Chapter 1 Introduction
1.9 Outline of the thesis
In this thesis, the pure LFO and Mg, Sr substituted LFO with different concentrations of Mg and Sr are synthesized. Several physical properties, such as dielectric, magnetic, and conductivity, apart from the structural and microstructural, are explored for the suitability of the material to be used for circulators and phase shifters (low loss with high magnetization and temperature stability). The temperature and frequency-dependent broadband dielectric and permeability response (1MHz‒1GHz) is studied. The effect of the incorporation of Mg and Sr on the structural, microstructural, dielectric loss, and magnetic response is investigated systematically. Further, the study is focused on preparing LFO- based ceramic composites with CB to be used as an efficient EMI shielding material. The EMI SE efficiency of LFO/CB composites with different wt% of CB and Dy substituted LFO/CB (10 wt%) are analyzed systematically. Also, the complex permittivity and
permeability in the X-band and Ku band, magnetization, and microstructure of all the composites are studied. The synergetic effect of different types of contribution, such as conduction loss, magnetic loss, and dielectric loss, towards the efficient shielding efficiency is obtained. It is found that strikingly excellent efficiency can compete with the recently used materials. Finally, I fabricated our in-house PLD target and developed a series of thin films of LFO by optimizing different deposition conditions. The effect of thickness on the microstructural, magnetic, dielectric, and electrical properties of LFO and Dy substituted LFO is analyzed and correlated. A chronological and brief description of each chapter is included below:
Chapter 1: introduces the different types of applications of microwave materials (circulator, phase shifter, microwave absorber, EMI shielding), the theory and underlying principle associated with it, and different types of ferrites used for microwave applications.
The importance of lithium ferrites and their suitability for microwave applications, the literature related to LFO, and the literature gap are also discussed. The research problem and the thesis organization are also included.
Chapter 2: describes the methods of preparation of LFO, LFO-based composites, and the deposition of LFO thin films by pulsed laser deposition system. It also includes experimental details of several characterization techniques such as XRD, Raman, FESEM, dielectric measurement (Impedance analyzer, LCR meter), VSM, VNA, and AFM and the basic details of the working principle associated with it.
Chapter 3: narrates the details regarding the formation of Mg and Sr substituted lithium ferrite and the effect of Mg and Sr on the structural, dielectric, and magnetic response of LFO. The incorporation of Mg and Sr led to lattice distortion that resulted in variations of lattice constant, bond length, and bond angles. The morphology of LMFO and LSFO ferrites exhibited a dense microstructure, and the average grain size is reduced with Mg and
Sr concentrations. The frequency and temperature-dependent permeability and permittivity are analyzed in the broadband frequency range of 1MHz–1GHz. The dielectric constant of lithium ferrite is improved with Mg substitution, x = 0.005 (εr = 3034, tan δ = 0.001at 1MHz) showed a maximum value at room temperature. The sample with x = 0.003 showed the best dielectric response (εr = 5986, tanδ = 1.17 @1MHZ, at RT) out of all the samples in the LSFO series. The relaxor behavior of the dipoles (the shift in the loss tangent with frequency) is observed in temperature-dependent dielectric response in the broadband. The activation energy of LMFO and LSMO is in the range of 1.39 – 0.35 eV and 0.142 eV – 0.07 eV, respectively. In case of LMFO, maximum permeability ~ 29 is obtained for x = 0.007 at 1 MHz, RT. The magneto-crystalline anisotropy, the thickness and the movement of the domain walls are the main contributers to the variation in permeability. The obtained Curie temperature for LFO is 873K, which is reduced with the substitution of Mg. The highest saturation magnetization (Ms = 54 emu/g) is obtained for x = 0.007 among all the LMFO samples. In the case of the LSFO series, randomness is observed in the values of Tc
with respect to the Sr concentration, and a maximum Ms of 61 emu/g is observed for x = 0.007. The magnetic properties are explained by considering Neel's two sub-lattice models.
The correlation between variation in structural parameters and the cationic distribution on the dielectric and magnetic response of substituted LFO is also discussed. Further, the applicability of Mg and Sr substituted LFO for microwave applications, such as circulators and phase shifters, is also analyzed (that require low dielectric loss, moderate permittivity and permeability, high magnetization, and high Curie temperature).
Chapter 4: This chapter aims to develop a highly efficient material for EMI shielding. It describes the preparation of LFO/CB (composites with different wt %) and Dy substitute LFO/CB (10 wt %) with varying concentrations of Dy and analyzes the magnetic, structural and microstructural behavior. Further, the addition of Dy caused the inhibition of grain
growth and the formation of irregular grains. Though saturation magnetization decreased with incorporating CB, it did not create any considerable decrement. The saturation magnetization is also enhanced with an increase in Dy concentration up to x = 0.1 (LD10FO/CB) and then decreased after that. The complex permittivity, permeability, and EMI shielding efficiency are studied in the X- and Ku band regimes. The 20 wt% of CB with LFO (3 mm thickness) showed highest total SE (28 dB), out of which absorption- based (SEA) is 23.17 dB and reflection-based shielding effectiveness (SER) is 4.58 dB. Both both LFO/CB (15) and LFO/CB (20) exhibited highest absorption efficiency ~ 99.68 % among the composites in the broad frequency range (8.2 ‒ 18 GHz). The maximum shielding effectiveness of 24 dB is obtained for LD10FO/CB ~ 17 ‒ 18 GHz. The Dy substitution enhances the magnetic as well as dielectric loss. Also, it is observed that absorption is the dominant mechanism in EMI shielding. The maximum absorption efficiency of 99.6 % is obtained for LD10FO/CB ~ 17 ‒ 18 GHz. with the incorporation of CB concentration, both real and imaginary parts of permittivity drastically increased (LFO:
ε' = 4, ε"= 0.11; LFO/CB (20): ε' = 46, ε"= 19.05 @8.2 GHz). Complex permeability is also enhanced with CB content. The values of complex permittivity and permeability for LD10FO/CB are in the range of (20 ‒ 40) and (2 ‒ 6), respectively. Incorporating Dy (rare earth ion) in spinel ferrite led to promoting 4f ‒ 3d coupling along with 3d ‒ 3d, which helped enhance the magnetic and electrical properties. The electron hopping mechanism explains the variation of permeability and permittivity. The synergic effect of dielectric and magnetic loss are the main contributors to the high SE. The above results suggest that LD10FO/CB can be used for EMI shielding applications. The results demonstrate that LFO-based ceramics can be used as a commercial microwave absorber.
Chapter 5: This chapter portrays the successful deposition of single-phase LFO and LDFO thin films using a pulsed laser deposition system. It also reports the various deposition
conditions such as background pressure, substrate temperature, the substrate to target distance, etc. The dielectric response is analyzed in the frequency range of (10kHz – 1MHz) and temperature range of (300 K – 523K). The dielectric constant improved, whereas dielectric loss decreased with an increase in thickness. Impedance spectra are studied using the equivalent circuit model, and the associated conduction mechanism is investigated using different conduction models. Conductivity is found to be improved, and the activation energy decreased with the increase in film thickness. A room temperature hysteresis loop with in-plane and out-of-plane configurations is studied. The saturation magnetization is reduced monotonically with an increase in thickness attributed to the decrease in the compressive strain. Coercivity enhanced with the film thickness, possibly due to the grain size enhancement. The influence of film thickness on the physical properties is correlated.
Further, it concludes that controlling the film thickness is the easiest and most effective method to tune the dielectric and magnetic response. The observed results suggest that LFO and LDFO films are promising materials for magnetic oxide semiconductor applications.
Chapter 6: summarizes the work done, future work, and the direction we can adopt for further research. The EMI shielding efficiency can be improved further by incorporating other carbon derivatives and nanomaterials of different structures. In the case of thin films, bilayer and multilayer films with dielectric seed layers can be deposited on single crystal and conductive substrates, and the resistive behavior can be studied broadly.