Synthesis of Ni base nanoparticles by W/O emulsion combustion
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To develop a process of fine particle production by spray pyrolysis, spray combustion of W/O (water-in-oil) emulsion, of which water phase was raw material solution and oil phase was fuel for heat source of high-temperature reaction field, was investigated. In this study, nickel oxide particles, which were a preliminary step of the nickel fine particle production, were synthesized and its structural characteristics were evaluated. Mixed solution of nickel nitrate and white kerosene was used as raw material. W/O emulsions were prepared using ultrasonic homogenizer and stirring these raw materials adding a surfactant. These emulsions were burning in a high temperature furnace to produce nickel oxide particles. The mean particle diameter of produced particles was less than 20 nm according to TEM observation. The diameter of the particles was much smaller than the estimated value based on the size distribution of dispersed solution phase in the emulsions and its concentration. Moreover, there is no effect of the concentration of the aqueous solution phase. On the other hand, X-ray diffraction pattern showed that the produced particles were complex of metal nickel with nickel oxide.
KeywordsNickel oxide nanoparticle W/O emulsion Spray combustion Ultrasonic homogenizer
List of symbols
Diameter of water phase in W/O emulsion (m)
Diameter of produced particle (m)
Flow rate of emulsion injected into an atomization nozzle (m3 s−1)
Flow rate of emulsion returned from an atomization nozzle (m3 s−1)
Molecular weight of produced particle (kg kmol−1)
Weight of sample (kg)
Initial weight of sample (kg)
Concentration of nickel salt solution (kmol m−3)
Density of produced particle (kg m−3)
Spray pyrolysis is one of the methods for fine particle production [1, 2, 3] and a droplet of precursor solution or slurry is a template of a produced particle. Many studies for development of fine particle production using the method have already been reported. A spherical droplet of precursor solution or slurry almost decides the size of the produced particle when these droplets do not break and/or unite themselves. Therefore, spray methods to produce fine and uniform droplets have been studied on and mass production methods of these droplets have been developed.  Electrostatic spray [5, 6, 7], ultrasonic spray [8, 9] and low-pressure spray methods [10, 11] are suitable for uniform nano-particle production since fine and uniformly sized droplets are obtained. On the other hand, it has been difficult to generate a large amount of fine droplets using these spray methods. Therefore, there is a need to produce a large amount of nano-particles more effectively and industrially. Then, we proposed production method for the ultrafine particle by the combustion of W/O emulsion prepared by ultrasonic homogenizer . In this method, a large amount of W/O emulsion that included fine and uniform droplets of a metallic salt solution can be fed to a reactor at a time. The emulsion is burned and high-temperature combustion field is formed in the reactor. As a result, fine water phases of the aqueous solution are dried, decomposed and sintered by thermal energy of combustion in a short time. Although particle production by W/O emulsion combustion has already been reported [13, 14, 15], the W/O emulsions have been prepared by simple stirring and the size of water phases is not small enough for nano-particle production of less than 1 m . Furthermore, there has been little reporting of the relationship between particle morphology and water phase structure in the emulsion.
In this work, nano-particle production due to spray combustion of a W/O emulsion prepared with an ultrasonic homogenizer was performed to obtain the nickel oxide ultrafine particle as the precursor of the nickel ultrafine particle. Based on microscopic observation of both the product and the raw material, the influence of the characteristics of the W/O emulsion and experimental conditions on the structure of produced particles was investigated.
As the raw material, Ni(NO3)2·6H2O (Kanto Chem. Co., Ltd.) was dissolved in water and an aqueous solution of a certain concentration was prepared. Kerosene dissolving sorbitan mono-palmitic acid ester (Sigma-Aldrich Japan Co., Ltd.), which was a surfactant with 6.7 of hydrophilic–lipophilic balance (HLB) value, was added in the water phase. The mass ratio of kerosene to aqueous solution was set to 7/3 and the surfactant was added 2.1 mass percent of aqueous solution and kerosene mixture.
Results and discussion
Weight loss of the hydrated metal salt progressed through almost three stages in any condition. The first two steps would be elimination reaction of hydration water since the mass ratio of the water to the hydrated metal salt (37%) was roughly consistent with the mass yield at the end of first two steps. Furthermore, the weight loss rate at the end of second step was almost same in any case. The third step would be decomposition of the metal salt and especially reduction reaction under hydrogen atmosphere. In fact, the mass ratios of both nickel oxide and metal nickel to the hydrated metal salt are 26 and 20%, respectively. These values were almost same to the mass yield at the end of third step; it was found that nickel oxide and/or metal nickel would be produced by decomposition and/or reduction reaction of the metal salt. Moreover, weight loss hardly appeared over 600 K. Based on these results, the above experimental conditions were determined.
Production of nickel oxide particle by W/O emulsion combustion
Median diameter of particles produced from W/O emulsions
Median diameter (nm)
Estimated median diameter (nm)
From these results, it is observed that the concentration of nickel salt solution had little influence on the size distribution of the synthesized particles although the predicted distributions of particle size shifted to smaller particle size with decrease in the concentration. In comparison with the estimated distributions, it was found that all actual distributions were proximate to the estimate based on the most dilute concentration of aqueous solution. When the concentration is high, solid phase such as metal salt will rapidly precipitate in the solution droplet. It is supposed that the solid phase may play a role in promoting evaporation such as boiling stone. Therefore, the water phase in the emulsion may explosively evaporate and cause secondary atomization both the emulsion and the water phase [21, 22] with increase in the concentration. As a result, the higher the concentration of the solution was, the smaller both primary and agglomerated particle would become. Moreover, crystals of nickel oxide (II) have face-centered cubic structure and that is responsible for cubic shape particles observed by TEM.
The diameter of primary particles produced from W/O emulsion was approximately 10 nm based on TEM observation. On the other hand, the size of agglomerated particles was almost less than 10 m due to SEM observation. The form of the primary particles was spherical or cubic.
The size distributions of primary particles were independent of the concentration of nickel salt solutions.
Not only nickel oxide but also metal nickel particles were produced by W/O emulsion combustion even under oxidative atmosphere according to TEM observation and XRD analysis.
This research was partially supported by JSPS KAKENHI Grant Number JP14750616 and Tanikawa Fund Promotion of Thermal Technology. Moreover, we thank Mr. Takanori Watanabe, Mr. Goshi Yokota and Mr. Shingo Kawagoe for assistance with experiments.
JK conceived of the presented idea. JK wrote the manuscript with support from NK and YI. MH supervised the project. All authors discussed the results and contributed to the final manuscript.
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