MPa Nitrogen Atmosphere Research Articles

Vat photopolymerization-engaged fabrication of high-performance, complex Si3N4 implants under rapid debinding efficiency

In this study, the vat photopolymerization (VP) additive manufacture technique was employed to fabricate Si3N4 ceramics implants. A photosensitive suspension with a high solid loading of 40 vol% and a low viscosity of 1.95 Pa•s at a shear rate of 30 s−1 was achieved by optimization of the composition. Particularly, as high as 50 vol% content of PPG was used in the photosensitive suspension, leading to a smooth printing and increased debinding efficiency with a heating rate up to 3 °C/min. The printed body was sintered at 1800 °C under 0.4 MPa nitrogen atmosphere, obtained the Si3N4 ceramic with a relative density of 96.08%, a fracture toughness of 5.88 MPa m1/2, a flexural strength of 677 MPa and a Vickers hardness of 16.26 GPa. A cylindric Si3N4 ceramic implant was made by the optimized suspension and printing parameters, and the implantation test in the histological section was conducted, showing a good biocompatibility after 6 weeks.

Effect of matrix−nanoparticle supramolecular interactions on the morphology and mechanical properties of polymer foams

AbstractIn this study, we report the fabrication of supramolecular polymer nanocomposite foams with a uniform cell structure, high cell density and high expansion ratio using a soft matrix of poly(methyl acrylate‐co‐2‐hydroxyethyl methacrylate) and silica nanoparticle fillers, both functionalized with ureido‐pyrimidinone (UPy) supramolecular groups. Microcellular structures were formed using a batch foaming process at 90 °C under a 9 MPa nitrogen atmosphere. Nanocomposites were characterized and compared before and after the foaming process to investigate the effect of supramolecular interactions on the thermomechanical properties and morphology of the foams. TEM images revealed that while strong inter‐filler supramolecular interactions do not have a positive effect on their dispersion state, matrix−filler interactions derived from hydrogen bonding UPy motifs result in a rather uniform distribution of nanoparticles. Competing filler−filler and matrix−filler supramolecular interactions can be balanced and optimized by adjusting UPy populations along the chains and on the surface of nanoparticles. At a given chain functionality, increasing the nanoparticle loading up to an optimum concentration improves the mechanical properties and formability of the system. Above such concentration strong interactions between fillers, which are not compensated by the matrix, result in large aggregates and consequently undermine the material performance. Supramolecular polymer foams illustrate a similar thermal and viscoelastic behavior to that of neat samples but after foaming, due to the formation of a cellular structure and rearrangement or dissociation of UPy dimers under the foaming conditions, the elastic modulus is reduced. © 2018 Society of Chemical Industry

Monitoring the progress of the acetylation reactions of 4-aminophenol and 2-aminophenol in acetonitrile modified supercritical fluid carbon dioxide and pure acetonitrile using on-line supercritical fluid chromatography and on-line liquid chromatography

The progress of acetylation reactions of 4-aminophenol and 2-aminophenol in acetonitrile modified SF-CO2 or pure acetonitrile were monitored using on-line SFC or a non-conventional type of on-line normal phase liquid chromatography. For each reaction solvent, acetylations using acetic anhydride were performed at 323K, 333K and 343K. Reactions in SF-CO2 were monitored using a single valve recirculation interface that coupled a high pressure reaction vessel operated at 17.5MPa. Reactions in acetonitrile were monitored using a two valve interface to couple a reaction vessel operated under 0.1MPa nitrogen atmosphere. For both aminophenol isomers, acetylation proceeded more rapidly in acetonitrile modified SF-CO2 compared with acetonitrile. Also for each reaction solvent, the rate of acetylation of 4-aminophenol exceeded that of 2-aminophenol. On-line chromatographic analysis rapidly established that: (i) 4-aminophenol produced a diacetylated product only in acetonitrile modified SF-CO2, whereas (ii) 2-aminophenol produced a diacetylated product only in acetonitrile.

Single-step combustion process for the synthesis of 1-D, 2-D, and 3-D hierarchically grown AlN structures

In this study, a single-step combustion process for the synthesis 1-D, 2-D, and 3-D micro and nanostructures of AlN was developed. The structures were grown by combustion of Al+kAlF3 (or ZnF2)+mNH4Cl reaction mixture under 4.0 MPa nitrogen atmosphere. Based on experimental data, the growth conditions, morphology, and composition, optical, and thermal properties of AlN micro and nanostructures were determined. The formation of structures was associated with the combustion temperature, liquid and gas-phase reactions, and the type of additives used in the experiments. The new insight, we gleaned from this study may help in the selective growth of AlN micro and nanostructures of different morphology, enhancing their functionality.

Partial nitrogen loss in SrTaO2N and LaTiO2N oxynitride perovskites

SrTaO2N heated in a helium atmosphere began to release nitrogen of approximately 30 at% at 950 °C while maintaining the perovskite structure and its color changed from orange to dark green. Then it decomposed above 1200 °C to a black mixture of Sr1.4Ta0.6O2.73, Ta2N, and Sr5Ta4O15. The second decomposition was not clearly observed when SrTaO2N was heated in a nitrogen atmosphere below 1550 °C. After heating at 1500 °C for 3 h under a 0.2 MPa nitrogen atmosphere, the perovskite product became dark green and conductive. Structure refinement results suggested that the product was a mixture of tetragonal and cubic perovskites with a decreased ordering of N3−/O2−. The sintered body was changed to an n-type semiconductor after a partial loss of nitrogen to be reduced from the originally insulating SrTaO2N perovskite lattice. LaTiO2N was confirmed to have a similar cis-configuration of the TiO4N2 octahedron as that of TaO4N2 in SrTaO2N. It also released some of its nitrogen at 800 °C changing its color from brown to black and then decomposed to a mixture of LaTiO3, La2O3, and TiN at 1100 °C. These temperatures are lower than those in SrTaO2N.

EFFECT OF REACTION TIME ON MICROSTRUCTURE, DIELECTRIC PROPERTY AND MICROWAVE ABSORPTION PROPERTY OF Cu-DOPED SiC NANOPOWDER

Cu -doped SiC nanopowders have been prepared via combustion synthesis of the silicon and carbon system in a 0.1 MPa nitrogen atmosphere under different reaction time, using copper as the dopant and PTFE as the chemical activator, respectively. X-ray diffraction, scanning electronic microscope and Raman spectra have been used to characterize the phase and morphology of prepared nanopowders. Results indicate that the lattice constant of prepared Cu -doped SiC nanopowder decreases with extending reaction time. The prepared nanopowders have fine spherical particles and narrow particle size distribution and the particle size increases with increasing reaction time. The electric permittivities of prepared Cu -doped SiC nanopowders in the frequency range of 8.2–12.4 GHz have been determined. The real part ε′, imaginary part ε′′ and dielectric loss tgδ of complex permittivity decrease with increasing reaction time. All prepared Cu -doped SiC nanopowder exhibits good microwave absorption property in the frequency range of 8.2–12.4 GHz.

Effect of Aluminum Content on Mechanical Properties and Thermal Conductivities of Sintered Reaction‐Bonded Silicon Nitride

A systematic study was conducted of the effect of Al impurities on the mechanical and thermal properties of sintered reaction‐bonded silicon nitrides (SRBSNs). 2 mol.% Y2O3 and 5 mol.% MgSiN2 were added as sintering aids to high‐purity raw Si powder, followed by respective additions of 0, 0.01, 0.1, 0.2, and 0.4 mass% of Al relative to Si. The prepared powder mixes were press formed and nitrided in a 0.1 MPa nitrogen atmosphere at 1400°C for 8 h. Postsintering of the nitrides was conducted under a pressurized nitrogen atmosphere of 0.9 MPa at 1900°C for 6 h. The SRBSN specimens exhibited hardly any difference in their four‐point bending strength, which ranged from 800 to 850 MPa. However, their fracture toughness clearly decreased with increasing additive Al content. This is thought to be due to a reduction in the proportion of coarse prismatic grains improving the toughness with increasing additive Al content. A significant reduction in the thermal conductivity was also observed with increasing additive Al content. This is conjectured to be due to the formation of SiAlON through a substituted solid solution of aluminum (Al) and oxygen (O) in β‐Si3N4 crystals. An inductively coupled plasma and hot gas extraction method were employed to directly analyze Al and O in β‐Si3N4 particles. Approximately half of the additive Al was confirmed to be in solid solution in β‐Si3N4 particles along with the solid solution of O in a chemical formula that nearly corresponds to β‐Si6–zAlzOzN8–z. The chemical analysis results give a Z value range of 0.0006–0.024.

Combustion synthesis and microwave absorption property of SiC(Fe) solid solution powder under different reaction time

SiC(Fe) solid solution powders were synthesized via combustion reaction of the Si and C system in a 0.1 MPa nitrogen atmosphere using iron as the dopant and PTFE as the chemical activator, under different reaction time. The microstructures of prepared powders have been characterized by the SEM, EDS, XRD and Raman spectra, respectively. Results indicate that the prepared powders have fine spherical particles and narrow particle size distribution. The electric permittivities of SiC samples were determined in the frequency range of 8.2–12.4 GHz. Results show that the permittivity of SiC decreases with the increasing reaction time. The Fe doped SiC powder with reaction time of 45 min with 2 mm or 2.5 mm thickness exhibit the best microwave absorption properties in the X-band range(8.2–12.4 GHz). A method to predict absorbing property in the other frequency range has been presented.

Preparation and microwave absorption properties of Fe-doped SiC powder obtained by combustion synthesis

Abstract Fe-doped SiC powders were synthesized via combustion reaction of the Si and C system in a 0.1 MPa nitrogen atmosphere using iron as the dopant. The prepared powders have fine spherical particles and narrow particle size distribution. The electric permittivities of SiC samples were determined in the frequency range of 8.2–12.4 GHz. Results show that the permittivity of SiC increases with the increasing iron contents. The 5% Fe-doped SiC powder with 2 mm or 2.5 mm thickness exhibits the best microwave absorption over the frequencies ranging from 8.2 to 12.4 GHz.

Blue Emission by Interstitial Site Occupation of Ce3+in AlN

An AlN phosphor that was coactivated by Ce3+ and Si4+ ions was synthesized by gas pressure sintering (GPS) at 2050 °C for 4 h in a 0.92 MPa nitrogen atmosphere. The phosphor emits blue color with the Commission International de l’Eclairage (CIE) chromaticity coordinate of (0.15, 0.07), which is close to the National Television Standard Committee (NTSC) blue coordinate. The crystallization and cell parameters of the samples were investigated by Rietveld refinement and high-resolution transmission electron microscopy (HRTEM). Since the ionic size of Ce3+ ions are much larger than that of Al3+ ions, the locations of rare earth (RE) ions in the AlN lattice are unclear. An octahedral interstitial site inside the wurtzite structure is proposed to be the site for RE ions. The effect of Si substitution was verified by energy-dispersive X-ray spectroscopy (EDX) and solid-state nuclear magnetic resonance (ssNMR). Cathodoluminescence (CL) was examined under electron bombardment at different accelerating voltages. Nitride phosphor exhibited high brightness and favorable stability. This study reveals the superior characteristics of nitride compounds in field emission display (FED) applications.

Decomposition of MgSiN 2 in nitrogen atmosphere

Magnesium silicon nitride MgSiN 2 was prepared by direct nitridation of Si/Mg 2Si/Mg/Si 3N 4 powder compact in a temperature range 1350–1420 °C. The thermal stability examination showed that MgSiN 2 is stable up to 1400 °C at 0.1 MPa N 2 pressure. The activation energy of decomposition of MgSiN 2 calculated from the temperature dependence of mass loss in the range of 1400–1650 °C is Δ H = 501 kJ mol −1. The time dependence and nitrogen pressure dependence of MgSiN 2 decomposition was also investigated at constant temperature. MgSiN 2 is stable at 1560 °C in 0.6 MPa nitrogen atmosphere. Using these experimental data together with the heat capacity published in a literature the Gibbs energy of formation of MgSiN 2 was calculated in a temperature range 25–2200 °C.

Synthesis and photoluminescence of a novel Sr-SiAlON:Eu 2+ blue-green phosphor (Sr 14Si 68− sAl 6+ sO sN 106− s:Eu 2+ ( s ≈ 7))

A novel Eu 2+ activated Sr-SiAlON oxynitride phosphor, with the chemical composition of Sr 14Si 68− s Al 6+ s O s N 106− s :Eu 2+ ( s ≈ 7), was synthesized by firing the powder mixture of SrO, SrSi 2, α-Si 3N 4, AlN and Eu 2O 3 at 1900 °C for 6 h under 1 MPa nitrogen atmosphere. The structure has a typical feature of SiAlON consisting of the host framework which is constructed by a three-dimensional MX 4 tetrahedral (M: Si or Al; X: O or N) network, and Sr or Eu 2+ ions as the guest ions. It has been shown that the Sr-SiAlON:Eu 2+ phosphor has the excitation band covering the range of the ultraviolet light region to 500 nm, and exhibits an intense blue-green color with the emission band centered at about 508 nm. The temperature dependent emission intensity of the Sr-SiAlON:Eu 2+ phosphor is better than that of a typical blue-green Ba 2SiO 4:Eu 2+ phosphor. It is demonstrated that Sr-SiAlON:Eu 2+ phosphor is very promising for use in white -LEDs.

Synthesis, crystal structure and photoluminescence of Eu-α-SiAlON

Eu-α-SiAlON (Eu m/2 Si 12− m− n Al m+ n O n N 16− n ) was synthesized with nominal compositions having small m and n values, by firing the powder mixture of Eu 2Si 5N 8, α-Si 3N 4, AlN, and Al 2O 3 at 1900 °C for 6 h under 1 MPa nitrogen atmosphere. The ratio of the oxidation state of Eu 2+/Eu 3+ was estimated from the X-ray absorption fine structure (XAFS) measurement. The observed X-ray absorption near edge spectrum (XANES) showed that the Eu ion in Eu-α-SiAlON was mainly in divalent state but also coexisted with a small amount of Eu in the trivalent state. The crystal structure of Eu-α-SiAlON was refined by the Rietveld analysis of the X-ray powder diffraction patterns. The lattice constants of the samples increased with increasing m and n values. The excitation band of Eu-α-SiAlON ranged from the ultraviolet to the visible light region and a broad emission band centered at about 590 nm were observed. The emission intensity decreased significantly with increasing the ratio of trivalent Eu ion, suggesting that the trivalent Eu could be one of the luminescent killer centers.

Photoluminescence and thermal stability of yellow-emitting Sr-α-SiAlON:Eu2+ phosphor

Novel α-SiAlON:Eu2+-based yellow oxynitride phosphors with the formula Sr0.375−xEux2+Si12−m−nAlm+nOnN16−n (m = 0.75, n = x = 0.004–0.04) have been prepared by firing the powder mixture of SrSi2, α-Si3N4, AlN, and Eu2O3 at 2,000 °C for 2 h under 1 MPa nitrogen atmosphere. The luminescence properties, the dependence of the activator concentration of Eu2+ and the thermal stability of Sr-α-SiAlON:Eu2+ phosphor have been investigated in comparison with Ca-α-SiAlON:Eu2+ phosphor. Similar to Ca-α-SiAlON:Eu2+ phosphor, Sr-α-SiAlON:Eu2+ phosphor has the excitation wavelength ranging from the ultraviolet region to 500 nm, and exhibit intense yellow light. The strongest luminescence was achieved at about x = 0.02 with the emission peak at 578 nm, slightly shorter than that of Ca-α-SiAlON:Eu2+ phosphor at 581 nm. Temperature-dependent emission intensity of Sr-α-SiAlON:Eu2+ phosphor is comparable to that of Ca-α-SiAlON:Eu2+ phosphor. The results suggest that the different position of the emission peak for Sr- and Ca-α-SiAlON:Eu2+ depends on the composition and the Stokes shift, and the thermal stability is nearly independent of Sr and Ca or fixed by the network of (Si, Al)–(O, N) in α-SiAlON at the same Eu2+ concentration.

Synthesis, Crystal Structure, and Photoluminescence of Sr‐α‐SiAlON:Eu2+

Sr‐containing α‐SiAlON (Srm/2Si12‐m–nAlm+nOnN16–n:Eu2+) phosphor was obtained as a major phase in compositions having small m and n values, by firing the powder mixture of SrSi2, SrO, α‐Si3N4, AlN, and Eu2O3 at 2000°C for 2 h under 1 MPa nitrogen atmosphere. The crystal structure of Sr‐α‐SiAlON was refined by the Rietveld analysis of the synchrotron X‐ray powder diffraction pattern. The crystal structure showed that the Sr–N2 bonding distance of Sr‐α‐SiAlON was fairly large compared with that of Ca‐α‐SiAlON. The displacement of N2 sites probably allow the interstices in α‐SiAlON to accommodate the introduction of the large Sr ion. Sr‐α‐SiAlON:Eu2+ phosphor has an excitation wavelength ranging from the ultraviolet region to 500 nm and emits a strong yellow light.

Improvement of permittivity of SiC with Al doping by combustion synthesis using Al2O3

Abstract Al-doped SiC powders were synthesized via combustion reaction of the Si/C system in a 0.1 MPa nitrogen atmosphere using alumina as the dopant. The prepared powders have fine spherical particles and narrow particle size distribution. The electric permittivities of SiC samples were determined in the frequency range of 8.2–12.4 GHz. Results show that the permittivity of SiC increases with the increasing alumina contents. The real part ɛ ′ and imaginary part ɛ ″ of permittivity of the sample increase by about 20% and 60%, respectively, when the alumina content is 5%. The improvement mechanism in permittivity by doping has been discussed.

Preparation and Characterization of Aluminum‐Doped Silicon Carbide by Combustion Synthesis

Al‐doped β‐SiC powders were synthesized via combustion reaction of the Si/C system in a 0.1 MPa nitrogen atmosphere, using polytetrafluoroethylene as the chemical activator and Al as the dopant. The β‐SiC powders produced have fine spherical particles and narrow particle size distribution. The impurity phase of Al2O3 is generated and the doped β‐SiC contains N component when Al content is up to 10%. The electric permittivities of β‐SiC samples were determined in the frequency range of 8.2–12.4 GHz. Results show that the β‐SiC doped with 10% Al has the highest real part ɛ′ and imaginary part ɛ″ of permittivity. The mechanism of dielectric loss by doping has been discussed.

Combustion synthesis of rod-like AlN nanoparticles

Rod-like aluminum nitride (AlN) nanoparticles were prepared by the nitriding combustion synthesis method. A charge composed of Al and AlN diluent powders (40/60 mol%) with 5 mass% MgO as additive was combusted under 1 MPa nitrogen atmosphere. The combustion temperature of the reaction was 2030°C and the speed was 1.58 mm/s. The FE-SEM observation showed that the rod-like AlN nanoparticles have diameters in the range of 90-300 nm and maximum length about 2 μm. It also showed evidences of the core-shell model of nitridation where the nanorods grew near the flow-out of eggshell-type AlN hollows. The EDX analyses showed the formation of Mg-Al-O-N droplets on the tips of AlN nanorods which strongly suggested that the vapor-liquid-solid (VLS) mechanism was the predominant growth mode. However, when the liquid droplets necessary for VLS were exhausted with continuous covering on the surface of the growing nanorods, the growth mode changed probably to vapor-solid (VS) mechanism. Furthermore, a growth model of the formation of rod-like AlN nanoparticles was proposed and discussed in accordance with experimental results.

Effect of Sintering Process on Fabrication of Porous Si<sub>3</sub>N<sub>4</sub>/SiC Composites

Porous SiC/Si3N4 composites were fabricated by reaction between Si3N4 and C powder in 0.5 MPa nitrogen atmosphere at 1700oC, 1800oC and 1900oC for 0.5h, 1h and 2h, respectively. The characteristics of in situ porous Si3N4/SiC composites were further investigated by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). Results show that the SiC particles are very fine. And the transformation from α-Si3N4 to β-Si3N4 is obvious at 1800oC for 2h and at 1900oC. Otherwise, at 1700oC, the formation of rod-like β-Si3N4 is inhibited. The compressive test was conducted at a cross-head speed of 0.5 mm/min to get the basic mechanical performance of the porous ceramics. The highest value of strength of these composites was obtained when sintered at 1800oC for 1h.

Blue emission of Ce3+ in lanthanide silicon oxynitride phosphors

Ce3+-activated lanthanide silicon oxynitride (La5Si3O12N, La4SiO7N2, LaSiO2N, and La3Si8O4N11) phosphors were prepared by firing the mixture of La2O3, Si3N4, SiO2, and CeO2 at 1500–1600 °C under a 0.5 MPa nitrogen atmosphere. Diffuse reflection spectrum, photoluminescence spectra, and temperature-dependent luminescence of these phosphors are presented in this work. A blue emission of Ce3+ in all lanthanide silicon oxynitrides was observed under ultraviolet irradiation, which is strongest in La3Si8O4N11:Ce3+. The concentration quenching and thermal quenching of the samples were discussed.