Structural, Morphological and Thermal Decomposition Studies of Calcium and Hafnium Modied BaTiO3 Electro Ceramics

Calcium and hafnium co-doped barium titanate could be used as a replacement for lead zirconate titanate, which is a lead-based ferroelectric material. Solid state reaction accompanied by the usual sintering technique is the classical ceramic-processing method, which demands a lot of time and effort. The present work aims to make the process a lot easier and quicker by employing a modied sol-gel combustion technique to synthesize polycrystalline Ba 0.85 Ca 0.15 Ti (1-x) Hf x O 3 (x=0.00, 0.05, 0.10, 0.15) electro ceramics . The molar ration is xed at 1:1 for metal and citric acid at pH ~ 1. It was found that Ba 0.85 Ca 0.15 Ti (1-x) Hf x O 3 (where x=0.00, 0.05, 0.10, 0.15) crystallizes completely at around 1000 °C which is much lower than traditional methods. The structure supposedly displays a tetragonal symmetry with the P4mm space group as conrmed through x-ray diffraction (XRD) and Raman spectroscopy.


Introduction
Life in the 21 st century is largely in uenced by Arti cial Intelligence (AI), which means we live in a datadriven world. Active Internet users worldwide is roughly 4.57 billion as of 2020, and each of them creates 1.7 MB of data, on average, every second [1][2][3][4][5]. With the growing number of Internet-of-Things (IoT) devices and rise of smart cities, complemented by 5G networks, data generation per day is projected to increase to 463 exabytes by 2025 [6][7][8][9][10][11][12]. Furthermore, the increasing number of remote jobs in the ongoing pandemic allows people to limit physical contact with co-workers and things (especially touch screen gadgets) [10]. As tasks are becoming largely data intensive, the necessity of increased computational cost to run capable hardware becomes a hurdle. In this scenario, the emergence of new ferroelectric materials appears as a light in the darkness [11][12][13][14][15].
Although ferroelectric materials have been around for a century, they have recently attracted more attention with the growing popularity of multifunctional materials. They are excellent options for lowpower sensors and non-volatile memories [9]. The ferroelectric materials market was largely dominated by lead zirconate titanate (PZT) ceramics for decades. Due to the hazards associated with lead for both humans and the environment, many nations have banned its usage across various industries [16][17][18][19][20].
Manufacturers of smart devices using the ferroelectric property of PZT, which was described as a possible carcinogen, were therefore forced to look for alternatives. Barium titanate is one of the few ceramics known to exhibit excellent ferroelectric properties. It was one of the most important ferroelectric oxides used in the electronic industry before the invention of PZT [21][22][23][24]. The d33 coe cients and ferroelectric properties of doped barium titanate are comparable to those of PZT. In 2008, with the discovery of high d33 coe cients in calcium and zirconium (Zr) co-doped barium titanate, scientists again started investigating the material [25][26][27][28][29][30]. This work is an attempt to obtain improved properties in doped barium titanate by replacing Zr with hafnium (Hf) using a novel sol-gel synthesis technique. Solid state methods used traditionally requires very high calcination temperatures of up to 1300 °C and extremely high sintering temperatures around 1600 °C. Both the metals possess similar atomic radii due to lanthanide contraction, and hence similar properties are expected.
In this work, a comparatively easy synthesis technique for the production of Ba 0.85 Ca 0.15 Ti (1-x) Hf x O 3 (x=0.00, 0.10, 0.15) ceramics is proposed. The structural characteristics were analysed using x-ray diffraction (XRD). Thermal decomposition studies of the as-prepared gel were carried out using thermogravimetric/differential thermal analysis (TG/DTA). Scanning electron microscopy (SEM) was used to study the morphology of the ceramic samples, and any unwanted functional groups were con rmed using Fourier transform infrared spectroscopy (FTIR). Raman spectroscopy was used to con rm structural symmetry as well as the presence of any multiple phases in the structure.

Materials And Methods
With a magnetic stirrer, citric acid anhydrous (99.5% AR-Sisco Research Laboratories Pvt. Ltd.) was stirred in de-ionized water until it was completely dissolved. Barium nitrate (99.0% AR-Sisco Research Laboratories Pvt. Ltd.) and titanium isopropoxide (98.0%-Sisco Research Laboratories Pvt. Ltd.) were then added to the solution, followed by calcium nitrate tetrahydrate (99.0% AR-Sisco Research Laboratories Pvt. Ltd.) and hafnium dichloride oxide octahydrate (99.9%-Alfa Aesar). Nitric acid (69.0%-Merck) was added dropwise to adjust the pH at around 1, and the solution was stirred continuously at 100 °C. After 4 hours, the solution would boil and froth before turning into a dark brown powder as a result of ignition. The powder was calcined in air at 1000 °C in a high-temperature mu e furnace to remove organic compounds and vaporous impurities. X-ray diffraction (Bruker D2 Phaser 2 nd generation diffractometer) was used to analyse the crystal phase of the material with Cu K α radiation of 1.54 Å.
TGA/DTA analysis was conducted between 0 °C and 100 °C. Fourier transform infrared spectroscopy (Bruker Alpha II) was done between 500 cm -1 and 4000cm -1 . Raman spectroscopy (EZ Raman highperformance series II) was used to analyse the structural phases in the material. Carl Zeiss EVO 18 scanning electron microscope (SEM) was used to obtain the microscopic images of all samples. For all the compositions, the structure crystallizes with the tetragonal symmetry having the P4mm space group. All the obtained peaks were compared with the reference le available in the Joint Committee on Powder Diffraction Standards (JCPDS) database with COD ID: 04-019-9410. It was observed that a pure perovskite structure without any secondary phase was obtained. A well-de ned peak was obtained at 30°w hich con rms the formation of the perovskite phase. It is very clear from the patterns that both Hf and Ca have completely diffused into the corresponding lattice sites of barium titanate. The additional peaks obtained for BCTH5 and BCTH15 may be due to trace amounts of BaO phase formed during calcination. Figure 2 shows the thermal decomposition behaviour of BCTH10. It portrays the differential thermal and thermogravimetric analyses of the BCTH10 sample at a heating rate of 20 °C/min. As seen in the gure, the curve presents more than one peak for both endothermic and exothermic effects. This is due to the dehydration of absorbed water present in the ceramic combined with the removal of other organic compounds. The DTA curve presents an exothermic peak at 375 °C, probably due to the removal of water and the resultant weight loss of around 13%. The peaks at 500 °C, 570 °C, 603 °C, and 630 °C, due to the decomposition of metal complex, removal of organic compounds, and phase formation, contributed to a further weight loss of 37%. The weight loss after 630 °C is negligible, as con rmed by the straight line in the TG curve after this point. Figure 3 shows the Fourier transform infrared spectra of the BCTH samples. A broad band at 570 cm -1 con rms the formation of metal oxide. The stretching vibration of Ti-O and Hf-O is con rmed by the peak at 565 cm -1 . The peaks at 2364 cm -1 for the BCTH10 and BCTH15 samples are due to carbon dioxide, which may have formed as a direct result of the measuring conditions. The BCTH0 curve has no peak at 1684 cm -1 whereas the Hf doped BCTH shows a new peak at the value mentioned and may probably be due to the presence of hafnium. The peak may have occurred for all other samples due to the presence of a C=O carbonyl group which formed during the synthesis as a result of decomposition of hafnium dichloride oxide octahydrate which is an alkoxide. The hygroscopic nature of the material increases with increase in hafnium concentration. The peaks at 3750 cm -1 and 3884 cm -1 further supports the argument which occurred due to O-H stretching mode of water molecule.

Morphological studies
The scanning electron microscopy (SEM) images obtained for all the samples are shown in Figure 4. The SEM images clearly show that the particles undergo agglomeration. All the grains in the sample possess polyhedral geometry. The BCTH10 sample has the largest grain size. Larger grain sizes potentially yield better ferroelectric and piezoelectric properties.

Raman spectroscopic analysis
The materials in the region of 150-1000 cm -1 were analysed using Raman spectroscopy, a well-known tool to understand the physics of ferroelectric materials from their lattice-dynamics coupling. Moreover, Raman spectroscopy has a lower time scale as opposed to x-ray diffraction and presents a good structural ngerprint of the material under consideration. All the studied samples exhibit a tetragonal structure. This is con rmed by the peaks at 170 cm -1 and 247cm -1 , as well as the prominent peak at 514 cm -1 . The broad peak around 836 cm -1 is due to the presence of several dissimilar atoms at the A and B sites.

Conclusions
BCTH ceramics were successfully synthesized using the sol-gel technique. The structural parameters, which were studied using x-ray diffraction (XRD), con rmed the presence of tetragonal symmetry in all the compositions with the P4mm space group. Thermogavimetric and differential thermal analysis (TG/DTA) identi ed the material phase formation temperature at 1000 °C. Fourier transform infrared spectroscopy (FTIR) analysis showed that the material is free of unwanted functional groups. The morphology and shape of all the grains in the samples appeared to be polyhedral. The BCTH10 sample has the greatest grain size among all samples. In addition to XRD analysis, Raman spectroscopy showed that all the samples possess tetragonal phase with the P4mm space group.

Declarations
Funding: This research did not receive any speci c grant from funding agencies in the public, commercial, or not-for-pro t sectors.