Mohd Saidi Norshafiqah, Abdullah Norli, Norizan Mohd Nurazzi, Janudin Nurjahirah, Mohd Kasim Noor Azilah, Osman Mohd Junaedy, Mohamad Imran Syakir, Mohd Rosli Mohd Afzanizam
Centre for Defence Foundation Studies, Universiti Pertahanan Nasional Malaysia, Kem Sungai Besi, Kuala Lumpur 57000, Malaysia.
Bioresource Technology Division, School of Industrial Technology, Universiti Sains Malaysia, Penang 11800, Malaysia.
Nanomaterials (Basel). 2022 Nov 7;12(21):3922. doi: 10.3390/nano12213922.
The reputation of nanofluids as a convenient heat transfer media has grown in recent years. The synthesis of nanofluids is often challenging, particularly carbon-based nanofluids, due to the rapid agglomeration of the nanoparticles and the instability of the nanofluids. In this regard, surface modification and surfactant addition are potential approaches to improve the physical and thermal properties of carbon-based nanofluids that have been studied and the structural, morphological, and thermal characteristics of surface-oxidised carbon nanofibre (CNF)-based nanofluids has been characterised. Commercial CNF was first subjected to three different acid treatments to introduce surface oxygen functional groups on the CNF surface. Following the physical and thermal characterisation of the three surface-oxidised CNFs (CNF-MA, CNF-MB, and CNF-MC), including Raman spectroscopy, Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), and field emission scanning electron microscopy (FESEM), the CNF-MB was selected as the best method to synthesise the surface-oxidised CNF-based nanofluid. A total of 40 mL of ultrapure water was used as a pure base fluid and mixed with the surface-oxidised CNF at a concentration range of 0.1-1.0 wt.%, with a fixed of 10 wt.% amount of polyvinylpyrrolidone (PVP). The thermal conductivity of CNF-based nanofluid was then characterised at different temperatures (6, 25, and 40 °C). Based on the results, surface oxidation via Method B significantly affected the extent of surface defects and effectively enhanced the group functionality on the CNF surface. Aside from the partially defective and rough surface of CNF-MB surfaces from the FESEM analysis, the presence of surface oxygen functional groups on the CNF wall was confirmed via the Raman analysis, TGA curve, and FTIR analysis. The visual sedimentation observation also showed that the surface-oxidised CNF particles remained dispersed in the nanofluid due to the weakened van der Waals interaction. The dispersion of CNF particles was improved by the presence of PVP, which further stabilised the CNF-based nanofluids. Ultimately, the thermal conductivity of the surface-oxidised CNF-based nanofluid with PVP was significantly improved with the highest enhancement percentage of 18.50, 16.84, and 19.83% at 6, 25, and 40 °C, respectively, at an optimum CNF concentration of 0.7 wt.%.
近年来，纳米流体作为一种便捷的传热介质，其声誉与日俱增。纳米流体的合成往往具有挑战性，尤其是碳基纳米流体，这是由于纳米颗粒的快速团聚以及纳米流体的不稳定性。在这方面，表面改性和添加表面活性剂是改善碳基纳米流体物理和热性能的潜在方法，且已对基于表面氧化碳纳米纤维（CNF）的纳米流体的结构、形态和热特性进行了表征。首先对商用CNF进行三种不同的酸处理，以在CNF表面引入表面氧官能团。在对三种表面氧化的CNF（CNF-MA、CNF-MB和CNF-MC）进行包括拉曼光谱、傅里叶变换红外（FTIR）、热重分析（TGA）和场发射扫描电子显微镜（FESEM）在内的物理和热表征后，选择CNF-MB作为合成基于表面氧化CNF的纳米流体的最佳方法。总共40 mL超纯水用作纯基础流体，并与表面氧化的CNF以0.1 - 1.0 wt.%的浓度范围混合，同时固定添加10 wt.%的聚乙烯吡咯烷酮（PVP）。然后在不同温度（6、25和40°C）下对基于CNF的纳米流体的热导率进行表征。基于结果，通过方法B进行的表面氧化显著影响了表面缺陷程度，并有效增强了CNF表面的基团官能度。除了FESEM分析显示CNF-MB表面存在部分缺陷和粗糙表面外，通过拉曼分析、TGA曲线和FTIR分析证实了CNF壁上存在表面氧官能团。肉眼沉降观察还表明，由于范德华相互作用减弱，表面氧化的CNF颗粒在纳米流体中保持分散状态。PVP的存在改善了CNF颗粒的分散性，进一步稳定了基于CNF的纳米流体。最终，含有PVP的基于表面氧化CNF的纳米流体的热导率得到显著提高，在最佳CNF浓度为0.7 wt.%时，在6、25和40°C下的最高增强百分比分别为18.50%、16.84%和19.83%。
