A novel microwave dielectric ceramic Li2NiZrO4 with rock salt structure

Low loss Li2NiZrO4 ceramics with rock salt structure were successfully prepared by the solid-phase reaction method. The relationship between sintering temperature, phase composition and dielectric properties of Li2NiZrO4 ceramics was reported for the first time. The grain size gradually increased and the porosity decreased with the sintering temperature increasing. When the sintering temperature exceeds 1300 °C, the grains grow abnormally and some grains begin to melt. The XRD patterns indicated the second phase ZrO2 appeared due to the volatilization of lithium. The grains grow abnormally and a second phase of ZrO2 increased the loss of Li2NiZrO4 ceramics. The samples sintered at 1300 °C possessed the best dielectric properties: εr = 12.3, Qf = 20000 GHz, τf = −23.4 ppm °C−1, which would make the ceramic a possible candidate for millimeter-wave applications.


Introduction
Due to the rapid development of wireless communication systems, dielectric ceramics have been widely investigated as resonators, dielectric substrates and dielectric waveguide circuits in contemporary communications. It performs a significant function in millimeter wave communication as the substrate materials of microwave integrated circuits. 1 To satisfy the demand of the fast-paced expansion of communications, these materials need to have a suitable dielectric constant to meet miniaturization of the device. In order to reduce the loss of the device at high frequencies, the quality factor is also required to be as large as possible. 2 At the same time, in order to ensure temperature stability, a temperature coefficient of the resonance frequency close to zero is also required. 3 In recent years, it has been extensively indicated that the mixed Li 2 O-AO-BO 2 (A ¼ Mg, Zn and Ni; B ¼ Ti, Zr and Sn) system is quite appropriate for microwave communication. Out of these microwave dielectric ceramics, The Li-containing Li 2 -MgTiO 4 ceramic is adopted as a perfect microwave dielectric material, which is an appropriate candidate for the component miniaturization and integration. 4-6 Li 2 MgTiO 4 with the microwave dielectric characteristics of 3 r ¼ 15.07, Q f ¼ 97629 GHz (at 8.2 GHz) and s f ¼ 3.81 ppm C À1 was reported by Pan et al. 7 Additionally, Zhang et al. investigated the phase composition of (1 À x)Li 2 TiO 3 -xNiO (0 # x # 0.5) ceramics and obtained excellent microwave dielectric characteristics: The Li 2 ZrO 3 -AO ceramic system was investigated. Ma et al. indicated the impact of ZnO addition on the microwave dielectric characteristics of Li 2 ZrO 3 ceramics and obtained microwave dielectric characteristics of 0.7Li 2 ZrO 3 -0.3ZnO ceramics: 3 r ¼ 14.8, Q f ¼ 26800 GHz and s f ¼ 1 ppm C À1 . 9 Bi et al. reported the microwave dielectric characteristics of Li 2 MgZrO 4 ceramics: 3 r ¼ 12.30, Q f ¼ 40900 GHz, besides s f ¼ À12.31 ppm C À1 when it was sintered at 1175 C for 4 h. 10 Cheruku et al. synthesized Li 2 NiZrO 4 materials with LiNO 3 , Ni(NO 3 ) 2 $6H 2 O, ZrN 2 O 7 and C 6 H 6 O 7 by solution combustion technique in phase pure nanocrystalline form for the rst time. They found the electrical relaxation is essentially non-Debye and temperature independent. This material exhibits considerable conductivity at room temperature and is a possible candidate for electrode material in solid-state batteries. 11,12 Nevertheless, there have no report about the microwave dielectric characteristics of Li 2 NiZrO 4 materials. In the present work, the sintering temperature, density as well as microwave dielectric properties of Li 2 NiZrO 4 ceramics were investigated. Besides, the relationship existing between phase composition, sintering temperature, microstructure and microwave dielectric characteristics of Li 2 NiZrO 4 ceramic was also investigated.

Experiment
The starting materials Li 2 CO 3 (99.99%, Aladdin), ZrO 2 (99.9%, Aladdin) and NiO (99.99%, Aladdin) were used to fabricate Li 2 NiZrO 4 ceramic by solid state reaction methodology. First, the prepared powder is put into a nylon can and ball milled for 5 h with alcohol as the medium. Thereaer, the powder was dried and pre-sintered at 1050 C for 4 hours. The calcined powder milling was carried out again for 7 h and followed by drying. In addition, the dried powder was mixed with polyvinyl alcohol for the purpose of forming a pellet. The pellets were pressed into a cylinder shape (diameter: 12 mm, thickness: 6 mm) under the pressure of 10 MPa. Eventually, the specimens were muffled with the same material powder and sintered at 1250-1350 C for 5 h.
The measurement of the density of the dielectric ceramic was carried out with the help of the Archimedes method. In addition, the testing for the phase formation was carried out by X-ray diffractometer (XRD) (Tongda TDM-20) with Cu-Ka radiation. The microstructure of the specimen was observed by scanning electron microscope (SEM) (AURA-100, Seron, South Korea). The measurement of the dielectric constant (3 r ) as well as quality factor (Q f ) was carried out by network analyzer (E5061B, KEYSIGHT) on the basis of the Hakki-Coleman dielectric resonator methodology. [13][14][15] The calculation of the temperature coefficient of resonant frequency (s f ) is performed in accordance with the formula: where f t 1 and f t 2 are the resonant frequencies at the temperature t 1 ¼ 25 C and t 2 ¼ 85 C, correspondingly.

Results and discussion
The XRD patterns of Li 2 NiZrO 4 ceramics sintered between 1250-1350 C are presented in Fig. 1. As seen, pure phase Li 2 -NiZrO 4 (PDF#40-0363) ceramics with rock salt structure was formed. When the sintering temperature reached 1325-1350 C, the presence of the ZrO 2 (PDF#86-1449) secondary phase is observed. 16 Y. Iida et al. indicated the fact that the volatilization of lithium is obvious upon sintering at 1000 C. 17 This situation was also observed in LiMO 3 (M ¼ Nb, Ta) crystals. [18][19][20] Therefore, it is taken into account that the volatilization of lithium gives rise to the formation of ZrO 2 with the sintering temperature exceeding 1300 C. 10 The SEM micrographs of Li 2 NiZrO 4 sample sintered at different temperatures for 5 h are shown in Fig. 2. It is seen that the grain size of Li 2 NiZrO 4 ceramics gradually increases as the sintering temperature is higher. A number of intergranular pores can be observed in the Fig. 2(a) and (b).With the sintering temperature growth, the grain size increased substantially and few pores are observed in Fig. 2(c). These pores are caused by lithium volatilizing. As the sintering temperature ranged from 1325 to 1350 C, the grains showed abnormal growth. Moreover, some grains start melting, and a small amount of pores is still existed owing to the lithium volatilization. 10,21,22 Fig . 3 presents the variation of the apparent density as well as relative density of Li 2 NiZrO 4 ceramics. With the sintering temperature increase from 1250 to 1325 C, the apparent density increased from 4.32 to 4.57 g cm À3 . When the sintering temperature was 1350 C, the apparent density was 4.52 g cm À3 . The theoretical density of Li 2 NiZrO 4 crystal is 4.9 g cm À3 . As the sintering temperature amounted to 1300 C, the relative density was 92.8%. As the sintering temperature becomes higher, there is a gradual growth of the grain size, together with the pore declining, which is in appropriate relation to the variation in density.
The variation in dielectric constant of Li 2 NiZrO 4 ceramics as a function of sintering temperature is given in Fig. 4. With the sintering temperature increasing from 1250 to 1300 C, the 3 r value continuously increased. The variation in 3 r value was  consistent with that in the apparent density. Ordinarily, many factors affect the dielectric constant, such as ionic polarizability, density and second phase. 23 In the present work, the dielectric constant increases with increasing sintering temperature, which is related to relative density, polarizability and the second phase. The relationship between relative permittivity, polarizability and relative density can be described by Clausius-Mosotti equation (see eqn (1)). 24 where a, 3 0 , and 3 r are the molecular polarizability, permittivity of the vacuum and dielectric ceramics, respectively. The theoretical dielectric polarizability (a theo ) of Li 2 NiZrO 4 ceramics can be obtained according to the Shannon's additive rule (see eqn (2)). 25 Where a (Li + ) ¼ 1.20Å 3 , a (Ni 2+ ) ¼ 1.23Å 3 , a (Zr 4+ ) ¼ 3.25Å 3 and a (O 2À ) ¼ 2.01Å 3 are the ionic polarizabilities. And as the porosity fraction increases, the dielectric constant will decrease, seen from eqn (1). In order to eliminate the inuence of porosity on dielectric, dielectric is corrected by eqn (3). 26,27 3 mea À 3 rc 1 À 3pð3 rc À 1Þ 23 rc þ 1 where p is porosity the fraction, 3 mea is measured permittivity and 3 rc is the porosity-corrected permittivity. The observed dielectric polarizability (a obs ) is calculated using eqn (2) (see eqn (4)). 28 where b and V m are the constant (4p/3) and the molar volume, respectively. According to eqn (1), The theoretical dielectric polarizability (a theo ) of Li 2 NiZrO 4 ceramics is 14.92. The observed dielectric polarizability (a obs ) is 14.88 by eqn (2) and (3) (sintered at 1300 C). Obviously, there is a very small difference of (D ¼ |(a theo À a rc )/a rc Â 100%|) #1%, indicating that the values of a theo and a obs are basically the same. As the sintering temperature continues to increase, the relative density decreases and the dielectric constant continues to increase. This phenomenon indicates that the dielectric constant is also affected by the second phase. When the sintering temperature reaching 1350 C, the ZrO 2 second phase is formed and possess a higher dielectric constant value of 23. 29 So the relative dielectric constant continues to increase while the relative density decreases. 30,31 In Fig. 5, the variations of the quality factor and temperature coefficient of the resonant frequency are presented. There is an increase in the Q f value from 16 000 to 20 000 GHz with the sintering temperature increasing from 1250 to 1300 C. The Q f value reached the topmost value of 20 000 at 1300 C. Nonetheless, the Q f value sharply declined with the sintering temperature ranging from 1325 to 1350 C and the Q f value is   merely 10 200 GHz at 1350 C. In general, dielectric loss can be divided into two categories. One component is a result of the bulk crystal phase, termed as intrinsic dielectric loss while the other component is termed as extrinsic dielectric loss, such as the grain boundaries, aws and so on. With the sintering temperature increasing from 1250 to 1300 C, there is an increase in the grain size as well as the apparent density, which shares a similarity with the variation of Q f value. When the sintering temperature kept rising, the abnormal grain growth and second phase precipitation led to the decline of Q f value. 32 The trend of s f with the sintering temperature is also shown in Fig. 5. With the sintering temperature ranging from 1250 and 1350 C, the s f value uctuates between À22.29 and À25.92 ppm C À1 .

Conclusion
The Li 2 NiZrO 4 ceramic was successfully prepared by solid-state reaction method. The phase composition, microstructure and microwave dielectric properties of Li 2 NiZrO 4 ceramics were studied. With the sintering temperature exceeding 1300 C, the ZrO 2 second phase was formed owing to the lithium volatilization. The 3 r is dependent on second phases and density. The 3 r gradually increases as the density increases with the sintering temperature ranging 1250-1325 C. When the sintering temperature reaching 1350 C, the ZrO 2 second phase possess a signicant effect on the 3 r . The Q f value was primarily constrained by the microstructure and second phase. The samples sintered at 1300 C showed the optimal dielectric characteristics: 3 r ¼ 12.3, Q f ¼ 20 000 GHz, s f ¼ À23.38 ppm C À1 that made the ceramic promising for millimeter-wave applications.

Conflicts of interest
There are no conicts to declare.