Formation of magnesium silicate hydrate in the Mg(OH)2-SiO2 suspensions and its influence on the properties of magnesia castables

Brucite is an alternative magnesium precursor for magnesia castables. The hydration process of brucite-microsilica suspensions at 50?°C was firstly studied to identify the magnesium-silicate-hydrate (M-S-H) formation. The existence of M-S-H was dependent on the characteristics of brucite. The brucite with smaller grain size exhibited higher reactivity and favored the formation of M-S-H. The early strength of magnesia castables with addition of the 1?wt% reactive brucite powder was increased, which was related to the modified microstructure via filling effect of brucite fine particles and the formation of M-S-H. Explosion resistance of castables was improved as well attributed to the enhanced early strength and the M-S-H phase in the presence of reactive brucite. Besides, the facilitated formation of forsterite bonding phase in the brucite containing castables during firing process and the thermal-mechanical strength such as hot modulus of rupture and refractoriness under load were significantly increased.     

Effect of microporous magnesia aggregates on microstructure and properties of periclase-magnesium aluminate spinel castables

The present study investigated three lightweight periclase-magnesium aluminate spinel castables containing microporous magnesia aggregates with a varying apparent porosity (12.8%, 30.8% and 39.3%). The effect of the apparent porosity of the aggregates on the phase composition, microstructure, fracture behavior and strength of the lightweight castables was investigated by XRD, SEM and three-point bending tests. Large cracks between the aggregates with an apparent porosity of 12.8% and the matrix reduced the strength of the castable. For the aggregates with an apparent porosity of 30.8%, an excellent interlocking interface with the matrix increased the strength considerably, but also reduced the fracture toughness. At the highest level of the apparent porosity of the aggregates of 39.3%, the formation of a small number of microcracks between the aggregates and matrix reduced the strength, while the fracture toughness was only slightly affected. The lightweight castables with the best combination of properties were achieved at an apparent porosity of the aggregates of 30.8% since they had a low bulk density of 2.63 g/cm³ as well as a high compressive and flexural strength of 70.2 MPa and 20.9 MPa, respectively.     

Enhancing mechanism of improved slag resistance of Al2O3-spinel castables added with pre-synthesized (Al,Cr)2O3 micro-powder

In order to enhance the slag resistance of Al₂O₃-spinel castables, (Al,Cr)₂O₃ is added into Al₂O₃-spinel system as a pre-synthesized micro-powder. Firstly, (Al,Cr)₂O₃ micro-powder is synthesized by sintering under reduction conditions to prevent the formation of hexavalent chromium. The Al₂O₃-spinel castables are prepared using tabular alumina, fused spinel, α-Al₂O₃ micro-powder, calcium aluminate cement and (Al,Cr)₂O₃ micro-powder as the raw materials. The bulk density, porosity, mechanical properties, and slag resistance of the samples are tested. Afterward, the effects of (Al,Cr)₂O₃ micro-powder (0–3 wt%) on the slag resistance and microstructures of the Al₂O₃-spinel castables are assessed by X-ray diffraction (XRD) and energy-dispersive (SEM-EDS) analysis. The results show that the addition of (Al,Cr)₂O₃ micro-powder can could inhibit the deteriorating effects of Cr³⁺ on the mechanical properties of the samples. The microstructure results also shows that with the addition of the (Al,Cr)₂O₃ micro-powder, a secondary solid solution of Ca(Al,Cr)₁₂O₁₉ formed, causing the unit cell to become larger. In the slag erosion area, CA₆ crystals formed with network-like interwoven structures, high density, and greater thickness. These characteristics significantly reduce the erosion and permeability indices of the castables, and improve the slag erosion resistance of the material.     

Role of Mg(OH)2 in pore evolution and properties of lightweight brine magnesia aggregates

Lightweight magnesia aggregates were fabricated using high-purity MgO agglomerates with the addition of Mg(OH)₂ as a pore former. The pore evolution and its relationship to the resulting properties were investigated. Mg(OH)₂ decomposition increased the number of inter-agglomerate pores, which subsequently affected the porosity and pore structure. When Mg(OH)₂ was 0–20 wt%, the inter-agglomerate pores were converted to both open and closed small pores, which effectively reduced the thermal conductivity and improved the thermal shock resistance (TSR) by accommodating thermal stress and inducing crack deflection. Small pores also favored the formation of a dense (Mg, Fe)O corrosion layer, preventing further slag penetration. However, large open pores occurred with further increasing Mg(OH)₂ content, which dramatically deteriorated the TSR and slag resistance. The specimen with 20 wt% Mg(OH)₂ exhibited the best overall performance, with a thermal conductivity of 16.6 W/(m·K) at 500 °C, and a residual flexural strength ratio of 32.3%; its slag resistance was comparable with that of dense magnesia.     

Preparation and characterization of magnesium aluminate (MgAl2O4) spinel ceramic foams via direct foam-gelcasting

Lightweight MgAl₂O₄ spinel ceramic foams with high mechanical strength and good dielectric properties were prepared with a direct foam-gelcasting method using MgAl₂O₄ and TiO₂ (rutile phase, as sintering aid) powders. The effects of calcination temperature and foam volume on bulk density, apparent porosity, and on the mechanical and dielectric properties of the ceramic foams were investigated. Tailored porosity (75.14–82.46%), pore size (10–200 μm), dielectric constant (1.66–2.05), and compressive strength (4.0–14.3 MPa), were obtained based on the change of the foam volume in the foamed slurries, and the calcination temperature of porous ceramics. The compressive strength and dielectric constant of the as-manufactured spinel foam with a porosity of ~75.14% was as high as 14.3 MPa and 2.05, respectively. The spinel ceramic foam which had a porosity of 81.84% was prepared with a foam volume of 350 mL and a sintering temperature of 1500 °C, and exhibited heterogeneous pore structures, whereby large and open spherical cells involved in small circular windows on the internal walls with a mean pore size of ~66.26 μm and a grain size of ~8 μm. The experimental dielectric constant matches well with that calculated by the modified Bruggeman model. The dependence of the mechanical strength on the relative density can be represented by the Gibson and Ashby model. The fitted index values of the power relationship were 3.504 and 3.533, compared to the theoretical value of 1.5. The ceramic foam can potentially become a new type of electromagnetic wave-transmitting radome material due to its low dielectric constant (1.66–2.05) and dielectric loss (0.0026–0.006) values.     

含3nm硅溶胶的苯丙杂化乳液的制备与性能研究

在单一乳化剂存在下 ,用原位分散聚合方法制备了含 3 nm硅溶胶的苯丙杂化乳液。通过 FTIR、粒径分析、ζ电位和流变性能测试 ,表征了杂化乳液中的杂化效应、微粒形态及乳液稳定性 ,并与分别由 3 nm和 1 0 nm硅溶胶与苯丙乳液直接共混形成的杂化乳液进行了比较。与直接共混的杂化乳液成膜后的力学性能相比 ,由原位分散聚合形成的 3 nm硅溶胶杂化乳液成膜后 ,显示出较高的拉伸强度、拉伸模量和断裂伸长率。     

Mechanistic investigation into the electrolytic formation of iron from iron(III) oxide in molten sodium hydroxide

Iron(III) oxide tablets were electrolytically reduced to iron in molten sodium hydroxide at 530 °C and recovered to produce iron with 2 wt.% oxygen suitable for re-melting. The cell was operated at 1.7 V and an inert nickel anode was used. The thermodynamics and mechanism of the process was also investigated. By controlling the activity of sodium oxide in the melt, the cell could be operated below the decomposition voltage of the electrolyte with the net sequence of events being the ionization of oxygen, its subsequent transport to the anode and discharge leaving behind iron at the cathode. A reduction time of 1 h was achieved for a 1 g oxide tablet (close to the theoretical reduction time predicted by Faraday’s laws) at a current density of 520 mA cm⁻² with iron phase yields of ∼90 wt.%. The energy consumption was 2.8 kWh kg⁻¹.     

Slow-crack-growth and indentation damage in calcium magnesium aluminosilicate (CMAS) glass from desert sand

Desert sand from a Middle East country was melted into calcium magnesium aluminosilicate (CMAS) glass. Its chemical composition was analyzed to be 25.2CaO-2.6MgO-8.2Al₂O₃-59.8SiO₂-1.6Fe₂O₃-1.5K₂O weight % using inductively coupled plasma-atomic emission spectrometry. The CMAS glass powder was hot pressed into billets. Slow-crack-growth (SCG) and indentation deformation/fracture of the CMAS glass was investigated. The SCG susceptibility parameter (n) was found to be 25 ± 3 which is within a range of n = 15–35 that has been observed in many silicate glasses and glass ceramics. A similarity in indentation hardness and toughness was found between the CMAS glass and the low-silica content (50–70%) glasses. However, an exception was that significant lateral cracking was typified in the CMAS glass, as quantified via stress analysis in the vicinity of an indent.     

Host lattice and solid solution formation in an octal-cation (NbTaZrTiHfVWMo)C high entropy carbide ceramic

High entropy carbide ceramics (HECCs) exhibit property peculiarity, such as higher hardness and higher toughness than conventional binary counterparts. The cation-sites of HECCs consist of 4 or more types of transitional metals with equiatomic ratios. Our work advocates that the individual constituent carbide in HECCs cannot be treated equally; instead, one serves as the host lattice and other metallic elements occupy its cation-site lattice substitutionally. urging sintering before the completion of solid solution, individual carbides tend to bind with each other to form a variety of intermediate products. Specifically, in the octal-cation (NbTaZrTiHfVWMo)C carbide, TaC serves as the host lattice and a few intermediate products, such as (Zr,Hf)C and (Ti,V,W,Mo)C, formed. A dense and single-phase carbide can be obtained through sintering at a low temperature of 2000°C and the so-obtained carbide has a nanohardness of 38.71 GPa and a fracture toughness of ∼ 4.67 MPa·m^1/2, outweighing a majority of the existing carbides.     

Effects of different types of sintering additives and post-heat treatment (PHT) on the mechanical properties of SHS-fabricated Si3N4 ceramics

Porous silicon nitride (Si₃N₄) ceramics were fabricated by self-propagating high temperature synthesis (SHS) using Si, Si₃N₄ and sintering additive as raw materials. Effects of different types of sintering additives with varied ionic radius (La₂O₃, Sm₂O₃, Y₂O₃, and Lu₂O₃) on the phase compositions, development of Si₃N₄ grains and flexural strength (especially high-temperature flexural strength) were researched. Si₃N₄ ceramics doped with sintering additive of higher ionic radius had higher average aspect ratio, improved room-temperature flexural strength but degraded high-temperature flexural strength. Besides, post-heat treatment (PHT) was conducted to crystallize amorphous grain boundary phase thus improving the creep resistance and high-temperature flexural strength of SHS-fabricated Si₃N₄ ceramics. Excellent high-temperature flexural strength of 140 MPa~159 MPa and improved strength retention were achieved after PHT at 1400 °C.     

Enhanced thermal and mechanical properties of PW‐based HTPB binder using polystyrene (PS) and PS–SiO2 microencapsulated paraffin wax (MePW)

Polystyrene (PS) microencapsulated paraffin wax (MePW) and PS–SiO₂ MePW were used to improve the form‐stability of PW in hydroxyl‐terminated polybutadiene‐derived polyurethane (HTPB) binder. HTPB matrix containing different contents of PS MePW, PS–SiO₂ MePW, and PW were prepared. The chemical composition, crystallinity, microstructure, heat capacities, thermal stabilities, thermal reliabilities, leakage, and mechanical properties of the composites were compared using Fourier transforms infrared spectroscope, X‐ray diffractometer, scanning electronic microscope, differential scanning calorimeter, thermo‐gravimetric analyzer, thermal cycling test, leaking test, compression, and tensile tests, respectively. The results showed that the MePW/PW/HTPB composites were prepared without chemical reaction. The thermal stability and mechanical properties of PS–SiO₂ MePW/PW/HTPB increased more dramatically than that of PS MePW/PW/HTPB. With the increasing contents of MePWs, the PW leakage of the composites decreased, especially for PS MePW/PW/HTPB. Consequently, the MePW/PW/HTPB composites possess a potential application for PW‐based polymer‐bonded explosive system. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46222.     

Structural and ion transport properties of sodium ion conducting Na2MTeO6 (M= MgNi and MgZn) solid electrolytes

Solid-state electrolytes Na₂MTeO₆ (M = MgNi and MgZn) were prepared via a conventional solid-state reaction method. Structural properties of the samples were investigated by using powder X-ray diffraction (XRD), Raman, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy (XPS) techniques. XPS and XRD studies indicate the phase pure hexagonal layered P2-type structure of samples. Raman and FTIR spectroscopy reveal the possible bending and stretching vibration modes for Te–O and other metal oxides. The ion transport properties of the solid electrolytes were investigated by using AC impedance spectroscopy. The electrical properties were examined by means of classical brick layer model. The specific grain conductivity (σ_g) is found to be 2.13 × 10⁻⁵ S cm⁻¹ and 0.90 × 10⁻⁵ S cm⁻¹ at 20 °C for Na₂MgNiTeO₆ and Na₂MgZnTeO₆ electrolytes, respectively. The activation energy of σ_g for Na₂MgNiTeO₆ and Na₂MgZnTeO₆ is found to be 0.59 eV and 0.36 eV respectively for the temperature below 30 °C. Summerfield AC conductivity scaling analysis of samples is performed. These electrolytes could be potential candidates in solid-state Na⁺ battery applications.     

Enhanced dielectric properties of poly(vinylidene fluoride)–surface functionalized BiFeO3 composites using sodium dodecyl sulfate as a modulating agent for device applications

In this work, we prepared a series of poly(vinylidene fluoride) (PVDF)–surface functionalized BiFeO₃ (h‐BFO)–Sodium dodecyl sulfate (SDS) composite films by solvent casting method to investigate the effect of SDS in the composites. The X‐ray diffraction confirmed that the structure of h‐BFO significantly changed in the PVDF‐(h‐BFO)‐SDS composite in comparison with the rhombohedral structure of pure BiFeO₃. The microscopic study illustrated that the composite with a higher percentage of SDS content facilitated the dispersion as well as proper distribution of ceramic particles in the polymer matrix. The presence of different functionalities of respective polymer and the modified fillers was confirmed by FTIR Spectrophotometer. The dielectric and electrical study done by Impedance Analyzer revealed that the SDS treated surface functionalized composites showed relatively higher dielectric properties than that of two phase composites and pure polymer. Finally, the ferroelectric properties of the composite films done by P‐E loop tracer revealed that the SDS‐treated composites showed an enhanced remanent polarization. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45040.     

Influence of ZrC addition on the microstructure,mechanical properties and oxidation resistance of Ti(C,N)-based cermets

In this study, Ti(C,N)-WC-NbC-ZrC-Co-Ni cermets were prepared by sintering-hip at 1450?°C. The effect of ZrC addition on the microstructure, mechanical properties, oxidation resistance and wear resistance of Ti(C,N)-WC-NbC-Co-Ni cermets were explored in detail. The results show that ZrC addition plays the role of inhibitor in the dissolution–reprecipitation process, which can increase the wear-resistant carbide phases and inhibit the precipitation of brittle (Ti,W,Nb)(C,N) rim phase. Therefore, the core-rim structures are refined and the Nb content in binder increases, which enhance mechanical properties and oxidation resistance of cermets. With the increasing ZrC content, the oxidation resistance of cermets can be improved constantly, while the transverse rupture strength, fracture toughness and wear resistance of these cermets increase first and then decrease. The cermet with 1?wt% ZrC exhibits the transverse rupture strength of 2549?MPa and highest fracture toughness of 13.0?MPa?m^1/2. The oxidation weight gain of cermets containing 5?wt% ZrC after holding 100?h at 750?°C in air is 2.8?×?10^?6 g?mm^?2, which is only 22% of that in the cermets without ZrC addition.     

Electrodeposition of poly(N-methylpyrrole) on stainless steel in the presence of sodium dodecylsulfate and its corrosion performance

Poly(N-methylpyrrole)-dodecylsulfate (PNMPy-DS) coating was electrosynthesized by potentiodynamic method on a stainless steel in oxalic acid solution containing sodium dodecylsulfate for the first time. The effects of electrochemical synthesis parameters, such as applied potential, scan rate and cycle number, on the protective behaviors of PNMPy-DS films were investigated and the optimum synthesis conditions were determined. The PNMPy-DS coating was characterized by the cyclic voltammetry, FT-IR spectroscopy and SEM methods. Corrosion protection behavior of this polymer-coated steel was investigated in 0.5 mol L⁻¹ HCl solution by potentiodynamic polarization and EIS methods. The results show that the PNMPy-DS coating provides effective protection for the stainless steel against to corrosion due to the fact that the large negatively charged dodecylsulfate dopant in the polymer structure electrostatically repels corrosive chloride ions and delays their access to metal surface.     

Photo-Crosslinked Poly(ε-caprolactone fumarate) Networks: Roles of Crystallinity and Crosslinking Density in Determining Mechanical Properties

We present a material design strategy of combining crystallinity and crosslinking to control the mechanical properties of polymeric biomaterials. Three polycaprolactone fumarates (PCLF530, PCLF1250, and PCLF2000) synthesized from the precursor polycaprolactone (PCL) diols with nominal molecular weights of 530, 1250, and 2000 g mol⁻¹, respectively, were employed to fabricate polymer networks via photo-crosslinking process. Five different amounts of photo-crosslinking initiator were applied during fabrication in order to understand the role of photoinitiator in modulating the crosslinking characteristics and physical properties of PCLF networks. Thermal properties such as glass transition temperature (T_g), melting temperature (T_m), and degradation temperature (T_d) of photo-crosslinked PCLFs were examined and correlated with their rheological and mechanical properties.     

Defect formation and mechanical stability of perovskites based on LaCrO3 for solid oxide fuel cells (SOFC)

Perovskites on the basis of LaCrO₃ are of interest as ceramic interconnect materials for the development of solid oxide fuel cells (SOFCs). The interconnects are exposed to oxidising and reducing atmospheres under operating conditions. Oxygen vacancy formation was determined as a function of oxygen partial pressure between 1 and 10⁻²² bar at temperatures between 900 and 1100 °C. Different perovskite compositions made of (La,Ca/Sr)CrO_3−δ, La(Cr,Mg)O_3−δ, La(Cr,Mg/Cu/Co,Al)O_3−δ, and (La,Ca)(Cr,Al)O_3−δ were investigated. Defect models were evaluated to describe the oxygen vacancy formation and the respective thermodynamic data were determined. The results are used to explain existing literature data on the isothermal expansion of LaCrO₃ based perovskites under reducing conditions. Complementary mechanical measurements with selected perovskite compositions revealed that lower oxygen partial pressure causes higher stiffness, strength and fracture toughness. The change in properties is discussed in terms of the observed ferroelastic domains and the interaction of the domain wall motion with the oxygen vacancies.     

Fabrication and properties of in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites

The in situ silicon nitride nanowires reinforced porous silicon nitride (SNNWs/SN) composites were fabricated via gelcasting followed by pressureless sintering. SNNWs were well distributed in the porous silicon nitride matrix. The tip-body appearance suggested a VLS growth mechanism. The flexural strength and elastic modulus of the prepared composites can achieve 84.3?±?3.9?MPa and 23.3?±?2.0?GPa respectively (25?°C), while the corresponding porosity was 40.7?vol.%. Remarkably, the strength retention rate of the composites at 1400?°C was up to 66.1%. This is due to the excellent thermal stability of SNNWs and silicon nitride matrix. Also, the fracture toughness of the composites was improved to ～42% larger than pure porous silicon nitride ceramics because of the bridging effect of the NWs and the interlocking effect of β-Si₃N₄ crystals. In addition, a good thermal shock resistance and dielectric properties were indicated. The good overall performance made SNNWs/SN composites promising candidate for advanced high-temperature applications.     

Effect of Ta addition on the microstructures and mechanical properties of in situ bi-phase (TiB2-TiCxNy)/(Ni-Ta) cermets

In this work, in situ bi-phase (TiB₂-TiC_xN_y)/(Ni-Ta) cermets were fabricated via a combined combustion synthesis and hot-pressing (CSHP) method in a Ni-Ti-BN-B₄C-Ta system. The effects of Ta addition on the reaction process, phase constituents, microstructures and mechanical properties of the (TiB₂-TiC_xN_y)/(Ni-Ta) cermets were studied. Ta is shown to dilute the system and lead to a small number of intermediate phases (Ni₂₀Ti₃B₆ and Ni₃Ti) that are retained in the products. Furthermore, the addition of Ta can markedly refine the ceramic particles and decrease the size and quantity of voids. The evaluation of the mechanical properties revealed that an increase in the Ta content resulted in increases in the compression strength (σ_UCS) and hardness and that the fracture strain (ε_f) increased first and then decreased. The cermet with the optimal addition of 5?wt% Ta possessed the best mechanical properties without decreasing the value of ε_f (2.9%). The addition of 5?wt% Ta resulted in a compressive strength of 3.37?GPa and the highest hardness of 1909 Hv, which is an increase of ~16% and ~22%, respectively, compared to cermets without added Ta.     

Dependence of chain conformation on degree of sulfonation and counterion dissociation of sodium poly(styrene sulfonate) in semidilute aqueous solution

The chain conformation of sodium poly(styrenesulfonate) (NaPSS) in the semidilute aqueous solution, estimated from the plots of reduced viscosity versus segmental concentration, was found to be dependent on the degree of sulfonation and dissociation extent of counterions. The latter, altered by the content of sodium chloride external salt, was estimated from the ionic conductivity measurements on the basis of the modified Manning's conductivity theories with consideration of the influence of chain overlapping in the semidilute regime. The results indicate that the increase of chain extension of NaPSS with counterion dissociation was further enhanced by increasing the degree of sulfonation.