ZrO2-doped Zn0.5Ti0.5TaO4 microwave dielectric ceramics with low dielectric loss and excellent temperature stability

Herein, the weight fraction of the tri-rutile and ixiolite structures in Zn0.5Ti0.5-xZr x TaO4 (ZTZTO x , 0 ≤ x ≤ 0.10) was successfully adjusted by the substitution of Ti4+ ions by Zr4+.Zn0.5Ti>0.48Zr0.02TaO4 ceramics exhibited excellent dielectric properties when sintered at 1290°C with τ f = -3.73 ppm/°C, εr = 33.22, and Q×f=79,266 GHz, respectively. As the amount of Zr4+- substitution increases, the tri-rutile structure content gradually decreases, and the tri-rutile structure is completely transformed into an ixiolite structure at 0.06 ≤x. This leads to the grain size showing a trend of decreasing and then increasing. Non-intrinsic factors suggest that ixiolite structures with lower εr and higher Q×f values are the main cause of the change in dielectric properties. Intrinsic factors suggest that an increase in the number of Raman characteristic waves leads to a gradual decrease in εr . The damping loss of the lattice vibrational behavior decreases, leading to a gradual increase in Q×f. In conclusion, Zn0.5Ti0.48Zr0.02TaO4 ceramics with aτf close to zero have good prospects for application in microwave communications.


Introduction
Microwave dielectric ceramics (MWDCs) are used as substrates for various electronic devices (e.g.antennas, duplexers, filters, resonators, etc.) and are increasingly in demand due to the rapid upgrading of microwave communication technology [1] .Low-loss ceramics with a suitable relative permittivity (εr) and a temperature coefficient of resonance frequency (τf) close to zero are generally preferred.MWDCs with a large εr can effectively reduce the size of the device, but they usually have a large dielectric loss (or a low-quality factor Q × f).MWDCs with a small εr have a smaller dielectric loss, but they increase the size of the device and usually have a poor τf.In addition to having a low dielectric loss, MWDCs with medium εr also have an excellent temperature stability, which has attracted extensive attention [2] .
Among the family of ceramics with intermediate dielectric constants, MWDCs with the chemical formula A0.5B0.5CO4 have gained a great deal of attention for their outstanding dielectric properties.Of these, Park et al. prepared Zn0.5Ti0.5TaO4ceramics with a tri-rutile structure displaying excellent dielectric properties (εr = 47.1, Q × f = 35,800 GHz, τf = 84 ppm/°C) using a solid-phase reaction method [3] .Wu et al. prepared nanocrystalline powders by sol-gel method and synthesized Zn0.5Ti0.5TaO4ceramics with ixiolite as the main crystalline phase after sintering, which exhibited better dielectric properties with εr = 35.7,Q × f = 57,550 GHz, τf = -24.7 ppm/°C [4] .It can be found that Zn0.5Ti0.5TaO4ceramics with a tri-rutile crystal structure have positive τf values, while Zn0.5Ti0.5TaO4ceramics with an ixiolite crystal structure have negative τf values.Therefore, the question of whether τf can be adjusted close to 0 by adjusting the two-phase occupation ratio is an urgent one.Furthermore, there are no reports so far on tuning the τf of Zn0.5Ti0.5TaO4close to 0 ppm/°C, nor on connecting its lattice vibrations to its dielectric properties.The advancement of the fundamental theory and engineering applications of Zn0.5Ti0.5TaO4ceramics have been hampered by these unfinished works.
Herein, the use of Zr 4+ ions substituted by Ti 4+ ions of Zn0.5Ti0.5TaO4enabled the two-phase occupation regulation and the successful adjustment of the τf close to 0 ppm/°C.The structure-property relationship is established in terms of intrinsic and non-intrinsic factors by X-ray diffraction, lattice vibrational mode analysis, and micro-morphological evolution, respectively.

2.
Experimental Zn0.5Ti0.5-xZrxTaO4(ZTZTOx, x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10) ceramics were prepared using a conventional solid phase reaction method.In accordance with stoichiometric ratios, ZnO (99%, Aladdin), TiO2 (99.8%,Aladdin), ZrO2 (99.99%,Aladdin), and Ta2O5 (99.5%, Rhawn) were weighed.The mixed powder was then ball milled for 10 h with zirconium dioxide balls and deionized water as the ball milling medium.The ball-milled slurry was dried, ground, and then calcined at 1200 °C for 4 h.The calcined powder was again ball-milled for 10 h.The resulting powder was granulated with 10 wt.% polyvinyl alcohol aqueous solution (PVA).The granulated particles were uniaxially pressed into cylindrical billets with a diameter of 12 mm and a thickness of 5-6 mm, and finally sintered in a muffle furnace at 1250-1330 °C for 6 h at a temperature change rate of 2 °C/min.The samples sintered at 1290 °C were annealed at 1270 °C for 1 h and then subjected to microscopic morphological analysis.
The Archimedes drainage method was used to determine the bulk density of the sinter-produced ceramics.X-ray diffraction (XRD, D8 Advance, Bruker, Germany) was used to analyze the crystal structure of the ceramics using Cu Kα radiation from 10°to 90°in steps of 0.01°, and the XRD data were refined using jade software.The lattice vibrations were analyzed by Raman spectroscopy (LabRAM HR evolution, Horiba Scientific, Japan) with a He-Ne laser (λ = 532 nm) as the excitation source.Field emission scanning electron microscopy (FESEM, Phenom, Pharos, Netherlands) was used to study the elemental distribution and microscopic morphology of the annealed ceramics.Using a vector network analyzer (Agilent N5230A, Agilent Technologies, USA), the Hakki-Coleman dielectric resonator method in TE011 mode was used to evaluate the microwave dielectric characteristics.τf was determined as: = (−)/[(−)×]×10 6 ppm/°C (1) where is the starting temperature (25 °C) and is the end temperature (85 °C). is the resonant frequency at and is the resonant frequency at .

3.
Results and Discussion Fig. 1 demonstrates the bulk densities of the different components of ZTZTOx ceramics at different sintering temperatures.ZTZTOx (x = 0.00, 0.02, 0.04) ceramics were sintered at 1290 °C to obtain maximum densities of 6.891 g/cm 3 , 6.962 g/cm 3, and 6.994 g/cm 3 , respectively.The trace amount of Zr 4+ -substitution did not lead to a change in the optimum sintering temperature.The maximum bulk densities of 7.005 g/cm 3 and 7.018 g/cm 3 were only obtained at 1310 °C for ZTZTO0.06 and ZTZTO0.08,due to the high sintering temperature of ZrO2.Finally, the bulk density of ZTZTO0.10 continues to increase and is 7.024 g/cm 3 at 1330 °C.The XRD patterns of the ZTZTOx ceramics are shown in Fig. 2(a).At x = 0.00-0.04, the ZTZTOx ceramics consist of tri-rutile and ixiolite crystal structures in a coexisting form.And as x increases from 0.06 to 0.10, the ZTZTOx ceramics are all ixiolite crystal structures.The increase in the amount of Zr 4+substitution causes the tri-rutile structure to shift towards the ixiolite structure.Fig. 2(b) illustrates enlarged patterns with 2θ lying between 26.5°and 30.8°, with the diffraction peak (111) of the ixiolite structure shifted towards a lower 2θ value due to the Zr ion (0.72 Å) having a larger radius than the Ti ion (0.605 Å), which increases in cell volume.In contrast, the diffraction peak (110) of the tri-rutile structure gradually decreases and disappears at x = 0.06.The XRD data were refined and analyzed by Jade software and the refined cell parameters, the weight fraction (wt.%) of the two phases, and the theoretical densities are listed in Table 1.The mass share of the tri-rutile structure decreases from 71.3% at x = 0.00 to 24.2% at x = 0.04 and finally disappears completely.In addition, the gradual increase in the theoretical density is due to the fact that the mass of the Zr atoms is greater than the mass of the Ti atoms.Fig. 2(c) plots the crystal structure schematically of ixiolite and tri-rutile.Raman spectra can reveal changes in the vibrational behavior of the lattice, thus reflecting structural transitions, and can also establish a relationship between structure and properties.Fig. 3(a) shows the Raman spectra of ZTZTOx ceramics.ZTZTO0.00ceramics detected a total of 9 Raman vibration modes, while ZTZTOx (x = 0.02, 0.04) determined 10 Raman vibration modes.When x > 0.04, a total of 11 vibration modes were observed.The Raman characteristic peaks of the tri-rutile structure (located around 236 cm -1 and 445 cm -1 ) fade until they disappear, confirming that a phase transition from the tri-rutile to the ixiolite structure has occurred.After a Gauss-Lorentz model was used to fit the Raman spectra, the fitted Raman spectra for x = 0.00, 0.04, and 0.06 are shown in Fig. 3(b)-(d).For the tri-rutile structure, the A1g and B2g vibrational modes result from the symmetric vibrations of the cation, the stretching motion of the oxygen ion and the stretching vibration of the oxygen octahedron.The vibration of the cation and the rotation of the oxygen octahedron lead to the Eg vibrational mode.For the ixiolite structure, the vibrations of the cation give rise to two vibrational modes with wavenumbers less than 200 cm -1 .The vibrational mode with a wave number between 200 cm -1 and 450 cm -1 is produced by the vibration of O-cation-O while the vibrational modes with wave numbers greater than 450 cm -1 result from the stretching vibrations of the cation-O bond [5,6] .The effect of lattice vibrations on the dielectric properties will be analysed later for intrinsic factors.
The microscopic morphology of the polished and thermally etched ZTZTOx ceramics is shown in Fig. 4, and Fig. 4(a)-(f) correspond to an increase in x from 0.00 to 0.10, respectively.Only a few grain boundary pores are visible in all ceramics, which all have good densities.The grains mostly show irregular polygonal shapes, while the grain sizes show significant variations.Fig. 5 depicts the statistical histogram of the particle size distribution.As x increases from 0.00 to 0.10, the average grain sizes are 14.6 μm, 14.5 μm, 9.8 μm, 8.3 μm, 8.4 μm and 15.0 μm respectively.As the amount of Zr 4+substitution increases, the tri-rutile structure gradually changes to an ixiolite structure, resulting in a gradual decrease in grain size.Interestingly, when only the ixiolite structure is present, the grain size gradually increases with increasing Zr 4+ -substitution.Fig. 6 shows the elemental distribution of ZTZTO0.02,which shows that Zr 4+ is uniformly distributed in the tri-rutile and ixiolite structures.The above results indicate that the Zr 4+ -substitution successfully achieved the transition from the tri-rutile structure to the ixiolite structure.The factors influencing microwave dielectric properties can be divided into intrinsic and nonintrinsic factors.Usually, non-intrinsic factors are the effects of the composition of the raw material, the preparation method, grain size, relative density, porosity and phase composition.In this work, the nonintrinsic factors include mainly variations in phase composition, grain size and porosity.Fig. 7(a)-(b) display the εr and Q×f values for different components.Overall, the εr decreases as the amount of Zr 4+ substitution increases, while Q×f shows a gradual increase.In addition, the εr and Q×f values of ZTZTOx (x=0.00-0.06)both obtain a maximum value at 1290 °C as the sintering temperature increases, while the εr and Q×f of ZTZTO0.08 and ZTZTO0.10show a gradual increase with increasing sintering temperature.Among them, the εr and Q×f values of ZTZTO0.00,ZTZTO0.02,ZTZTO0.04, and ZTZTO0.06 after sintering at 1290 °C were 35.62, 33.22, 30.81, 26.67 and 55,575 GHz, 79,266 GHz, 84,119 GHz, 82,890 GHz, respectively.This is due to the change of crystal structure from tri-rutile to ixiolite structure due to the occurrence of Zr 4+ -substitution, ixiolite structure has relatively low εr and high Q×f values.The εr and Q×f values for ZTZTO0.08 and ZTZTO0.10 at 1290 °C were 28.44, 27.65, and 86,104 GHz, 84,892 GHz respectively.This is due to the high sintering temperature of ZrO2, which does not reach the optimum sintering temperature at 1290 °C, resulting in no significant increase in Q×f values.
The presence of pores leads to the accumulation of carriers, which in turn leads to an increase in conductivity and an increase in losses.The relative densities of the ceramics at different sintering temperatures are shown in Fig. 7(c).x = 0.00-0.06, the best densities were obtained at 1290°C with relative densities of 96.70%, 97.07%, 96.98%, and 96.49%, respectively.ZTZTO0.08 and ZTZTO0.10 have relative densities of 96.66% and 96.24% at 1290 °C.The relative densities of all ceramics were above 95% so densification was not the main cause of variation in dielectric properties in this work.In addition, samples with smaller grain sizes are less ordered, which leads to an increase in dielectric losses.Zr 4+ -substitution leads to a trend of decreasing and then increasing grain sizes, but the Q×f values do not show a trend of decreasing and then increasing, mainly due to the higher Q×f values of the ixiolite structure, which plays a dominant role.For intrinsic factors, the lattice vibrational behavior is closely related to the dielectric properties.Fig. 8(a) plots the observed permittivity (εr-obs.), the corrected permittivity (εr-corr.),and the shift of Raman characteristic peaks for ZTZTOx ceramics as a function of x value.The εr-corr. is calculated by the Bosman-Havenga equation [7] : −.=−.(1+1.5) (2) where is porosity.As the amount of Zr 4+ -substitution increases, εr decreases approximately linearly.Furthermore, the εr-obs.values are similar to the εr-corr.values.The shift of the Raman characteristic peak can be reflected in its relation to εr according to the following equation [8] : where ΩP and are the plasma frequency and the phonon frequency.εr shows an inverse relationship with the wavenumbers.Fig. 8(a) also plots the wavenumber variation of the Raman signature peak with wavenumber located near 850 cm -1 .Its wave number increases approximately linearly with increasing x value, which is intrinsic to the linear decrease in εr.
The overall trend of gradually decreasing FWHM indicates that the damping loss for this type of vibration mode is gradually decreasing, which is intrinsically responsible for the gradual increase in Q×f.τf is an indication of the temperature stability of the MWDC, the closer the value is to 0 ppm/°C the more stable it is under extreme conditions.Fig. 9 shows the τf values for ZTZTOx ceramics at different sintering temperatures.Since ceramics with a tri-rutile structure have a positive τf value and ceramics with an ixiolite structure have a negative τf value, the τf changes from positive to negative as the tri-rutile structure changes to an ixiolite structure.As can be seen in Fig. 9, when x ≥ 0.02, the ceramics all have negative τf values, with only ZTZTO0.00 ceramics having a positive τf value, which is because the tri-rutile structure in the ZTZTO0.00ceramic has a percentage of 71.
In addition, with Zr 4+ -substitution, the complete transition from tri-rutile structure to ixiolite structure leads to a decrease in grain size followed by an increase in grain size.In terms of non-intrinsic factors, the ixiolite structure has lower εr and higher Q×f values resulting in a gradual decrease in εr and an increase in Q×f.For the intrinsic factors, an increase in the number of Raman characteristic waves leads to a gradual decrease in εr.The damping loss of the lattice vibrational behavior decreases, resulting in a gradual increase in Q×f.The excellent dielectric properties of Zn0.5Ti0.48Zr0.02TaO4indicate that it has promising applications in the field of microwave communications.

Fig. 8 (
b) demonstrates the Q×f values of the ZTZTOx ceramics after sintering at 1290 °C and the full width at half maxima (FWHM) values of the Raman characteristic peak located near 850 cm -1 .According to equations (4) and (5), lower dielectric loss is associated with more dramatic long-term attenuation of microwave energy propagation[9].From a lattice vibration point of view, a lower FWHM value means that a certain type of vibration mode has a lower damping behavior, which leads to a lower intrinsic dielectric loss.= 0 2

Table 1 .
The refined lattice parameters, weight fractions, and theoretical density.