Microstructure and mechanical properties of Al2O3 composites with surface-treated carbon nanotubes (CNTs): dispersibility of modified carbon nanotubes (CNTs) on Al2O3 matrix

Aluminum oxide (Al2O3) matrix have been reinforced by the multi-walled carbon nanotubes (MWCNTs) to overcome the inherent brittleness of Al2O3 matrix. In order to increase mechanical properties of MWCNTs-Al2O3 composites, MWCNTs need to be well dispersed and individually incorporated in Al2O3 matrix. In this work, aluminum hydroxide (Al(OH)3) used as a Al2O3 precursor and MWCNTs were mixed in an aqueous solution for the homogeneous mixing of hetero-particles, as functions of the content of MWCNTs and the potential hydrogen (pH) of Al(OH)3 suspension. Firstly, MWCNTs were purified and modified by an acid reagent, inducing that the dispersibility of MWCNTs is increased in an aqueous solution by carboxylic group given on the surface of MWCNTs. The modified MWCNTs were added in the Al(OH)3 suspension, and then the mixture was filtered at room temperature. The filtered powders were formed using an uniaxial pressing and then densified by a pressureless heat treatment. As the pH is decreased the Al(OH)3 particles are well dispersed in an aqueous solution, due to the increment of repulsive force between particles with a same surface charge. MWCNTs are individually incorporated into Al2O3 matrix up to 1 vol.% MWCNTs, whereas MWCNTs are aggregated at the composite with 3 vol.% MWCNTs. Therefore, control of the pH and the MWCNTs content are key factors to be considered for the fabrication of MWCNTs-Al2O3 composites with high functional properties.     

Synthesis of composite material of cobalt oxide (Co3O4) with hydroxide functionalized multi-walled carbon nanotubes (MWCNTs) for electrochemical determination of uric acid

The gout is mainly found due to accumulation of uric acid crystals into the joints which produces the inflammatory symptoms. Thus, it is highly demanded to detect uric acid from our body. Herein, we prepare a composite material of cobalt oxide (Co₃O₄) with hydroxide functionalized multi-walled carbon nanotubes (MWCNTs) by hydrothermal method. The composite material is used for the modification of glassy carbon electrode (GCE) and investigated for the electrochemical determination of uric acid (UA). The analytical techniques such as scanning electron microscopy (SEM), powder X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and Fourier Infra-red spectroscopy (FTIR) are used to characterize the composite material. The Co₃O₄ exhibits a dendrite morphology and very well chemically coupled with MWCNTs. The elemental analysis confirms the presence of cobalt (Co), oxygen (O) and carbon (C) as main constituent of the composite material. The Co₃O₄ exhibitsa cubic unit cell crystallography in the composite system. The FTIR study reveals the characteristic bands of Co–O bands in the composite material. The cyclic voltammetry isused to study the electrochemical properties of prepared materials. The composite sample with highest percentage of MWCNTs shows an excellent electrochemical activity towards the oxidation of uric acid in phosphate buffer solution pH 7.3. The enzyme free uric acid sensor possesses a linear range of 0.1 mM to 3 mM with a quantified limit of detection of 0.005?±?0.0023 mM. The modified electrode is stable, selective, and very sensitive towards uric acid, therefore it may be used for the monitoring of uric acid from clinical samples. The proposed composite material can be of great interest for energy and biomedical fields.

     

A highly-sensitive l-lactate biosensor based on sol-gel film combined with multi-walled carbon nanotubes (MWCNTs) modified electrode

A new type of amperometric l-lactate biosensor based on silica sol-gel and multi-walled carbon nanotubes (MWCNTs) organic–inorganic hybrid composite material was developed. The sol-gel film was used to immobilize l-lactate oxidase on the surface of glassy carbon electrode (GCE). MWCNTs were used to increase the current response and improve the performance of biosensor. The sol-gel film fabrication process parameters such as H₂O : TEOS and pH were optimized, Effects of some experimental variables such as applied potential, temperature, and pH on the current response of the biosensor were investigated. Analytical characteristics and dynamic parameters of the biosensors with and without MWCNTs in the hybrid film were compared, and the results showed that analytical performance of the biosensor could be improved greatly after introduction of the MWCNTs. Sensitivity, linear range, limit of detection (S / N = 3) were 2.097 μA mM^− 1, 0.3 to 1.5 mM, 0.8 × 10^− 3 mM for the biosensor without MWCNTs and 6.031 μA mM^− 1, 0.2 to 2.0 mM, 0.3 × 10^− 3 mM for the biosensor with MWCNTs, respectively. This method has been used to determine the l-lactate concentration in real human blood samples.     

Potentiometric urea biosensor based on multi-walled carbon nanotubes (MWCNTs)/silica composite material

A novel potentiometric urea biosensor has been fabricated with urease (Urs) immobilized multi-walled carbon nanotubes (MWCNTs) embedded in silica matrix deposited on the surface of indium tin oxide (ITO) coated glass plate. The enzyme Urs was covalently linked with the exposed free –COOH groups of functionalized MWCNTs (F-MWCNTs), which are subsequently incorporated within the silica matrix by sol–gel method. The Urs/MWCNTs/SiO₂/ITO composite modified electrode was characterized by Fourier transform infrared (FTIR) spectroscopy, thermal gravimetric analysis (TGA) and UV–visible spectroscopy. The morphologies and electrochemical performance of the modified Urs/MWCNTs/SiO₂/ITO electrode have been investigated by scanning electron microscopy (SEM) and potentiometric method, respectively. The synergistic effect of silica matrix, F-MWCNTs and biocompatibility of Urs/MWCNTs/SiO₂ made the biosensor to have the excellent electro catalytic activity and high stability. The resulting biosensor exhibits a good response performance to urea detection with a wide linear range from 2.18 × 10^? 5 to 1.07 × 10^? 3 M urea. The biosensor shows a short response time of 10–25 s and a high sensitivity of 23 mV/decade/cm².     

碳纳米管的表面修饰对Co-Mo催化剂HDS性能影响的研究

采用程序升温脱附(TPD)、Boehm滴定以及质量滴定(Masstitration)等方法分析了多壁碳纳米管(MWCNT)经过浓硝酸改性处理前后的表面酸性和酸性分布;同时运用XRD技术研究了碳纳米管负载Co Mo催化剂的表面物种和活性组分的分散;并在高压微型反应器中,以二苯并噻吩(DBT)为模型化合物,对碳纳米管负载的Co Mo催化剂体系进行了加氢脱硫(HDS)活性评价,以期揭示碳纳米管的表面修饰对其负载Co Mo催化剂HDS性能的影响。研究结果表明:MWCNT表面呈一定的碱性,通过浓硝酸化学修饰以后,反而呈现较强的酸性,并且随着酸处理时间的延长,表面的酸性进一步增加。如,M MWCNT 6(酸化6h)表面总酸量为1.29×10-3mol/g,M MWCNT 24(酸化24h)表面总酸量为2.46×10-3mol/g。酸改性提高了Co Mo催化剂在MWCNT表面的分散程度,以致Co Mo/MWCNT催化剂的HDS活性和选择性明显高于Co Mo/γ Al2O3催化体系。然而,Co Mo/M MWC NT 6的HDS活性以及选择性与Co Mo/MWCNT相差不多,略有降低,但是仍比Co Mo/γ Al2O3催化体系的HDS活性高。Co Mo/M MWCNT 24催化剂的HDS活性降低很多,并且催化性能发生了很大改变,产物中出现了大量的副产物1,2 二苯基乙烷。表明,深度酸化改性反而不利于提高碳纳米管负载Co Mo催化体系的HDS活性。     

Determination of Pb2+ ions by a modified carbon paste electrode based on multi-walled carbon nanotubes (MWCNTs) and nanosilica

A novel carbon paste ion selective electrode for determination of trace amount of lead was prepared. Multi-walled carbon nanotubes (MWCNTs) and nanosilica were used for improvement of a lead carbon paste sensor response. MWCNTs have a good conductivity which helps the transduction of the signal in carbon paste electrode. The electrode composition of 20 wt% paraffin oil, 57% graphite powder, 15% ionophore (thiram), 5% MWCNTs, and 3% nanosilica showed the stable potential response to Pb²⁺ ions with the Nernstian slope of 29.8 (±0.2) mV decade^?1 over a wide linear concentration range of 10^?7–10^?2 mol L^?1. The electrode has fast response time, and long term stability (more than 2 months). The proposed electrode was used to determine the concentration of lead ions in waste water and black tea samples.     

Rheological and mechanical properties of surface modified multi-walled carbon nanotube-filled PET composite

Poly(ethylene terephthalate) (PET)/multi-walled carbon nanotube (MWCNT) composites were prepared by in situ polymerization. To improve the dispersion of MWCNTs in PET matrix, the surface modified MWCNTs having acid groups (acid-MWCNT) and diamine groups (diamine-MWCNT) were used. The functional groups on the surface of modified MWCNTs were confirmed by infrared (IR) spectrometry. SEM analysis showed better dispersion of diamine-MWCNTs as compared to pristine-MWCNTs and acid-MWCNTs in the PET. The reaction between PET and diamine-MWCNTs was evidenced by the shifting of the G band to a higher frequency in Raman spectroscopy and an increase of the complex viscosity in rheological properties. The composites containing functionalized MWCNTs showed a large increase in the tensile strength and modulus. The PET/diamine-MWCNT composites showed maximum tensile strength and modulus increases by 350% and 290% at 0.5 and 2.0 wt%, respectively, as compared to pure PET.     

聚酰亚胺/多壁碳纳米管杂化薄膜力学性能研究

采用不同长径比的多壁碳纳米管通过原位聚合法制备一系列多壁碳纳米管改性的聚酰胺酸胶液,并经热亚胺化途径制备聚酰亚胺杂化薄膜。利用扫描电子显微镜(SEM)观察薄膜的断面形貌,采用红外光谱仪(FT-IR)分析酸化前后多壁碳纳米管表面官能团的变化,并采用电子万能试验机对薄膜的力学性能进行测试,分析了多壁碳纳米管的含量和长径比对杂化薄膜力学性能的影响。结果表明,小掺杂量下,长径比大的多壁碳纳米管更有利于增强PI杂化薄膜的拉伸强度;而长径比小的多壁碳纳米管使杂化薄膜拉伸强度提高的碳纳米管掺杂量范围更宽。     

Determination of Cu2+ using poly(2-aminothiazole)/ multi-walled carbon nanotubes composite film modified glassy carbon electrodes

2-Aminothiazole was electropolymerized by cyclic voltammetry (CV) on the multi-walled carbon nanotubes (MWCNTs) modified glassy carbon electrode (GCE) surface. Poly(2-aminothiazole)/MWCNTs/GCE was used for determination of copper ions. The anodic peak currents of copper ions evaluated by differential pulse stripping voltammetry (DPSV) are linear with the concentrations in the range from 1.0 x 10(-7) M to 2.0 x 10(-5) M with a linear coefficiency of 0.9985. The detection limit is 2.0 x 10(-9) M calculated for a signal-to-noise ratio of 3 (S/N = 3). The proposed method was applied successfully to the determination of copper ions in drinking water, and the recovery was 96%.     

Mechanical and thermal properties of epoxy nanocomposites reinforced with amino-functionalized multi-walled carbon nanotubes

The functionalized multi-walled carbon nanotubes (MWNTs) with amino groups were prepared after such steps as oxidation, the addition of carboxyalkyl radicals, acylation and amidation. Besides oxidated-MWNTs/epoxy nanocomposites, amino-functionalized MWNTs/epoxy nanocomposites, in which MWNTs with amino groups acted as a curing agent and covalently attached into the epoxy matrix, were fabricated. Subsequently, the effects of MWNT content on the mechanical and thermal properties for the two systems were investigated. It is found that both the tensile strength and impact strength enhance with the increase of MWNT addition, and the most significant improvement of the tensile strength (+51%) and especially impact strength (+93%) is obtained with amine-treated MWNTs at an 1.5 wt.% content. Moreover, the thermal stability of the nanocomposites also distinctly improves. The improvement of the properties of the amine-treated MWNTs system is more remarkable than those of o-MWNTs system. The reasons for these changes were discussed.     

Mechanical properties of Al2O3 particle-Y-TZP matrix composite and its toughening mechanism

Dense Al₂O₃ particle-Y-TZP matrix (Al₂O₃<40 vol%) composite was prepared by pressureless sintering at 1550°C. Composites with 10–30 vol% Al₂O₃ particles showed enhanced fracture toughness, bending strength and Vicker's hardness as compared to single-phase Y-TZP. The highest strength (1150 MPa) and highest toughness (12.4 MPa m^1/2) were obtained for the composite containing 10 vol% Al₂O₃. It was found that, in addition to the contribution by the crack-deflection effect, the enhanced phase transformation from tetragonal to monoclinic during fracture was the main toughening mechanism in operation in the composites.     

多壁碳纳米管/聚己内酯超细复合纤维的制备及性能

通过静电纺丝法制备出多壁碳纳米管(MWCNTs)增强聚己内酯(PCL)超细复合纤维膜。用扫描电镜(SEM)、透射电镜(TEM)、拉曼光谱仪、差示扫描量热仪(DSC)和X射线衍射仪(XRD)对MWCNTs/PCL超细复合纤维进行表征,并进行了拉伸测试。结果表明,MWCNTs分散于PCL纤维中,MWCNTs的加入降低了PCL的结晶度。当PCL中MWCNTs的含量为0.5 wt%时,其结晶度最低,但此时MWCNTs/PCL超细复合纤维具有最好的力学性能。     

Surfactant-associated electrochemical properties of ferrocene adsorbed on a glassy carbon electrode modified with multi-walled carbon nanotubes

The electrochemical properties of ferrocene (Fc) on a glassy carbon (GC) electrode modified by multi-walled carbon nanotubes (MWNTs) in the presence and absence of surfactants have been investigated by progressively voltammetric sweeping. Dihexadecyl phosphate (DHP) and hexadecyl trismethyl ammonium chloride (HTAC) are found to impact the redox reactions of Fc adsorbed on MWNT surfaces. An excess amount of DHP dispatches Fc from MWNTs surfaces, leading to weakly adsorbed configuration of Fc. The formal potential of the adsorbed Fc in the presence of DHP shifts to a lower potential. Cationic surfactant HTAC on MWNT surfaces depresses the redox reactions corresponding to the weakly adsorbed configuration of Fc. It becomes evident that the configuration and hence redox reactions of Fc depend strongly on the presence and concentrations of surfactants on the electrode surfaces and in the buffer solutions.     

Changes in material properties and surface fractality of multi-walled carbon nanotubes modified by heat and acid treatments

Heat treatment and acid oxidation of multi-walled carbon nanotubes (MWNTs) were conducted in this study. Fractal analysis was carried out to study the surface properties of the MWNTs after modifications. The results of the present work show that there was a contraction in the hysteresis loop at P/P_o around 0.80, which differentiated the capillary condensation in the internal pores from that in the aggregated pores forming by the entangled MWNTs. The maximum of the pore size distribution (PSD) curves in the mesopore range did not shift after modification because it described the major inner diameters of the MWNTs. A smooth and ordered surface could be formed after heat treatment, but a much rougher and less homogeneous surface was produced on acid-treated MWNTs. Moreover, an excellent linear increase in the surface fractal dimension (d _s ) values was observed as the micropore surface area (S_mi) increased, which implies that the generation of micropores would contribute to the surface roughness.     

Photoactive multi-walled carbon nanotubes: synthesis and utilization of benzoin functional MWCNTs

This study describes a facile method for the synthesis of functionalized multi-walled carbon nanotubes (MWCNTs) carrying photoactive group. The synthesis of MWCNTs-based macro-photoinitiator was achieved by the esterification reaction between benzoin moiety and acyl chloride functional MWCNTs. Synthesized MWCNT-based photoinitiator (MWCNTs–benzoin) was used in the photopolymerization of styrene to yield polystyrene (PS)-grafted MWCNTs (MWCNTs–PS) by “grafting from” method. The efficiency of MWCNTs–benzoin photoinitiator was determined by evaluating the effect of initiator to monomer ratio and reaction period on photopolymerization of styrene. Fourier transform infrared spectroscopy, Raman and X-ray photoelectron spectroscopy analyses confirmed the covalent bonding for functionalization of MWCNTs and determined the final structures. Thermogravimetric analysis, gel permeation chromatography and UV spectroscopy were performed to evaluate the grafting efficiency of PS that covalently grafted to MWCNTs, and high efficiency of MWCNTs–benzoin as a macro-photoinitiator was also confirmed. Scanning electron microscopy was used to determine the surface morphology of functionalized MWCNTs and MWCNTs–PS.     

Mechanical properties of in-situ toughened Al2O3/Fe3Al

Mechanical properties of in-situ toughened Al₂O₃/Fe₃Al nano-/micro-composites were measured. Effects of Fe₃Al content, sintering temperature and holding time on properties and microstructure of the composites were investigated. The addition of Fe₃Al nano-particles decreased the aspect ratio and grain size of Al₂O₃, and changed the fracture mode of composites. The maximum bending strength and fracture toughness were 832 MPa and 7.96 MPa m^1/2, which were obtained in Al₂O₃/5 wt.% Fe₃Al sintered at 1530 °C and Al₂O₃/10 wt.% Fe₃Al sintered at 1600 °C, respectively. Compared to monolithic alumina, the strength increased by 132% and the toughness increased by 73%. The improvement in the mechanical properties of the composites was attributed to the change in fracture mode from intergranular fracture to transgranular fracture, the “in-situ reinforced effect” arising from the platelet grains of Al₂O₃ matrix, refined microstructure by dispersoids, as well as crack deflection and bridging of intergranular and intragranular Fe₃Al.     

Hydrogen sensing properties of multi-walled carbon nanotubes

The hydrogen sensing properties of multi-walled carbon nanotubes (MWNTs) synthesized by a hot filament CVD process are reported in this paper. The MWNTs were synthesized by a hot filament assisted chemical vapor deposition method using cobalt oxide nanoparticles as the catalyst on SiO₂ surfaces. The MWNTs were characterized with Raman spectroscopy and scanning electron microscopy. Two-terminal test devices were fabricated by depositing a layer of MWNTs between prefabricated gold electrodes on SiO₂ surfaces. The diameter of these MWNTs was in the 5–8 nm range. The sensitivity of carbon nanotubes was measured for different gas concentrations at different temperatures. It was found that the MWNTs were sensitive to H₂ in low temperature regions of 140–350 °C and had a maximum sensitivity (80%) at 230 °C. No sensitivity was observed at a temperature lower than 140 °C or higher than 400 °C. Though bare MWNTs are not sensitive to H₂ at room temperature, they exhibited very good sensing characteristics in the 140–300 °C range.     

Fabrication of conductive porous alumina (CPA) structurally modified with carbon nanotubes (CNT)

A novel binary porous composite nano-carbon networks (NCNs)/alumina, which is denoted as electrically conductive porous alumina (CPA), was structurally modified by carbon nanotubes (CNT) pre-treated with mixed concentrated acids at 60 °C for 6 h in this study. This conductive ceramics (CCs) was fabricated by combination of gelcasting and high temperature reductive sintering (HTRS) in novel atmosphere. CNT pre-treatment leading to the increased hydrophilicity makes it possible to make uniformly dispersed CNT/alumina slurry. And by HTRS in Ar at 1700 °C for 2 h, well-gelled polymer net-paths in green body prepared by gelcasting technology were totally converted to nano-carbon networks (NCNs) without destruction of CNT. NCN with graphitic crystal structure was evaluated by Raman spectroscopy in sintered ceramic body. Moreover, comparing with as-received CNT, the decreased surface defect of detected composite also supported the further graphitization of CNT via HTRS in Ar instead of burning out. With the aid of field-emission scanning electronic microscopy (FE-SEM) observation, the increased alumina grains in sintered ceramic body CNT/NCN/alumina was valid. Moreover, it was demonstrated that there were three components in this composite, which is carbon filler with two different forms (CNT and NCN) and alumina matrix. And these three components CNT covered with Al₂O₃ particles (Al₂O₃/CNT), NCN and alumina grains (alumina) co-exist in four different situations as follows: (a) Al₂O₃/CNT–alumina co-junction, (b) Al₂O₃/CNT–NCN co-junction, (c) Al₂O₃/CNT–alumina–NCN and (d) Al₂O₃/CNT mesh between alumina boundaries. Furthermore, by comparing with binary composite NCN/alumina (CPA), the increased flexural strength of ternary composite CNT/NCN/alumina (CNT/CPA) up to 38 MPa was attributed to the reinforcement CNT acting as elastic bridge in composite.     

Mechanical properties of sinter-forged Al2O3-ZrO2 ceramics

Two kinds of composites, Al₂O₃-25 wt% ZrO₂(2 mol% Y₂O₃) (Y-ZTA), Al₂O₃-25 wt% ZrO₂(8 mol% CeO₂) (Ce-ZTA) were produced by the sinter-forging process. The effect of presintering temperature on the mechanical properties of the composites was examined. The sinter-forging process increased the room-temperature bending strength in comparison with pressureless sintering, owing to the smaller grain size in sinter-forged bodies than in pressureless sintered ones. It was found necessary to keep the presintering temperature considerably lower than sinter-forging temperature in order to improve the room-temperature strength. The strength of sinter-forged Ce-ZTA was higher than that of sinter-forged Y-ZTA. The residual surface compressive stress induced by the phase transition during grinding in Ce-ZTA was found to be effective to further improve the strength and fracture toughness.