**2. Experimental procedure**

materials due to their unique electrical, dielectric, electronic, mechanical, magnetic, optical and catalytic properties. These ferrites are characterized by good magnetic properties [1], low

eddy current loss for high-frequency electromagnetic wave propagation [2], chemical stability and fairly high mechanical hardness [4], low dielectric losses and high Curie temperature [5].

The most significant and most popular use of ferrites is in optics, electronics, mechanics and other technical fields [9]. Ferrites also play a major role in medicine, biomedical applications, as chemical catalysis and special coatings (antistatic, electromagnetic shielding). Scientific articles contain extensive information on hyperthermia. This method introduces ferrite nanoparticles into living organisms and, under controlled conditions, nanoparticles are transported to the cancerous areas of the body, and cancer cells are destroyed in a magnetic field

Ferrites have become suitable for many technological applications such as microwave devices [11] and telecommunication devices, electric motors and generators, as excellent core material for power transformers in electronics, antenna rods, loading coils and read/write heads for high speed digital tape [1], tape recorders and discs [3], high-density information storage and recording devices and as permanent magnets [11], sensors [12], and so on. Magnetic nanoparticles and in particular magnetic fluids (ferrofluids) are particularly important in biotechnology and biomedicine—the supply of biomedical drugs and as contrast media [12], in medical diagnostics [13]. Ferrite materials are widely used in catalysts [12]. In recent years, ferrite materials have been used to prevent and eliminate radio frequency interference in audio systems [4], as polarized ferroelectric ceramics in acoustic elements in underwater converters [14] and microwave absorbing materials [15], including ferrite-containing radar absorbing paints for masking military aircraft [16]. Lately, it has been discovered that cobalt ferrite nanoparticles can also act as photomagnetic material that shows interesting light-induced

Ferrites in a nanocrystalline state (i.e., below single domain sizes [9]) are often found to have unique physical and mechanical properties compared to coarse-grained polycrystalline materials [18]. It is known that the properties of nanocrystalline ferrite materials, including dielectric constant, conductivity, permeability, and other magnetic properties are determined by their microstructure [19], which, in turn, is influenced by the method of their production [8], that is, the synthesis methods [1]. It is well known that the microstructure, in particular the crystallite size, essentially determines the parameters of the hysteresis loop of soft ferromagnetic materials [20]. Samples obtained with different synthesis methods show different electrical and magnetic properties [4]. Therefore, many new nanoparticle production techniques

Ferrites, as the majority of ceramic materials, are obtained by reactions of solid phase from various oxides [21]. The development of nanotechnological processes has resulted in the development of several liquid phase and gas phase synthesis methods—chemical co-precipitation

has a high permeability in the radio frequency range [6], high thermal stability [7],

) [3] magnetic coercivity, high electrical resistivity and negligible

(NiFe2 O4

98 Powder Technology

CoFe2 O4

by heat treatment [10].

coercivity changes [17].

have been developed in recent years.

) [2] or high (CoFe2

O4

moderate saturation magnetization [3] and electrical conductivity [8].

In research, nickel and cobalt ferrite nanopowders are obtained by the chemical sol-gel self-propagating combustion ("combust.") method [36], the co-precipitation technology in combination with hydrothermal synthesis ("hydrotherm.") [37] or spray-drying ("spray") [38] method and high-frequency plasma chemical synthesis ("plasma") [39]. The obtained nanopowders have been studied for mechanical and magnetic properties.

The synthesis of cobalt and nickel ferrites by the sol-gel self-propagating combustion method was carried out using reagent grade chemicals: Co(NO<sup>3</sup> ) 2 ·6H<sup>2</sup> O, Ni(NO<sup>3</sup> )·6H<sup>2</sup> O, Fe(NO<sup>3</sup> )3 ·9H<sup>2</sup> O, glycine, nitric acid [36]. A 100 ml 0.1 M cobalt (or nickel) nitrate solution was added to a 200 ml 0.1 M iron nitrate solution. The glycine was separately dissolved in 100 ml of distilled water, nitric acid added and both added to the nitrate mixture. Glycine (Gly) was used as a self-combustion agent with a molar ratio Me/Gly = 1: 0.8 and Gly/Nitr. = 1:4. The mixture was evenly stirred until the mixture has congealed. Then the mixture was heated until it ignited, and the heating was continued at 300°C for 4 h.

By the co-precipitation method, cobalt and nickel ferrites were synthesized using reagent grade chemicals: FeCl<sup>3</sup> . 6H<sup>2</sup> O, urea, Co(NO<sup>3</sup> )2 . 6H<sup>2</sup> O or Ni(NO<sup>3</sup> )2 . 6H<sup>2</sup> O, NaOH [37]. The precursor was obtained as follows: urea was hydrolyzed for 3 h in a FeCl3 . 6H<sup>2</sup> O solution (molar ratio of 3: 1) at 70–75°C. Cobalt or nickel nitrate was added the cooled reaction mixture. The molar ratio FeCl<sup>3</sup> . 6H<sup>2</sup> O: Co(NO<sup>3</sup> )2 . 6H<sup>2</sup> O or Ni(NO<sup>3</sup> )2 . 6H<sup>2</sup> O corresponds to the metal ion stoichiometry in ferrite. Continually stirring the suspension with 40% NaOH solution, cobalt or nickel hydroxide was slowly precipitated until the pH of the suspension reached 9–10. Then the suspension was placed in an ultrasonic bath for 20 min and then treated for 24 h at 40°C. The sediment was then washed with distilled water by decantation until the presence of Cl ions was no longer detected. Next are two processing options:

**A.** by the hydrothermal method, the volume of the hydroxides mixture is reduced by decanting to 250 ml, poured into the reaction vessel and placed in an autoclave. The hydroxide mixture was then treated hydrothermally at different temperatures (200–250° C, 1–3 h, p = 17–17.5 MPa). After hydrothermal treatment, the formed precipitate was filtered with a water jet pump using a 5 μm membrane filter and washed with distilled water and dried at 40°C;

**B.** for spraying the hydroxide mixture with the spray-drying method, the pelleting machine was used developed by RTU Institute of Inorganic Chemistry. Main parameters of the suspension spray: hot air temperature and consumption of 370°C and 24 m3 /h, temperature in evaporating chamber 120–130°C.

Technological equipment developed by the Institute of Inorganic Chemistry of the Riga Technical University [35] was used for the production of ferrites by means of high-frequency (HF) plasma chemical synthesis. Commercial metals and metal oxides (Ni, Co, NiO, CoO and FeO) powders were evaporated in HF plasma to obtain ferrites. All raw materials in stoichiometric ratios (to obtain NiFe2 O4 and CoFe2 O4 ) were injected into nitrogen plasma at an average temperature of 5800–6200 K. After evaporation of the raw materials, the vapor was cooled very quickly with the cooling gas (air) and the product condensed on the filter in the form of nanosized ferrite particles.

Ferrite nanopowders for sintering were prepared as follows: the ferrite nanopowder samples were mechanically mixed for 1 h in a planetary mill with 3% by weight of stearic acid (400 rpm, ZrO2 container, ZrO2 ball material) using isopropanol as a dispersing medium. Stearic acid was used for better pressing. After mixing, the samples were dried in an oven at 80°C and sieved through a 200 μm sieve. For sintering without pressure samples were pressed (200 MPa) as tablets with a diameter of 12 mm and a height of 4–6 mm. Stearic acid was burned out at 600°C. Samples were sintered at 900–1300°C in an air atmosphere at a rate of 10°C/min in an oven LHT-08/18 (Nabertherm GmbH) for 2 h.

All samples were analyzed using the X-ray diffractometer Advance 8 (Bruker AXS). The size of the crystallites was calculated using the Scherer's equation. The magnetic properties of the synthesized ferrites were analyzed using vibrating sample magnetometry (VSM Lake Shore Cryotronics, Inc., Model 7404 VSM). The SSA was measured using the BET single point method. The size and morphology of the particles as well as the microstructure of the sintered material were studied using transmission electron microscope JEM-100S (JEOL) and a scanning electron microscope Mira/Tescan and Tescan Lyra-3 on the fracture surfaces. The density and open porosity of the sintered samples were determined by the Archimedes method.
