CaV2O6: A highly effective sintering aid for 0.61CaTiO3-0.39LaAlO3 ceramics


 The 0.61CaTiO3-0.39LaAlO3 (CTLA) ceramics are widely used in relay station, aerospace and radar systems for their superior properties: remarkable quality factor and excellent thermal stability. However, some deficiencies remain, such as low permittivity and excessively high sintering temperature, which limit their development in microwave communication. Herein, we introduce CaV2O6 to the CTLA ceramics to solve these problems and systematically investigate its effects on sintering temperature, phase constitution, microstructure and microwave dielectric properties. The CTLA ceramics added with 0–2.0 wt% CaV2O6 were prepared by the traditional solid-state reaction procedure. The XRD patterns indicated that the pure phase Ca0.61La0.39Al0.39Ti0.61O3 (PDF #52-1773) was obtained from all samples, which revealed that the CaV2O6 was dissolved into CTLA lattice to form a solid solution. As the CaV2O6 content increased, the strongest X-ray diffraction peaks gradually shifted toward low angles, which manifested the increase of cell volume of the solid solution. When the additive amount was 1.0 wt%, the CaV2O6 could high-effectively lower the sintering temperature from 1450 ℃ to 1290 ℃ and obviously promote the growth of grains. Meanwhile, the εr slightly increased, the Q×f significantly improved and the τf favorably decreased to closer to zero, then the prominent microwave dielectric performance was exhibited, with εr = 40.6, Q×f = 48,800 GHz (at 4.5 GHz), and τf = 0.78 ppm/℃. Such CTLA ceramics are expected to promote the development of high-performance and temperature-stable microwave components.


Introduction
With the rapid development of wireless communication technologies such as millimeter-wave communication, internet of things (IoT), smart cities, self-driving cars, the low-delay, highquality, high-integration and temperature-stable microwave components such as antenna, diplexers, filters and dielectric resonators (DRs) are increasingly the key to their applications.
Due to the advantages of miniaturization, low loss, high stability and simple fabrication, microwave dielectric ceramics (MWDCs) are widely used as the pivotal resonant materials to achieve information functions in these components, and finally determine their qualities and sizes [1][2][3]. Therefore, high-performance MWDCs materials are becoming increasingly important [4][5][6].
Considering that the radius limit percentage (ΔR) and the electronegativity criterion (ΔS) of V 5+ in CaV 2 O 6 versus Ti 4+ and Al 3+ in CTLA satisfy the conditions of ion substitution and forming a solid solution [21][22][23], hence, in this study, we chose CaV 2 O 6 as a sintering aid for CTLA ceramics and studied its effects on the sintering behavior, crystal structure and microwave dielectric performance.

Experimental procedure
The 0.61CaTiO 3 -0.39LaAlO 3 ceramics added with CaV 2 O 6 were prepared via the traditional solid-state reaction route. Firstly, the high-purity La 2 O 3 (≥99.90%), TiO 2 (≥99.90%) and CaCO 3 (≥99.90%) were stoichiometrically poured into a nylon tank filled with ethanol and zirconia balls, and ground for 6 h using a planetary ball mill. Secondly, the dried mixture was pre-fired at 1050 ℃ for 10 h. Subsequently, the calcined powders were added with 0-2.0 wt% CaV 2 O 6 , secondary ground, dried, secondary pre-fired and then added with 6.0 wt% polyvinyl alcohol (PVA). Later, the powders were pressed into cylindrical green bulks with the radius and thickness both in 7.5 mm under the pressure of 25 MPa. Ultimately, the green bulks were sintered into samples at 1170-1480 ℃ with the conventional heating rate of 5 ℃/min.
According to Archimedes method, the bulk density could be directly obtained using a densitometer (GF-300D, A&D Company Limited, Japan). The relative density was calculated as: where ρ bulk and ρ theory are the bulk density and theoretical density, respectively. The theoretical density ρ theory was calculated as: where Z, W m , N A and V cell are the number of atoms associated with each unit cell, atomic weight, Avogadro constant and unit cell volume, respectively. According to the Goldschmidt rule, the radius limit percentage (ΔR) was as follows: where R A and R B are the radii of ions A and B, respectively. Substitution between ions A and B is usually favorable when ΔR < 15% [24]. In addition, the electronegativity criterion (ΔS) proposed by Ringwood was as follows: Where S A and S B are the electronegativities of ions A and B, respectively. When ΔS < 0.1, the potential for ion substitution exists [25].
The phase constitution was investigated using X-ray diffraction (XRD) (Philips, X'pert Pro MPD, Holland). The microtopography was obtained by scanning electron microscopy (SEM) (Zeiss, MERLIN Compact, Germany). The microwave dielectric performance were studied by the Hakki-Coleman method using a network analyzer (Agilent Technologies, E5071C, USA). The temperature coefficient of the resonance frequency (τ f ) was calculated as: where f A and f B are the respective resonant frequencies at temperatures 25 °C and 85 °C.  was obtained from all samples at all additive amounts and sintering temperatures, and there was no any secondary phase could be observed. This indicates the formation of a pure-phase solid solution. Table 1 lists the electronegativities and radii of the ions in CTLA and CaV 2 O 6 , as well as their electronegativity criterions (ΔS) and radius limit percentages (ΔR) based on V 5+ [26,27]. As shown in the table, the radius limit percentage (ΔR < 15%) and the electronegativity criterion (ΔS < 0.1) of V 5+ in CaV 2 O 6 versus Ti 4+ and Al 3+ in CTLA satisfied the conditions of ion substitution. And this led to the forming of the solid solution [24,25]. It is worth noting that, as shown in the inset graph in Fig. 1, the strongest X-ray diffraction peaks gradually shifted toward the lower angles as the CaV 2 O 6 content increased, which demonstrates the increase in cell volume [19]. As shown in the table 1, the ΔS of Al 3+ and Ti 4+ were approximately equal (1.5), but Al 3+ had a much smaller ΔR (1.9) than Ti 4+ (13.0), then the bigger V 5+ preferentially and mainly replaced the smaller Al 3+ . And this led to the increase of cell volume of the solid solution.     Table 2 and their variation trends are depicted in Fig. 4. It can be seen that the cell volume increased stepwise with the augment of CaV 2 O 6 , which conforms to the shifts in the strongest X-ray diffraction peaks.   5 (a)). At the CaV 2 O 6 content of 0.2 wt%, the grains grew bigger and fuller with sizes of 3.0-5.0 μm, and the grain boundaries became distinct (Fig. 5 (b)). With 0.5 wt% CaV 2 O 6 added, continuously-grown grains began to flatten with sizes of about 4.0-6.0 μm, the compactness increased, and the number of pores decreased (Fig. 5 (c)). When the CaV 2 O 6 content increased to 1.0 wt%, the grains further grew bigger and exhibited more flat morphology with sizes of 5.0-7.0 μm, and the grain boundaries became significantly clearer. An excellent microstructure with relatively high density and low porosity was achieved (Fig. 5 (d)). However, as the CaV 2 O 6 content continued to increase, grain growth stopped and some of the grain boundaries became fuzzy, which indicates a slight deterioration in morphology (Fig. 5 (e)).  temperatures increase continuously and slightly. The dielectric constant exhibited the same trend as relative density. This phenomenon is attributed to the permittivity of pure-phase ceramics being positively correlated with relative density at extrinsic level [28]. In addition, at intrinsic level, molecular polarizability is also the primary factor affecting permittivity [29].

Results and discussion
Molecular polarizability of the CTLA solid solutions can be calculated by Shannon's additive rule [30][31][32]: Where the x is molar amount of CaV 2 O 6 . And according to the ion polarizabilities listed in   relative density, which is ascribed to the fact that relative density is the pivotal extrinsic factor affecting Q×f [28]. At intrinsic level, there is a relationship between packing fraction and dielectric loss. The packing fraction was calculated as [33]: Where ∑V atoms is the sum of the volume of each atom, and V molar−volume is the molar volume.   showed a trend of first decreasing and then increasing. At intrinsic level, the bond energy represents the energy required to break chemical bonds, which greatly determines the thermal stability and affects the τ f value [34][35][36]. The bond energy is composed of complete ionic energy (E i u ) and non-polar covalent energy (E c u ), and is calculated as: Where d is the bond length, r A and r B are the covalent radii of ions A and B, and E A and E B are their homonuclear bond energies. The bond length and covalent radii in the CTLA lattice were obtained by refinement in GSAS software [37], and the homonuclear bond energies were taken from the literature [34].

Supplementary Files
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