Studies on highly dense pure YIG polycrystalline ceramics fabricated by tape-casting method

Pure phase Y 3 Fe 5 O 12 (YIG) ceramics was successfully produced by tape-casting forming process and one-step solid-state reaction method. With the sintering temperature above 1100 ºC, the pure phase YIG ceramics was synthesized with no YIP or Fe 2 O 3 phase in XRD patterns. YIG ceramic sintering at 1400 ºC for 10 h showed a clear grain structure with an obvious grain boundary, and no pores were observed in the SEM images. YIG ceramics in this paper has a high relative density which was 99.8% and the saturation magnetization was 28.2 emu/g at room temperature. The hysteresis loss at temperatures of 230-360 K was smaller than 10 mJ/kg. The tan 𝛿 𝜀 was nearly zero at 6~7 GHz and 11~12 GHz, showing that it can be used as a good material for microwave applications. In addition, the low values of tan 𝛿 𝜀 and tan 𝛿 𝜇 indicates that it may have a good electromagnetic wave absorption ability.


Introduction
Yttrium iron garnet, Y 3 Fe 5 O 12 (YIG) has a bcc cubic structure and belongs to the space group Ia-3d (230). Furthermore, per unit cell (1895.6 Å 3 ) contains 8 formula units, where Y 3+ ions occupy the eight-coordinate (O 2-) dodecahedral sites, while Fe 3+ ions partially take up six-coordinate octahedral and three-coordinate tetrahedral sites. YIG materials and its modifications by doping as soft ferrites are suitable for magnetic microwave devices with high frequency and low microwave loss [1,2]. Microwave filters and oscillators based on YIG have a wider electric tuning range in frequency than other candidates including ferroelectric and varactors [3]. Multiferroic magnetoelectric materials developed on YIG exhibit both ferroelectricity and ferromagnetism, and typically yield much larger magnetoelectric coupling response than natural multiferroic single-phase compounds, making them suitable for magneto-electric applications [4]. Faraday rotators with substituted YIG films or bulk crystals own large Faraday Rotation capacity, low saturation magnetization and low optical absorption loss, showing high potential applications in magneto-optical devices [5,6].
Pure polycrystalline Bi-substituted YIG thin films were grown on Corning glass substrates by an original CVD technique and its Faraday rotation properties were studied [7]. Bi-substituted Y-Yb mixed iron garnet single crystals were grown by the flux method and the specific Faraday Rotation, optical absorption coefficient and the saturation magnetization were investigated [5]. Nowadays, polycrystalline YIG (powder or bulk ceramic) are generally synthesized through conventional solid-state reaction (CSSR) method. For YIG ceramics, properties of density, purity and microstructure have great impact on the magnetic properties and therefore the performance of its microwave devices.
However, some secondary phases (Fe 2 O 3 , YFeO 3 , YFe 2 O 4 ) are usually coexisted with YIG fabricated by CSSR method [8,9]. The growth kinetic and reaction mechanism of Y 2 O 3 -Fe 2 O 3 system were studied by different diffusion couples. And, the results showed that the formation of YIG phases were highly depended on pre-formed YIP phases and the diffusion of Fe 3+ cations [10][11][12]. By adding excess 8-10 wt% Fe 2 O 3 , 99% of YIG ceramic phase was achieved. What's more, YIG with properly excess Fe 2 O 3 could be used for high frequency tunable dielectric resonator antenna (DRA) [13,14]. Recently, YIG ceramics with about 98-99% of theoretical density were fabricated by solid-state reaction method [15,16]. While, new fabrication methods are pursued for much higher dense and purity fine-grained YIG ceramics.
In this work, high-dense pure YIG polycrystalline ceramics were fabricated by tape-casting forming process and one-step solid-state reaction method. Phase purity, relative density and micro-morphology depends on sintering temperature and holding time were studied. The activation energy was calculated from the linear Arrhenius plots. XPS was performed to check valance states of Fe and O atoms, as well as EPR spectrum was introduced to check O vacancy. Thereafter, magnetic and dielectric properties were chractarized for the sample sintered at 1400 ℃ for 8h.

Material and methods
Commercially available powders of Y 2 O 3 (99.99%) and Thereafter, the tape-casting forming process was carried out. The distance between the blade and tape was 500 μm and the casting speed was 500 mm/min. The prepared slurry on the tape was dried at room temperature, and then cut into slices. A proper number of layers of slices were stacked and compressed under 20 Mpa at 120 ℃ to form into green body. The green body was calcined at 800 ℃ in an oxygen atmosphere to remove organic additives, and then sintered at various temperatures in a furnace under air atmosphere. Finally, the obtained YIG ceramics were prepared for characterization.
The phase identification of the fabricated ceramics was performed on an XRD system (Bruker, Germany) using Cu Kα radiation (λ=1.5406 Å). The surface micro-morphology was carried on a field-emission SEM (JSM-6700F, JEOL, Japan).
The XPS was conducted by using an X-ray photoelectron spectrometer (ESCALAB 250X, Thermo Scientific, USA). The EPR was conducted by using an electron paramagnetic resonance instrument (A300-10/12, Bruker, Germany). The bulk density was measured using the Archimedes' method. The magnetic hysteresis loops were obtained by a vibrating sample magnetometer (VSM, QD, USA). The magnetic and dielectric properties were determined with a microwave network analyzer (PNA-N5244A, Agilent, USA). Fig. 1 shows the XRD patterns of YIG ceramics fabricated by combining tape-casting forming process and solid-state reaction method at various sintering temperatures in air atmosphere. It can be clearly observed that the ceramics with sintering temperatures above 1100 ºC are all YIG pure phases (PDF# 71-2150). According to Ali's report, YIG ceramics prepared through dry-pressing forming process generally contain unwanted phase of YIP even with 1250 ºC sintering temperature [10]. It is suggested that YIP is formed as an intermediate phase and will further react with Fe 2 O 3 to produce single-phase YIG by diffusion process [12]. It is suggested that Fe 3+ cations diffuse and initiate the Fe-Y atomic bonding. Then, with Fe 3+ cations continuously diffusing, the YIP layer covered on the surface of Y 2 O 3 particles turned to be YIG while the inner Y 2 O 3 changed into YIP.

XRD
Considering the saturated vapor pressure of Fe and Y calculated by Eq. (1) and listed in Table 1, it can be seen that Fe has a larger saturated vapor pressure than Y, which signifies that the iron is more volatile in this system. If Fe 2 O 3 is inhomogeneous in this system, high concentration of Fe element means more evaporation at high temperature. So, the existence of YIP impurity is highly depended on the inhomogeneous and evaporation of Fe 2 O 3 .
Compared with dry-pressing forming method, our tape-casting forming method provide much better homogeneous mixture of Y 2 O 3 and Fe 2 O 3 particles, thus leading to the highly pure YIG ceramics.
Here, coefficients in the equation can be obtained from reference [17] and listed in Table 1.    According to Chen's report, Fe 2+ ions and oxygen vacancies may exist in ceramics fabricated by high temperature sintering process due to the low oxygen conditions [16], and this can be verified by XPS measurement. Fig. 3 Fig 3(a). In Fig. 3(b), based on the Lorentzian-Gaussian fitting, Fe  Fig.   3(c) was divided into two peaks centered at 529.9 eV and 531.6 eV. And the latter peak indicated the existence of oxygen vacancy which can also be proved by the EPR spectrum, as shown in the illustration in Fig. 3(c).

Fig. 3. XPS spectra in (a) the survey range, (b) Fe ion and (c) O ion in YIG ceramics
sintered at 1400 ℃ for 10 h.

Relative density and activation energy
The sintering kinetics, namely densification kinetics, of YIG ceramics could be characterized by the relationship between the relative density and sintering conditions (temperatures and holding times). Densities of YIG ceramics sintered at various temperatures and holding times were measured by Archimedes' method and were plotted in Fig. 4(a). Obviously, higher sintering temperature and longer holding time did lead to higher relative density. Besides, the relative density increased significantly along with the temperature below 1375 ℃, while it increased slowly when the temperature further raised.
For samples sintered at 1450 ℃ for 8 h, the relative density reached the highest value of 99.8%, which was bigger than that values reported in references [15] and [18]. The high relative density indicated extremely low residual porosity and highly dense microstructure.
Densification kinetics can be interpreted by Arrhenius equation: [10,18,19] lnK=-E a /RT+C where, E a is the activation energy for YIG ceramic densification, R is the gas constant (8.314 J/mol/K), T is the absolute temperature, C is the constant, and K is the rate constant depending on temperature. Densification rate versus reciprocal temperature was plotted in Fig. 4(b), where rate constant K was related to relative density and calculated from Fig.   4(a). By linear fitting, the value of E a was determined to be 183.81 kJ/mol, which was comparable with the activation energy of 169 kJ/mol for lattice diffusion or grain boundary formation [18] and much smaller than the energy of 484 kJ/mol for YIG formation from YIP [10]. It can be drawn that the mechanism of YIG phase formation and ceramics densification was the result of homogeneous diffusion probably assisted by the sintering aids of SiO 2 (products of TEOS decomposition) located at grain boundaries.

Intrinsic magnetic property
The magnetic properties of YIG ceramics sintered at 1400 ℃ were studied by the static hysteresis loops under various temperatures, as shown in Fig. 5(a). The well-defined hysteresis loop can be clearly observed at 30 K, while turned to be narrower along with the increase of temperature. The saturation magnetization (M s ) and hysteresis loss (area of the hysteresis loop) depending on temperature were plotted in Fig. 5(b). The value of M s decreased from 38.9 emu/g down to 25.3 emu/g as the temperature increased from 30 K up to 360 K. And, the trend could be ascribed to the enhanced thermal vibrations of atoms and lattices under high temperatures, and hence magnetic dipoles were difficult to be aligned [20]. It was worth mentioning that the room-temperature (300 K) value of M s was 28.2 emu/g, which was a little higher than that value (27.4 emu/g) reported in previous literature [15]. The result indicated that our YIG ceramics exhibited much higher relative density and more compact microstructures. Hysteresis loss, characterized by the area of the hysteresis loop, generally decreased with temperature. However, in this work, there was an upturn point at 230 K (4 mJ/kg), and the mechanism needed to be further studied. Besides, hysteresis loss at 30 K was as high as 238.8 mJ/kg, while values of that at temperatures of 230-360 K was smaller than 10 mJ/kg.  processes, as shown in Fig.6 (a). [21] In addition, the dielectric loss tangent tan determined by ε''/ε' fluctuates with the frequency, and interestingly, tan at about 6~7

Electromagnetic parameters
GHz and 11~12 GHz is extremely low suitable for practical microwave devices, as shown in Fig. 6(b). As we know, the dielectric loss is ascribed to conduction loss and polarization relaxation loss. And, polarization relaxation loss can be characterized by the Cole-Cole semicircle, that is, a simple semicircle in the ε'ε''-coordinate plane means a polarization relaxation process. As shown in Fig. 6(c), there are three semicircles suggesting that the dielectric loss is determined by multiple dielectric relaxation losses, and this result is in accordance with relaxation peaks in Fig. 6(a). [22] Future work will be focused on dielectric properties of YIG ceramics under applied magnetic field in a wide range of frequency. For complex permeability, the real part ′ (the ability for the storage of magnetic energy) and the imaginary part ′′ (the dissipation capacity of magnetic energy) have the same changing trend along with the rise of frequency from 2 to 18 GHz, as shown in Fig. 7(a).
Apparently, ′ and ′′ exhibit three distinct resonance peaks located at about 6.2 GHz, 11.6 GHz and 16.6 GHz in turn, meaning perfect magnetic loss ability at these frequencies, which is well agree with the result of magnetic loss tangent tan determined by ′′ / ′ , as shown in Fig. 7(b). Generally, the magnetic loss contains exchange resonance, natural resonance and eddy current loss in the investigated frequency from 2 GHz to 18 GHz.
According to previous report, if the eddy current loss is the only reason for the magnetic loss, frequency dependence of 0 = ′′ ′ 2 should be a constant. [23] In Fig. 7(c), C 0 is nearly constant with little fluctuations at the range of 5~18 GHz, suggesting that the eddy current loss dominates the magnetic loss but also the exchange resonance may exist. As for lower frequency between 2 GHz and 5 GHz, the natural resonance will always dominate.
The electromagnetic wave (EMW) dissipation is both rely on the dielectric loss and magnetic loss. For our YIG ceramics, values of tan and tan were pretty low, indicating that this material may have a good EMW absorption ability, on which future work will be focused.

Conclusion
In this study, pure phase YIG ceramics were successfully produced by tape-casting forming process and one-step solid-state reaction method. The microstructure of the YIG ceramics sintering at 1400 ºC for 10 h showed a clear grain structure with an obvious grain boundary, and no pores were observed in the SEM images. At 1450 ºC with a holding time of 8 h, the relative density of the YIG ceramic was nearly 99.8%. The activation energy was calculated to be 183.81 kJ/mol. The saturation magnetization was 28.2 emu/g at 300 K.
And, hysteresis loss at 30 K was as high as 238.8 mJ/kg, while values of that at temperatures of 230-360 K was smaller than 10 mJ/kg. The tan was nearly zero at 6~7 GHz and 11~12 GHz. The low values of tan and tan indicates that it may have a good electromagnetic wave absorption ability. This work provided a new fabrication method for much higher dense and purity fine-grained YIG ceramics. And the future work will be focused on the study of the uptown point (230 K) on the hysteresis loss chart, the dielectric properties under applied magnetic field in a wider range of frequency and the EMW absorption of the dense YIG ceramics.