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Synthesis, characterization, and microwave dielectric properties of ixiolite-structure ZnTiNb2O8 ceramics through the aqueous sol–gel process
Journal of Alloys and Compounds 626 (2015) 217–222
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Synthesis, characterization, and microwave dielectric properties of ixiolite-structure ZnTiNb2O8 ceramics through the aqueous sol–gel process Q.J. Mei, C.Y. Li, J.D. Guo, H.T. Wu ⇑ Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Received 18 September 2014 Received in revised form 30 November 2014 Accepted 8 December 2014 Available online 15 December 2014 Keywords: ZnTiNb2O8 Nanopowder synthesis Sol–gel Microwave dielectric properties
a b s t r a c t Microwave dielectric ceramics based on ixiolite-structure ZnTiNb2O8 were prepared by an aqueous sol–gel process. Highly reactive nanosized ZnTiNb2O8 powders with particle sizes of 30–50 nm were successfully obtained at 700 °C as precursors. Sintering characteristics and microwave dielectric properties of ZnTiNb2O8 ceramics were studied depending on sintering temperatures ranging from 950 °C to 1100 °C. The dielectric properties were strongly dependent on the compositions, the densifications and the microstructures of the specimens. With the increase of sintering temperature, density, er and Q f values were increased and saturated at 1050 °C with excellent microwave properties with an er of 35.3, an high Q f of 66,700 GHz, and a sf of 55.4 ppm/°C. The correlations of microstructure and dielectric properties for ZnTiNb2O8 ceramics were also investigated. The relatively low sintering temperature and high dielectric properties in microwave range would make these ceramics promising for application in electronics. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Microwave dielectric materials have attracted increasing interest because of their potential applications in mobile and satellite communications. Specifically, they have been investigated as various components for wireless communications, including duplexers, resonators, antennas, and oscillators [1–3]. Among of several kinds of dielectric ceramics, ixiolite-structure ZnTiNb2O8 (ZTN) ceramics became a kind of well-known dielectric ceramics promisingly to be used as resonators, filters and antennas for communication operating at millimeter wave frequencies due to high quality factors and appropriate dielectric constant and till now much attention was paid to research on its microwave dielectric properties by the conventional solid-state method [4–10]. For instance, the sintering temperature of ZTN ceramics was firstly reported to be about 1250 °C with a dielectric constant of 34, a Q f of 42,500 GHz and a sf of 52 ppm/°C by Kim et al. [4,5]. Liao and Li [9] reported that the ZTN precursor was synthesized 900 °C for 3 h and then sintered at 1120 °C for 6 h with dielectric constant (e) = 34.4, Q f = 56,900 GHz, sf = 47.94 ppm/°C. In addition, Yin et al. [10] also reported that the ZTN powders could be obtained at 1050 °C via traditional solid-state method and microwave ⇑ Corresponding author. Tel.: +86 531 82769782; fax: +86 531 87974453. E-mail address: [email protected] (H.T. Wu). http://dx.doi.org/10.1016/j.jallcom.2014.12.031 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
dielectric ceramics ZnTiNb2O8 which sintered at 1150 °C exhibited the best microwave properties: high permittivity (er = 34.2), high quality factor (Q f = 28,696 GHz), but very negative temperature coefficient of resonant frequency (sf = 75.841 ppm/°C). However, the sintering temperatures of dielectric ceramics via traditional solid-state method seemed to be higher and not helpful for the application of wireless communication systems practically. So it was of great significance to decrease the sintering temperature of ZTN dielectric ceramics by other attempts. Usually there were several approaches to reduce the sintering temperature of the ceramics: (i) the usage of smaller particle size of starting materials synthesized by chemical processes and (ii) the addition of lowmelting glasses, oxides or mixed-oxides. As we known, adding glass flux usually caused the detrimental effect on the microwave properties of ceramics reported on many other ceramic systems, such as doping Li2O–ZnO–B2O3 (LZB) glass, BaCu(B2O5) additive on ZTN dielectric ceramics [11–14], or Li2O–V2O5, Li2TiO3, ZnO– B2O3–SiO2, MgO–LiF, Bi2O3, etc. reported on other ceramic systems [15–19]. Now in order to reduce the sintering temperature and improve the sintering ability there were many other investigations of the chemical processing or special methods, which were developed as alternatives to the conventional solid-state reaction of mixed oxides for producing ceramics by using starting materials with smaller particle sizes. Among of all these methods, the sol–gel
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was undoubtedly one of useful process for producing powders with good control over stoichiometry and homogeneity, yielding nanosized particles and widely used in many other ceramics system [20–23]. However, at present few researches on the preparation of ZTN ceramics by the aqueous sol–gel process were reported in the present literatures. The primary objectives of present research were to synthesize pure and nano-sized ZTN powders as precursors for the preparation of ceramics by the simple and cheap aqueous sol–gel method. The whole process involved complexation of aqueous metal ions by non-toxic poly functional carboxyl acids such as citric acid and ethylene glycol, and avoided complex steps such as refluxing of alkoxides, resulting in less time consumption compared with other techniques. In this work, ZTN nano-particles were synthesized at relatively low temperatures. Additionally the microstructures, microwave dielectric properties of ZTN ceramics as a function of sintering temperatures were investigated in detail. The results showed that the preparation of ZTN ceramics with excellent microwave properties could be obtained at low sintering temperatures by the aqueous sol–gel process. 2. Experimental Analytical-grade TiO2, Nb2O5, K2CO3, Zn(NO3)2, HNO3, citric acid (CA) and ethylene glycol (EG) were used as raw materials to synthesize the ZTN nanopowders as shown in Fig. 1. Firstly, the mixture of Nb2O5 and K2CO3 were co-melted at 900 °C in order to obtain the K3NbO4 phase according to the phase diagram. Subsequently, the K3NbO4 phase was dissolved in distilled water, then the solution was set at pH 2 to form precipitate of Nb(OH)5. The whole formation process of Nb(OH)5 phase could be formulated from Eqs. (1)–(4). Thirdly, the Nb(OH)5 precipitate was filtered off and washed with distilled water for six times to remove the K+ ions and then dissolved completely in citric acid water solution by continuous magnetic stirring at 300 rpm for 15 min. Similarly, Ti(OH)4 dissolved in citric acid water solution could be obtained. Meanwhile, a stoichiometric amount of Zn(NO3)2 was added to the mixing citric acid solution of Ti and Nb, and then the solution was stirred for another 30 min. Finally, the ethyl alcohol (60–80 ml) was added to the as-prepared mixed solution in drops and stirred for 1 h to form a transparent and stable sol. pH of the solution was maintained in the range of 3.5–5 by adding buffering agents. The sol was heated at 80–90 °C for 1 h to obtain a xerogel. The xerogel was decomposed at various temperatures ranging from 600 °C to 900 °C in a muffle furnace for crystallization. The as-prepared powers were ball milled in a polyethylene jar for 4 h using ZrO2 balls in ethanol medium to reduce the conglobation phenomena. The powders were then mixed with polyvinyl alcohol as a binder, granulated and pressed into cylindrical disks of 10 mm diameter and about 5 mm height at a pressure of about 200 MPa. These pellets were preheated at 600 °C for 4 h to expel the binder and then sintered at selected temperatures for 2 h in air at a heating rate of 5 °C/min.
3K2 CO3 þ Nb2 O5 ! 2K3 NbO4 þ CO2
Nb2O5
K2CO3
K3NbO4 Nb2O5•xh2O Nb-CA
ð1Þ
TiO2
6K3 NbO4 þ 5H2 O ! 18Kþ þ Nb6 O8 19 þ 10OH
ð2Þ
þ 4Kþ þ Nb6 O8 19 þ 4OH þ 8H ! K4 H4 Nb6 O19 # þ4H2 O
ð3Þ
K4 H4 Nb6 O19 þ 15Hþ þ 11OH ! 6NbðOHÞ5 # þ4Kþ
ð4Þ
In order to analyze the evolution of ZTN phase, the as-formed Zn–Ti–Nb xerogel was characterized using thermogravimetry (TG) and differential thermal analysis (DTA) (Model STA 449 F3, NETZSCH Co., Germany). Phase analysis of ZTN powder was conducted with the help of a Rigaku diffractometer (Model D/MAX-B, Rigaku Co., Japan) using Ni filtered Cu Ka radiation (k = 0.1542 nm) at 40 kV and 40 mA settings. Rietveld refinements of the crystal structures were performed using the Topas program [24]. The reliability of the refinement result was judged by the pattern R factor (Rp), the weighted pattern R factor (Rwp) and the goodness of fit indicator (v2). The zero shift, individual scale factor, unit-cell parameters, and phase profile parameters (U, V, and W) along with symmetry parameter were refined until the apparent convergence of XRD patterns was reached. Based on XRD analysis, the morphology and particle sizes were examined using a transmission electron microscopy (Model JEOL JEM-2010, FEI Co., Japan). An network analyzer (N5234A, Agilent Co., America) was used for the measurement of microwave dielectric properties. Dielectric constants were measured using Hakki–Coleman post-resonator method by exciting the TE011 resonant mode of dielectric resonator by using an electric probe as suggested by Hakki and Coleman [25]. Unloaded quality factors were measured using TE01d mode by the cavity method [26]. All measurements were made at room temperature and in the frequency of 8–12 GHz. Temperature coefficients of resonant frequency were measured in the temperature range of 25–85 °C.
3. Results and discussion TG–DTA curves of the Zn–Ti–Nb xerogel at a heating rate of 10 °C/min were shown in Fig. 2. It was indicated that all chemical reactions involving weight losses, such as decomposition of the organic polymeric network, finished completely below 600 °C. According to TG curves the total weight loss was about 90% and occurred in two steps. Firstly obvious weight loss began at 200 °C and initial weight loss was below 400 °C, resulting from the evaporation of residual solvent and the decomposition of the organic polymeric network with evolution of CO2 and H2O. Secondly weight loss occurred in TG curves, combined with a significantly exothermal peak in the temperature region of 400–600 °C, which was attributed to the oxidation of metal–organic groups. TG results of Zn–Ti–Nb xerogel were also similar with these of other ceramic xerogel synthesized by sol–gel methods [27,28]. It was obvious that no further significant weight loss and thermometric peaks were observed above 600 °C in TG–DTA curves, which indicated the minimum firing temperature to synthesize the ZTN phases. The XRD patterns of ZTN xerogel calcined at temperatures ranging from 600 to 900 °C for 60 min in air atmosphere were shown in Fig. 3. At 600 °C there were not any significant diffraction peaks
Zn(NO3)2
K4TiO4 Ti(OH)x Ti-CA
Zn-CA
Zn-Ti-Nb precursor Zn-Ti-Nb xerogel ZnTiNb2O8 nanopowders Fig. 1. Chart for the synthesis of ZnTiNb2O8 nanopowders by the aqueous sol–gel processing.
Fig. 2. TG–DTA curves of Zn–Ti–Nb xerogel in air atmosphere at a heating rate of 10 °C/min ranging from room temperature to nearly 1200 °C.
Q.J. Mei et al. / Journal of Alloys and Compounds 626 (2015) 217–222
Fig. 3. X-ray diffraction patterns of Zn–Ti–Nb xerogel calcined at 600–900 °C for 60 min.
existing in XRD patterns and it meant amorphous state of ZTN phase. However, it was pleasantly found that the crystallization of ixiolite-structure ZTN as predominant phase took place at 700 °C in view of diffraction peaks from significant lattice planes, such as (1 1 1), (1 1 0), and (0 2 3), which was in agreement with the XRD pattern of JCPDS No. 48-0323. The XRD patterns of the xerogel fired at 800 °C and 900 °C still consisted of predominant peaks of ZTN with sharper peaks free from any second phases. Therefore, according to XRD result it was indicated that calcination temperature of synthesizing ixiolite-structure ZTN phase was remarkably decreased to 700 °C by the sol–gel process, which was lower than the conventional mixed oxide route reported earlier [9,10]. For example, Liao and Li [9] reported that the ZTN precursor was synthesized 900 °C for 3 h and Yin et al. [10] also reported that the ZTN powders could be obtained at 1050 °C via. By comparison, the sol–gel process used in this work showed obvious advantages over the traditional solid-state method. In order to characterize as-prepared nanopowders, a typical TEM micrograph of a cluster of well crystallized ZTN nanoparticles calcined at 700 °C was illustrated in Fig. 4. The specimen was composed of large amorphous clusters containing nanosized crystalline particles. It was worth noting that the particles were well-distributed with high uniformity and basically regular in shape. The particle sizes were measured by the liner intercept method [29] and the
Fig. 4. TEM micrograph of raw ZnTiNb2O8 nanopowders calcined at 700 °C for 60 min.
219
range of particle sizes was just about 30–50 nm. A close-up of a typical ZTN nanocrystal was presented in Fig. 5 showing well resolved {1 1 1} lattice planes. The crystals were terminated with well defined {1 1 1} dominating pinacoidal faces. Accordingly, its presence was determined by the measurement of d values. The inset in Fig. 5 was the corresponding SAED pattern for the whole particle. The SAED pattern from these clusters corresponded to the ixiolite-structure ZTN phase (JCPDF #48-0323) well. Diffuse diffraction spots clearly indicated the crystallization character of this powder. The strongest SAED diffraction spots in the pattern corresponded to: D1 = {1 1 0}, D2 = {1 1 1}, D3 = {0 2 0}, and D4 = {0 0 2} inside and out. Therefore, it should also be stated that due to both SAED patterns and high-resolution crystal lattice image the ixiolite-structure ZTN phase were present with well crystallinity. Curves of apparent densities and diametric shrinkage ratio of ZTN ceramics sintered for 4 h depending on sintering temperatures from 950 °C to 1100 °C were plotted in Fig. 6, through which the optimized sintering temperature could be determined. Here ZTN ceramic had a theoretical density of 5.336 g/cm3 and its shrinkage tendency was characterized by the ratio of diametric size before and after the ceramic sintering. It was found that apparent densities increased from 4.3 to 5.125 g/cm3 with sintering temperatures increased from 950 °C to 1100 °C. At 1050 °C a saturated value of apparent densities was found to be nearly 5.3 g/cm3 and the curve
Fig. 5. High-resolution crystal lattice image of ZnTiNb2O8 nanocrystals on crystal indices of (1 1 1) with the inset of corresponding selected area electron diffraction pattern (SAED).
Fig. 6. Curves of apparent densities and diametric shrinkage ratio of ZnTiNb2O8 ceramics sintered for 4 h depending on sintering temperatures from 950 °C to 1100 °C.
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of diametric shrinkage ratio also showed the similar tendency. Based on the result of sintering characteristics, it was concluded that sintered at 1050 °C for 4 h the ceramics of ZTN had nearly full density. In order to characterize the microstructure of ZTN ceramics sintered at different temperatures, SEM micrographs from 950 °C to 1100 °C were illustrated in Fig. 7(a)–(d). It was easily found that much pores were exist in samples sintered at 950 °C shown in Fig. 7(a). Also all pores almost disappeared at 1050 °C on the surface of ZTN samples. With the increase of sintering temperature from 1050 °C to 1100 °C shown in Fig. 7(c) and (d), grain sizes increased rapidly and average grain sizes measured was less than 2 lm at 1050 °C as shown in Fig. 7(c). In addition, abnormal grain growth could be found as shown in Fig. 7(d) when sintering
temperature arise to 1100 °C. Therefore, it was obvious that ZTN ceramics were successfully prepared with full density through the sol–gel process at 1050 °C for 4 h, which was lower than that by solid-state reaction methods. By comparison, Kim et al. [4,5] reported that the ceramics were sintered at 1250 °C for 2 h by the solid-state method firstly. In addition, some results of ZTN ceramics sintered at 1120 °C and at 1150 °C were also reported by the solid-state method [9,10]. The results showed that the preparation of ZTN ceramics could be obtained at lower sintering temperatures by the aqueous sol–gel process. In addition, EDS analysis about grains chosen randomly from the samples sintered at 1050 °C was shown in Fig. 7(e). The inset in Fig. 7(e) presented a quantitative result about elementary
Fig. 7. FE-SEM micrographs of ZnTiNb2O8 ceramics sintered at different sintering temperature for 2 h ((a)–(d) corresponding to 950 °C, 1000 °C, 1050 °C,1100 °C) and EDS analysis.
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composition. The concentrations of Zn, Ti, Nb and O ions in the grain were analyzed to be 8.65, 6.63, 16.57 and 68.14 at%, respectively. The ratio of Zn/Ti/Nb/O was approximately corresponding to the formula of ZTN phase. The XRD patterns of ZTN ceramics sintered from 950 °C to 1100 °C were shown in Fig. 8. The predominant phases were identified as the ixiolite-structure with the space group of Pbcn(60) and just minor secondary phases of Zn3Nb2O8 with monoclinic structure were observed at 28–29°. The X-ray diffraction patterns of ZTN ceramics did not have significant change throughout the sintering temperatures ranging from 950 °C to 1100 °C. Fig. 9 showed a typical pattern plot of ZTN sample sintered at 1050 °C after refinement of all parameters of interest. The crystal cell parameters was a = 4.674 Å, b = 5.659 Å, c = 5.017 Å with the same a, b, c angle of 90° and direct cell volume was 132.7208 Å3. The ZTN (ICSD #40710) reported by Baumgarte and Blachnik [30], was adopted as the starting model. The calculated pattern was overlaid on the measured pattern and the differences between two profiles were plotted along the bottom. According to the inset in Fig. 9, it was found that the measured pattern of ZTN phase fitted well with the calculated pattern with the structure of ICSD #40710 and the phase with all XRD peaks existed with nearly 100% ixiolite-structure by quantitative analysis. Changes of er, Q f and sf values as a function of sintering temperatures were shown in Fig. 10. It was found that er values of ZTN ceramics steadily increased from 26.8 to 35.3 with the increase of sintering temperature from 950 to 1050 °C, and then saturated at 35 during the temperatures of 1050–1100 °C. Based on results of sintering characteristic curves and microstructure shown in Figs. 6 and 7, it was obvious that low er values were mainly caused by pores (er 1) at sintering temperatures lower than 1050 °C. The curve of er values showed a similar tendency with those of
Fig. 10. Curves of er, Q f and sf values as a function of sintering temperatures for ZnTiNb2O8 ceramics in the temperature region of 950–1100 °C.
apparent density and shrinkage ratio, which were sensitive to dense degree of ceramics significantly. The er value obtained at 1050 °C was 35.3, which was similar with the results reported by the solid-state method [5,9,10]. For example, Kim et al. [5] reported a dielectric constant of 34, Liao and Li [9] reported that the ZTN ceramic was sintered at 1120 °C for 6 h with dielectric constant of 34.4 and Yin et al. [10] also reported that microwave dielectric ceramics ZnTiNb2O8 which sintered at 1150 °C exhibited the best high permittivity (er = 34.2). To clarify effects of crystal structure on dielectric constant, theoretical dielectric polarizability (atheo.) was calculated to be 28.99 according to additive rule with ionic polarizability of composing ions or oxides [31] as formulated in Eq. (5). While observed dielectric polarizability (aobs.) was calculated to be 29.12 by Clausius–Mossotti equation as formulated in Eq. (6) with measured dielectric constant at microwave frequencies [32]. By comparison values of atheo. and aobs. were in good agreement with each other, and the minor deviation from the aobs. and atheo. could attribute to relative density because the aobs. value depended on specimens and fabrication process.
atheo: ¼ aðZnTiNb2 O8 Þ ¼ aðZnOÞ þ aðTiO2 Þ þ aðNb2 O5 Þ 1 b
aobs: ¼ V m
Counts
e1 eþ2
ð6Þ
where a (ZnO), a (Nb2O5), and a (TiO2) represented oxides polarizabilities reported by Shannon [32]. Moreover, Vm, e and b indicated the molar volume of samples, dielectric constant and constant value (4p/3), respectively. With the increase of sintering temperatures from 950 to 1050 °C, Q f values increased from 38,600 to 67,200 GHz with saturated Q f values 66,000 GHz in the sintering temperature region of 1050–1100 °C. The remarkable increase in Q f values ranging from 950 to 1000 °C was also related to the reduction of
Fig. 8. X-ray diffraction patterns of ZnTiNb2O8 ceramics sintered at different sintering temperature from 950 °C to 1100 °C for 2 h.
40,000 35,000 30,000 25,000 20,000 15,000 10,000 5,000 0 -5,000 -10,000 10
ð5Þ
Ixiolite 100.00 %
20
30
40
50
60
70
80
90
100
110
120
2Th Degrees Fig. 9. The profile fits for the Rietveld refinement of ZnTiNb2O8 ceramics sintered at 1050 °C using the program of Topas. The agreement indices: Rwp (%) = 9.36, Rp (%) = 7.14, and GOF = 2.12. The inset show that the measured pattern of ZnTiNb2O8 phase fitted well with the calculated pattern with the structure of (ICSD # 40710).
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porosity according to results of SEM microstructures shown in Fig. 7(a) and (b). Additionally it was found that the Q f value slightly decreased to be 62,500 GHz when the samples were sintered at 1100 °C. According to the SEM results as shown in Fig. 7(d), it was believed that the non-uniform microstructure caused by abnormal grain growth would have an harmful influence on the Q f value when adopting excessively sintering process. The similar phenomenon was also found in another ceramic system [21]. ZTN ceramics sintered at 1050 °C with increased density had a Q f value of 66,700 GHz, which was significantly higher than the results reported by the solid-state method [5,9,10]. By comparison, Kim et al. [5] reported a Q f value of 33,000 GHz by the solidstate technique, Liao and Li [9] reported its best result of Q f = 56,900 GHz, Yin et al. [10] also reported that the ZTN ceramic exhibited the best microwave properties of high quality factor (Q f = 28,696 GHz). As for Q f values of dielectric ceramics, it was well known that porosity, secondary phase and structure defect of ceramics as extrinsic factors usually produced a deterioration in Q f values [33]. Among these factors, the porosity was suggested to affect Q f values obviously below 1050 °C. It was found that relative density was one of the most important factors in controlling dielectric loss, which was demonstrated on many other microwave dielectric materials. Once the as-prepared samples were of nearly full density, Q f values were mainly affected by intrinsic factors, such as crystal structure and lattice defects. As for lattice defects, dielectric loss tangent at microwave frequency was mainly determined by the anharmonic terms in the crystal’s potential energy according to by Schlömann’s theory [34,35]. Dielectric ceramics have many kinds of structural or lattice defects such as grain boundaries, voids, dislocations, point defects and substitutional ions. Rustum Roy reported that using the solution-sol–gel route had been developed to make a variety of ceramic materials, which provided major advantages in lowering sintering temperatures, refining microstructure, and controlling morphology and final phase composition [36]. The ixiolite-structure ZTN precursors were provided with well crystalline and less lattice defects for the fabrication of ceramics by the solution-sol–gel route in this work, which should contribute to the improvement of Q f values. Moreover, remarkable changes in sf values of ZTN ceramics fluctuated around 60 ppm/°C with the increase of sintering temperatures from 950 °C to 1100 °C and these values were ranged from 54.5 to 63.5 ppm/°C, which was similar with the results by other reports [5,9,10]. For example, Kim et al. [4,5] reported a similar sf value of 52 ppm/°C, Liao and Li [9] reported a slightly better value of sf = 47.94 ppm/°C. In addition, Yin et al. [10] reported a very negative temperature coefficient of resonant frequency (sf = 75.84 ppm/°C). Thus, it was considered that additional improvement in sf value and lowing sintering temperature for further by adding sintering additives were required for dielectric resonator applications at high frequency. 4. Conclusions The ixiolite-structure ZTN powders with particle sizes of 30– 50 nm were obtained successfully by the aqueous sol–gel synthetic route, which showed major advantages over reported solid-state
methods. A considerable decrease in synthesis temperature (at 700 °C) was obtained in air atmosphere for the formation of ZTN nanopowders with well crystallinity. Moreover, sintering ability and microwave properties of ZTN ceramics depending on sintering temperatures were systematically investigated. ZTN samples with nearly full densities were obtained at 1050 °C and had excellent microwave dielectric properties of er = 35.3, Q f = 66,700 GHz and sf = 55.4 ppm/°C. Acknowledgments This work was supported by the Project Development Plan of Science and Technology of Ji’nan City (No. 201303061), Ji’nan City Youth Science and Technology Star Project (No. 2013035), the National Training Plan Innovation Project of College Students (No. 201310427004) and National Natural Science Foundation (No. 51472108). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Synthesis, characterization, and microwave dielectric properties of ixiolite-structure ZnTiNb2O8 ceramics through the aqueous sol–gel process Q.J. Mei, C.Y. Li, J.D. Guo, H.T. Wu ⇑ Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China
a r t i c l e
i n f o
Article history: Received 18 September 2014 Received in revised form 30 November 2014 Accepted 8 December 2014 Available online 15 December 2014 Keywords: ZnTiNb2O8 Nanopowder synthesis Sol–gel Microwave dielectric properties
a b s t r a c t Microwave dielectric ceramics based on ixiolite-structure ZnTiNb2O8 were prepared by an aqueous sol–gel process. Highly reactive nanosized ZnTiNb2O8 powders with particle sizes of 30–50 nm were successfully obtained at 700 °C as precursors. Sintering characteristics and microwave dielectric properties of ZnTiNb2O8 ceramics were studied depending on sintering temperatures ranging from 950 °C to 1100 °C. The dielectric properties were strongly dependent on the compositions, the densifications and the microstructures of the specimens. With the increase of sintering temperature, density, er and Q f values were increased and saturated at 1050 °C with excellent microwave properties with an er of 35.3, an high Q f of 66,700 GHz, and a sf of 55.4 ppm/°C. The correlations of microstructure and dielectric properties for ZnTiNb2O8 ceramics were also investigated. The relatively low sintering temperature and high dielectric properties in microwave range would make these ceramics promising for application in electronics. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Microwave dielectric materials have attracted increasing interest because of their potential applications in mobile and satellite communications. Specifically, they have been investigated as various components for wireless communications, including duplexers, resonators, antennas, and oscillators [1–3]. Among of several kinds of dielectric ceramics, ixiolite-structure ZnTiNb2O8 (ZTN) ceramics became a kind of well-known dielectric ceramics promisingly to be used as resonators, filters and antennas for communication operating at millimeter wave frequencies due to high quality factors and appropriate dielectric constant and till now much attention was paid to research on its microwave dielectric properties by the conventional solid-state method [4–10]. For instance, the sintering temperature of ZTN ceramics was firstly reported to be about 1250 °C with a dielectric constant of 34, a Q f of 42,500 GHz and a sf of 52 ppm/°C by Kim et al. [4,5]. Liao and Li [9] reported that the ZTN precursor was synthesized 900 °C for 3 h and then sintered at 1120 °C for 6 h with dielectric constant (e) = 34.4, Q f = 56,900 GHz, sf = 47.94 ppm/°C. In addition, Yin et al. [10] also reported that the ZTN powders could be obtained at 1050 °C via traditional solid-state method and microwave ⇑ Corresponding author. Tel.: +86 531 82769782; fax: +86 531 87974453. E-mail address: [email protected] (H.T. Wu). http://dx.doi.org/10.1016/j.jallcom.2014.12.031 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
dielectric ceramics ZnTiNb2O8 which sintered at 1150 °C exhibited the best microwave properties: high permittivity (er = 34.2), high quality factor (Q f = 28,696 GHz), but very negative temperature coefficient of resonant frequency (sf = 75.841 ppm/°C). However, the sintering temperatures of dielectric ceramics via traditional solid-state method seemed to be higher and not helpful for the application of wireless communication systems practically. So it was of great significance to decrease the sintering temperature of ZTN dielectric ceramics by other attempts. Usually there were several approaches to reduce the sintering temperature of the ceramics: (i) the usage of smaller particle size of starting materials synthesized by chemical processes and (ii) the addition of lowmelting glasses, oxides or mixed-oxides. As we known, adding glass flux usually caused the detrimental effect on the microwave properties of ceramics reported on many other ceramic systems, such as doping Li2O–ZnO–B2O3 (LZB) glass, BaCu(B2O5) additive on ZTN dielectric ceramics [11–14], or Li2O–V2O5, Li2TiO3, ZnO– B2O3–SiO2, MgO–LiF, Bi2O3, etc. reported on other ceramic systems [15–19]. Now in order to reduce the sintering temperature and improve the sintering ability there were many other investigations of the chemical processing or special methods, which were developed as alternatives to the conventional solid-state reaction of mixed oxides for producing ceramics by using starting materials with smaller particle sizes. Among of all these methods, the sol–gel
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was undoubtedly one of useful process for producing powders with good control over stoichiometry and homogeneity, yielding nanosized particles and widely used in many other ceramics system [20–23]. However, at present few researches on the preparation of ZTN ceramics by the aqueous sol–gel process were reported in the present literatures. The primary objectives of present research were to synthesize pure and nano-sized ZTN powders as precursors for the preparation of ceramics by the simple and cheap aqueous sol–gel method. The whole process involved complexation of aqueous metal ions by non-toxic poly functional carboxyl acids such as citric acid and ethylene glycol, and avoided complex steps such as refluxing of alkoxides, resulting in less time consumption compared with other techniques. In this work, ZTN nano-particles were synthesized at relatively low temperatures. Additionally the microstructures, microwave dielectric properties of ZTN ceramics as a function of sintering temperatures were investigated in detail. The results showed that the preparation of ZTN ceramics with excellent microwave properties could be obtained at low sintering temperatures by the aqueous sol–gel process. 2. Experimental Analytical-grade TiO2, Nb2O5, K2CO3, Zn(NO3)2, HNO3, citric acid (CA) and ethylene glycol (EG) were used as raw materials to synthesize the ZTN nanopowders as shown in Fig. 1. Firstly, the mixture of Nb2O5 and K2CO3 were co-melted at 900 °C in order to obtain the K3NbO4 phase according to the phase diagram. Subsequently, the K3NbO4 phase was dissolved in distilled water, then the solution was set at pH 2 to form precipitate of Nb(OH)5. The whole formation process of Nb(OH)5 phase could be formulated from Eqs. (1)–(4). Thirdly, the Nb(OH)5 precipitate was filtered off and washed with distilled water for six times to remove the K+ ions and then dissolved completely in citric acid water solution by continuous magnetic stirring at 300 rpm for 15 min. Similarly, Ti(OH)4 dissolved in citric acid water solution could be obtained. Meanwhile, a stoichiometric amount of Zn(NO3)2 was added to the mixing citric acid solution of Ti and Nb, and then the solution was stirred for another 30 min. Finally, the ethyl alcohol (60–80 ml) was added to the as-prepared mixed solution in drops and stirred for 1 h to form a transparent and stable sol. pH of the solution was maintained in the range of 3.5–5 by adding buffering agents. The sol was heated at 80–90 °C for 1 h to obtain a xerogel. The xerogel was decomposed at various temperatures ranging from 600 °C to 900 °C in a muffle furnace for crystallization. The as-prepared powers were ball milled in a polyethylene jar for 4 h using ZrO2 balls in ethanol medium to reduce the conglobation phenomena. The powders were then mixed with polyvinyl alcohol as a binder, granulated and pressed into cylindrical disks of 10 mm diameter and about 5 mm height at a pressure of about 200 MPa. These pellets were preheated at 600 °C for 4 h to expel the binder and then sintered at selected temperatures for 2 h in air at a heating rate of 5 °C/min.
3K2 CO3 þ Nb2 O5 ! 2K3 NbO4 þ CO2
Nb2O5
K2CO3
K3NbO4 Nb2O5•xh2O Nb-CA
ð1Þ
TiO2
6K3 NbO4 þ 5H2 O ! 18Kþ þ Nb6 O8 19 þ 10OH
ð2Þ
þ 4Kþ þ Nb6 O8 19 þ 4OH þ 8H ! K4 H4 Nb6 O19 # þ4H2 O
ð3Þ
K4 H4 Nb6 O19 þ 15Hþ þ 11OH ! 6NbðOHÞ5 # þ4Kþ
ð4Þ
In order to analyze the evolution of ZTN phase, the as-formed Zn–Ti–Nb xerogel was characterized using thermogravimetry (TG) and differential thermal analysis (DTA) (Model STA 449 F3, NETZSCH Co., Germany). Phase analysis of ZTN powder was conducted with the help of a Rigaku diffractometer (Model D/MAX-B, Rigaku Co., Japan) using Ni filtered Cu Ka radiation (k = 0.1542 nm) at 40 kV and 40 mA settings. Rietveld refinements of the crystal structures were performed using the Topas program [24]. The reliability of the refinement result was judged by the pattern R factor (Rp), the weighted pattern R factor (Rwp) and the goodness of fit indicator (v2). The zero shift, individual scale factor, unit-cell parameters, and phase profile parameters (U, V, and W) along with symmetry parameter were refined until the apparent convergence of XRD patterns was reached. Based on XRD analysis, the morphology and particle sizes were examined using a transmission electron microscopy (Model JEOL JEM-2010, FEI Co., Japan). An network analyzer (N5234A, Agilent Co., America) was used for the measurement of microwave dielectric properties. Dielectric constants were measured using Hakki–Coleman post-resonator method by exciting the TE011 resonant mode of dielectric resonator by using an electric probe as suggested by Hakki and Coleman [25]. Unloaded quality factors were measured using TE01d mode by the cavity method [26]. All measurements were made at room temperature and in the frequency of 8–12 GHz. Temperature coefficients of resonant frequency were measured in the temperature range of 25–85 °C.
3. Results and discussion TG–DTA curves of the Zn–Ti–Nb xerogel at a heating rate of 10 °C/min were shown in Fig. 2. It was indicated that all chemical reactions involving weight losses, such as decomposition of the organic polymeric network, finished completely below 600 °C. According to TG curves the total weight loss was about 90% and occurred in two steps. Firstly obvious weight loss began at 200 °C and initial weight loss was below 400 °C, resulting from the evaporation of residual solvent and the decomposition of the organic polymeric network with evolution of CO2 and H2O. Secondly weight loss occurred in TG curves, combined with a significantly exothermal peak in the temperature region of 400–600 °C, which was attributed to the oxidation of metal–organic groups. TG results of Zn–Ti–Nb xerogel were also similar with these of other ceramic xerogel synthesized by sol–gel methods [27,28]. It was obvious that no further significant weight loss and thermometric peaks were observed above 600 °C in TG–DTA curves, which indicated the minimum firing temperature to synthesize the ZTN phases. The XRD patterns of ZTN xerogel calcined at temperatures ranging from 600 to 900 °C for 60 min in air atmosphere were shown in Fig. 3. At 600 °C there were not any significant diffraction peaks
Zn(NO3)2
K4TiO4 Ti(OH)x Ti-CA
Zn-CA
Zn-Ti-Nb precursor Zn-Ti-Nb xerogel ZnTiNb2O8 nanopowders Fig. 1. Chart for the synthesis of ZnTiNb2O8 nanopowders by the aqueous sol–gel processing.
Fig. 2. TG–DTA curves of Zn–Ti–Nb xerogel in air atmosphere at a heating rate of 10 °C/min ranging from room temperature to nearly 1200 °C.
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Fig. 3. X-ray diffraction patterns of Zn–Ti–Nb xerogel calcined at 600–900 °C for 60 min.
existing in XRD patterns and it meant amorphous state of ZTN phase. However, it was pleasantly found that the crystallization of ixiolite-structure ZTN as predominant phase took place at 700 °C in view of diffraction peaks from significant lattice planes, such as (1 1 1), (1 1 0), and (0 2 3), which was in agreement with the XRD pattern of JCPDS No. 48-0323. The XRD patterns of the xerogel fired at 800 °C and 900 °C still consisted of predominant peaks of ZTN with sharper peaks free from any second phases. Therefore, according to XRD result it was indicated that calcination temperature of synthesizing ixiolite-structure ZTN phase was remarkably decreased to 700 °C by the sol–gel process, which was lower than the conventional mixed oxide route reported earlier [9,10]. For example, Liao and Li [9] reported that the ZTN precursor was synthesized 900 °C for 3 h and Yin et al. [10] also reported that the ZTN powders could be obtained at 1050 °C via. By comparison, the sol–gel process used in this work showed obvious advantages over the traditional solid-state method. In order to characterize as-prepared nanopowders, a typical TEM micrograph of a cluster of well crystallized ZTN nanoparticles calcined at 700 °C was illustrated in Fig. 4. The specimen was composed of large amorphous clusters containing nanosized crystalline particles. It was worth noting that the particles were well-distributed with high uniformity and basically regular in shape. The particle sizes were measured by the liner intercept method [29] and the
Fig. 4. TEM micrograph of raw ZnTiNb2O8 nanopowders calcined at 700 °C for 60 min.
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range of particle sizes was just about 30–50 nm. A close-up of a typical ZTN nanocrystal was presented in Fig. 5 showing well resolved {1 1 1} lattice planes. The crystals were terminated with well defined {1 1 1} dominating pinacoidal faces. Accordingly, its presence was determined by the measurement of d values. The inset in Fig. 5 was the corresponding SAED pattern for the whole particle. The SAED pattern from these clusters corresponded to the ixiolite-structure ZTN phase (JCPDF #48-0323) well. Diffuse diffraction spots clearly indicated the crystallization character of this powder. The strongest SAED diffraction spots in the pattern corresponded to: D1 = {1 1 0}, D2 = {1 1 1}, D3 = {0 2 0}, and D4 = {0 0 2} inside and out. Therefore, it should also be stated that due to both SAED patterns and high-resolution crystal lattice image the ixiolite-structure ZTN phase were present with well crystallinity. Curves of apparent densities and diametric shrinkage ratio of ZTN ceramics sintered for 4 h depending on sintering temperatures from 950 °C to 1100 °C were plotted in Fig. 6, through which the optimized sintering temperature could be determined. Here ZTN ceramic had a theoretical density of 5.336 g/cm3 and its shrinkage tendency was characterized by the ratio of diametric size before and after the ceramic sintering. It was found that apparent densities increased from 4.3 to 5.125 g/cm3 with sintering temperatures increased from 950 °C to 1100 °C. At 1050 °C a saturated value of apparent densities was found to be nearly 5.3 g/cm3 and the curve
Fig. 5. High-resolution crystal lattice image of ZnTiNb2O8 nanocrystals on crystal indices of (1 1 1) with the inset of corresponding selected area electron diffraction pattern (SAED).
Fig. 6. Curves of apparent densities and diametric shrinkage ratio of ZnTiNb2O8 ceramics sintered for 4 h depending on sintering temperatures from 950 °C to 1100 °C.
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of diametric shrinkage ratio also showed the similar tendency. Based on the result of sintering characteristics, it was concluded that sintered at 1050 °C for 4 h the ceramics of ZTN had nearly full density. In order to characterize the microstructure of ZTN ceramics sintered at different temperatures, SEM micrographs from 950 °C to 1100 °C were illustrated in Fig. 7(a)–(d). It was easily found that much pores were exist in samples sintered at 950 °C shown in Fig. 7(a). Also all pores almost disappeared at 1050 °C on the surface of ZTN samples. With the increase of sintering temperature from 1050 °C to 1100 °C shown in Fig. 7(c) and (d), grain sizes increased rapidly and average grain sizes measured was less than 2 lm at 1050 °C as shown in Fig. 7(c). In addition, abnormal grain growth could be found as shown in Fig. 7(d) when sintering
temperature arise to 1100 °C. Therefore, it was obvious that ZTN ceramics were successfully prepared with full density through the sol–gel process at 1050 °C for 4 h, which was lower than that by solid-state reaction methods. By comparison, Kim et al. [4,5] reported that the ceramics were sintered at 1250 °C for 2 h by the solid-state method firstly. In addition, some results of ZTN ceramics sintered at 1120 °C and at 1150 °C were also reported by the solid-state method [9,10]. The results showed that the preparation of ZTN ceramics could be obtained at lower sintering temperatures by the aqueous sol–gel process. In addition, EDS analysis about grains chosen randomly from the samples sintered at 1050 °C was shown in Fig. 7(e). The inset in Fig. 7(e) presented a quantitative result about elementary
Fig. 7. FE-SEM micrographs of ZnTiNb2O8 ceramics sintered at different sintering temperature for 2 h ((a)–(d) corresponding to 950 °C, 1000 °C, 1050 °C,1100 °C) and EDS analysis.
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composition. The concentrations of Zn, Ti, Nb and O ions in the grain were analyzed to be 8.65, 6.63, 16.57 and 68.14 at%, respectively. The ratio of Zn/Ti/Nb/O was approximately corresponding to the formula of ZTN phase. The XRD patterns of ZTN ceramics sintered from 950 °C to 1100 °C were shown in Fig. 8. The predominant phases were identified as the ixiolite-structure with the space group of Pbcn(60) and just minor secondary phases of Zn3Nb2O8 with monoclinic structure were observed at 28–29°. The X-ray diffraction patterns of ZTN ceramics did not have significant change throughout the sintering temperatures ranging from 950 °C to 1100 °C. Fig. 9 showed a typical pattern plot of ZTN sample sintered at 1050 °C after refinement of all parameters of interest. The crystal cell parameters was a = 4.674 Å, b = 5.659 Å, c = 5.017 Å with the same a, b, c angle of 90° and direct cell volume was 132.7208 Å3. The ZTN (ICSD #40710) reported by Baumgarte and Blachnik [30], was adopted as the starting model. The calculated pattern was overlaid on the measured pattern and the differences between two profiles were plotted along the bottom. According to the inset in Fig. 9, it was found that the measured pattern of ZTN phase fitted well with the calculated pattern with the structure of ICSD #40710 and the phase with all XRD peaks existed with nearly 100% ixiolite-structure by quantitative analysis. Changes of er, Q f and sf values as a function of sintering temperatures were shown in Fig. 10. It was found that er values of ZTN ceramics steadily increased from 26.8 to 35.3 with the increase of sintering temperature from 950 to 1050 °C, and then saturated at 35 during the temperatures of 1050–1100 °C. Based on results of sintering characteristic curves and microstructure shown in Figs. 6 and 7, it was obvious that low er values were mainly caused by pores (er 1) at sintering temperatures lower than 1050 °C. The curve of er values showed a similar tendency with those of
Fig. 10. Curves of er, Q f and sf values as a function of sintering temperatures for ZnTiNb2O8 ceramics in the temperature region of 950–1100 °C.
apparent density and shrinkage ratio, which were sensitive to dense degree of ceramics significantly. The er value obtained at 1050 °C was 35.3, which was similar with the results reported by the solid-state method [5,9,10]. For example, Kim et al. [5] reported a dielectric constant of 34, Liao and Li [9] reported that the ZTN ceramic was sintered at 1120 °C for 6 h with dielectric constant of 34.4 and Yin et al. [10] also reported that microwave dielectric ceramics ZnTiNb2O8 which sintered at 1150 °C exhibited the best high permittivity (er = 34.2). To clarify effects of crystal structure on dielectric constant, theoretical dielectric polarizability (atheo.) was calculated to be 28.99 according to additive rule with ionic polarizability of composing ions or oxides [31] as formulated in Eq. (5). While observed dielectric polarizability (aobs.) was calculated to be 29.12 by Clausius–Mossotti equation as formulated in Eq. (6) with measured dielectric constant at microwave frequencies [32]. By comparison values of atheo. and aobs. were in good agreement with each other, and the minor deviation from the aobs. and atheo. could attribute to relative density because the aobs. value depended on specimens and fabrication process.
atheo: ¼ aðZnTiNb2 O8 Þ ¼ aðZnOÞ þ aðTiO2 Þ þ aðNb2 O5 Þ 1 b
aobs: ¼ V m
Counts
e1 eþ2
ð6Þ
where a (ZnO), a (Nb2O5), and a (TiO2) represented oxides polarizabilities reported by Shannon [32]. Moreover, Vm, e and b indicated the molar volume of samples, dielectric constant and constant value (4p/3), respectively. With the increase of sintering temperatures from 950 to 1050 °C, Q f values increased from 38,600 to 67,200 GHz with saturated Q f values 66,000 GHz in the sintering temperature region of 1050–1100 °C. The remarkable increase in Q f values ranging from 950 to 1000 °C was also related to the reduction of
Fig. 8. X-ray diffraction patterns of ZnTiNb2O8 ceramics sintered at different sintering temperature from 950 °C to 1100 °C for 2 h.
40,000 35,000 30,000 25,000 20,000 15,000 10,000 5,000 0 -5,000 -10,000 10
ð5Þ
Ixiolite 100.00 %
20
30
40
50
60
70
80
90
100
110
120
2Th Degrees Fig. 9. The profile fits for the Rietveld refinement of ZnTiNb2O8 ceramics sintered at 1050 °C using the program of Topas. The agreement indices: Rwp (%) = 9.36, Rp (%) = 7.14, and GOF = 2.12. The inset show that the measured pattern of ZnTiNb2O8 phase fitted well with the calculated pattern with the structure of (ICSD # 40710).
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porosity according to results of SEM microstructures shown in Fig. 7(a) and (b). Additionally it was found that the Q f value slightly decreased to be 62,500 GHz when the samples were sintered at 1100 °C. According to the SEM results as shown in Fig. 7(d), it was believed that the non-uniform microstructure caused by abnormal grain growth would have an harmful influence on the Q f value when adopting excessively sintering process. The similar phenomenon was also found in another ceramic system [21]. ZTN ceramics sintered at 1050 °C with increased density had a Q f value of 66,700 GHz, which was significantly higher than the results reported by the solid-state method [5,9,10]. By comparison, Kim et al. [5] reported a Q f value of 33,000 GHz by the solidstate technique, Liao and Li [9] reported its best result of Q f = 56,900 GHz, Yin et al. [10] also reported that the ZTN ceramic exhibited the best microwave properties of high quality factor (Q f = 28,696 GHz). As for Q f values of dielectric ceramics, it was well known that porosity, secondary phase and structure defect of ceramics as extrinsic factors usually produced a deterioration in Q f values [33]. Among these factors, the porosity was suggested to affect Q f values obviously below 1050 °C. It was found that relative density was one of the most important factors in controlling dielectric loss, which was demonstrated on many other microwave dielectric materials. Once the as-prepared samples were of nearly full density, Q f values were mainly affected by intrinsic factors, such as crystal structure and lattice defects. As for lattice defects, dielectric loss tangent at microwave frequency was mainly determined by the anharmonic terms in the crystal’s potential energy according to by Schlömann’s theory [34,35]. Dielectric ceramics have many kinds of structural or lattice defects such as grain boundaries, voids, dislocations, point defects and substitutional ions. Rustum Roy reported that using the solution-sol–gel route had been developed to make a variety of ceramic materials, which provided major advantages in lowering sintering temperatures, refining microstructure, and controlling morphology and final phase composition [36]. The ixiolite-structure ZTN precursors were provided with well crystalline and less lattice defects for the fabrication of ceramics by the solution-sol–gel route in this work, which should contribute to the improvement of Q f values. Moreover, remarkable changes in sf values of ZTN ceramics fluctuated around 60 ppm/°C with the increase of sintering temperatures from 950 °C to 1100 °C and these values were ranged from 54.5 to 63.5 ppm/°C, which was similar with the results by other reports [5,9,10]. For example, Kim et al. [4,5] reported a similar sf value of 52 ppm/°C, Liao and Li [9] reported a slightly better value of sf = 47.94 ppm/°C. In addition, Yin et al. [10] reported a very negative temperature coefficient of resonant frequency (sf = 75.84 ppm/°C). Thus, it was considered that additional improvement in sf value and lowing sintering temperature for further by adding sintering additives were required for dielectric resonator applications at high frequency. 4. Conclusions The ixiolite-structure ZTN powders with particle sizes of 30– 50 nm were obtained successfully by the aqueous sol–gel synthetic route, which showed major advantages over reported solid-state
methods. A considerable decrease in synthesis temperature (at 700 °C) was obtained in air atmosphere for the formation of ZTN nanopowders with well crystallinity. Moreover, sintering ability and microwave properties of ZTN ceramics depending on sintering temperatures were systematically investigated. ZTN samples with nearly full densities were obtained at 1050 °C and had excellent microwave dielectric properties of er = 35.3, Q f = 66,700 GHz and sf = 55.4 ppm/°C. Acknowledgments This work was supported by the Project Development Plan of Science and Technology of Ji’nan City (No. 201303061), Ji’nan City Youth Science and Technology Star Project (No. 2013035), the National Training Plan Innovation Project of College Students (No. 201310427004) and National Natural Science Foundation (No. 51472108). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
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