Optimization of the spark plasma sintering processing parameters affecting the properties of polyimide

The present study deals with the optimization of polyimide (PI) mechanical properties, obtained by Spark Plasma Sintering (SPS), by using a method combining Design of Experiments (DOE) with physical, structural, and mechanical characterizations. The effects of SPS parameters such as temperature, pressure, dwell time, and cooling rate on the density, mechanical properties, and structure of PI were investigated. The experimental results revealed that the mechanical properties of the material were optimized by raising the sintering temperature up to 350°C. The optimized SPS processing parameters were a temperature of 350°C, a pressure of 40 MPa, and a dwell time of 5 min. Under these conditions, a relative density of 99.6% was reached within only a few minutes. The corresponding mechanical properties consisted of Young's modulus of 3.43 GPa, a Shore D hardness of 87.3, and a compressive strength of 738 MPa for a maximum compressive strain of 61.8%. Moreover, when working at 320°C and at 100 MPa, an increase in the dwell time was necessary to enhance the properties. Contrary to the other parameters, the cooling rate appeared to be a non‐significant parameter. Finally, correlations between the PI structure and the mechanical properties were made to demonstrate the densification mechanisms. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41542.     

Proton-conducting ceramics as electrode/electrolyte materials for SOFC's—part I: preparation,mechanical and thermal properties of sintered bodies

The aim of these investigations was to prepare and to examine compounds of a high temperature solid oxide fuel cell with a proton conducting electrolyte in view of the mechanical and thermal properties. The powders were made by the conventional solid reaction of carbonates and oxides. The stoichiometry of the electrolyte Ba,Ca niobate (BCN) was varied with x=0, x=0.12 and x=0.18. As potential cathode material SrCeO₃ and SrZrO₃ stabilised with 5% Yb was prepared, and as anode material cermet of BCN and Ni with 50:50 wt.% was synthesised. The mechanical properties like bending strength (room and high temperature), Young modulus (E), modulus of rigidity (G), Poison's ratio, micro hardness and fracture toughness were measured on sintered samples. The highest values for bending strength, E and G could be found for BCN12 (156 MPa, 160 GPa, 63 GPa) and the cerate (175 MPa, 145 GPa, 56 GPa), the lowest for the cermet BCN/Ni (72 MPa, 68 GPa, 29 GPa). The investigation of the thermal properties of the bulk material showed a thermal stability to a temperature of 1400 °C. The thermal expansion coefficient measured at 1000 °C was found to be in the range of 10–12×10⁻⁶/K. Further investigations with respect to the mechanical and thermal properties have to be made for the whole system of cathode–electrolyte–anode.     

Elevated Temperature Strength Enhancement of ZrB2–30 vol% SiC Ceramics by Postsintering Thermal Annealing

The mechanical properties of dense, hot‐pressed ZrB₂–30 vol% SiC ceramics were characterized from room temperature up to 1600°C in air. Specimens were tested as hot‐pressed or after hot‐pressing followed by heat treatment at 1400°C, 1500°C, 1600°C, or 1800°C for 10 h. Annealing at 1400°C resulted in the largest increases in flexure strengths at the highest test temperatures, with strengths of 470 MPa at 1400°C, 385 MPa at 1500°C, and 425 MPa at 1600°C, corresponding to increases of 7%, 8%, and 12% compared to as hot‐pressed ZrB₂–SiC tested at the same temperatures. Thermal treatment at 1500°C resulted in the largest increase in elastic modulus, with values of 270 GPa at 1400°C, 240 GPa at 1500°C, and 120 GPa at 1600°C, which were increases of 6%, 12%, and 18% compared to as hot‐pressed ZrB₂–SiC. Neither ZrB₂ grain size nor SiC cluster size changed for these heat‐treatment temperatures. Microstructural analysis suggested additional phases may have formed during heat treatment and/or dislocation density may have changed. This study demonstrated that thermal annealing may be a useful method for improving the elevated temperature mechanical properties of ZrB₂‐based ceramics.     

Effects of reactive end-capper on mechanical properties of chemical amplified photosensitive polyimide

Photosensitive polyimide (PSPI) was synthesized and characterized to replace the conventional polyimide buffer layer because direct patterning with PSPI could reduce the processing procedure to the half. Since PSPI should be dissolved in alkaline aqueous solution and have good mechanical properties after imidization, low molecular weight of PSPI was synthesized with reactive end-capper, which could extend the chain length of PSPI during imidization. Therefore norbornene end-capped PSPI precursor was synthesized with various 5-norbornene-2,3-dicarboxylic anhydride (NDA) content.Although molecular weight of PSPI decrease with increasing NDA content, the elongation at break and the glass transition temperature (T_g) of PSPI films imidized at 300 °C increased with increasing NDA content. On the other hand, elongation at break of PSPI films imidized at 350 °C decreased but T_g of those increased with increasing NDA content. Above T_g, thermal expansion coefficient decreased dramatically by introducing NDA end-capper. From mechanical and thermal properties of PSPI, it appears that low molecular weight of PSPI can be chain-extended and crosslinked during imidization.     

Poly(arylene ether nitrile) copolymers containing pendant phenyl: Synthesis and properties

A series of poly(arylene ether nitrile) copolymers (PENAPs) were synthesized with bisphenol A (BP-A), bisphenol AP (BP-AP) and 2,6-Dichlorobenzonitrile (DCBN) via a nucleophilic substitution polycondensation reaction. FTIR and ¹H-NMR were used to confirm the structure of PENAPs. Glass transition temperature (T_g) of PENAPs determined by differential scanning calorimetry (DSC) ranged from 154.2 to 200.8°C. The 5% weight lost temperature (T_5%) of PENAPs were 418.9–447.7°C. The tensile and DMA test indicated that PENAPs possessed excellent mechanical properties with tensile strength more than 92.8 MPa and storage modulus more than 1.0 GPa at about 150°C. The melt flowability was measured by rheology properties testing ranging from 80 to 1639 GPa at 290°C and under shear frequency 100 Hz, which indicated the copolymers had good flowability and thermal stability. Additionally, PENAPs could be dissolved in many solutions, which meant PENAPs had good solubility and can be processed by solution method.     

Rigid thermosetting liquid molding resins from renewable resources. II. Copolymers of soybean oil monoglyceride maleates with neopentyl glycol and bisphenol A maleates

Soybean oil monoglycerides (SOMG), obtained by the glycerolysis of soybean oil, were reacted with maleic anhydride to produce SOMG maleate half esters. The copolymers of the SOMG maleates with styrene produced rigid thermosetting polymers. The dynamic mechanical analysis (DMA) of this polymer showed a glass‐transition temperature (T_g) around 133°C and a storage modulus (E′) value around 0.94 GPa at 35°C. The tensile tests performed on this polymer showed a tensile strength of 29.36 MPa and a tensile modulus of 0.84 GPa. Mixtures of SOMG with neopentyl glycol (NPG) and SOMG with bisphenol A (BPA) were also maleinized under the same reaction conditions and the resulting maleates were then copolymerized with styrene. The resulting polymers were analyzed for their mechanical properties. The T_g of the copolymers of the SOMG/NPG maleates with styrene was 145°C and the E′ value at 35°C was 2 GPa. The tensile strength of this polymer as calculated from the stress–strain data was 15.65 MPa and the tensile modulus was 1.49 GPa. The T_g of the copolymers of SOMG/BPA maleates, on the other hand, was found to be around 131°C and the E′ value was 1.34 GPa at 35°C. The changes observed in the mechanical properties of the resulting polymers with the introduction of NPG maleates and BPA maleates to the SOMG maleates can be explained by the structural changes on the polymer backbone. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 972–980, 2002     

Characterization of two precursor polyblends: Polyhydroxyamide and poly(amic acid)

Blends of two precursor polymers, polyhydroxy amide (PHA) and poly(amic acid) (PAA), were studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The presence of PHA enhanced thermal and mechanical properties of the polyblends. All of the polyblended films showed large endothermic peaks that decreased monotonically with increasing heat treatment temperature. The cyclization onset temperature (T₁), initial decomposition temperature (T₂), and weight residue at 900°C of the polyblends were shown to be in the ranges of 144–146°C, 532–540°C, and 44–45%, respectively. Also, the thermal stabilities were enhanced consistently with increasing annealing temperature from 25 to 250°C. The ultimate strength and initial modulus of the polyblends increased from 84 to 136 MPa and from 2.93 to 5.34 GPa, respectively, with increasing PHA content. Similar to the trend of thermal stability, increasing the annealing temperature of the polyblends increased the tensile properties of the films. The observed tensile properties are discussed in terms of the morphology of the fractured films as studied by scanning electron microscopy (SEM). The degree of crystallinity of the polyblends was characterized as a function of heat treatment temperatures by wide angle X‐ray diffractometry (WAXD).     

Phase evolution and sinterability of lanthanum phosphate – Towards a below 600 °C Spark Plasma Sintering

Lanthanum phosphate, due to its interesting thermal and mechanical properties is a widely studied material for refractory applications. Sintering processes have already been proposed to densify this material and drive its microstructure. Inspired by recent progress on low temperature sintering, we investigate a low temperature Spark Plasma Sintering (LowT-SPS) using hydrated precursor. First, lanthanum phosphate thermal behaviour was studied using TGA/DTA and XRD analysis on various heat-treated powders. Their SPS behaviour were explored by in situ dilatometry measurements. As hydrated precursor showed a low temperature densification, samples were sintered at temperatures from 160 °C to 350 °C under 400 MPa. Even if dense and nano-scaled microstructures were obtained, a residual hydration was observed. Finally, a well densified and fine-grained monazite type lanthanum phosphate was obtained at 550 °C and under 200 MPa. Its mechanical properties are then compared to conventional and Spark Plasma Sintered materials.     

Effects of water on drawability,structure, and mechanical properties of poly(vinyl alcohol) melt‐spun fibers

Poly(vinyl alcohol) (PVA) melt‐spun fibers with circular cross‐section and uniform structure, which could support high stretching, were prepared by using water as plasticizer. The effects of water content on drawability, crystallization structure, and mechanical properties of the fibers were studied. The results showed that the maximum draw ratio of PVA fibers decreased with the increase of water content due to the intensive evaporation of excessive water in PVA fibers at high drawing temperature. Hot drying could remove partially the water content in PVA as‐spun fibers, thus reducing the defects caused by the rapid evaporation of water and enhancing the drawability of PVA fibers at high drawing temperature. The decreased water content also improved the orientation and crystallization structure of PVA, thus producing a corresponding enhancement in the mechanical properties of the fibers. When PVA as‐spun fibers with 5 wt % water were drawn at 180 °C, the maximum draw ratio of 11 was obtained and the corresponding tensile strength and modulus reached ～0.9 GPa and 24 GPa, respectively. Further drawing these fibers at 215 °C and thermal treating them at 220 °C for 1.5 min, drawing ratio of 16 times, tensile strength of 1.9 GPa, and modulus of 39.5 GPa were achieved. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45436.     

Preparation and properties of an oxide fiber‐reinforced silica matrix composite

Oxide (Nextel? 440) fiber‐reinforced silica composites, with the density and porosity of 1.97 g/cm³ and 21.8%, were prepared through sol‐gel. Their average flexure strength, elastic modulus, shear strength, and fracture toughness at room temperature were 119.7 MPa, 25.6 GPa, 10.8 MPa, and 4.0 MPa·m^1/2, respectively. The composites showed typical toughened fracture behavior, and distinct pullout fibers were observed at the fracture surface. Their mechanical properties were performant up to 1000°C, with the maximum flexural strength of 132.2 MPa at 900°C. Moreover, the composites showed good thermal stability, even after thermal aging and thermal shock at elevated temperatures.     

Production of nepheline/quartz ceramics from geopolymer mortars

This research has investigated the mechanical properties and microstructure of metakaolin derived geopolymer mortars containing 50% by weight of silica sand, after exposure to temperatures up to 1200 °C. The compressive strength, porosity and microstructure of the geopolymer mortar samples were not significantly affected by temperatures up to 800 °C. Nepheline (NaAlSiO₄) and carnegieite (NaAlSiO₄) form at 900 °C in the geopolymer phase and after exposure to 1000 °C the mortar samples were transformed into polycrystalline nepheline/quartz ceramics with relatively high compressive strength (～275 MPa) and high Vickers hardness (～350 HV). Between 1000 and 1200 °C the samples soften with gas evolution causing the formation of closed porosity that reduced sample density and limited the mechanical properties.     

Thermo-mechanical and high-temperature dielectric properties of cordierite-mullite-alumina ceramics

Heterogeneous ceramics made of cordierite (55–56 wt%), mullite (22–33 wt%) and alumina (23–11 wt%) were prepared by sintering non-standard raw materials containing corundum, talc, α-quartz, K-feldspar, kaolinite and mullite with small amounts of calcite, cristobalite and glass phases. The green specimens prepared by PVA assisted dry-pressing were sintered within the temperature range of 950–1500 °C for different dwelling times (2–8 h). The effects of sintering schedule on crystalline phase assemblage and thermomechanical properties were investigated. The sintered ceramics exhibited low coefficients of thermal expansion (CTE) (3.2–4.2×10⁻⁶ °C⁻¹), high flexural strength (90−120 MPa and high Young modulus (100 GPa). The specimens sintered at 1250 °C exhibited the best thermal shock resistance (∆T~350 °C). The thermal expansion coefficients and thermal shock resistance were studied using Schapery model, the modelling results implying the occurrence of non-negligible mechanical interactions between the phases in bulk. The dielectric properties characterized from room to high temperature (RT– HT, up to 600 °C) revealed: (i) noticeable effects of sintering schedule on dielectric constant (5–10) and dielectric loss factor (~0.02–0.04); (ii) stable dielectric properties until the failure of the electrode material. The thermomechanical properties coupled with desirable dielectric properties make the materials suitable for high density integrated circuitry or high temperature low-dielectric materials engineering.     

Effect of interphase on mechanical properties and microstructures of 3D Cf/SiBCN composites at elevated temperatures

Interphase between the fibers and matrix plays a key role on the properties of fiber reinforced composites. In this work, the effect of interphase on mechanical properties and microstructures of 3D C_f/SiBCN composites at elevated temperatures was investigated. When PyC interphase is used, flexural strength and elastic modulus of the C_f/SiBCN composites decrease seriously at 1600°C (92 ± 15 MPa, 12 ± 2 GPa), compared with the properties at room temperature (371 ± 31 MPa, 31 ± 2 GPa). While, the flexural strength and elastic modulus of C_f/SiBCN composites with PyC/SiC multilayered interphase at 1600°C are as high as 330 ± 7 MPa and 30 ± 2 GPa, respectively, which are 97% and 73% of the values at room temperature (341 ± 20 MPa, 41 ± 2 GPa). To clarify the effect mechanism of the interphase on mechanical properties of the C_f/SiBCN composites at elevated temperature, interfacial bonding strength (IFBS) and microstructures of the composites were investigated in detail. It reveals that the PyC/SiC multilayered interphase can retard the SiBCN matrix degradation at elevated temperature, leading to the high strength retention of the composites at 1600°C.     

Elaboration of performance of tea dust–polypropylene composites

In this research work, dynamic, mechanical, and thermophysical properties of untreated and 5, 7, and 10 wt % styrene treated tea dust (TD):polypropylene (PP) composites prepared by injection‐molding machine were elaborated. There were distinctive and significant improvement observed in mechanical properties (tensile strength, tensile modulus, and elongation at break), physical analysis (water swelling), dynamic mechanical properties (storage modulus, loss modulus, and tan δ), and thermal behavior and surface morphological properties of styrene treated TD:PP (40:60) composites as compared to that of untreated one. The tensile strength (from 7.00 to 9.95 MPa), tensile modulus (from 350 to 715 MPa), storage modulus (from 8500 to ～10,500 MPa), and loss modulus (from ～150 to ～200 MPa) increased on 10 wt % styrene treatment of TD over the untreated TD:PP (40:60) composites. The styrene treated TD:PP (40:60) composites behaved as more elastic than their pure counterpart. Styrene treated TD:PP (40:60) composites were more thermally more stable (85 °C difference) as compared to virgin PP. Overall, this research also indicates the use of TD waste. An improvement in dispersion of styrene treated TD particles in PP was also observed in the preparation of the PP composites due to good compatibility of styrene with PP. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44750.     

Improving the thermal and mechanical properties of poly(arylene ether nitrile) films through blending high- and low-molecular-weight polymers

In present work, novel phthalonitrile end-capping poly(arylene ether nitrile)-phenyl (PEN-Ph) films with excellent mechanical properties as well as high glass transition temperature were prepared through blending high-molecular-weight PEN-Ph (HMW PEN-Ph) and low-molecular-weight PEN-Ph (LMW PEN-Ph). Then, the thermal and mechanical properties of the samples with different mass ratio of HMW PEN-Ph and LMW PEN-Ph were studied, and the effect of heat-treatment temperature on the performance of films was also investigated. The analysis results indicate that the crosslinking density as well as film formation can be controlled by adjusting the mass ratio of HMW PEN-Ph to LMW PEN-Ph. Besides, when the mass ratio of HMW PEN-Ph to LMW PEN-Ph is 5:2, the film treated at 340 °C possesses the best thermal and mechanical properties, with T_g of 218.9 °C and tensile strength of 104.8 MPa, increased by 10.9 °C and 16.6 MPa than pure HMW PEN-Ph film, respectively. Thus, the presence of LMW PEN-Ph makes the thermal and mechanical properties of the films improve dramatically, providing the possibility for the application in the electronics and high-temperature resistant fields. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48457.     

Mechanical and thermal enhancements of benzoxazine‐based GF composite laminated by in situ reaction with carboxyl functionalized CNTs

An easy and efficient approach by using carboxyl functionalized CNTs (CNT‐COOH) as nano reinforcement was reported to develop advanced thermosetting composite laminates. Benzoxazine containing cyano groups (BA‐ph) grafted with CNTs (CNT‐g‐BA‐ph), obtained from the in situ reaction of BA‐ph and CNT‐COOH, was used as polymer matrix and processed into glass fiber (GF)‐reinforced laminates through hot‐pressed technology. FTIR study confirmed that CNT‐COOH was bonded to BA‐ph matrices. The flexural strength and modulus increased from 450 MPa and 26.4 GPa in BA‐ph laminate to 650 MPa and 28.4 GPa in CNT‐g‐BA‐ph/GF composite, leading to 44 and 7.5% increase, respectively. The SEM image observation indicated that the CNT‐COOH was distributed homogeneously in the matrix, and thus significantly eliminated the resin‐rich regions and free volumes. Besides, the obtained composite laminates showed excellent thermal and thermal‐oxidative stabilities with the onset degradation temperature up to 624°C in N₂ and 522°C in air. This study demonstrated that CNT‐COOH grafted on thermosetting matrices through in situ reaction can lead to obvious mechanical and thermal increments, which provided a new and effective way to design and improve the properties of composite laminates. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Low-temperature thermally modified fir-derived biomorphic C–SiC composites prepared by sol-gel infiltration

In order to solve the problems (i.e. low infiltration efficiency, cracks, interface separation and poor mechanical properties) in the process of wood-derived C–SiC composites, the thermal modification of fir at low temperatures (300 °C ～ 350 °C) combined with sol-gel infiltration was used to successfully produce biomorphic ceramics. The prepared materials were comprehensively characterized and exhibited improved interfacial bonding between C and SiC and mechanical properties. The weight gain per unit volume (0.123 g/cm³) of SiO₂ gel in the fir thermally modified at 300 °C is 167.4%, higher than that (0.046 g/cm³) of the unmodified fir. A well-bonded interface was formed between the SiO₂ gel and the pore wall of the fir thermally modified at 300 °C. With the increase of modification temperature from 300 °C to 350 °C, the distance between SiO₂ gel and the pore wall increases, and a gap (1–3 μm) is observed between SiO₂ gel and the pore wall of the fir carbonized at 600 °C. The C–SiC composites sintered at 1400 °C exhibited the highest compressive strength and bending strength of 40.8 ± 5.8 MPa and 11.7 ± 2.1 MPa, respectively, owing to the well-bonded interface between C of fir thermally modified at 300 °C and SiC. However, the composites sintered at 1600 °C for 120 min exhibited the lowest compressive strength and bending strength of 28.1 ± 13.4 MPa and 5.7 ± 1.6 MPa, respectively, which are 31.1% and 51.3% lower than those sintered at 1400 °C for 120 min, respectively. This might result from the porous structure formed by the excessive consumption of fir-derived carbon during the reaction between C and SiO₂ at 1600 °C for 120 min. Therefore, thermal modification in the preparation of biomorphic C–SiC composites can promote slurry infiltration and the formation of a well-bonded interface between C and SiC, thus improving the mechanical properties of the composites.     

Hyperbranched-polysiloxane-based hyperbranched polyimide films with low dielectric permittivity and high mechanical and thermal properties

A series of hyperbranched polysiloxane (HBPSi)-based hyperbranched polyimide (HBPI) films with low dielectric permittivity and multiple branched structures are fabricated by copolymerizing 2,4,6-triaminopyrimidine (TAP) with 4,4′-(hexafluoroisopropylidene)diphthalic anhydride, 4,4′-diaminodiphenyl ether, and HBPSi via the two-step polymerization method. The dielectric permittivity of HBPSi hyperbranched polyimide films decreases with increasing TAP fraction, namely, from 3.28 for sample PI-1 to 2.80 for PI-4, mainly owing to the enlarged free volume created by the incorporation of multiple branched structures. Moreover, HBPSi HBPI possesses desirable solubility and good mechanical properties and thermal stability. PI-4 not only has low dielectric permittivity (2.80, 1 MHz), excellent solubility (soluble in several common organic solvents), and remarkable thermal properties (glass-transition temperature of 273 °C, 5% weight loss temperature of 498 °C in N₂ and 486 °C in O₂), but it also demonstrates admirable mechanical properties with a tensile strength of 103 MPa, elongation at break of 7.3%, and a tensile modulus of 2.16 GPa. HBPSi HBPI might have potential applications in interlayer dielectrics and other microelectronics fields. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47771.     

Modification of Free Volume in Epoxy Adhesive Formulations

Additives are described which modify the free volume available for segmental motion in epoxy adhesives. Such a mechanism can produce an increase in the tensile modulus of conventional epoxy-amine systems of>60% (e.g. to>4.1 GPa) and in tensile strength of>50% (e.g. to 125 MPa), while also producing a ductile mode of failure (stress-strain curve has negative slope before failure). At low strains, a reduction in free volume hinders polymer segmental motion and so increases the modulus. However, these materials also exhibit a very low Poisson's ratio and strains of ca. 5% cause a sufficient increase in free volume that ductile failure can occur. Improvements in low temperature cure properties (e.g. 118 MPa tensile strength at 60°C cure) together with reductions in the coefficient of thermal expansion and water uptake are also reported. These improvements in bulk adhesive properties are shown to translate into improved adhesive joint performance.     

Effects of high-temperature annealing on the microstructure and properties of C/SiC–ZrC composites

C/SiC–ZrC composites were prepared by a combining slurry process with precursor infiltration and pyrolysis, and then annealed from 1200 °C to 1800 °C. With rising annealing temperature, their mass loss rate increased, and the flexural strength and modulus decreased from 227.9 MPa to 41.3 MPa and from 35.3 GPa to 22.7 GPa, respectively. High-temperature annealing, which elevated thermal stress and strengthened interface bonding, was harmful to the flexural properties. However, it improved the ablation properties by increasing the crystallization degree of SiC matrix. The mass loss rate and linear recession rate decreased with increasing annealing temperature and those of the samples annealed at 1800 °C were 0.0074 g/s and 0.0011 mm/s respectively. Taking mechanical and ablation properties into consideration simultaneously, the optimum annealing temperature was 1600 °C.