Stability and thermophysical characterization of ZrO2 nanoparticles by surface-modified
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摘要: 纳米流体因具有优良的热物性被广泛用于换热设备中，但较差的稳定性限制了工业化应用。为了提高纳米流体的稳定性和传热性能，采用真空干燥法将β-环糊精(β-CD)接枝到ZrO2纳米颗粒表面，比对修饰前后纳米颗粒的形貌特征、表面官能团及分子结构变化情况。此外，采用两步法制备体积分数为0.06 vol.% 的ZrO2/乙二醇(EG):去离子水(DI)和β-ZrO2/EG:DI纳米流体，通过沉降观察法和透射电镜(TEM)共同表征修饰前后纳米流体的稳定程度，并研究了纳米流体在20-60 ℃温度下的黏度和导热系数的变化规律。实验结果表明，表面附着了聚合物的β-ZrO2纳米颗粒之间产生了空间位阻，削弱了颗粒的团聚趋势，有利于保持纳米流体的长期稳定。与室温下的ZrO2纳米流体相比，β-ZrO2纳米流体中颗粒分散更加均匀，静置2天后团聚体的沉降速度减小了57.90%。在60 ℃时，β-ZrO2/EG:DI的黏度与ZrO2/EG:DI纳米流体相比无明显变化，而导热系数增大了10.25%。这是因为，包裹在β-ZrO2纳米颗粒表面的聚合物形成了弹性层，使得已修饰颗粒间产生弹性碰撞，从而引起微对流达到强化换热的效果。因此，在不影响纳米流体热物性的前提下，纳米颗粒表面修饰是改善纳米流体稳定性的有效方式之一。Abstract: Nanofluids have attracted considerable interest in advanced thermal properties; however, poor stability limits practical application. Then, vacuum drying methods were used to graft β-cyclodextrin (β-CD) onto ZrO2 nanoparticles to improve stability of nanofluids. The morphological features, surface functional groups, and molecular structure of the nanoparticles before and after modification were compared. A two-step method was used to prepare ZrO2/EG:DI and β-ZrO2/EG:DI nanofluids at volume fraction of 0.06 vol.%. The stability of the nanofluids were characterized by TEM and sedimentation observation method, and the viscosity and thermal conductivity were evaluated in the temperature range of 20–60 ℃. The results indicated that when β-ZrO2 nanoparticles are modified with β-CD, the polymer attachment introduces spatial site resistance between the nanoparticles. This resistance mitigates particle agglomeration and promotes the long-term stability of the nanofluid. Therefore, the modified β-ZrO2 nanoparticles were more uniformly dispersed, which had a 57.90% lower settling velocity in base fluid than ZrO2 nanofluid at room temperature. Besides, the thermal conductivity of β-ZrO2 nanofluid was10.25% higher than that of ZrO2 nanofluid at 60 ℃ with the constant viscosity. The enhancement in nanofluid thermal conductivity results from the polymer wrapped around the β-ZrO2 nanoparticles, forming an elastic layer and leading to elastic collisions, as well as micro-convection. Therefore, surface modification of nanoparticles is one of the effective methods to improve the stability of nanofluids without affecting the thermal conductivity.
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Key words:
- nanoparticles surface modification /
- β-cyclodextrin /
- nanofluids /
- stability /
- thermal conductivity
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图 1 纳米颗粒表面修饰及纳米流体制备示意图
Figure 1. Schematic diagram of nanoparticle surface modification and nanofluid preparation
图 4 (a) ZrO2纳米颗粒；(b) ZrO2纳米流体；(c) β-ZrO2纳米颗粒；(d) β-ZrO2纳米流体的SEM
Figure 4. SEM images of (a) ZrO2 nanoparticles; (b) ZrO2 nanofluid; (c) β-ZrO2 nanoparticles; (d) β-ZrO2 nanofluid
图 5 (a) ZrO2/EG:DI (b) β-ZrO2/EG:DI纳米流体的TEM及粒径分布图
Figure 5. TEM images and agglomerate particle size distributions of (a) ZrO2/EG:DI (b) β-ZrO2/EG:DI nanofluids
图 6 纳米流体中颗粒沉降速度随温度变化情况
Figure 6. Settling velocity of different nanofluids as a function of temperature.
图 7 纳米流体(a)黏度 (b)导热系数随温度变化情况
Figure 7. Variation of (a) viscosity and (b) thermal conductivity of nanofluids with temperature.
图 8 改性纳米颗粒稳定性及热物性增强机理 (a)弹性层分子结构；(b)空间位阻；(c) SEM；(d)沉降图；(e)弹性碰撞；(f)导热系数示意图
Figure 8. Mechanism of enhanced stability and thermophysical property of modified nanofluids: (a) molecular structure of the elastic layer; (b) steric potential resistance; (c) SEM; (d) sedimentation diagram; (e) elastic collision; (f) diagram of thermal conductivity.
图 9 纳米流体在 (a)层流区 (b)湍流区的对流换热性能
Figure 9. Convective heat transfer performance of nanofluids in (a) laminar flow region (b) turbulent flow region
表 1 纳米颗粒及基液的物理参数
Table 1. Physical Parameters of Nanoparticles and Base fluids
Material Density/
(kg·m−3)Specific heat capacity/
(J·kg−1·K−1)Thermal conductivity/
(W·m−1·K−1)Manufacturers ZrO2 nanoparticles 5850 713 2 Beijing Deke Daojin Science and Technology Ethylene glycol 1110 2395 0.256 Shanghai Aladdin Biochemical Technology Deionized water 998.2 4185 0.599 Labs 
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