The NaCa₄V₅O₁₇ ceramics owing low sintering temperature had been prepared via conventional solid state reaction method using V₂O₅ as vanadium source. The sinterability, microwave dielectric characterisations and compatibility with Ag were investigated. Pure phase NaCa₄V₅O₁₇ with triclinic structure was confirmed by Rietveld refinement and Raman spectrum. The permittivity (ε_r) and quality factor (Qxf) values mainly depended on the relative density, whereas the temperature coefficient of resonant frequency (τ_f) value was closely connected with the tetrahedral distortion of V₍₁₎O₄. The NaCa₄V₅O₁₇ ceramics sintered at 800 ^oС owned high densification and moderate microwave dielectric performances under 10.7 GHz: ε_r = 9.5, Q×f = 34,200 GHz, τ_f = -90.0 ppm/^oС, but poor chemical compatibility with Ag paste.

Keywords: Ceramics, microwave dielectric properties, NaCa₄V₅O₁₇, tetrahedral distortion

In the future 10 years, 5G wireless communications will become the dominant wireless protocol for applications like Artificial Intelligence and the Internet of Things [1]. The rapid evolution of 5G wireless communications gives rise to higher requirement of microwave dielectric ceramics [2]. To address the requirements of 5G wireless communications, the dielectric ceramics must satisfy following primary dielectric characteristics: low ε_r (ε_r ≤ 10) to increase signal transmission velocity, near-zero τ_f to ensure thermal stabilization at different operating temperatures, and high Qxf or low dielectric loss to decrease the power dissipation [3, 4]. Meanwhile, low sintering temperatures should be ensured to meet low-temperature co-sintered ceramic technology, which can be used to reduce the size of electronic devices [5, 6]. Therefore, the dielectric materials with superior performances and inherent lower sintering temperature received prodigious attention.
Recently, vanadium host compounds are followed with interest once again since their low sintering temperature and superior dielectric performances. Lots of vanadium-basic ceramics have been investigated for LTCC substrate applications, for example Ca₅Co₄(VO₄)₆, (CaBi)(MoV)O₄, LiMgVO₄, (Bi, Ce)VO₄, etc [7-11]. Quite recently, a novel NaCa₄V₅O₁₇ compounds with triclinic structure had been fabricated by Xie et al. [12]. Later, Fang et al. [13] first presented the microwave dielectric performances (e_r = 9.72, Q×f = 51,000 GHz, τ_f = -84 ppm/^oС) of NaCa₄V₅O₁₇ ceramics. Considering the microwave dielectric performances of the ceramics are strongly depended on the craft parameters, including primary materials, ball milling and sintering conditions etc. [14]. The variation in one or more these parameters greatly influence the dielectric properties of such ceramics [15]. However, there is no related microwave dielectric properties study about NaCa₄V₅O₁₇ ceramics using V₂O₅ as vanadium source. Here, we reported the effects V₂O₅, as the vanadium source, on sinterability, microstructure, microwave dielectric performances as well as compatibility with silver of NaCa₄V₅O₁₇ ceramics.

We fabricated NaCa₄V₅O₁₇ samples through the traditional solid-state route. V₂O₅ (99%), Na₂CO₃ (99.8%) and CaCO₃ (99%) were weighed according to stoichiometric NaCa₄V₅O₁₇, and then were ball milled for 8 h using alcohol and ZrO₂ balls for grinding media. The resultant milled powders were dried under 80 ^oC, followed by presintered under 600 ^oC/4 h, and then regrinded for further 8 h. The above presintered powders were mixed with 5wt % PVA, granulated and sieved through a No. 80 sieve. The granules were pressed at 200 MPa into compacts (Ф10 mm×5 mm). These compacts were heated at 500 ^oC for 2 h to exclude PVA, and then fired in air ambient under 750-850 ^oС dwelling for 4 h.
The phase constitutions in sintered specimens had been studied by X-ray powder diffraction (XRD, Smartlab, Japan) and Raman spectra (Jobin Yvon, Longjumeau, France) equipped with He-Ne laser and an output of 30 mW. Rietveld refinement of XRD patterns was carried out with the GSAS program [16]. Microstructures of NaCa₄V₅O₁₇ ceramics were observed through a scanning electron microscope (SEM, Hitachi, Tokyo, Japan). Archimedes' principle was carried out to determine the bulk densities. The ε_r and Qxf values of samples were evaluated by resonant cavity method using Rohde & Schwarz ZVB20 vector network analyzer under about 10-12 GHz. The τ_f value of samples was calculated between 25 and 85 °C according to Reference [17].

Fig. 1 exhibits the refined fit of NaCa₄V₅O₁₇ sintered under diverse temperatures to their diffraction patterns. The Rietveld refined lattice parameters, reliability factors and V₍₁₎O₄ tetrahedral distortion for all specimens are enumerated in Table 1. As shown in Fig. 1, the calculated XRD profiles based on NaCa₄V₅O₁₇ structural model closely fitted those of experimental ones, indicating the NaCa₄V₅O₁₇ ceramics crystallized in a triclinic structure with P-1(2) space group. Pure phase NaCa₄V₅O₁₇ without an obvious secondary phase was obtained for all samples sintered in the range of 750-850 ^oC. Moreover, as seen in Table 1, the achieved reliability factors (Rp, Rwp) are less than 15, suggesting the refined results are credible. Meanwhile, no obvious changes of unit cell volume were observed under different sintering temperatures.
The Raman spectrum of NaCa₄V₅O₁₇ sample fired under 800 ^oC is displayed in Fig. 2. Three distinct regions of Raman modes can be distinguished. The first region located at 900-940 cm^-1 is associated with terminal V-O (V = O) symmetric stretching vibrations [12]. The second region located at 550-880 cm^-1 is related to the V-O-V antisymmetric or symmetric stretching vibrations [18]. The third region below the 400 cm^-1 is ascribed to the peripheral modes [19].
Fig. 3 gives the typical SEM micrographs of NaCa₄V₅O₁₇ ceramics heated at distinct temperatures. From Figs. 3(a)-(c), the number of residual open pores was reduced accompanied by average grain size growth with an elevated temperature. And the 800 ^oC-sintered sample presented a relative compact and uniform microstructure with a mean grain size around 3.5 um, which is helpful to improve the dielectric properties of the NaCa₄V₅O₁₇ ceramics [20]. When the sintering temperature rose to 825 ^oC, excessive grain growth, uneven grain distribution and crack formed, these would deteriorate the dielectric performances of the present ceramics [21].
Fig. 4 presents the variations in relative density, pore volume, microwave dielectric properties and tetrahedral distortion of NaCa₄V₅O₁₇ ceramics heated at 750-850 ^oC. As shown in Fig. 4(a), the relative density was enhanced to the maximum (94.9%) under 800 ^oC and thereafter declined with an elevated temperature, which was agree with the above morphology analysis. In general, the Qxf value at microwave region of dielectric ceramics is predominated by extrinsic parameters like relative density, second phase and oxygen vacancies [22-24]. Considering the similar variation tendency between Qxf value and relative density of NaCa₄V₅O₁₇ ceramics, suggesting the Q×f value was predominated by the relative density. The ε_r is determined by density, dielectric polarizability and molecular volume [25, 26]. For a given compounds, its ε_r mainly depends on the relative density or porosity owing to the invariant dielectric polarizabilities and chemical formula [26]. Therefore, the changes of ε_r and ε_corr of NaCa₄V₅O₁₇ ceramics exhibited a tendency inverse or similar to the variation in porosity or relative density, and the corrected dielectric constant (ε_corr) was calculated according to reference [27]. It was reported that the τf value could be determined by the distortion of oxygen polyhedral and phase composition [28, 29], which was also well represented in the present work. As seen in Fig. 4(c), the change of τ_f of NaCa₄V₅O₁₇ ceramics exhibited a tendency similar to that of the variation of V₍₁₎O₄ tetrahedral distortion (δ_V(1)O4), indicating the τ_f is greatly affected by δ_V(1)O4. The δ_V(1)O4 calculated based the Rietveld refinement data and Shannon equation [30]. It is noted that the NaCa₄V₅O₁₇ ceramics in our cases possessed compatible ε_r (9.5) and τ_f (-90.0 ppm/^oC) but inferior Qxf (34,200 GHz) value than that of NaCa₄V₅O₁₇ (ε_r = 9.72, τ_f = -84 ppm/^oC, Qxf = 51,000 GHz) reported by Yin et al. [13], which may be due to the different vanadium source and processing conditions [31].
Fig. 5 depicts the XRD patterns of NaCa₄V₅O₁₇ presintered powder with 20 wt% Ag powder co-fired under 800 ^oC. As seen from Fig. 5, a new main phase AgCa₁₀(VO₄)₇ (JCPDS#54-1149) formed except for NaCa₄V₅O₁₇ phase, which was also not agree with the previous report [13]. This result showed that a chemical reaction between the basic phase NaCa₄V₅O₁₇ and Ag happened, which impeded its further application for LTCC.

The NaCa₄V₅O₁₇ ceramics owing low firing temperature had been prepared via the route of conventional solid state reaction. The impacts of V₂O₅ on the sinterability, microwave dielectric characterizations as well as chemical compatibility with Ag of present ceramics were investigated. The XRD and Raman spectrum revealed that the NaCa₄V₅O₁₇ ceramics crystallized in a triclinic structure with P-1(2) space group. The ε_r and Qxf values were dominated by the relative density, whereas the τ_f value was closely connected with the tetrahedral distortion of V₍₁₎O₄. Typically, the 800 ^oС-sintered NaCa₄V₅O₁₇ ceramics owned moderate microwave dielectric performances at measured frequency of 10.7 GHz: e_r = 9.5, Q×f = 34,200 GHz, τ_f = -90.0 ppm/^oС, whereas its poor chemical compatibility with Ag paste affected its practical viability for LTCC applications.

This study was financially supported by China Post-doctoral Science Foundation (2015M582696), Shaanxi Province Postdoctoral Science Foundation, Education Department of Shaan xi Province (18JK0711), Science and technology plan project of Xi'an Bureau of Science and technology (GXYD17.19), and Innovation Funds of Graduate Programs of Xi'an Post and Telecom-munications University (Grant Nos. CXJJLA2018008, CXJJLD2019020).