Control of layer stacking in CVD graphene under quasi-static condition

The type of layer stacking in bilayer graphene has a significant influence on its electronic properties because of the contrast nature of layer coupling. Herein, different geometries of the reaction site for the growth of bilayer graphene by the chemical vapor deposition (CVD) technique and their effects on the nature of layer stacking are investigated. Micro-Raman mapping and curve fitting analysis confirmed the type of layer stacking for the CVD grown bilayer graphene. The samples grown with sandwiched structure such as quartz/Cu foil/quartz along with a spacer, between the two quartz plates to create a sealed space, resulted in Bernal or AB stacked bilayer graphene while the sample sandwiched without a spacer produced the twisted bilayer graphene. The contrast difference in the layer stacking is a consequence of the difference in the growth mechanism associated with different geometries of the reaction site. The diffusion dominated process under quasi-static control is responsible for the growth of twisted bilayer graphene in sandwiched geometry while surface controlled growth with ample and continual supply of carbon in sandwiched geometry along with a spacer, leads to AB stacked bilayer graphene. Through this new approach, an efficient technique is presented to control the nature of layer stacking

Synergistic effect on static and dynamic mechanical properties of carbon fiber-multiwalled carbon nanotube hybrid polycarbonate composites

Carbon fiber (CF) and multiwalled carbon nanotube (MWCNT)-reinforced hybrid micro-nanocomposites are prepared through melt mixing followed by injection moulding. The synergistic effect on both the static and dynamic mechanical properties with MWCNT/aMWCNT and CF reinforcement in a polycarbonate matrix is investigated by utilizing dynamic mechanical analysis, and flexural and tensile measurements. The enhancement in the flexural modulus and strength of the composite specimens as compared to pure PC for maximum loading of CF is 128.40% and 39.90%, respectively, which further improved to 142.94% and 42.60%, respectively, for CF-functionalized MWCNTs. Similarly, the storage modulus of the composite specimens reinforced with a maximum loading of CF and CF-functionalized MWCNTs show an increment of 176.57% and 203.33%, respectively over pure PC at 40 degrees C. Various types of parameter such as the coefficient C factor, degree of entanglement and adhesion factor have been calculated to analyze the interaction between fillers and the polymer matrix. Composite specimens containing 2 wt% of functionalized MWCNTs show a lower C value than the as-synthesized MWCNTs, which is indicative of a higher effectiveness of functionalized MWCNT-containing composite specimens. These results are well supported by optical microscopy and Raman spectroscopy by confirming the distribution of reinforcement

Melt-Spun SiGe Nano-Alloys: Microstructural Engineering Towards High Thermoelectric Efficiency

Silicon-germanium (SiGe) alloys are prominent high-temperature thermoelectric (TE) materials used as a powering source for deep space applications. In this work, we employed rapid cooling rates for solidification by melt-spinning and rapid heating rates for bulk consolidation employing spark plasma sintering to synthesize high-performance p-type SiGe nano-alloys. The current methodology exhibited a TE figure-of-merit (ZT) approximate to 0.94 at 1123 K for a higher cooling rate of similar to 3.0 x 10(7) K/s. This corresponds to approximate to 88% enhancement in ZT when compared with currently used radioisotope thermoelectric generators (RTGs) in space flight missions, approximate to 45% higher than pressure-sintered p-type alloys, which results in a higher output power density, and TE conversion efficiency (eta) approximate to 8% of synthesized SiGe nano-alloys estimated using a cumulative temperature dependence (CTD) model. The ZT enhancement is driven by selective scattering of phonons rather than of charge carriers by the high density of grain boundaries with random orientations and induced lattice-scale defects, resulting in a substantial reduction of lattice thermal conductivity and high power factor. The TE characteristics of synthesized alloys presented using the constant property model (CPM) and CTD model display their high TE performance in high-temperature regimes along with wide suitability of segmentation with different mid-temperature TE materials

Nanoparticles-decorated coal tar pitch-based carbon foam with enhanced electromagnetic radiation absorption capability

In the present study, to replace existing high-density radar-absorbing materials (RAM) for civil and military aerospace applications, lightweight coal tar pitch-based carbon foam (CFoam) was developed by a sacrificial template technique. The CFoam was decorated with Fe3O4 and ZnO nanoparticles to improve electromagnetic (EM) radiation absorption to make it useful as RAM. To ascertain the effect of the decorated nanoparticles on the CFoam, it was characterized by scanning electron microscopy, X-ray diffraction, a vector network analyzer and a vibration sample magnetometer. It was observed that Fe3O4 and Fe3O4-ZnO nanoparticles have a positive effect on the overall properties of CFoam. The compressive strength of CFoam increases by 22% and its thermal stability increases by 100 degrees C, whereas its electrical conductivity decreases by almost 25%. The total shielding effectiveness (SE) of CFoam increases from -25 dB to -54 and -56 dB, respectively, for Fe3O4- and Fe3O4-ZnO nanoparticles-decorated CFoam. The enhancement in total SE for Fe3O4- and Fe3O4-ZnO-coated CFoam is basically due to the contribution of absorption losses by -42 and -45 dB. The Fe3O4 and Fe3O4-ZnO coatings increase surface resistance and magnetic properties because the ferromagnetic nanoparticles act as tiny dipoles, which become polarized in the presence of an EM field and result in the better absorption of EM radiation. This clearly demonstrates that decorated nanoparticles on conducting lightweight CFoam are useful as RAM for different applications to attenuate EM radiation

Synthesis and characterization of multiwalled CNT-PAN based composite carbon nanofibers via electrospinning

Electrospun fibrous membranes find place in diverse applications like sensors, filters, fuel cell membranes, scaffolds for tissue engineering, organic electronics etc. The objectives of present work are to electrospun polyacrylonitrile ( PAN) nanofibers and PAN-CNT nanocomposite nanofibers and convert into carbon nanofiber and carbon-CNT composite nanofiber. The work was divided into two parts, development of nanofibers and composite nanofiber. The PAN nanofibers were produced from 9 wt% PAN solution by electrospinning technique. In another case PAN-CNT composite nanofibers were developed from different concentrations of MWCNTs (1-3 wt%) in 9 wt% PAN solution by electrospinning. Both types of nanofibers were undergone through oxidation, stabilization, carbonization and graphitization. At each stage of processing of carbon and carbon-CNT composite nanofibers were characterized by SEM, AFM, TGA and XRD. It was observed that diameter of nanofiber varies with processing parameters such as applied voltage tip to collector distance, flow rate of solution and polymer concentrations etc. while in case of PAN-CNT composite nanofiber diameter decreases with increasing concentration of CNT in PAN solution. Also with stabilization, carbonization and graphitization diameter of nanofiber decreases. SEM images shows that the minimum fiber diameter in case of 3 wt% of CNT solution because as viscosity increases it reduces the phase separation of PAN and solvent and as a consequence increases in the fiber diameter. AFM images shows that surface of film is irregular which give idea about mat type orientation of fibers. XRD results show that degree of graphitization increases on increasing CNT concentration because of additional stresses exerting on the nanofiber surface in the immediate vicinity of CNTs. TGA results shows wt loss decreases as CNT concentration increases in fibers

Chitosan–SiO2–multiwall carbon nanotubes nanocomposite: A novel matrix for the immobilization of creatine amidinohydrolase

A matrix made up of chitosan–SiO2–multiwall carbon nanotubes (CHIT–SiO2–MWCNTs) nanocomposite was fabricated to investigate the immobilization of creatine amidinohydrolase (CAH). CAH enzyme was covalently immobilized with the CHIT–SiO2–MWCNTs matrix using glutaraldehyde as a linker. The resulting CAH/CHIT–SiO2–MWCNTs biomatrix was characterized with Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and cyclic voltammetry (CV) taking CHIT–SiO2–MWNTs as a reference. The influence of various parameters on CAH enzyme activity within the matrix was investigated including pH, temperature, and time. The Michaelis–Menten constant and apparent activities for the CAH enzyme were calculated to be 0.58 mM and 83.16 mg/cm2, respectively; indicating CHIT–SiO2–MWCNTs nanocomposite matrix has a high affinity to immobilize CAH enzyme

Development of structurally stable electrospun carbon nanofibers from polyvinyl alcohol

Continuous carbon nanofibers are of great interest in research nowadays due to their outstanding properties. But synthesis of cost effective, stable and straight carbon nanofibers from low cost polymeric precursor is still a challenge. In this direction, in the present study efforts are made to prepare stable and straight carbon nanofibers from low cost polyvinyl alcohol (PVA). The PVA nanofibers are drawn by electrospinning technique from the solution of PVA and stabilization of nanofibers was carried out in the presence of iodine vapors for different time interval with applying the load. The stabilized nanofibers were carbonized at 1000 degrees C and semi-graphitized at 2200 degrees C. The resultant nanofibers were characterized by analytical and spectroscopic techniques. The microscopic study reveals that there is not much difference in stabilized, carbonized and semi graphitized nanofiber diameter, derived from PVA nanofiber stabilized with the application of load. The XRD and TEM study reveals the presence of amorphous as well as crystalline phases in semi graphitized nanofibers with different interlayer spacing. The higher value of electrical conductivity obtained as a consequence of alignment of carbon layer in the case of nanofiber stabilized with applying load and further increases in conductivity due to crystallinity and decreases in sp(3) content

Improved electromagnetic interference shielding effectiveness of light weight carbon foam by ferrocene accumulation

The influence of nanosized iron particles (NSIP) derived from an organometallic compound, i.e. ferrocene, on the properties of light weight carbon foam (CF) derived from coal tar pitch was investigated. It was observed that NSIP acts as a catalyst, resulting in an improved degree of graphitization of CF with increasing NSIP content and hence, higher electrical and thermal conductivity values. The higher value of conductivity has a positive effect on the electromagnetic interference (EMI) shielding effectiveness (SE) of the CF. The EMI SE increased with increasing NSIP content in CF. The specific SE of the light weight CF was 130 dB. cm(3)/g at 10 wt% of ferrocene in CF, which is the highest value reported so far for CF, particularly at such a low thickness (2.75 mm). Besides, it was thermally stable up to 600 degrees C in an oxidizing atmosphere and exhibits high specific thermal conductivity up to 121 W cm(3) g(-1) m(-1) K-1

Depression in glass transition temperature of multiwalled carbon nanotubes reinforced polycarbonate composites: effect of functionalization

Functionalized multiwalled carbon nanotubes (a-MWCNTs) and non-functionalized MWCNTs were melt mixed with polycarbonate polymer by utilizing twin screw micro compounder having a back flow channel to obtain nanocomposites with varying composition from 0.5 to 10 wt% MWCNT and 2 wt% a-MWCNT. Mechanical properties of composite samples were studied using dynamic mechanical analyzer, flexural and tensile tests. Both DMA and flexural and tensile tests suggest formation of continuous network of CNT-polymer that is supported by measured storage modulus for different loading of MWCNT and a-MWCNT. The composite sample showed lower glass transition temperature (T-g) as compared to pure PC. Effect of functionalization of MWCNTs on T-g of its of polycarbonate composites is studied and showed higher T-g depression in functionalized MWCNTs compared to non functionalized MWCNTs based composites over pure polycarbonate. In DMA, lowering of height of tan delta peak indicates that polymer in composite material participating in T-g was reduced along with loading of MWCNT, consistent with immobilization of polymer material present at the CNT interface. Effect of functionalization on morphology was investigated using scanning electron microscope and confirms the better interaction in case of a-MWCNTs compare to MWCNTs based composites. Further, Raman spectroscopic analysis indicates higher interaction between a-MWCNT and PC matrix as compared to as synthesized MWCNT