Synthesis of one dimensional isolated Ni-Zn ferrite microtubes for induction heating applications Текст научной статьи по специальности «Химические науки»

DOI: 10.25702/KSC.2307-5252.2018.9.1.100-104 УДК 666.653

СИНТЕЗ ОДНОМЕРНЫХ ИЗОЛИРОВАННЫХ МИКРОТРУБОК НА ОСНОВЕ Ni-Zn-ФЕРРИТОВ ДЛЯ ПРАКТИЧЕСКИХ ПРИЛОЖЕНИЙ С ИСПОЛЬЗОВАНИЕМ ИНДУКЦИОННОГО НАГРЕВА

Е. В. Ребров12, П. Гао3

1 Университет Уорвик, г. Ковентри, Великобритания

2 Тверской государственный технический университет, г. Тверь, Россия

3 Университет Хунань, г. Чанша, Китай

Аннотация

На сегодняшний день основным методом производства современных функциональных материалов является керамическая технология. При изготовлении многокомпонентных оксидных соединений с применением керамической технологии трудно достичь высокой однородности химического состава. В данной работе для синтеза одномерных изолированных трубок микронного размера (микротрубок) на основе Ni0,5Zn0,5Fe2O4 использовался метод темплатного золь-гель синтеза. Представлены результаты по исследованию физико-химических и магнитных свойств полученных материалов методами рентгенофазового анализа, адсорбции азота, сканирующей и просвечивающей электронной микроскопии и магнитометрии с вибрирующим зондом. Исследовано влияние температуры прокалки на электромагнитные свойства полученных ферритов. Показано, что увеличение температуры прокалки c 873 до 1273 К уменьшает удельную поверхность образцов с 80,7 до 17,0 м2/г за счет увеличения размера зерна. Максимальное значение коэрцитивности (88,1 Э) получено при прокалке при температуре 973 К. Данные образцы обладают максимальной скоростью нагрева (4,36 Вт/г) в поле СВЧ. Ключевые слова:

N-Zn-ферриты, керамические трубки микронного размера, магнитные свойства, СВЧ-нагрев.

SYNTHESIS OF ONE DIMENSIONAL ISOLATED Ni-Zn FERRITE MICROTUBES FOR INDUCTION HEATING APPLICATIONS

E. V. Rebrov12, Pengzhao Gao3

1 School of Engineering, University of Warwick, Coventry, UK

2 Tver State Technical University, Tver, Russia

3 Hunan University, Changsha, China

Abstract

Ceramic technology is currently the main method for industrial production of advanced functional materials. However this method does not allow to achieve highly uniform chemical composition. In this study, one-dimensional isolated Ni0,5Zn0,5Fe2O4 microtubes have been prepared via a template assisted sol-gel method. Temperature dependence of the structural and magnetic properties was studied via XRD, N2 adsorption, SEM, TEM, and VSM. The effect of calcination temperature on magnetic properties of the resulting materials has been studied. An increase in calcination temperature from 873 to 1273 K caused a decrease in the specific surface area from 80,7 to 17,0 m2/g due to an increase of the grain size. The microtubes calcined at 973 K have the highest coercivity of 88,1 Oe and demonstrated the largest specific heating rate of 4,36 W/g in a radiofrequency field at 295 kHz. Keywords:

Ni-Zn ferrite, ceramic microtubes, magnetic properties, radio frequency heating. Introduction

Recently, the unusual morphologies of metal oxides, such as ordered porous particles, fibers, rods, and hollow structures have attracted much attention as their performance strongly depends on their chemical composition and their textural properties such as morphology, surface area, and pore size [1, 2]. In particular, hollow metal oxides with tubular structures have been extensively investigated because they offer advantages over other shapes, including a high surface area, narrow pore size distribution, and enhanced mass transfer rate [3]. Especially, microtubes have a large importance in the miniaturization of components and devices because of their small diameter and high aspect ratio, which makes them ideal candidates to load guest species such as biomolecules and catalysts [4]. In recent year, Ni-Zn ferrites were applied in high-frequency device applications due to their high values of chemical stability and saturation magnetization. When exposed to an AC magnetic field, Ni-Zn ferrites undergo a magnetization reversal process leading to energy losses which generate heat. The current methods for preparation of hollow structures include template approach, dry or wet spinning and electrospinning [4]. As the microstructure and magnetic

properties of ferrite materials are rather sensitive to their preparation method, many methods are not commercially viable at a meaningful scale due to complexity, long synthesis time, or cross contamination with impurities. In this way, the template approach is very simple and cost-effective preparation method compared to the other synthesis methods.

The goal of this study was to develop a template assisted sol-gel method for the synthesis of one dimensional isolated Ni-Zn ferrite microtubes (NZF microtubes) with high specific heating rate using natural cotton fiber templates. The influence of calcination temperature on the structural, magnetic and RF heating properties of the materials obtained was investigated.

Experimental

Ni-Zn ferrite microtubes with a nominal composition of Ni0,5Zn0,5Fe2O4 were prepared by the template assisted sol-gel synthesis following the approach described in our prevous study [5]. Solution A was prepared by dissolution of the corresponding metal nitrates in ethanol (all from Aldrich Co., ACS grade). Citric acid was dissolved in ethanol in a separate vessel to produce solution B which was added into solution A and the resulting mixture was stirred for 4 h. Then an ammonia solution was added dropwise till a pH of 2,4, the mixture was stirred for 24 h and then it was absorbed by cotton fibers. These cotton fibers were dried in 353 K. The samples were calcined at a desired temperature in the 873-1273 K range for 1 h to produce the corresponding ferrite microtubes. The microtubes are labeled as NZF-T hereafter, where index T represents the calcination temperature in K.

The phase composition of the samples was determined using an X-ray diffractometer (X'Pert PRO) with Cu Ka radiation produced at 40 kV and 27,5 mA, at a scanning rate of 5 o 2theta /min and a step of 0,02 o. The morphology was characterized by scanning electron microscopy (JSM-6700F, Jeol, Oxford) equipped with an energy dispersive spectrometer (EDS). The specific surface area was determined by nitrogen adsorption at 77 K on a nitrogen adsorption apparatus (Micromeritics NOVA 1000E). The magnetization curves of the as-prepared samples were measured by a vibrating sample magnetometer (Princeton Measurements Corporation MicroMag 3900 VSM) equipped with a 2 Tesla electromagnet at several temperatures in the range from 298 to 833 K. The saturation magnetization (Ms), remnant magnetization (Mr), coercivity (Hc) and hysteretic losses were evaluated from the magnetization curves.

The RF properties of the samples were characterized by their specific absorption rate (SAR). The SAR is determined by the thermal energy released by the material in a external magnetic field with a frequency of295 kHz and an intensity of 500 Oe.

Results and discussion

XRD spectra of Ni0,5Zn0,5Fe2O4 microtubes calcined at different temperatures are shown in Figure 1. It can be seen that the spinel phase is formed in the entire temperature range studied (873-1273 K).

03

c

IU

jH

-J

JUL

_L

_L

H Hematite

NZF-1273 _,_U_^

NZF-1173

NZF-1073

NZF-973

NZF-873

Standard NZF _i_i_i_

20

30

70

40 50 60 20(degree)

Fig. 1. XRD patterns of the Nio,5Zno,5Fe2O4 microtubes

80

The unit cell parameter, interplanar spacing, average crystal size and specific surface area of the microtubes calcined at different temperatures are listed in Table 1. As the temperature increases, the specific surface area decreases by a factor of nearly five due to progressive aggregation of small crystallites into larger particles.

Figure 2 shows characteristic SEM images of the template and Nio,5Zno,5Fe2O4 microtubes calcined at different temperatures. It can be seen that one-dimensional isolated ferrite microtubes ans obtained. The microtubes with a low degree of crystallinity are obtained after calcination at 873 K which agrees with the XRD data. Their mean diameter is 6 ^m (Figure 2, a). The microtubes with a mean diameter of 3,7 ± 0,2 ^m and with a higher degree of crystallinity were obtained after calcination at 1073 K (Figure 2, b). They are formed by rather uniform nanoparticles with a size of 80 nm and they have a higher length-to-diameter ratio of 12. After calcination at 1273 K, the microtubes with a mean diameter of 2,5 ± 0,2 ^m were obtained. A characteristic feature of the fibers is the presence of small channels along the radial direction.

H

H

H

Table 1

Physical properties of Ni0,5Zn0,5Fe2O4 microtubes calcined at different temperatures

Sample NZF-873 NZF-973 NZF-1073 NZF-1173 NZF-1273

J311 interplanar spacing (nm) 2,5392 2,5391 2,5378 2,5363 2,5330

Unit cell parameter (nm) 0,8430 0,8429 0,8423 0,8423 0,8413

Average crystal size (nm) 25,3 37,2 73,6 95,4 112

Specific surface area (m2/g) 80,7 63,7 44,3 29,4 17,0

Fig. 2. SEM images of (a) NZF-873; (b) NZF-1073; (c) NZF-1273 microtubes. Scale bar is 1 ^m

In order to study the structure transition during calcination, characteristic SEM images of the microtubes calcined at different temperatures were taken. As the temperature increases, the template and the gel absorbed in the surface layer of the template are decomposed causing an interfacial solid-state reaction which yields interconnected ferrite particles (Fig. 3, a). The tube diameter decreases after calcination at higher temperatures due to increased size of individual ferrite nanoparticles. The individual nanoparticles can be seen in a TEM image (Fig. 3, b). They have the shape of irregular polyhedrons with a mean size between 50 and 80 nm. The observed particle size is somehow larger than the crystallite size obtained from the XRD study due to the formation of connecting necks between the two neighboring particles during calcination. No large aggregated nanoparticles was observed confirming much higher degree of dispersion as compared with a non-templated synthesis method [6]. Due to this fact, the specific area of the Ni0,5Znc,5Fe2O4 microtubes was considerably enhanced in this study,

Fig. 3. Characteristic SEM image of cross sectional view (scale bar is 4 ^m) (a) and TEM image of NZF-1073 microtubes (b)

Room temperature hysteresis loops of the Ni0,5Zn0,5Fe2O4 microtubes calcined at different temperatures are shown in Figure 4 and their magnetic parameters are listed in Table 2. The maximum value of saturation magnetization exceeds that of the bulk Ni0,5Znc,5Fe2O4 ferrite (56 emu/g) in all samples. The saturation magnetization in nanoparticles is influenced by both the intrinsic (composition, preferential site occupancy of the cations, exchange effect) and extrinsic factors (microstructure and grain size) [6]. The increased mean crystal size increases saturation magnetization. However the exchange interaction between the Ni-Zn ferrite and the hematite impurity decreases the saturation magnetization in the samples calcined at higher temperatures [6]. The highest coercivity of 88.1 Oe is observed in NZF-973 as its grain size (37,2 nm, Table 1) is only slightly above the critical domain size (22,2 nm, Table 2). In the multidomain region, the coercivity decreases as the grain size increases from 37.2 to 112 nm. As the calcination temperature increases, the Curie temperature of the microtube increases from 532 to 549 K (Table 2). A similar trend was observed by Sepelak et al. for nanostructured Mg ferrites [7].

Fig. 4. Magnetization curves of Nio,5Zno,5Fe2O4 microtubes calcined at different temperatures (a), hysteresis loops at a larger magnification (b)

Table 2

Magnetic properties of Nio,5Zno,5Fe2O4 microtubes calcined at different temperatures

Sample NZF-873 NZF-973 NZF-1073 NZF-1173 NZF-1273

Saturation magnetization (emu/g) 56,6 62,8 67,8 67,1 66,6

Coercivity (Oe) 56,9 88,1 56,6 31,4 26,1

Curie temperature (K) 532 534 535 537 549

Critical domain size (nm) 27,3 22,2 19,1 19,5 20,1

Specific heating rate (W/g) 2,45 4,36 3,10 2,30 2,10

Figure 5, a shows the temperature dependence of the coercivity of the Nio.5Zno.5Fe2O4 microtubes. The coercivity shows a non-monotonous temperature dependence with a local maximum in the range between 573 and 823 K, which is slightly above their Curie temperature. Below the Curie temperature, the coercivity decreases with an increase in temperature in accordance with the ferromagnetism theory as the degree of atomic thermal vibration increases. Above the Curie temperature, an internal induced magnetic field is formed via a preferred orientation of the magnetic moments in the microtubes in the direction opposite to that of the applied magnetic field. The magnitude of this effect is proportional to the magnetic susceptibility, which decreases with temperature resulting in a decrease of coercivity. However such coercivity behavior cannot be accounted for by exclusively considering the temperature dependence. The characteristic internal stress in the isolated microtubes may result in the anomalous coercivity behavior since the internal stress induces anisotropy in the microtubes. The presence of temperature-dependent mechanical stresses acting on the domain walls must be considered as a cause of the observed behavior.

300 400 500 600 700 800 300 400 500 600 700 800 900

Tested temperature (K) Tested temperature(K)

Fig. 5. Coercivity(a) and hysteresis loss (b) of Nio,5Zno,5Fe2O4 microtubes as a function of temperature

Figure 5, b shows hysteresis loss of the microtubes as a function of tested temperature. Due to a non-zero saturation magnetization and a moderate coercivity, the microtubes could be heated by RF heating in the temperature range above their Curie temperature. This is a unique feature of these materials which was not observed in the nanoparticles of the same phase composition.

From the viewpoint of application of the obtained materials in catalytic reactors under radiofrequency heating, it is important that they possess high specific surface area and demonstrate high specific heating rate which allows to maintain desired temperature inside the reactor. It can be seen that the sample with a mean particle size of 37 nm demonstrated the best combination of specific surface area and the largest specific heating rate in RF field of 295 kHz.

To conclude, we report here the synthesis of a new generation of one-dimensional isolated Ni-Zn ferrite microtubes displaying heating properties at elevated temperatures considerably larger than any previously described material. The unique heating properties of these materials was shown to result from the formation of an isolated one-dimensional ferromagnetic tubular structure with a diameter in the 4-6 micron range and a mean particle size close to the critical domain size. These materials can be

coated with various catalysts and they have been used for fine chemicals synthesis a continuous-flow reactor working under RF heating [8, 9]. This process offers the possibility to perform catalytic transformations in small and decentralized units, taking profit of the high energy efficiency and a possibility for fast scale-up [10]. Acknowledgements

The financial support from the Science and Technology Planning Project of Hunan Province, China (2012WK3023), and the Russian Science Foundation (project 15-13-20015) is gratefully acknowledged.

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Сведения об авторах Ребров Евгений Викторович

доктор технических наук, кандидат химических наук, Университет Уорвик, г. Ковентри, Великобритания; Тверской государственный технический университет, г. Тверь, Россия e.rebrov@warwick.ac.uk Пенжао Гао

кандидат химических наук, Университет Хунань, г. Чанша, Китай gaopengzhao7602@hnu.edu.cn

Rebrov Evgeny Victorovich

Dr. Sc. (Engineering), Ph.D (Chemistry), University of Warwick, Coventry, UK; Tver State Technical University, Tver, Russia

e.rebrov@warwick.ac.uk

Pengzhao Gao

PhD (Chemistry), Hunan University, Changsha, China gaopengzhao7602@hnu.edu.cn

DOI: 10.25702/KSC.2307-5252.2018.9.1.104-109 УДК 544.02 : 544 - 971

ОСОБЕННОСТИ ТЕРМОДИНАМИЧЕСКОГО ОПИСАНИЯ СИСТЕМ НА ОСНОВЕ ОКСИДОВ ГАФНИЯ И РЕДКОЗЕМЕЛЬНЫХ ЭЛЕМЕНТОВ ПРИ ВЫСОКИХ ТЕМПЕРАТУРАХ

В. Л. Столярова, В. А. Ворожцов, С. И. Лопатин

Санкт-Петербургский государственный университет, г. Санкт-Петербург, Россия Аннотация

Керамика на основе оксидов гафния и редкоземельных элементов представляет интерес для создания новых материалов высшей огнеупорности. В обзоре обсуждаются термодинамические свойства, полученные в рассматриваемых системах различными методами высокотемпературной химии.