Carbon nanotubes (CNT) represent one of the most unique materials in the field of nanotechnology. CNT are the allotrope of carbon having sp2 hybridization. CNT are considered to be rolled-up graphene ...with a nanostructure that can have a length to diameter ratio greater than 1,000,000. CNT can be single-, double-, and multi-walled. CNT have unique mechanical, electrical, and optical properties, all of which have been extensively studied. The novel properties of CNT are their light weight, small size with a high aspect ratio, good tensile strength, and good conducting characteristics, which make them useful for various applications. The present review is focused on the structure, properties, toxicity, synthesis methods, growth mechanism and their applications. Techniques that have been developed to synthesize CNT in sizeable quantities, including arc discharge, laser ablation, chemical vapor deposition, etc., have been explained. The toxic effect of CNT is also presented in a summarized form. Recent CNT applications showing a very promising glimpse into the future of CNT in nanotechnology such as optics, electronics, sensing, mechanical, electrical, storage, and other fields of materials science are presented in the review.
•High heat load, a barrier in industries can be overcome by using nanofluids.•Nanofluids have higher thermal conductivity compared to conventional fluids.•For heat transfer applications, nanofluids ...must have long term stability.•Preparation of a long term stable nanofluids is challenging.•Study focuses on preparation, analysis & application challenges of nanofluids.
High heat load is becoming a barrier in industrial development. This high heat load can be overcome by increasing the rate of heat transfer. Heat transfer rate can be increased by increasing temperature gradient, area of heat transfer or by improving thermo physical properties of heat transfer fluids. Emergence of modern technology provides a great opportunity to process and produce particles in the size range of 1–100nm called nanoparticles having high specific surface area. Colloidal suspension of nanoparticles into the conventional fluid called nanofluid has higher thermal conductivity compared to conventional fluids. Long term stability of nanofluid is one of the basic requirements for its better utilization in heat transfer applications. Preparation of a long term stable nanofluid is one of the main technical challenge. The main focus of this study is to review the work carried out by various researchers in the last two decades and to summarize the preparation and analytical techniques used for preparation of stable nanofluids. The paper also discusses some new challenging issues that need to be solved for better industrial application of nanofluids.
Activated carbon (AC) supported monometallic Pd and bimetallic Pd-M (M = Co, Ni, and Cu) catalysts were prepared by impregnation-reduction method and investigated for carbon monoxide (CO) oxidation. ...X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed highly disperse Pd-M nanoparticles (~ 3–5 nm) on the AC support. The addition of the second transition metal in Pd/AC resulted in an increase in the specific surface area and smaller average particle size. CO oxidation experiments were carried out in a vertical fixed-bed quartz reactor at gas hourly space velocity (GHSV) of 30,000 h
− 1
under atmospheric pressure. The catalytic order is as follows: Pd–Cu/AC > Pd–Ni/AC > Pd–Co/AC > Pd/AC. The T
50
(50% CO conversion) value of Pd–Cu/AC and Pd/AC catalysts are 30 and 70 °C. BET, XRD and TEM analysis of the used Pd-M/AC and Pd/AC catalysts were performed in order to find any change in the specific surface area, structure, morphology, and average particle size of the catalysts after CO oxidation. The results showed that the bimetallic Pd-M/AC catalysts had better catalytic activity and stability than the monometallic Pd/AC catalyst. Pd–Cu/AC showed excellent time-on-stream stability of 50 h. The apparent activation energy of the Pd–Cu/AC is found to be 69.32 kJ mol
− 1
. Thus, bimetallic Pd-M/AC catalysts have a potential for practical CO oxidation reaction.
Graphical Abstract
CNT nanofluids are getting attention in heat transfer applications due to their very high thermal conductivity in comparison with conventional fluids. For commercial exploitation of CNT nanofluids as ...heat transfer media, they must have long-term stability. In this study, the two-step method was modified to prepare dynamically stable CNT nanofluids by utilizing commercial grade multiwalled carbon nanotubes and SDBS as a surfactant. The modified technique consists of separation of coarse agglomerates of CNT from the CNT nanofluids by applying centrifugal action after its preparation. The effect of relative centrifugal force was also studied for the very first time on the stable concentration of CNT nanofluids. The stability of CNT nanofluids was analyzed by measurement of their CNT concentration and Zeta potential. Results showed that CNT nanofluids possess good stability and remain stable for more than 15 months. In addition to stability, thermo-physical properties such as thermal conductivity, density, and viscosity of CNT nanofluids were also measured. The results of this study elucidated the effect of RCF on the stable concentration of CNT nanofluids. It is expected that the results obtained in this study may significantly contribute to the proper tailoring of CNT nanofluids, by providing long-term stable CNT nanofluids which are suitable for industrial heat transfer applications.
Zirconia (ZrO2) nanoparticles were synthesized by solution combustion using urea as an organic fuel. Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), ...transmission electron microscopy (TEM), UV–vis and Fourier transform infrared (FTIR) measurements were performed in order to characterize the catalyst. The calculated crystallite size of ZrO2, calculated with the help of the Scherrer equation, was around 30.3 nm. The synthesized ZrO2 was scrutinized regarding its role as catalyst in the oxidation of carbon monoxide (CO). It showed 100% CO conversion at 240 °C, which is the highest conversion rate reported for ZrO2 in literature to date. It is found that through solution combustion, Pt2+ ions replace Zr4+ ions in the ZrO2 lattice and because of this, oxygen vacancies are formed due to charge imbalance and lattice distortion in ZrO2. 1% Pt was doped into ZrO2 and yielded excellent CO oxidation. The working temperature was lowered by 150 °C in comparison to pure ZrO2. Further, it is highly stable for the CO reaction (time-on-stream ≈ 40 h). This is because of a synergic effect between Pt and Zr components, which results in an increase of the oxygen mobility and oxygen vacancies and improves the activity and stability of the catalyst. The effects of gas hourly space velocity (GHSV) and initial CO concentration on the CO oxidation over Pt(1%)-ZrO2 were studied.
This paper reports the hydrodynamic characteristics of CNT nanofluids prepared using mixed surfactant system flowing through helically coiled tubes under laminar conditions. The curvature ratio was ...varied from 0.036 to 0.067 and CNT concentration from 0 to 0.051 vol%. Results showed that friction factor of CNT nanofluid was higher than water for all range of parameters studied. Friction factor was found to be dependent on the volumetric concentration of CNT as well as on curvature ratio of helical coil. A maximum of 69% enhancement in friction factor was calculated with respect to water when CNT nanofluid of 0.051 vol% flowing through helical coil of curvature ratio;
0.067. On increasing the curvature ratio;
from 0.036 to 0.067, friction factor increased by 46% at CNT concentration 0.051 vol%. Based on the experimental data, an empirical correlation has been proposed to evaluate friction factor of CNT nanofluids flowing through helical coils. The proposed correlation explains the present experimental data within ±11%.
This article proposes a better alternative method to prepare CNT antifreeze nanofluid in EG/water by modifying the conventional method that requires long hours of sonication. Sonicating a sample for ...long hours is time and energy consuming and may deform the structure of CNT. In the modified method, the nanofluid preparation was carried out by dispersion of CNT in EG via sonication followed by adding water and again sonication. The study shows that nanofluid could be prepared in less sonication time of 1.5 h compared to the 5 h required in the conventional method. FTIR spectroscopy revealed that interaction of EG with CNT occurs via trans conformation resulting in greater stabilization and better interaction of nanofluid prepared by this method (85 days) as compared to nanofluid prepared by the conventional method (50 days). The nanofluid prepared by this method has better physical–chemical properties compared to nanofluid prepared by the conventional method. The nanofluid prepared by this method showed higher stability and better physical–chemical properties at a lower sonication time. Hence it is a more effective and cost efficient technique for preparing CNT (EG/water) nanofluid.
Zirconia (ZrO2) nanoparticles co-doped with Cu and Pt were applied as catalysts for carbon monoxide (CO) oxidation. These materials were prepared through solution combustion in order to obtain highly ...active and stable catalytic nanomaterials. This method allows Pt2+ and Cu2+ ions to dissolve into the ZrO2 lattice and thus creates oxygen vacancies due to lattice distortion and charge imbalance. High-resolution transmission electron microscopy (HRTEM) results showed Cu/Pt co-doped ZrO2 nanoparticles with a size of ca. 10 nm. X-ray diffraction (XRD) and Raman spectra confirmed cubic structure and larger oxygen vacancies. The nanoparticles showed excellent activity for CO oxidation. The temperature T 50 (the temperature at which 50% of CO are converted) was lowered by 175 °C in comparison to bare ZrO2. Further, they exhibited very high stability for CO reaction (time-on-stream ≈ 70 h). This is due to combined effect of smaller particle size, large oxygen vacancies, high specific surface area and better thermal stability of the Cu/Pt co-doped ZrO2 nanoparticles. The apparent activation energy for CO oxidation is found to be 45.6 kJ·mol−1. The CO conversion decreases with increase in gas hourly space velocity (GHSV) and initial CO concentration.
Ceria (CeO
2
) nanocatalysts were synthesized by three different methods via a simple sol–gel method (SGM), precipitation method (PPM), and solution combustion method (SCM) for CO oxidation reaction. ...The results show that the CeO
2
catalysts prepared by SGM possess small crystallite size (9 nm), more lattice defects (active sites), high specific surface area (188 m
2
g
–1
), and better thermal stability. Raman spectra showed that large number oxygen vacancies were possessed by CeO
2
-SGM catalysts as compared to CeO
2
-PPM and CeO
2
-SCM. It was observed that the catalytic activity for CO oxidation reaction is greatly influenced by the preparation method. The order of activity and stability of CeO
2
material prepared by different methods for CO oxidation process is as follows: CeO
2
-SGM > CeO
2
-PPM > CeO
2
-SCM, respectively.
The depletion of non-renewable resources and rise in global warming has caused great concern to humankind. With a view to use renewable source of energy and to eliminate hazardous chemical compounds ...from air, soil, and water, photocatalysis utilizing solar energy is becoming a rapidly expanding technology. Semiconductor nanoparticles have the ability to undergo photoinduced electron transfer to an adsorbed particle governed by the band energy positions of the semiconductor and the redox potential of the adsorbate. A brief overview of metal oxides and sulphides that can act as sensitizers for light-induced redox processes due to their electronic structure is presented here.
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