Hot-pressing and characterization of TiB2–SiC composites with different amounts of BN additive

This research aimed to study the influence of different amounts of hBN additive on the mechanical properties and microstructure of TiB₂-15 vol% SiC samples. All ceramics, containing 0, 3.5, and 7 vol% hBN, were sintered at 2000 °C using a hot-pressing route and reached their near full densities. Thanks to two different chemical reactions among the SiC reinforcement and the TiB₂ surface oxides (B₂O₃ and TiO₂), the in-situ phases of SiO₂ and TiC were generated over the sintering process. The intergranular mode was identified as the predominant fracture type in all three composite samples. The hBN additive could contribute to grain refining of composites so that the sample containing 7 vol% hBN reached the finest microstructure. Finally, the highest Vickers hardness of 25.4 HV_0.5 kg and flexural strength of 776 MPa were attained for the TiB₂–SiC and TiB₂–SiC-7 vol% hBN samples, respectively.     

Processing of different alumina–zirconia composites by slip casting

Two Al₂O₃–ZrO₂ mixture preparation routes: classical powder mixing and addition of a Zr (IV) precursor solution to a well dispersed Al₂O₃ suspension, were used to produce alumina (Al₂O₃)–zirconia (ZrO₂) slip cast composites. For the conventional powder mixing route, two commercial 3 mol% yttria-partially stabilized zirconia powders, 0.3 wt% Al₂O₃-doped (Al-doped Y-PSZ) and without Al₂O₃ (Y-PSZ), were employed. The influence of the zirconia content and the solid loading on the rheological properties of concentrated aqueous Al₂O₃–ZrO₂ slips were investigated. The density of green samples was studied and related to the degree of slip dispersion. In addition, the influence of the processing conditions on the density and microstructure development of sintered samples were investigated. By using the Zr (IV) precursor route, nano-sized ZrO₂ (ZN) particles homogeneously distributed on the Al₂O₃ particle surfaces were obtained; however, it let to aggregates of some Al₂O₃ particles with very fine ZrO₂ uniformly distributed. The viscosity and yield stress values of Al₂O₃–ZN suspensions were markedly higher than those of Al₂O₃–Al-doped Y-PSZ and Al₂O₃–Y-PSZ ones, for all the compositions and solid loading studied and resulted in a less dense packing of cast samples. However, for the composite with 10.5 vol% ZN a high sintered density and a smaller ZrO₂ grain size distribution compared with the conventional powder mixing route could be obtained.     

Influence of TiB2 content on the properties of TiC–SiCw composites

The impact of various volume percentages of TiB₂ additive (0, 10, 20, and 30) on the microstructure, relative density (RD), Vickers hardness, flexural strength, and thermal conductivity of as-sintered TiC-10 vol% SiC_w-based composite samples were scrutinized. All four samples were sintered using the SPS method under the following circumstances; sintering temperature of 1900 °C, dwell time of 7 min, and external pressure of 40 MPa. The best relative density of 98.73% was achieved for the sample with no TiB₂ additive, indicating the negative effect of TiB₂ additive on the RD and formation of porosity. The microstructural observations and XRD results confirmed the chemical interaction of TiO₂ and B₂O₃ oxide layers and SiC_w and in-situ formation of the TiSi brittle phase and TiC. The most significant values of flexural strength (511 MPa) and hardness (27.67 GPa) were related to TiC-10 vol% SiC_w and TiC-10 vol% SiC_w-30 vol% TiB₂ samples, respectively. On the contrary, the specimens with 30 vol% and 10 vol% TiB₂ as additive presented the poorest qualities of flexural strength (234 MPa) and Vickers hardness (22.12 GPa). Finally, the influence of the TiB₂ content on the thermal conductivity was evaluated, indicating the positive impact of this secondary phase on this characteristic, so with adding 30 vol% TiB₂ to TiC-10 vol% SiC_w, a thermal conductivity of 30.7 W/m.K was obtained.     

Sintering,microstrusture and hardness of different alumina–zirconia composites

Two commercial 3 mol% yttria-partially stabilized zirconia powders, 0.3 wt% Al₂O₃-doped (Al-doped Y-PSZ) and without Al₂O₃ (Y-PSZ), were used to produce alumina (Al₂O₃)-zirconia (ZrO₂) slip cast composites. The influence of the substitution of Al₂O₃ either by different Al-doped Y-PSZ contents or 50 vol% Y-PSZ on the sintering kinetic at the intermediate stage was investigated. In addition, the microstructure of Al₂O₃ and the different composites at temperatures in the range of 1100–1600 °C was studied and related to the sample hardness. An increase in the sintering rate was observed when Al-doped Y-PSZ increased from 22 to 50 vol% or when 50 vol% Y-PSZ was substituted by 50 vol% Al-doped Y-PSZ. 50 vol% ZrO₂ was the most effective concentration to reduce the rate of Al₂O₃ grain growth in the final sintering stage; the Al₂O₃ grain growth began at lower temperatures and became greater with decreasing the Al-doped Y-PSZ content. On the contrary, the ZrO₂ grain growth slightly increased with increasing the Al-doped Y-PSZ concentration. However, for 50 vol% Al-doped Y-PSZ a smaller ZrO₂ grain size distribution compared with 50 vol% Y-PSZ could be achieved. As the average Al₂O₃ grain size of the sintered samples became greater than about 1 µm a markedly decrease in the hardness was found; this occurred at temperatures higher than 1400 °C and 1500 °C for Al₂O₃ and the composite with 10.5 vol% Al-doped Y-PSZ, respectively.     

Synergized poly(lactic acid)–hydroxyapatite composites: Biocompatibility study

In vitro biocompatibility of impact modified composites produced from poly(lactic acid) (PLA) and hydroxyapatite (HA) is reported in this study. Surface modification was previously used to facilitate the dispersion of HA in PLA, whereas impact property of the PLA-HA composites was deliberately enhanced as it was necessary. Herein, osteoblast cell culture assay was used to assess the possible effects of HA surface modification and impact modification on the cell behavior in physiological media. Furthermore, antimicrobial properties of the HA were assessed. Evidence of HA modification was confirmed through elemental and spectroscopic analysis. Incorporation of HA offered better cell attachment and proliferation to the PLA matrix, with significant increase in the cell viability (%). Also, modification of HA did not present obvious cytotoxicity to the PLA-HA composite. Conversely, incorporation of impact modifier slowed down the rate of cell proliferation on the composite surface but facilitates increased wettability. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47400.     

Effect of impact angle on wear behavior of Mo2NiB2–Ni cermets with different Ni content

Herein, the influence of the impact angle and Ni content on the wear behavior of Mo₂NiB₂–Ni cermets was studied using an erodent-carrying slurry comprising artificial seawater and SiO₂ sands. The results reveal that the material loss may be attributed to the wear damage caused by SiO₂ sands because cermets are expected to exhibit good corrosion resistance in artificial seawater. The relative density of cermets markedly influences their resistance to wear damage, and the material loss experienced by cermets with poor relative density is 2–4 times higher than that of cermets with good relative density; this occurs because a higher relative density can markedly enhance the mechanical properties and reduce the defects in the cermets. Moreover, the results indicate that as the impact angle increases from 0° to 60°, the manifestation of the wear mechanism changes from damaging the Ni binder phase (caused by single cutting wear) to damaging both the Mo₂NiB₂ ceramic and Ni binder phases due to the combination of cutting wear and impact wear. The wear damage is dominated by the cutting wear and impact wear from SiO₂ sand at the low and high impact angles, respectively. Furthermore, the severe deterioration of the single ceramic skeleton at high impact angles indicates that the synergistic influence of the Mo₂NiB₂ ceramic and Ni binder phases on enhancing the wear resistance of the cermets intensifies at high impact angles.     

Dependence of magnetoelectric properties on the magnetostrictive content in 0–3 composites

The magnetoelectric response in composites of barium titanate (BTO) and cobalt ferrite (CFO) has been determined by measuring the magnetoelectric susceptibility coefficient. This was done by two different methods: magnetocapacitance measurements and magnetoelectric voltage measurement using a lock-in technique. These composites were prepared by the sol–gel method. Four different compositions with different molar ratios of the magnetostrictive phase (CFO) embedded in a piezoelectric matrix of BTO were studied to investigate the effect of the magnetostrictive content and the number density of interfaces on the magnetoelectric response. It was found from both techniques that the magnetoelectric coupling effect increases with the increase of applied field and it had a non-linear dependence on the percentage of magnetostrictive content in the composites.     

Thermodegradative study of HDPE–HA nanocomposites: IKP and E 2 function

IKP and E ₂ function methods were used to study thermal stability of HDPE–HA nanocomposites synthesized by in situ ethylene polymerization at different volumes of solvent and temperatures. Thermal analysis was carried out at five different heating rates, β = 3, 5, 7, 10, and 13 °C/min, under N₂ atmosphere. Kinetics parameters calculated by IKP method presented a slight increase on activation energy when HA was incorporated in HDPE. A similar tendency was observed in the results obtained from the E ₂ function method, where the activation energy of the nanocomposites increased 100 kJ/mol with respect to unfilled polymer (420–460 kJ/mol). These results implied higher stability of HDPE due to HA incorporation. HDPE and HDPE–HA degradation mechanisms are represented by a set of functions, those with the highest probability were: nucleation and nucleus growth (S3) 23 %, reaction order (S5) 16 %, reaction in the interface (S6. S7, S8) 11–14 %, and potential law (S14, S17) 3 %.     

Effects of different silane coupling agents on structure and properties of starch–chitosan–kaolin composites

Effects of different silane coupling agents on clay surface modification were studied. Herein, functionalized superfine kaolin was compounded with starch–chitosan (SCS) to prepare starch–chitosan-functionalized superfine kaolin composite. The characterization results showed that kaolin (K) was successfully modified; the composite formed a dense intercalated structure. The glass-transition temperature (T _g) was measured by differential scanning calorimetry and dynamic mechanical analysis. It decreased by 60 °C which attested the crystallinity of SCS. The results of thermogravimetric analysis showed that the fastest weight-loss temperature (T_max) was elevated by over 50 °C for composites. Mechanical properties of the composites were explored by electronic universal testing machine. Tensile strength and elongation of composites were improved by 4.7 and 10.9 times. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48050.     

Development of SiC–Si composites with fine-grained SiC microstructures

This work summarises the influence of the original particle-size of the SiC powder on the mechanical properties of silicon infiltrated SiC (SiC-Si) composite. These composites are based on a defined SiC particle-size structure. Using α-SiC powders with a mean particle-size of 12·8, 6·4, 4·5 and 3 μm, a clear linear enhancement of the bending strength with decrease of SiC-particle-size was observed. However, a further decrease of the SiC particle-size (from 3 to 0·5 μm) brought no increase of the strength and toughness, respectively. ©     

The effect of ethyl-lactate and ethyl-acetate plasticizers on PCL and PCL–HA composites foamed with supercritical CO2

The present study reports foaming of polycaprolactone (PCL) and PCL nano- and micro-composites with dispersed hydroxyapatite (HA) particles by means of binary mixtures of supercritical CO₂ (scCO₂) and either ethyl lactate (EL) or ethyl acetate (EA) as plasticizer. The effect of the size and concentration of HA particles, as well as the effects of the plasticizer type and the incorporation route were investigated aiming to fabricate porous scaffolds with uniform morphology and controlled pore size distribution. For this purpose, foaming experiments were carried out by selecting two operating temperatures, 40 and 45 °C, and two soaking times, 1 and 17 h. Furthermore, a double step of depressurization was used to promote the development of a double-scale pore size structure in porous scaffolds useful for tissue engineering.The results of this study indicated that supercritical foaming of PCL and PCL–HA composites is enhanced when the selected operating temperature and time are 45 °C and 17 h, respectively. Furthermore, although both EL and EA plasticizers enhanced the low temperature foaming of the materials, we demonstrated that the route of incorporation of the plasticizer is a critical aspect for enhancing composite foaming and scaffold fabrication. From this point of view, the best results were achieved when EA was pre-mixed with the polymeric powder for preparing a dough for the foaming process.     

Effects of SiC content on thermal shock behavior and elastic modulus of cordierite–mullite composites

Cordierite (Mg₂Al₄Si₅O₁₈) is a commercially available ceramic with low fracture toughness that hampers its broad industrial applications. Although several studies have reported the mechanical improvement of cordierite using various reinforcements, modulating its mechanical and thermal shock characteristics is not explored precisely. In the present research, we investigated the manufacturing of cordierite–mullite ceramics and the role of SiC on their thermomechanical properties. The in-situ formed mullite particles were obtained by mixing andalusite-talc-alumina and addition of SiC. It was found that thermal shock behavior and elastic moduli are dependent on SiC content and retained porosity. Furthermore, the addition of SiC to cordierite-based ceramics could enhance the thermal shock resistance via proper activation of the crack bridging mechanism in the matrix of the prepared composite.     

Oxidation of ZrB2–SiC ultra-high temperature composites over a wide range of SiC content

The oxidation performance of ZrB₂–SiC ultra-high temperature ceramics with SiC content ranging from 20 to 80 vol% has been evaluated at 1773 K for 50 h and at 2073 K for 20 min. Oxidation reaction pathways were interpreted using volatility diagrams of the ZrB₂–SiC system. At 1773 K for 50 h, all ZrB₂–SiC composites from 20 to 80 vol% SiC formed a protective SiO₂ surface coating. Samples with ≤50 vol% SiC developed a distinguishable SiC-depleted layer at 1773 K and 2073 K. High temperature torch testing for 20 min at approximately 2073 K revealed that samples with ≥65 vol% SiC exhibit a depression under the torch flame. Samples rich in ZrB₂ were dominated by a ZrO₂ layer after a similar exposure. The overall weight density of ultra-high temperature ceramics can be reduced with improved oxidation performance at 1773 K by adding at least 65 vol% SiC.     

Properties of cement–sand-based piezoelectric composites with carbon nanotubes modification

Carbon nanotubes (CNTs) were dispersed in a cement–sand-based piezoelectric composite as conductive fillers to improve its poling efficiency. Specimens were prepared by mixing PZT powders, cement and sand with CNTs. The effect of CNTs ranging from 0 to 0.9 vol% on properties of the composite, including its piezoelectric coefficient, dielectric constant and loss, and sensing characteristic, were characterized. It was found that the addition of CNTs facilitated effective poling under a low electric field of 1 MV/m at room temperature and improved the piezoelectric and dielectric properties of the composite. The composite modified by CNTs achieved optimal properties when the CNTs content was 0.6 vol% and this was verified by the investigation of sensing effects of the composite through compressive tests.     

Preparation of SiO2–SiC composites with a precursor method

SiO₂–SiC composite particles were prepared through a hybrid sol–gel precursor process. Compacts were prepared by using a conventional sintering process. The techniques of DSC–TG, SEM and XRD were use to characterize the composite particles and the sintered compacts. It was found that a core–shell structure was constructed in the composite particles with cores of SiC and shells of amorphous SiO₂. Nucleation of SiO₂ occurred at about 1200 °C. The optimized sintering temperature for 30SiO₂–70SiC (vol.%) composites was about 1400 °C with a relatively homogeneous microstructure. The maximum density was about 2.03 g cm^?3.     

Morphology and anti-ablation properties of PIP Cf/C–SiC composites with different CVI carbon content under 4.2 MW/m2 heat flux oxy-acetylene test

In this work, the needled carbon fiber preforms were used to make seven groups of carbon/carbon composite billets with different matrix carbon contents by controlling the processing time of chemical vapor infiltration (CVI). C_f/C–SiC composites were prepared by infiltration of SiC into these C/C composites billets using polycarbosilane (PCS) through precursor infiltration and pyrolysis (PIP). After oxy-acetylene torch testing (heat flux of 4.2 MW/m²) for 200s, 300s and 400s, respectively, it revealed that the anti-ablation properties of the C_f/C–SiC composite samples were enhanced by a higher content of SiC matrix. Additionally, specimens bearing longer duration tests showed a trend of lower average ablation rates. The lowest linear ablation rate is 0.008 mm/s and the mass ablation rate is 0.0019 g/s for those high SiC content samples tested for 400s. The SEM images of the tested samples showed the mechanism and the non-linear process of ablation resistance progression.     

Dielectric and electrical conductivity properties of multi-stage spark plasma sintered HA–CaTiO3 composites and comparison with conventionally sintered materials

One of the different issues limiting the wider application of monolithic hydroxyapatite (HA) as an ideal bone replacement material is the lack of reasonably good electrical transport properties. The comprehensive electrical property characterization to evaluate the efficacy of processing parameters in achieving the desired combination of electroactive properties is considered as an important aspect in the development of HA-based bioactive material. In this perspective, the present work reports the temperature (RT-200 °C) and frequency (100 Hz–1 MHz) dependent dielectric properties and AC conductivity for a range of HA–CaTiO₃ (HA–CT) composites, densified using both conventional pressureless sintering in air as well as spark plasma sintering in vacuum. Importantly, the AC conductivity of spark plasma sintered ceramics [～upto 10^?5 (Ω cm)^?1] are found to be considerably higher than the corresponding pressureless sintered ceramics [～upto 10^?8 (Ω cm)^?1]. Overall, the results indicate the processing route dependent functional properties of HA–CaTiO₃ composites as well as related advantages of spark plasma sintering route.     

Oxidation kinetics of MgO–C in air with varying ash content

Abstract

MgO–C refractories are extensively used in metallurgical vessels in the steel industry. The graphite plays a vital role owing to its non-wettability with slag. Though quality is very important for graphite, little published data is available on the effect of varying ash content on oxidation. Since MgO–C bricks manufactured from graphite with different ash contents are regularly used for lining converters and steel ladles at SAIL plants, a study has been conducted to select an optimum graphite with low cost. Graphites with 2·8, 4·4, 12 and 20% ash content were used to make MgO–C samples in the laboratory, and oxidation properties were evaluated using an electrically heated tube furnace. Kinetic analysis shows that a chemical reaction mechanism operates in the earlier part of oxidation, followed by a diffusion mechanism in the latter part. MgO–C manufactured from graphite with 4·4% ash content was observed to have the best oxidation resistance, with an activation energy of 43·4 kJ mol^?1 during the earlier stage followed by 34·2 kJ mol^?1 in the latter.     

Bond strength of hybrid sol–gel coatings with different additives

The hybrid sol–gel coating on Al 2024-T3 was modified by adding polyaniline, TiO₂, or γ-Al₂O₃ nanoparticles in the formulation separately. The coating was then used as an adhesive to bond Al 2024-T3 alloys, forming a single lap joint. The bond strength of the sol–gel coating was investigated using a universal tensile test machine. The lap shear strength of the original sol–gel coating was about 1.38 MPa and it was increased up to 2.26 MPa after the modification by adding 0.05 wt% PANI microparticles in the sol–gel coating. The small increase in strength was attributed to an improvement in its adhesive flexibility because of incorporation of the long-chain organic polymer in its structure. Furthermore, the addition of different amounts of TiO₂ nanoparticles in the unmodified sol–gel coating also led to an increase in shear strength compared to the undoped sol–gel coating. Typically, a sol–gel coating containing 2.0 wt% of TiO₂ recorded the highest adhesive strength of about 4.0 MPa. A similar increase in strength was observed when doping γ-Al₂O₃ nanoparticles into the original hybrid sol–gel coating. Adding 0.5 wt% of γ-Al₂O₃ in the sol–gel coating increased the adhesive bonding strength up to 4.48 MPa. The fracture surface of the specimen was separately observed by SEM and Optical Microscopy in order to examine potential evidences of mechanism and nature of failure. The reason why the adhesive strength increased after the modification of the sol–gel coating is discussed in this article.     

Transformation of ethylbenzene–m-xylene mixture on zeolites with different structures

Hierarchical zeolite ZSM-5 synthesized by applying amphiphilic organosilane as mesopore template, nanosized zeolite Beta and zeolite MCM-22 have been studied, for the first time in the conversion of mixed ethylbenzene–m-xylene feed. The effects of the channel structure, nanosizing and presence of mesopores in these zeolite materials with close Si/Al molar ratio on the catalytic activity and selectivity have been discussed. It was found that the diverse zeolites have different advantages and disadvantages in dependence on their structure and morphology. MCM-22 zeolite provides promising ethylbenzene conversion at low xylene loss with high production of the p-isomer among xylene.