Absorption performance enhancement by nano-particles and chemical surfactants in binary nanofluids

The objectives of this paper are to visualize the bubble behavior during the NH₃/H₂O absorption process with chemical surfactant and nano-particles and to study the effect of nano-particles and surfactants on the absorption characteristics. Binary nanofluid which means binary mixture with nano-sized particles is tested to apply nanofluid to the absorption system. Cu, CuO and Al₂O₃ nano-particles are added into NH₃/H₂O solution to make the binary nanofluids, and 2-ethyl-1-hexanol, n-octanol and 2-octanol are used as the surfactants. The concentration of ammonia in the basefluid, that of nano-particles in the nanofluid, and that of surfactants in the nanofluid are considered as the key parameters. The results show that the addition of surfactants and nano-particles improves the absorption performance up to 5.32 times. It can be concluded that the addition of both surfactants and nano-particles enhances significantly the absorption performance during the ammonia bubble absorption process.     

An experimental and theoretical study of the influence of surfactant on the preparation and stability of ammonia-water nanofluids

Two types of nanofluids are obtained by adding the mixture of carbon black nanoparticles with emulsifier OP-10, and Al₂O₃ nanoparticles with sodium dodecyl benzene sulfonate (SDBS) in the ammonia-water solution, respectively. The dispersion stability of the prepared nanofluids in different mass fractions of surfactants is investigated by the light absorbency ratio index methods. The results show that with the increase of mass fraction of surfactant, the stability of carbon black nanofluid is improved firstly and then is exacerbated, while the stability of Al₂O₃ nanofluid is exacerbated firstly, then is improved, and then is exacerbated again. The influences of surfactant on the stability of ammonia-water nanofluids abide by the monolayer adsorption theory or electric double layer adsorption theory. Finally, the theoretical surfactant mass fractions required in the preparation of ammonia-water nanofluids are calculated by simplifying the dispersion models and the results are in accordance with experimental results.     

Numerical design of ammonia bubble absorber applying binary nanofluids and surfactants

The objectives of this paper are to analyze the combined heat and mass transfer characteristics for the ammonia bubble absorption process and to study the effects of binary nanofluids and surfactants on the absorber size. The ammonia bubble absorbers applying binary nanofluids and surfactants are designed and parametric analyses are performed. In order to express the effects of binary nanofluids and/or surfactants on the absorption performance, the effective absorption ratios for each case are applied in the numerical model. The values of the effective absorption ratio are decided from the previous experimental correlations. The kinds and the concentrations of nano-particles and surfactants are considered as the key parameters. The considered surfactants are 2-ethyl-1-hexanol (2E1H), n-octanol, and 2-octanol and nano-particles are copper (Cu), copper oxide (CuO), and alumina (Al₂O₃). The results show that the application of binary nanofluids and surfactants can reduce the size of absorber significantly. In order to reach 16.5% ammonia solution under the considered conditions, for example, the addition of surfactants (2E1H, 700 ppm) can reduce the size of absorber up to 63.0%, while the application of binary nanofluids (Cu, 1000 ppm) can reduce it up to 54.4%. In addition, it is found that the effect of mass transfer resistance is more dominant than that of heat transfer resistance. That is, the enhancement of mass transfer performance is more effective than that of heat transfer performance.     

铁酸锌纳米强化氨水发生过程实验研究

本文提出一种可直接测试有、无纳米氨水溶液氨气发生量的氨水纳米降膜发生实验装置,通过实验对比分析了0%~0.5%质量浓度的纳米和分散剂、25%~40%质量浓度的氨水以及加热水温度对氨气发生率和相对发生率的影响。研究表明:添加合适质量分数的铁酸锌(ZnFe_2O_4)和十二烷基苯磺酸钠(SDBS)配制的氨水纳米流体可以增加氨气发生率。当氨水基液质量浓度为25%~40%时,添加质量分数分别为0.1%的ZnFe_2O_4和0.05%的SDBS,氨气发生率比原相应浓度的氨水基液约增加60%。但只添加分散剂SDBS会对氨气发生产生一定的抑制作用。选择分散剂用量时需兼顾分散稳定性和对发生起正或副作用,以达到最优效果。因此将纳米应用于吸收式制冷系统以提高系统的COP具有较广阔的前景。     

The preparation of Zn-ferrite epitaxial thin film from epitaxial Fe3O4:ZnO multilayers by ion beam sputtering deposition

A new method to grow a well-ordered epitaxial ZnFe₂O₄ thin film on Al₂O₃(0001) substrate is described in this work. The samples were made by annealing the ZnO/Fe₃O₄ multilayer which was grown with low energy ion beam sputtering deposition. Both the Fe₃O₄ and ZnO layers were found grown epitaxially at low temperature and an epitaxial ZnFe₂O₄ thin film was formed after annealing at 1000 °C. X-ray diffraction shows the ZnFe₂O₄ film is grown with an orientation of ZnFe₂O₄(111)//Al₂O₃(0001) and ZnFe₂O₄(1-10)//Al₂O₃(11-20). X-ray absorption spectroscopy studies show that Zn²⁺ atoms replace the tetrahedral Fe²⁺ atoms in Fe₃O₄ during the annealing. The magnetic properties measured by vibrating sample magnetometer show that the saturation magnetization of ZnFe₂O₄ grown from ZnO/Fe₃O₄ multilayer reaches the bulk value after the annealing process.     

Synthesis of dual Z-scheme photocatalyst ZnFe2O4/PANI/Ag2CO3 with enhanced visible light photocatalytic activity and degradation of pollutants

The ZnFe₂O₄/PANI/Ag₂CO₃ photocatalyst was synthesized by the co-precipitation method. The composition, morphology and optical properties of the synthesized photocatalyst were characterized. Compared with pure Ag₂CO₃, ZnFe₂O₄, PANI/Ag₂CO₃ and ZnFe₂O₄/Ag₂CO₃, ZnFe₂O₄/PANI/Ag₂CO₃ has the best photocatalytic ability of bisphenol A can reach 86.36% under 40 min of light, and it has a certain ability to be reused. At the same time, after 1 h of light, the degradation rate of Nitrobenzene can reach 90%. The reason for the increased catalytic ability of ZnFe₂O₄/PANI/Ag₂CO₃ can be attributed to the extended absorption capacity of the visible light region and the efficient separation of electron-hole pairs.     

A comparative study on the optical limiting properties of different nano spinel ferrites with Z-scan technique

We report the optical limiting properties of five different spinel ferrites, NiFe₂O₄, Ni_0.5Zn_0.5Fe₂O₄, ZnFe₂O₄, Ni_0.5Co_0.5Fe₂O₄, and CoFe₂O₄ with an average particle grain size of 8 nm. The optical limiting properties are investigated using the open aperture Z-scan technique. The obtained nonlinearity fits to a two-photon like absorption process. Except for NiFe₂O₄, the observed nonlinearity has contributions from excited state absorption. The optical limiting response is also studied against particle size and the nonlinearity is found to increase with increasing particle size within the range of our investigations. On comparing the optical limiting properties, ZnFe₂O₄ is found to be a better candidate for the optical limiting applications. To the best of our knowledge, this is the first report where the optical limiting properties of spinel ferrites are compared.     

Solvent thermal synthesis and gas-sensing properties of Fe-doped ZnO

In this study, pure ZnO microbullets, ZnO–ZnFe₂O₄ composite, and ZnO–Fe₂O₃–ZnFe₂O₄ composite with micron structured balloons, rods, and particles were prepared by a simple solvent thermal process using methanol or ethanol as solvents. The influence of solvents on the composition and morphology of the products was studied, and their gas-sensing properties were also investigated. The morphology of ZnO microbullets synthesized in ethanol is similar to but more uniform than that of ZnO microbullets synthesized in methanol. The Fe-doped ZnO synthesized in ethanol contains many micron particles homogeneously dispersing on the surface of the microbullets, which is composed of hexagonal wurtzite ZnO and franklinite ZnFe₂O₄, while Fe-doped ZnO prepared in methanol consists of micron structured balloons, rods, and particles, which is composed of hexagonal wurtzite ZnO, hematite Fe₂O₃, and franklinite ZnFe₂O₄. Compared with pure ZnO and ZnO–ZnFe₂O₄ composite, the ZnO–Fe₂O₃–ZnFe₂O₄ composite presented high response, rapid response/recovery characteristics, good selectivity, and excellent stability to acetone at relatively low operating temperature of 190 °C. This sensor could detect acetone in wide range of 1–1000 ppm, which was expected to be a promising gas sensor for detecting acetone.     

Photoelectrochemical property of ZnFe2O4/TiO2 double-layered films

ZnFe₂O₄/TiO₂ double-layered films on indium-tin oxide (ITO) substrate were prepared by a dip-coating method, and the optical absorption and photocurrent of the as-prepared films were measured. In the double-layered films, the onset of fundamental absorption edge shifts to a longer wavelength, and even shifts to a longer wavelength than that of ZnFe₂O₄-only film as the ZnFe₂O₄ layer thickness increases. Application of the coupled photoanodes double-layered films composed of ZnFe₂O₄ and TiO₂ can obviously increase the photocurrent. It was found that the photocurrent density of ZnFe₂O₄/TiO₂ double-layered films first increased and then decreased with increasing the ZnFe₂O₄ layer thickness. A five-fold increase in the photocurrent density was obtained compared with TiO₂-only films under optimum condition.     

A Study of the Effect of Nanometer Fe3O4 Addition on the Properties of Silicone Oil-based Magnetorheological Fluids

With the aim to improve the performance of magnetorheological fluids (MRFs) for mechanical transmission system, a process to prepare silicone oil-based MRFs with the addition of nanometer Fe₃O₄ particles is presented and five MRFs samples with different mass fraction of nanometer Fe₃O₄ particles have been prepared. The experimental materials, the preparation process, and test methods are elaborated. Moreover, the microstructures of soft magnetic carbonyl iron particles, nanometer Fe₃O₄ particles, and carbonyl iron/nano-sized Fe₃O₄ composites have been characterized via scanning electron microscope (SEM). Finally, test experiments of sedimentation stability, zero field viscosity, and shear yield stress have been carried out. The experimental results show that adding a certain amount of nanometer Fe₃O₄ particles (4 and 6 wt%) into MRFs can improve the performance of MRFs.     

大压差下静止氨水表面吸收特性研究

基于对大压差下静止氨水溶液表面吸收氨蒸气过程中热质传递现象的分析,建立了该吸收过程传热传质相互耦合的数学物理模型。在氨蒸气压力不变的情况下,推导出氨水溶液温度场、浓度场以及表征相界面传质的无量纲准则数的理论表达式,结果证明:相界面处氨浓度、温度均为定值,该值只取决于吸收的初始条件。在引入氨水相平衡方程的前提下,拟合出了传质准则数与初始压差、氨水溶液初始参数的半经验关联式,获得了一定初始条件下时均传质量随时间的变化曲线。曲线显示:在吸收开始时,时均传质量最大,随着吸收时间的增加,时均传质量迅速下降。     

Room temperature ferromagnetism in laser ablated Zn ferrite thin films

Nanocrystalline ZnFe₂O₄ was synthesized using sol–gel method. The room temperature ferromagnetic behavior was observed in ZnFe₂O₄ thin films fabricated by pulsed laser deposition (PLD) technique. The ZnFe₂O₄ nanoparticles and target material used for the fabrication of thin films was observed to have paramagnetic behavior at room temperature. The possible ferromagnetic behavior observed in ZnFe₂O₄ thin films has been explained in terms of random distribution of Zn²⁺ and Fe³⁺ ions at tetrahedral (A) and octahedral [B] sites. The blocking temperature T_B was observed in the case of both nanoparticles and thin films.     

The magnetic behaviour of nanostructured zinc ferrite

We have investigated a series of nanostructured ZnFe₂O₄ samples produced by mechanical activation (mean particle sizes d ～50-8 nm) by variable temperature neutron diffraction measurements (2-535 K) supported by DC magnetisation measurements (4.2-300 K). The systematic increase in the mean inversion parameter (c ～0.04-0.43) with increasing milling time is accompanied by a gradual decrease in the occurrence of the long range antiferromagnetic ordering observed in the starting ZnFe₂O₄ material, as well as a gradual decrease in the related diffuse short range order peak. The neutron diffraction patterns of particles with d < ～15 nm and c> ～0.2 are consistent with the occurrence of ferrimagnetic order and exchange interactions of the type Fe³⁺A—O^2?—Fe³⁺ [B]. Diagrams summarising the magnetic regions of nanostructured ZnFe₂O₄ are presented. The magnetic behaviour overall agrees well with the enhanced magnetisation and ferromagnetic behaviour reported for nanostructured, ultrafine and thin films of ZnFe₂O₄ by other groups using mainly magnetisation and Mössbauer spectroscopy measurements.     

ZnFe2O4 Leaves Grown on TiO2 Trees Enhance Photoelectrochemical Water Splitting

TiO₂ has excellent electrochemical properties but limited solar photocatalytic performance in light of its large bandgap. One important class of visible‐wavelength sensitizers of TiO₂ is based on ZnFe₂O₄, which has shown fully a doubling in performance relative to pure TiO₂. Prior efforts on this important front have relied on presynthesized nanoparticles of ZnFe₂O₄ adsorbed on a TiO₂ support; however, these have not yet achieved the full potential of this system since they do not provide a consistently maximized area of the charge‐separating heterointerface per volume of sensitizing absorber. A novel atomic layer deposition (ALD)‐enhanced synthesis of sensitizing ZnFe₂O₄ leaves grown on the trunks of TiO₂ trees is reported. These new materials exhibit fully a threefold enhancement in photoelectrochemical performance in water splitting compared to pristine TiO₂ under visible illumination. The new materials synthesis strategy relies first on the selective growth of FeOOH nanosheets, 2D structures that shoot off from the sides of the TiO₂ trees; these templates are then converted to ZnFe₂O₄ with the aid of a novel ALD step, a strategy that preserves morphology while adding the Zn cation to achieve enhanced optical absorption and optimize the heterointerface band alignment.     

MgO nanofluids: higher thermal conductivity and lower viscosity among ethylene glycol-based nanofluids containing oxide nanoparticles

Five kinds of oxides, including MgO, TiO₂, ZnO, Al₂O₃ and SiO₂ nanoparticles were selected as additives and ethylene glycol (EG) was used as base fluid to prepare stable nanofluids. Thermal transport property investigation demonstrated substantial increments in the thermal conductivity and viscosity of all these nanofluids with oxide nanoparticle addition in EG. Among all the studied nanofluids, MgO–EG nanofluid was found to have superior features, with the highest thermal conductivity and lowest viscosity. The thermal conductivity enhancement ratio of MgO–EG nanofluid increases nonlinearly with the volume fraction of nanoparticles. In the experimental temperature range of 10–60°C, thermal conductivity enhancement ratio of MgO–EG nanofluids appears to have a weak dependence on the temperature. Viscosity measurements showed that MgO–EG nanofluids demonstrated Newtonian rheological behaviour, and the viscosity significantly decreases with the temperature. The thermal conductivity and viscosity increments of the nanofluids are much higher than the corresponding values predicted by the existing classical models for the solid–liquid mixture.     

Magnetization study of Fe-doped ZnO co-doped with Cu: Synthesized by wet chemical method

Fe- and Cu-doped ZnO of nominal compositions Zn_0.95Fe_0.05O and Zn_0.94Fe_0.05Cu_0.01O were synthesized by a wet chemical route. X-ray diffraction analysis of the samples annealed at 575 K showed that they are single phase without any secondary phases. DC magnetization measurements of Cu co-doped samples (Zn_0.94Fe_0.05Cu_0.01O) as a function of field at room temperature showed ferromagnetic signature while the samples without Cu co-doping (Zn_0.95Fe_0.05O) are paramagnetic in nature. On increasing the temperature of annealing from 575 K to 1,075 K an impurity phase emerges in both the samples, which has been identified as a variant of ZnFe₂O₄. Both the samples heated at and above 1,075 K are found to be paramagnetic at room temperature. These observations, the absence of room temperature ferromagnetism in Zn_0.95Fe_0.05O and the disappearance of ferromagnetism in Zn_0.94Fe_0.05Cu_0.01O on raising the temperature of annealing clearly rules out the likelihood of room temperature ferromagnetism arising from the impurity phases like γ-Fe₂O₃ and/or ZnFe₂O₄ that might have been formed during the synthesis. Our results strongly suggest that room temperature ferromagnetism in Zn_0.94Fe_0.05Cu_0.01O can be attributed to the formation of a secondary phase of Cu-doped ZnFe₂O₄.     

Experimental investigation on the influence of high temperature on viscosity,thermal conductivity and absorbance of ammonia–water nanofluids

To select the optimal ammonia–water nanofluids and apply to ammonia–water absorption refrigeration systems (AARS), this paper investigated the influence of heating on viscosity, thermal conductivity and absorbance of binary nanofluids. The hysteresis phenomenon was observed after heating at high temperature which is rarely reported in the literature. Experimental results show that most of nanofluids' thermal conductivity increased by about 3–12% after heating. However, their viscosities increased by as much as 15% to 25% except the γ-TiO₂ ammonia–water nanofluid, which was reduced by 2% to 7%. This study also shows that the trend of viscosity is consistent with the absorbance. Due to fact that the thermal conductivity of γ-TiO₂/NH₃–H₂O mixture increased after heating, while the viscosity decreased, even if the concentration of the base liquid is 12.5% or 25%, therefore it is the optimal choice for practical research in AARS at present.     

Fabrication and microstructure of in situ toughened Al2O3/Fe3Al

Fe₃Al nano-particles and commercial purity Al₂O₃ powders were used as raw materials to fabricate in situ reinforced Al₂O₃/Fe₃Al nano/micro-composites. Densification and microstructure were studied. The Al₂O₃ matrix grains were characterized by platelet grains. The Fe₃Al particles inhibited the grain growth of Al₂O₃ grains and limited the densification of the composites. In Al₂O₃/Fe₃Al composites, the Fe₃Al particles were uniformly dispersed in the Al₂O₃ matrix. The major Fe₃Al micro-particles, about 1 μm in average size, existed at Al₂O₃ grain boundaries, and the Fe₃Al nano-particles were found embedded in the matrix grains. The grain size of the intragranular particles ranged from several to several hundred nanometers. The grain size and aspect ratio of Al₂O₃ platelet grains and distribution of intragranular Fe₃Al could be optimized by controlling the Fe₃Al contents and sintering process. The in situ formed Al₂O₃ platelet grains, as well as Fe₃Al dispersoids, were beneficial to the increase of the mechanical properties of alumina.     

Facile synthesis,morphological, structural,photocatalytic, and optical properties of ZnFe2O4 nanostructures

Spinel ferrite ZnFe₂O₄ nanostructures have been prepared as sunlight responsive photocatalysts via facile co-precipitation method. The structural, morphological, and optical responses were diligently characterized using XRD, Raman spectroscopy, FESEM, and UV–Vis absorption spectroscopy, respectively. FESEM studies revealed nanoparticles and porous-like nanoparticle aggregates, found to be of cubic spinel ZnFe₂O₄ from XRD and Raman studies. Crystallite size varied from 5 to 13.6 nm, whereas band gap changed from 1.89 to 1.95 eV with CTAB concentration variation. ZnFe₂O₄ nanostructures were employed for sunlight-assisted photodegradation of organic pollutants such as MB, MG, and MO dyes in water. The synthesized ZnFe₂O₄ nanoparticle aggregates with porous-like morphology with crystallite size of 9.2 nm showed superior photocatalytic response and decomposed 80.4% of MB dye in only 40 min. The superiority of the porous-like ZnFe₂O₄ nanoparticle aggregates was mainly ascribed to its optimal crystallite size, narrower band gap, and improved sunlight utilization efficiency. A plausible mechanism of photocatalytic oxidation of dye supported by scavenger studies has also been proposed. The synthesized ZnFe₂O₄ nanostructures have easy magnetic recycling property along with excellent photocatalytic capability and hold potential for the treatment of contaminated water.

     

Synthesis and photocatalytic activity of cobalt oxide doped ZnFe2O4-Fe2O3-ZnO mixed oxides

Novel cobalt oxide doped ZnFe₂O₄-Fe₂O₃-ZnO mixed oxides with the Zn/Fe molar ratio of 1/2 were synthesized with a citric acid complex method. The effects of cobalt oxide and calcination temperature on phase composition and photocatalytic activity of the mixed oxides were investigated. X-ray diffraction (XRD) analysis revealed that there were mainly ZnFe₂O₄, α-Fe₂O₃, amorphous ZnO and Fe₂O₃ in the 6 mol% cobalt oxide doped products calcined at 500 °C. 5-10 mol% cobalt oxide doping could significantly enhance the formation of ZnFe₂O₄ and altered the phase composition of the mixed oxides. Experimental results showed that cobalt oxide doping could remarkably improve the photocatalytic activity of the mixed oxides for phenol degradation. The 6 mol% cobalt oxide doped mixed oxides calcined at 500 °C exhibited better photocatalytic activity as compared with other samples.