Novel ultra-low temperature co-fired microwave dielectric ceramic at 400 degrees and its chemical compatibility with base metal

A novel NaAgMoO4 material with spinel-like structure was synthesized by using the solid state reaction method and the ceramic sample was well densified at an extreme low sintering temperature about 400°C. Rietveld refinement of the crystal structure was performed using FULLPROF program and the cell parameters are a = b = c = 9.22039 Å with a space group F D −3 M (227). High performance microwave dielectric properties, with a permittivity ~7.9, a Qf value ~33,000 GHz and a temperature coefficient of resonant frequency ~−120 ppm/°C, were obtained. From X-ray diffraction (XRD) and Energy Dispersive Spectrometer (EDS) analysis of the co-fired sample, it was found that the NaAgMoO4 ceramic is chemically compatible with both silver and aluminum at the sintering temperature and this makes it a promising candidate for the ultra-low temperature co-fired ceramics technology. Analysis of infrared and THz spectra indicated that dielectric polarizability at microwave region of the NaAgMoO4 ceramic was equally contributed by ionic displasive and electronic polarizations. Its small microwave dielectric permittivity can also be explained well by the Shannon's additive rule.

behavior, microwave dielectric properties and chemical compatibility with both silver and aluminum were studied in detail. Figure 1 (a) shows XRD patterns of the NaAgMoO 4 samples calcined at 350uC/4 h, sintered at 400uC/4 h, co-fired with 30 wt. % silver at 400uC/4 h and 30 wt. % aluminum at 450uC/4 h. It is seen that single phase with spinel-like structure was formed after calcinations at 350uC. However, a weak trace of silver is observed at 38.1u and it may be caused by decomposition of the Ag 2 CO 3 during calcinations process. As seen from XRD patterns of the co-fired samples, only peaks of the metal and NaAgMoO 4 phases were revealed, which means that both silver and aluminum are chemically compatible with the NaAgMoO 4 ceramic at the sintering temperature. To study crystal structure details of the NaAgMoO 4 ceramic, refinements were carried out using Fullprof software based on the fine XRD data. Both the situations of single and composite phases (spinel and silver) were considered. The observed and calculated XRD patterns are shown in Fig. 1 (b). The refined values of lattice parameters are a 5 b 5 c 5 9.22039 Å , with a space group F D 23 M (227), according to the data (ICSD #159740) reported by Bouhemadou et al. 9 The R p 5 12.1, R wp 5 14.8 and R exp 5 6.82 were obtained for single phase case as shown in Table I, while the R p 5 11.2, R wp 5 13.6 and R exp 5 6.79 were obtained for the composite phase and the ratio of silver phase is around 1.99%. In fact, due to the limited sensitivity of the XRD technique, the details of the remnant silver needs to be studied further. Schematic crystal structure of the NaAgMoO 4 was presented in insert of Fig. 1 (b). Na and Ag cations arrange in the 8coordinated site with a disordered distribution, while Mo cations occupy the 4-coordianted site.
Apparent densities of the NaAgMoO 4 ceramics are shown in Fig. 2  (a). It is seen that as sintering temperature was increased from 350uC to 380uC, bulk density of the NaAgMoO 4 ceramic increased from 4.45 to 4.77 g/cm 3 and reached saturated at about 400uC with a relative density about 97% (the theoretical density calculated from XRD patterns was 4.928 g/cm 3 ). SEM images of the NaAgMoO 4 ceramic sintered at 400uC/2 h and BSE images of co-fired ceramics with 30 wt. % Ag are presented in the insert of Fig. 2. It is seen that dense and homogeneous microstructure can be observed from the as-fired surface while a few of pores can be observed from the fractured surface. The grain size scattered between 2 , 5 mm and both the as-fired and fractured surfaces correspond well with each other. The co-fired ceramics were found to be composed of both NaAgMoO 4 and metal grains coupled with EDS analysis, which support the XRD results discussed above and further confirms that there were no intermediate phases and the desired chemical compatibility between NaAgMoO 4 and silver powders was obtained.
Microwave dielectric permittivity and Qf value of the NaAgMoO 4 ceramic as a function of sintering temperature are shown in Fig. 3 (a). As sintering temperature was increased from 350uC to 380uC, the dielectric permittivity increased from 7.0 to around 7.9 and kept stable. The Qf value first increased with sintering temperature, reached a maximum value at 400uC and then decreased slightly with further increase of sintering temperature. To further understand the temperature dependence of microwave dielectric properties of the NaAgMoO 4 ceramic, the resonant frequency, the microwave dielectric permittivity and Qf value measured in the temperature range 20 , 125uC are demonstrated in Fig. S1 in the Supplementary Information. It can be seen that microwave dielectric permittivity of the NaAgMoO 4 ceramic linearly increased with temperature slightly without any abnormity and temperature coefficient of resonant frequency is about 2120 ppm/uC. The Qf value decreased slightly from 33,000 GHz at 20uC to 26,000 GHz at 125uC. In conclusion, the best microwave dielectric properties were obtained in ceramic sintered at 400uC with a permittivity ,7.9, a Qf value ,33,000 GHz and a temperature coefficient of resonant frequency ,2120 ppm/uC.
To further study the intrinsic microwave dielectric properties, infrared reflectivity spectra of the NaAgMoO 4 ceramics were analyzed by using a classical harmonic oscillator model as follows: where e*(v) is complex dielectric function, e ' is the dielectric constant caused by the electronic polarization at high frequencies, c j , v oj ,  and v pj are the damping factor, the transverse frequency, and plasma frequency of the j-th Lorentz oscillator, respectively, and n is the number of transverse phonon modes. The complex reflectivity R(v) can be written as: Fitted infrared reflectivity values, complex permittivities and phonon parameters are shown in Fig. 3 (b) and Table II. It is seen that the calculated dielectric permittivity and dielectric loss values are almost equal to the measured ones using TE 01d method, which implies that majority of the dielectric contribution for this system at microwave region was attributed to the absorptions of structural phonon oscillation in infrared region and very little contribution was from defect phonon scattering. The optical dielectric constant calculated from the infrared spectra is about 3.07, which is almost 39% percent of the polarizability contribution at microwave region, and this implies that the contribution from the electronic polarizability can not be ignored in the low k (,10) microwave dielectric materials. The dielectric polarizability contribution of the strongest mode at 809.2 cm 21 is only 0.622, about 8% percentage, and this is due to its much higher frequency than the microwave region. The calculated dielectric loss is almost the same with the measured value and this means that there is no much space for the increase of Qf value by improving sintering process. The small microwave dielectric permittivity of NaAgMoO 4 ceramic can also be explained by the Shannon's additive rule. At microwave region, the polarizability is the sum of both ionic and electronic components. Shannon 10 suggested that molecular polarizabilities of complex substances could be estimated by summing the polarizabilities of constituent ions. Then the polarizabilities a could be obtained as follows: where the ionic polarization of Ag 1 was set to be 2.25 Å 3 as suggested in our previous work 11 . The ionic polarization of Na 1 , Mo 61 and O 22 were set to be 1.80 Å 3 , 3.28 Å 3 and 2.01 Å 3 , respectively 10,12 . Considering the Clausius-Mosotti relation as follow: where the V is the cell volume, 783.88/8 Å 3 . The calculated dielectric permittivity is about 6.75, which is a little smaller than the measured value 7.9 and the extrapolated value 7.85. A 14.5 percent deviation of permittivity from the measured value is considered acceptable con-sidering the simplicity of additive rule and the uncertainty of ionic polarization of Ag 1 .

Summary
In conclusion, a novel NaAgMoO 4 ceramic with high microwave dielectric performance, with a permittivity ,7.9, a Qf value ,33,000 GHz and a temperature coefficient of resonant frequency ,2120 ppm/uC, can be well densified at 400uC with grain size lying between 2 , 5 mm. From XRD and EDS analysis of the co-fired sample, it was found that the NaAgMoO 4 ceramic is chemically compatible with both silver and aluminum at the sintering temperature and this makes it a candidate for the ultra-low temperature cofired ceramics technology. Specifically, its densification temperature is almost half of that of the most popular low-fired Al 2 O 3 material with glass addition and it might be promising in the dielectric substrate application.

Methods
Reagent-grade Na 2 CO 3 , Ag 2 CO 3 , and MoO 3 (.99%, Fuchen Chemical Reagents, Tianjin, China) were weighted according to the stoichiometric formulation NaAgMoO 4 . Powders were mixed and milled for 4 h by using a planetary mill. The powder mixture was then dried and calcined at 350uC for 4 h. The calcined powders were ball milled for 5 h to obtain fine powders. Then the powders were pressed into cylinders (10 mm in diameter and 4 , 5 mm in height) at 100 MPa. Samples were sintered at temperatures from 350uC to 420uC for 2 h. Room temperature X-ray diffraction (XRD) was performed by using a XRD with Cu Ka radiation (Rigaku D/ MAX-2400 X-ray diffractometry, Tokyo, Japan). Diffraction pattern was obtained between 2h of 10-80u at a step size of 0.02u. The results were analyzed by the Rietveld profile refinement method, using FULLPROF program. As-fired and fractured surfaces were observed by using a scanning electron microscopy (SEM, FEI, Quanta 250 F   (ADVAVTEST TAS7500SP, Japan). A passive mode-lock fiber laser is used to pump and gate respectively two GaAs photoconductive antennas for the generation and detection of THz wave. Dielectric properties at microwave frequency were measured with the TE 01d dielectric resonator method with a network analyzer (HP 8720 Network Analyzer, Hewlett-Packard) and a temperature chamber (Delta 9023, Delta Design, Poway, CA). The temperature coefficient of resonant frequency TCF (t f ) was calculated with the following formula: where f T and f T0 are the TE 01d resonant frequencies at temperature T and T 0 , respectively.