional solubility parameter range of 18.5-48 MPa^1/2.The uniform dispersion of BNNTs is maintained even after the mixing with diverse polymers.Finally,composites incorporating uniformly-distributed BNNTs have been realized,and extraordinary property enhancements have been observed.The thermal conductivity of 20 wt.%Py-BNNT/epoxy composite has been significantly improved by 69.6%and the tensile strength of 2 wt.%Py-BNNT/PVA has been dramatically improved by 75.3%.Our work demonstrates a simple and facile route to dispersing BNNTs in diverse solvents,consequently leading to selective utlization of BNNT dispersed solvents in various application fields.

Solution-based processing of carbon nitride composite for boosted photocatalytic activities

Graphite-phase polymeric carbon nitride （CN） was reported to be a promising material in photoelectrochemical solar energy conversion. However, its high recombination rate of photogenerated carriers limits its potential applications. In this article, a heterojunction of CN and sulfur-doped CN （CNS） was constructed through a solution-based processing way. Interestingly, it was observed that the photocatalytic hydrogen production of the as-prepared composite was 32.6 times higher than that of bulk carbon nitride and 2.3 times higher than that of the composites by conventional impregnating method. This study opens a new avenue to construct heterojunction of CN for large-scale industrial applications in environmental remediation.