**Abstract**

The CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticles are synthesized by solution combustion method. The influence of Eu3+ on the structural, morphological, dielectrical, and humidity sensing study is recorded. The XRD pattern peaks of the as-prepared CuFe(2x)EuxO4 (where x = 0.00, 0.01, 0.02, 0.03) nanoparticle confirm the polycrystalline spinel cubic structure with a small amount of CuO impurity phase at 38.87° and 48.96°. Surface morphology of the samples was studied by scanning electron microscope (SEM) images of the nanoparticles, and their respective average grain size was estimated using Image software. Chemical composition of all prepared samples was analyzed by EDS spectra. The dielectric parameters of AC conductivity, electric modulus, and impedance of the samples were measured over a range of frequencies from 0.1 KHz to 1 MHz at room temperature. Europium-doped copper ferrite samples showed good humidity sensing response, response and recover times, and stability over a %RH range of 11–91%. These types of samples are very useful for sensor application, battery applications, electronic applications, and automotive applications.

**Keywords:** ferrite, Eu-doping, dielectric, impedance, humidity sensing

### **1. Introduction**

In recent years, the great developments in the applications of larger ionic rare earth-doped ferrites have been effectively realized in many prominent fields mainly in sensors, communication, and electronics. Copper ferrite metal oxide is a *p*-type semiconductor metal oxide and has been used in organic synthesis as a catalyst. Recently, many research groups have explored the importance of copper ferrite metal oxide in wide advanced applications because of their unique chemical, biocompatible, and physical properties [1]. The CuFe2O4 ferrite has spinel cubic

structure with general formula Fd3m. The CuFe2O4 ferrite belongs to a general formula AB2O4 in which A is a divalent ion site occupied by the Cu2+ ions and B is a trivalent ion site occupied by Fe3+ ions. The rare earth-doped CuFe2O4 ferrites have been used in many fields such as electronic devices, drug delivery systems, cancer therapy, and magnetic recording [1]. Rare earth-doped ferrites are also useful in high voltage electronics due to its negligible eddy current losses, high electrical resistivity, high permeability, magneto-optical, and magnetoresistive properties [2–4]. Generally, the trivalent ions are lesser in size than a divalent ion, and hence exchange of cations among the A and B sites plays a vital role in studying the structural, morphological, dielectric, and humidity sensing behavior of spinel copper ferrites. Copper ferrite has considerable good attraction of potential applications in various devices, like cores in transformers and microwave absorption [3, 4]. Copper ferrite nanoparticles can be prepared by various techniques like sol-gel method, coprecipitation method, solution combustion method, etc. [5, 6]. Many researchers have explored the properties of spinel ferrites by doping with different larger rare earth ions like samarium, terbium, gadolinium, and cerium in it. The europium-doped copper ferrites can be used in high-frequency applications due to the rare earth-doped ferrites showing low dielectric loss, good sensing response, and low value of conductivity. Europium-doped copper ferrites are usually considered as inverse spinel ferrites.

In the present work, spinel copper ferrites were prepared by solution combustion method and then in Eu3+ ions will be incorporated to investigate its structural, morphological, dielectric, and humidity sensing behavior at room temperature.

### **2. Experimental details**

#### **2.1 Required oxidizers and fuels**

The required oxidizers (metal nitrates), viz., copper nitrate [Cu(NO3)23H2O], europium nitrate [Eu(NO3)35H2O], and ferric nitrate [Fe(NO3)29H2O], and reducing agents, viz., carbamide [NH2CONH2] and glucose [C6H12O6], all were purchased from S.D. Fine Chemicals, Mumbai, India.
