Recycled carbon fibre-reinforced polypropylene thermoplastic composites

Comingled carbon fibre (CF)/polypropylene (PP) yarns were produced from chopped recycled carbon fibres (reCF) (20 mm in length, 7-8 μm diameter) blended with matrix polypropylene staple fibres (60 mm in length, 28 μm diameter) using a modified carding and wrap spinning process. Microscopic analysis showed that more than 90% of the reCF were aligned along the yarn axis. Thermoplastic composite test specimens fabricated from the wrap-spun yarns had 15-27.7% reCF volume content. Similar to the yarn, greater than 90% of the reCF comprising each composite sample made, showed a parallel alignment with the axis of the test specimens. The average values obtained for tensile, and flexural strengths were 160 MPa and 154 MPa, respectively for composite specimens containing 27.7% reCF by volume. It was concluded that with such mechanical properties, thermoplastic composites made from recycled CF could be used as low cost materials for many non-structural applications.     

The effects of carbon nanotubes on mechanical and thermal properties of woven glass fibre reinforced polyamide-6 nanocomposites

This paper presents a preliminary investigation on the effects of incorporating carbon nanotubes (CNT) into polyamide-6 (PA6) on mechanical, thermal properties and fire performance of woven glass reinforced CNT/PA6 nanocomposite laminates. The samples were characterized by tensile and flexural tests, thermal gravimetric analysis (TGA), heat distortion temperature (HDT) measurements, thermal conductivity and cone calorimeter tests. Incorporation of up to 2 wt% CNT in CNT/PA6/GF laminates improved the flexural stress of the laminates up to 36%, the thermal conductivity by approximately 42% and the ignition time and peak HRR time was delayed by approximately 31% and 118%, respectively.     

Sintering and dielectric properties of BaTiO3 prepared by a composite-hydroxide-mediated approach

High purity tetragonal BaTiO₃ powders consisted of uniform particles of ca. 150 nm in diameter were synthesized by a composite-hydroxide-mediated approach at 240 °C using a novel hydrothermal reaction apparatus with a rolling system. The product showed sinterability superior to the commercial powder, i.e., it could be sintered to full theoretical density at 1200 and 1100 °C without an additive and with 0.3 wt% of Li₂CO₃-0.04 wt% of V₂O₅ mixed sintering aid, respectively. The sintered body of the product also showed piezoelectric properties superior to the commercial one.     

Preparation of LiFePO4/C composite powders by ultrasonic spray pyrolysis followed by heat treatment and their electrochemical properties

LiFePO₄ powders could be successfully prepared from a precursor solution, which was composed of Li(HCOO)·H₂O, FeCl₂·4H₂O and H₃PO₄ stoichiometrically dissolved in distilled water, by ultrasonic spray pyrolysis at 500 °C followed by heat treatment at sintering temperatures ranging from 500 to 800 °C in N₂ + 3% H₂ gas atmosphere. Raman spectroscopy revealed that α-Fe₂O₃ thin layers were formed on the surface of as-prepared LiFePO₄ powders during spray pyrolysis, and they disappeared after sintering above 600 °C. The LiFePO₄ powders prepared at 500 °C and then sintered at 600 °C exhibited a first discharge capacity of 100 mAh g⁻¹ at a 0.1 C charge-discharge rate. To improve the electrochemical properties of the LiFePO₄ powders, LiFePO₄/C composite powders with various amounts of citric acid added were prepared by the present method. The LiFePO₄/C (1.87 wt.%) composite powders prepared at 500 °C and then sintered at 800 °C exhibited first-discharge capacities of 140 mAh g⁻¹ at 0.1 C and 84 mAh g⁻¹ at 5 C with excellent cycle performance. In this study, the optimum amount of carbon for the LiFePO₄/C composite powders was 1.87 wt.%. From the cyclic voltammetry (CV) and AC impedance spectroscopy measurements, the effects of carbon addition on the electrochemical properties of LiFePO₄ powders were also discussed.     

CF/EP composite laminates with carbon black and copper chloride for improved electrical conductivity and interlaminar fracture toughness

An experimental study was conducted to improve the electrical conductivity of continuous carbon fibre/epoxy (CF/EP) composite laminate, with simultaneous improvement in mechanical performance, by incorporating nano-scale carbon black (CB) particles and copper chloride (CC) electrolyte into the epoxy matrix. CF/EP laminates of 65 vol.% of carbon fibres were manufactured using a vacuum-assisted resin infusion (VARI) technique. The effects of CB and the synergy of CB/CC on electrical resistivity, tensile strength and elastic modulus and fracture toughness (K_IC) of the epoxy matrix were experimentally characterised, as well as the transverse tensile modulus and strength, Mode I and Mode II interlaminar fracture toughness of the CF/EP laminates. The results showed that the addition of up to 3.0 wt.% CB in the epoxy matrix, with the assistance of CC, noticeably improved the electrical conductivity of the epoxy and the CF/EP laminates, with mechanical performance also enhanced to a certain extent.     

Synthesis and microwave dielectric properties of CaO-MgO-SiO2 submicron powders doped with Li2O-Bi2O3 by sol-gel method

Using Ca(NO₃)₂·4H₂O, Mg(NO₃)₂·6H₂O, Si(OC₂H₅)₄, LiNO₃ and Bi(NO₃)₃·5H₂O as raw materials, CaO-MgO-SiO₂ submicron powders were prepared at low temperature by sol-gel method. The crystallization temperature was decreased enormously by the introduction of Li-Bi liquid phase sintering aids into Ca-Mg-Si sol, and the powders with average particle sizes of 80-100 nm and 200-400 nm were obtained at the calcining temperature of 750 °C and 800 °C, respectively. The sintering characteristic and dielectric properties of powders calcined at 750 °C with different content of powders calcined at 800 °C were studied. When the content of powders calcined at 800 °C was 10 wt%, the dielectric ceramic sintered at 890 °C had compact structure, and possessed excellent microwave dielectric properties: ?_r = 7.16, Q × f = 25630 GHz, τ_f = −69.26 ppm/°C.     

Effect of sintering temperature on dielectric properties of Ba0.6Sr0.4TiO3–MgO composite ceramics prepared from fine constituent powders

Composite ceramics of Ba_0.6Sr_0.4TiO₃ + 60 wt.% MgO were prepared from fine constituent powders by sintering at 1200–1280 °C. The composite specimens sintered at the relatively low temperatures showed satisfactory densification due to fine morphology of the constituent powders. The elevation of sintering temperature promoted the incorporation of Mg²⁺ into the lattice of the Ba_0.6Sr_0.4TiO₃ phase and grain growth of the two constituent phases. The dependence of the dielectric properties on sintering temperature was explained in relation to the structural evolution. Controlling the sintering temperature of the composite was found to be important to achieve the desired nonlinear dielectric properties. Sintering at 1230 °C was determined to be preferred for the composite in terms of the nonlinear dielectric properties. The specimen sintered at the temperature attained a tunability of 17.3% and a figure of merit of 127 at 10 kHz and 20 kV/cm.     

On the green density, sintering behavior and electrical property of tape cast Ce0.9Gd0.1O1.95 electrolyte films

Gadolinia doped ceria (Ce_0.9Gd_0.1O_1.95, GDC) electrolyte films were tape cast from oxalate coprecipitated GDC powders, gelcast GDC powders and their mixtures, respectively, to evaluate the effects of the original particle size and distribution on the properties of the green and sintered GDC cast tapes. The apparent density of different original powders, as well as the green density, sintered behavior, and electrical conductivity of tapes cast from the various starting powders were investigated. Mixing the coprecipitated and the gelcast GDC powders not only results in a higher packing efficiency of particles in the loose powders, but also results in higher green and sintered densities of cast tapes. Furthermore, tapes cast from the 50/50 powder mixtures can be sintered to 96.2% of theoretical density at relatively low sintering temperature of 1400°C, whereas those from the oxalate coprecipitated and from the gelcast powders were only 89.7 and 94.1% dense, respectively. The ac impedance measurement shows that GDC films cast from the 50/50 powder mixture exhibit good electrical conductivity (4.2 and 6.0 S m⁻¹ at 700 and 800°C in air, respectively). The test results have revealed that high-density GDC films can be fabricated by tape casting technique at relatively low sintering temperature by optimizing the particle size distribution of the starting powders.     

聚苯乙烯/聚酰胺-12合金的选择性激光烧结成形

在选择性激光烧结(SLS)成形中,聚苯乙烯(PS)成形件精度高但其力学性能差,而聚酰胺-12(PA12)成形件力学性能高但其尺寸精度较差。为此,通过添加增容剂合成了PS/PA12合金粉末并用于SLS成形,研究了PS/PA12合金、PS及PA12成形件的力学性能、精度及断口微观形貌。结果表明,PS/PA12合金成形件的拉伸强度、断裂伸长率、冲击强度、弯曲强度及弯曲模量分别是PS成形件的4.72倍、1.94倍、4.92倍、4.57倍及1.37倍;PS/PA12合金粉末成形件在x、y及z方向的尺寸误差比PA12分别降低了46.9%,47.3%及20.3%。说明PS/PA12合金粉末成形件兼顾了PS成形件精度高及PA12成形件力学性能好的优点,具有较高的推广使用价值。     

Influence of small scale melt mixing conditions on electrical resistivity of carbon nanotube-polyamide composites

Polyamide 6 (PA6) and polyamide 6.6 (PA66) were filled with multiwalled carbon nanotubes (MWNT) using small scale melt mixing under variation of processing conditions, including temperature, rotation speed, and mixing time. In PA66 an electrical percolation threshold of 1 wt% MWNT was found which is lower than that of PA6 at 2.5–4 wt%. In both cases mixing conditions influenced strongly the dispersion and distribution of CNT and the electrical volume resistivity, whereas crystallisation behaviour was only slightly changed. With increasing mixing energy input remaining agglomerates were less in number and smaller, leading to better dispersion. On the other hand, in samples containing 5 wt% MWNT in PA6 electrical volume resistivity showed a minimum at a quite low energy input and then increased considerably with further input of mixing energy. This increase may be related to MWNT breaking during mixing and encapsulation of MWNT by the polyamide chains.     

Porous alumina/zirconia layered composites with unidirectional pore channels processed using a tertiary-butyl alcohol-based freeze casting

Symmetric three-layer porous alumina/zirconia composites with controlled “designer” pore structure have been prepared by a tertiary-butyl alcohol-based freeze casting and sintering at 1400–1500 °C in air. Unidirectional aligned macropore channels were developed over a long range by controlling the solidification direction of tertiary-butyl alcohol solvent; in this case, they were surrounded by more dense structured walls. In layered composite, the bottom layer consisted of small sized pore channels (∼11 μm in diameter) compared with the middle and the top layer, due to the comparatively rapid velocity of the TBA crystal growth during solidification. The compressive strength (63–376 MPa) of the sintered porous layered composite remarkably increased as the porosity decreased (64–32%).     

The effects of Clay on fire performance and thermal mechanical properties of woven glass fibre reinforced polyamide 6 nanocomposites

The effects of small amount of organically modified Clay (Clay) in polyamide 6 (PA6) on fire performance and thermal mechanical properties of Clay/PA6/woven glass fibres (GF) laminates are investigated by cone calorimeter test, dynamic mechanical thermal analysis (DMTA), and heat distort temperature (HDT) measurement. The mechanical properties, such as tensile and flexural properties of Clay/PA6 composites and Clay/PA6/GF laminates were also measured. Up to 3 wt.% Clay in a Clay/PA6/GF laminate with fibre volume fraction of 30 vol.% delayed the ignition time and peak heat release rate (PHRR) time by 55% and 118%, respectively, even though the value of the PHRR or the HDT was not significantly affected. 2 wt.% Clay increased flexural modulus and strength of the Clay/PA6/GF laminate by 10% and 16%, respectively, but more Clay did not increase the mechanical properties accordingly. Small amount of Clay does not affect glass fibre dominated properties, such as HDT, but do affect matrix dominated properties, and significantly affect the fire performance in terms of delaying ignition time and PHRR time. Optimization of laminate making process could benefit from additions of more Clay, therefore further improve fire performance and enhance mechanical properties.     

Effect of sintering temperature on magneto-transport properties of La0.67Ba0.33MnO3/Ba0.7Sr0.3TiO3 composites

La_0.67Ba_0.33MnO₃-20 wt.%-Ba_0.7Sr_0.3TiO₃ composites were sintered at different temperatures in order to explore the possibility of improving the magneto-transport properties of the composites. Detail studies on the magnetic and electrical transport properties for the sintered composite samples have been performed. Results show that the sintered composites have identical ferromagnetic to paramagnetic transition temperature and filamentary feature of metallic phase. When sintering temperature higher than 1300 °C, the composites show Efros-Shklovskii-like variable-range hopping in the temperature range lower than Curie temperature. For samples sintered lower than 1100 °C, a dome-like resistance peak appears at a temperature well below the Curie temperature. Magnetoresistance behavior indicates the existence of spin polarized tunneling in the low temperature range. Considering the contributions from Efros-Shklovskii-like variable-range hopping and spin polarized tunneling, the resistance peak can be well fitted.     

Preparation, microstructure and properties of reaction-bonded AlN ceramics

AlN ceramics were prepared via a reaction-bonding technique, using Al and AlN powders as the starting materials. The effects of processing parameters and CaO as an additive were investigated. An AlN ceramic sintered at 1800°C with 0.5 wt.% CaO is highly densified and its dielectric constant and dielectric loss tgδ are 9.4 and 0.002 at 1 MHz, respectively. Its electrical resistivity and Vickers hardness are 2.88×10¹² Ω cm, 1210 kg/mm², respectively.     

Preparation and properties of negative thermal expansion Zr2WP2O12 ceramics

Using solid-state reaction method, Zr₂WP₂O₁₂ powder was synthesized for this study. The optimum heating condition was 1200 °C for 4 h. The obtained powder was compacted and sintered. The relative density of the Zr₂WP₂O₁₂ ceramics with no sintering additive was 60%. That of samples sintered with more than 0.5 mass% MgO was about 97%. The average grain size (D₅₀), as estimated from the polished surface of a sample sintered at 1200 °C for 4 h was about 1 μm. The obtained ceramics showed a negative thermal expansion coefficient of about −3.4 × 10⁻⁶ °C⁻¹. Young's modulus, Poisson's ratio, three-point bending strength, Vickers microhardness, and fracture toughness of the obtained ceramics were, respectively, 74 GPa, 0.25, 113 ± 13 MPa, 4.4 GPa and 2.3 MPa m^1/2.     

Synthesis,microstructure and mechanical properties of Yttria Stabilized Zirconia (3YTZP) – Multi-Walled Nanotube (MWNTs) nanocomposite by direct in-situ growth of MWNTs on Zirconia particles

In this research, Yttria Stabilized Zirconia (3YTZP) – carbon nanotube (CNT) composites are fabricated by direct in-situ growth of CNTs on the Zirconia particles, followed by densification via the Spark Plasma Sintering (SPS) technique. Scanning electron microscopy analysis of the 3YTZP-CNT powders shows uniform distribution of CNTs without the formation of agglomerates frequently seen with the traditional ex-situ mixing of CNTs in ceramic compositions. The samples were sintered to nearly 100% theoretical density and with a finer grain size microstructure. High Resolution Transmission Electron Microscopy (HRTEM) and Raman Spectroscopy confirm CNT retention in the sintered nanocomposites up to 1600 °C. The flexural strength increases from ∼260 MPa for samples without CNTs sintered at 1600 °C to ∼312 MPa for samples with ∼4 wt.% CNTs sintered at the same temperature. A corresponding increase in the indentation fracture toughness is also observed for samples with ∼4 wt.% CNTs sintered at 1600 °C as compared to samples sintered at the same temperature without CNTs.     

激光烧结尼龙12/ 累托石复合材料的结构与性能

将尼龙12 与有机累托石的混合粉末进行激光烧结成型, 利用X 射线衍射(XRD) 、差示扫描量热分析(DSC) 、扫描电镜(SEM) 等手段对烧结试样的材料结构进行了表征, 并对其力学性能及热性能进行了研究。结果表明: 尼龙12 在激光烧结过程中插入到累托石层间, 形成了尼龙12/ 累托石纳米复合材料。复合材料的力学性能及热稳定性能同尼龙12 烧结试样相比均有不同程度的提高, 有机累托石质量分数为10 %时, 复合材料的拉伸强度、弯曲强度和冲击强度分别提高了10.7 %、15.9 %和38.7 % , 热分解温度提高了27 ℃。      

Filler dispersion and electrical properties of polyamide 12/MWCNT-nanocomposites produced in reactive extrusion via anionic ring-opening polymerization

The reactive extrusion of lauryl lactam to polyamide 12 (PA12) of controlled molar mass was successfully performed in a microcompounder. The maximum residual monomer content was less than 1%. The in-situ polymerization in the presence of 1–5 wt.% multiwalled carbon nanotubes (MWCNTs) was studied and the processing conditions were optimized with respect to the electrical resistivity and MWCNT dispersion. Runs which yielded in higher molar mass PA12 resulted in better dispersion of MWCNTs, whereas nanocomposites with lower molar mass PA12 had lower electrical percolation thresholds (MWCNT concentration ∼1 wt.%). A high screw speed of 200 rpm was identified to cause best dispersion and the lowest percolation threshold.     

Low-temperature sintering of temperature-stable LaNbO4 microwave dielectric ceramics

We demonstrate the correlation between sintering behavior and microstructural observations in low-temperature sintered, LaNbO₄ microwave ceramics. Small CuO additions to LaNbO₄ significantly lowered the sintering temperature from 1250 to 950 °C. To elucidate the sintering mechanism, the internal microstructure of the sample manipulated by a focused ion beam (FIB) was investigated using transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS). LaNbO₄ with 3 wt% CuO sintered at 950 °C for 2 h possessed the following excellent microwave dielectric properties: a quality factor (Qxf) of 49,000 GHz, relative dielectric constant (?_r) of 19.5, and temperature coefficient of resonant frequency (τ_f) of 1 ppm/°C. The ferroelastic phase transformation was also investigated using in situ X-ray diffraction (XRD) to explain the variation of τ_f in low-temperature sintered LaNbO₄ as a function of CuO content.     

Mechanical and microstructural properties of Nylon-12/carbon black composites: Selective laser sintering versus melt compounding and injection molding

Composites of Nylon-12 reinforced with 4 wt.% carbon black (CB) manufactured by selective laser sintering (SLS) are compared in terms of flexural strength and flexural modulus, tensile strength and tensile modulus, and impact strength to composites made by extrusion and injection molding (Ex-IM). The Nylon-12 system made by SLS had 25% and 35% higher flexural and tensile modulus, respectively, compared to the Nylon-12 system made by Ex-IM and ∼10% higher strength. However, upon addition of CB both the modulus and the strength of the composites made by SLS were significantly lower compared to composites made by Ex-IM. This is due to the poor dispersion of nanoscale CB and due to the higher porosity of the composites made by SLS, which also explains the relatively low impact strength observed. Based on XRD and DSC studies, it is concluded that the composites made by the two processing methods did not differ significantly in their crystallization characteristics such as the degree of crystallinity, crystal type, and lamellar thickness. However, it was found that CB acted as a nucleating agent for Nylon-12 when Ex-IM was used, leading thus to smaller but more numerous polymer crystals.