**2. Experimental procedure**

### **2.1. Materials**

All the chemical reagents were of analytical pure grade (99.99%) and used without further purification. The versatile chemical coprecipitation–microwave sintering process [15] em‐ ployed in present investigation is two-step process which consists of coprecipitation method is the first step of synthesis followed by microwave sintering in second half of experiment. High-purity La(NO3)3.5H2O (Merck), Ni(NO3)2.6H2O (Sigma-Aldrich), Co(NO3)2.6H2O (Sigma-Aldrich), and Mn(NO3)3.4H2O (Sigma-Aldrich) were used as starting materials. In a typical experimental process, the high purity stoichiometric amounts of La(NO3)3.5H2O, Ni/ CO(NO3)2.6H2O, and Mn(NO3)3.4H2O were dissolved in appropriate amounts of deionized water and magnetically stared vigorously for 2 h at 80°C. The ammonia solution was used until to get 8.5 PH value. The stirring will continue for about 30 min, and the suspension was ball milled for about 24 h with ethanol as a milling media. The reactants were to be mixed well and then dried at 80°C in a cabinet drier for 24 h to obtain precursor powder sample. Then the powder was subjected to microwave sintering under uniform heating to get dense ceramics.

### **2.2. Microwave sintering setup**

Microwave processing systems usually consist of a microwave source, for generation of microwaves, a circulator, an applicator to deliver the power to the load, and systems to control the heating and the experimental diagram of the microwave sintering set up used is shown in Figure 3. Most applicators are multimode, where different field patterns are excited simulta‐ neously.

Further, In order to achieve pure double perovskite phases, the precursor samples were put into 2.45 GHz, 6 kW continuously adjustable microwave equipment (HAMiLab-HV3, Syno-Therm), the maximum operating temperature up to 1400°C, and 0.5–3 kW. The multimode microwave furnace consists of a cubical stainless steel chamber with a side of 30 cm. Two magnetrons (microwave source), each with a maximum rated power of 1100 watts, are situated opposite to each other. A box made of alumina, zirconia, and silica mixed cardboard is used as a thermal insulator. The material is positioned in the center of box and is surrounded by silicon carbide (susceptor) plates.

Microwave sintering (MWS) technique has gained a lot of significance in recent times for materials (metals, composites, ceramics/nanoparticles) synthesis and sintering mainly because of its intrinsic advantages [5] such as rapid heating rates, reduced processing times, substantial energy savings novel and improved properties, finer microstructures, and being environmen‐ tally more clean. Therefore, it is viewed as one of the most advanced sintering techniques in material processing [5, 48] and improved physical and mechanical properties [7]. It has been shown that microwave sintering technique may provide enhanced densification in sintering

However, to the best of our knowledge in the open literature, there have been only a few reports so far on the fabrication of double perovskite nanoparticles by microwave sintering approach [5, 51]. The purpose of the current chapter will focus on fabrication of the double perovskite La2MMnO6 (M = Ni, Co) ceramics and in order to further improve their magnetic and dielectric properties for practical spintronic applications through microwave sintering approach.

All the chemical reagents were of analytical pure grade (99.99%) and used without further purification. The versatile chemical coprecipitation–microwave sintering process [15] em‐ ployed in present investigation is two-step process which consists of coprecipitation method is the first step of synthesis followed by microwave sintering in second half of experiment. High-purity La(NO3)3.5H2O (Merck), Ni(NO3)2.6H2O (Sigma-Aldrich), Co(NO3)2.6H2O (Sigma-Aldrich), and Mn(NO3)3.4H2O (Sigma-Aldrich) were used as starting materials. In a typical experimental process, the high purity stoichiometric amounts of La(NO3)3.5H2O, Ni/ CO(NO3)2.6H2O, and Mn(NO3)3.4H2O were dissolved in appropriate amounts of deionized water and magnetically stared vigorously for 2 h at 80°C. The ammonia solution was used until to get 8.5 PH value. The stirring will continue for about 30 min, and the suspension was ball milled for about 24 h with ethanol as a milling media. The reactants were to be mixed well and then dried at 80°C in a cabinet drier for 24 h to obtain precursor powder sample. Then the powder was subjected to microwave sintering under uniform heating to get dense ceramics.

Microwave processing systems usually consist of a microwave source, for generation of microwaves, a circulator, an applicator to deliver the power to the load, and systems to control the heating and the experimental diagram of the microwave sintering set up used is shown in Figure 3. Most applicators are multimode, where different field patterns are excited simulta‐

Further, In order to achieve pure double perovskite phases, the precursor samples were put into 2.45 GHz, 6 kW continuously adjustable microwave equipment (HAMiLab-HV3, Syno-Therm), the maximum operating temperature up to 1400°C, and 0.5–3 kW. The multimode

of metal, oxides and non-oxide ceramics [5, 48, 49, 50].

**2. Experimental procedure**

**2.2. Microwave sintering setup**

neously.

**2.1. Materials**

8 Advanced Ceramic Processing

**Figure 3.** Experimental setup of multimode microwave furnace [3] (a) and Ssusceptor (b).

During the sintering process, the microwave sintering chamber was filled with high purity nitrogen gas flow (99.999%). An adjustable electrical control system was used to control the energy to be delivered to the sample at a programmed rate. The heating chamber was a double walled, stainless steel, water cooled tubular cavity that stayed cool to touch, even when processing temperature was ∼1400°C. Inside the chamber was a high purity quartz crystal cylinder where samples were loaded for processing. During the MW sintering of the samples, temperature was measured using an infrared pyrometer. The crucible was surrounded by SiC plates, which act as susceptors to provide initial heating of the compact disk samples. Once the materials are sufficiently hot they will couple/absorb MW effectively and will get heated directly, including the core. The secondary purpose of SiC is to maintain the surface temperature. The crucible was positioned at the center of the furnace, where the During the sintering process, the microwave sintering chamber was filled with high purity nitrogen gas flow (99.999%). An adjustable programmed electrical control system was used to deliver the required energy to the sample. The employed heating chamber was made up with stainless steel double walled tubular cavity with water-cooled facility, and the maintained processing temperature is about 1400°C. A high purity quartz crystal cylinder arrangement is available inside the chamber, where the samples were loaded for processing; the temperature of the sample was measured using infrared pyrometer during the MW sintering. The SiC plates surrounded in the crucible were served as susceptors and provide initial heating to be compact disc samples. Once the materials received absorb sufficient MW heat including the core and will get uniform heating. The secondary purpose of SiC is to maintain the surface temperature. The crucible was positioned at the center of the furnace so it provides strong MW radiation. The green compacted disks for heated at 900°C for 10 min in atmospheric N2 ambient temper‐ ature and heating rate of 20°C/min is maintained by varying magnetron power between 1000 and 2500 W followed by normal frequency cooling.

Figure 3. Experimental setup of multimode microwave furnace (a) and Susceptor (b).

#### ambient at temperature of 900°C for 10 min at a heating rate of 20°C/min by varying the **2.3. Characterization and property measurements of La2MMnO6 (M = Ni, Co) ceramics**

MW radiation is the strongest. The green compacted disks were heated in atmospheric N2

magnetron power between 1000 and 2500 W followed by normal furnace cooling. *Characterization and property measurements of La2MMnO6 (M = Ni, Co) ceramics*  The crystal structure of the microwave sintered products was characterized by X-ray diffrac‐ tion (XRD) using a Shimadzu X-ray diffractometer with Cu-Kα radiation 2*θ* range of 20 to 80°. Raman spectra were carried out on an RM-1000 micro-Raman spectrometer with the 514.53

11

nm line of an argon laser under ambient conditions. The composition, morphology, and microstructures of the products were characterized by transmission electron microscope (TEM FEI Tecnai F20 microscope, Japan) and field emission scanning electron microscope (FESEM, Hitachi S-4800, Japan) equipped with an energy-dispersive X-ray spectrometer (EDS). Fourier transform infrared spectroscopy (FTIR) was performed on a Nicolet 5700 spectrometer in the wave number range of 400–4000 cm–1. The spectroscopic grade KBr pellets were used for collecting the spectra with a resolution of 4 cm–1 performing 32 scans. X-ray photoelectron spectroscopy (XPS) was performed on an ESCA-UK XPS system with an Mg K<sup>α</sup> excitation source (hν = 1486.6 eV), where the binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. The magnetic properties were measured using a physical property measurement system (PPMS-9, Quantum Design, Inc., San Diego, CA, USA) at room temperature under a maximum field of 20 kOe. Silver paste was applied on both sides of the pellet for the electrical measurements. The variation of dielectric constant and dielectric loss as a function of frequency at room temperature and as a function of temperature at different frequencies was measured using computer interfaced HIOKI 3532-50 LCR-HITESTER.
