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Improvements in the sintering behavior and microwave dielectric properties of fergusonite-type NdNbO4 ceramics
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 907–912 www.elsevier.com/locate/ceramint
Improvements in the sintering behavior and microwave dielectric properties of fergusonite-type NdNbO4 ceramics Q.J. Mei, C.Y. Li, J.D. Guo, L.P. Zhao, H.T. Wun Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China Received 8 August 2014; received in revised form 29 August 2014; accepted 1 September 2014 Available online 6 September 2014
Abstract Microwave dielectric ceramics based on fergusonite-type NdNbO4 were prepared by an aqueous sol–gel process. The precursor powders and dielectric ceramics were characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and microwave methods. Highly reactive nano-sized magnesium titanate powders with particle sizes of 20–40 nm were successfully obtained at 900 1C as precursors. The sintering characteristics and microwave dielectric properties of NdNbO4 ceramics were studied depending on sintering temperatures ranging from 950 1C to 1300 1C. With the increase of sintering temperature, density, εr and Qƒ values increased and saturated at 1150 1C with excellent microwave properties of εr ¼ 29.8, Qƒ ¼49,010 GHz and τƒ ¼53 ppm/1C. The correlations of microstructure and dielectric properties for NdNbO4 ceramics were also investigated. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: NdNbO4; Nanopowder synthesis; Sol–gel; Microwave dielectric properties
1. Introduction Recently microwave and millimeterwave dielectric materials have been developed for wide applications in mobile and satellite communication systems such as miniaturization for mobile phones, transmitters and receivers with high performance [1–3]. The family of ortho-niobates materials has similar fergusonitetype structure (monoclinic, C2/C) and properties with RENbO4, where RE refers to lanthanoid atoms, from La to Lu, as well as Y. These materials have many exceptional characteristics making them promising in applications as multi-functional materials. Among them, the NdNbO4 ceramic with high quality factors, and an appropriate dielectric constant, became a type of well-known dielectric ceramic promisingly used as resonators, filters and antennas for communication operating at millimeter wave frequencies [4] and more recently, much attention has paid to research on its microwave dielectric properties [5–8]. For example, Kim reported that it possessed the quality factor Qf of 33,000 GHz, dielectric constant εr of 19.6, and temperature n
Corresponding author. Tel.: þ86 531 82765473; fax: þ 86 531 87974453. E-mail address: [email protected] (H.T. Wu).
http://dx.doi.org/10.1016/j.ceramint.2014.09.008 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
coefficients of the resonant frequency τƒ of 24 ppm/1C [4]. At present the synthesis of NdNbO4 phase was performed by traditional solid-state reaction methods. The mixed powders were usually calcined at 1100 1C for 2 h and then sintered at temperature over 1250 1C. Obviously high sintering temperature of the ceramics would limit their applications for practical cases. Usually it was believed that lowering the sintering temperature could be achieved by many methods such as chemical processing, adding glass flux, and using starting materials with smaller particle sizes. As we know, adding glass flux usually caused a detrimental effect on the microwave properties of ceramics [9,10]. In order to reduce the sintering temperature and improve the sintering ability there were many other investigations of the chemical processing or other special methods [11,12], which had been developed as alternatives to the conventional solid-state reaction of mixed oxides for producing ceramics by using starting materials with smaller particle sizes. Among all these methods, the sol–gel was undoubtedly one of a useful process for producing powders with good control over stoichiometry and homogeneity, yielding nano-sized particles and widely used in many other ceramics systems [13–16]. However, few researches about the preparation of NdNbO4 ceramics by the aqueous sol–gel process are reported in the present literature.
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Q.J. Mei et al. / Ceramics International 41 (2015) 907–912
The primary objectives of present research are to synthesize pure and nano-sized NdNbO4 powders as precursors for the preparation of ceramics by the simple and unexpensive aqueous sol–gel method. The whole process involved complexation of aqueous metal ions with 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, NdNbO4 nano-particles were synthesized at relatively low temperatures. Additionally microwave dielectric properties of NdNbO4 ceramics as a function of sintering temperatures were investigated in detail. The results showed that the preparation of NdNbO4 ceramics with excellent microwave properties could be obtained at low sintering temperatures by the aqueous sol–gel process. 2. Experimental Analytical-grade Nb2O5, K2CO3, Nd(NO3)3, HNO3, citric acid (CA) and ethylene glycol (EG) were used as raw materials to synthesize the NdNbO4 nanopowders. Firstly, the mixture of Nb2O5 and K2CO3 was co-melted at 900 1C in order to obtain the K3NbO4 phase according to the phase diagram. Secondly, the K3NbO4 phase was dissolved in distilled water; then, the solution was set at pH 2 to form a precipitate of Nb(OH)5. The whole formation process of the Nb(OH)5 phase could be formulated from Eqs. (1) to (4). Thirdly, the Nb(OH)5 precipitate was filtered off and washed with distilled water six times to remove the K þ ions and then dissolved completely in citric acid water solution by continuous magnetic stirring at 300 rpm for 15 min.. Meanwhile, a stoichiometric amount of Nd(NO3)3 was added to the above solution and then the solution was stirred for another 30 min. Finally, ethylalcohol (20–40 ml) was added to the as-prepared mixed solution in drops and stirred for 1 h to form a transparent and stable sol. The pH of the solution was maintained in the range of 3.5–5 by adding buffering agents. The sol was heated at 80–90 1C for 1 h to obtain a xerogel. The xerogel was decomposed at various temperatures ranging from 700 1C to 900 1C 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 1C for 4 h to expel the binder and then sintered at selected temperatures for 2 h in air at a heating rate of 5 1C/min. 3K2 CO3 þ Nb2 O5 -2K3 NbO4 þ CO2
ð1Þ
6K3 NbO4 þ 5H2 O-18K þ þ Nb6 O819 þ 10OH
ð2Þ
449 F3, NETZSCH Co., Germany). Phase analysis of NdNbO4 powder was conducted with the help of a Rigaku diffractometer (Model D/MAX-B, Rigaku Co., Japan) using Ni filtered CuKα radiation (λ=0.1542 nm) at 40 kV and 40 mA settings. Based on XRD analysis, the morphology and particle sizes were examined using a transmission electron microscope (Model JEOL JEM2010, FEI Co., Japan). A network analyzer (Model N5234A, Agilent Co., America) was used for the measurement of microwave dielectric properties. Dielectric constants were measured using the Hakki–Coleman post-resonator method by exciting the TE011 resonant mode of dielectric resonator using an electric probe as suggested by Hakki and Coleman [17]. Unloaded quality factors were measured using the TE01d mode by the cavity method [18]. All measurements were made at room temperature and in the frequency of 8–10 GHz. Temperature coefficients of resonant frequency were measured in the temperature range of 25–85 1C. 3. Results and discussion TG–DTA curves of the NdNbO4 xerogel at a heating rate of 10 1C/min are shown in Fig. 1. It was indicated that all chemical reactions involving weight loss, such as decomposition of the organic polymeric network, finished completely below 500 1C. According to TG curves, the total weight loss was about 90% and occurred in two steps. Firstly obvious weight loss began at 200 1C and initial weight loss (about 50%) was below 400 1C, resulting from the evaporation of residual solvent and the decomposition of the organic polymeric network with evolution of CO2 and H2O. Secondly weight loss (about 20–40%) occurred in TG curves, combined with a significantly exothermal peak in the temperature region of 400–550 1C, which was attributed to the oxidation of metalorganic groups. TG results of the NdNbO4 xerogel were also similar to those of other ceramic xerogels synthesized by sol– gel methods [19–21]. It was obvious that no further significant weight loss and thermometric peaks were observed above 600 1C in TG-DTA curves, which indicated the minimum firing temperature to synthesize the magnesium titanate phases. The XRD patterns of NdNbO4 xerogel calcined at temperatures ranging from 700 to 900 1C for 60 min in air atmosphere are shown in Fig. 2. It was found that the crystallization of the
4K þ þ Nb6 O819 þ 4OH þ 8H þ -K4 H4 Nb6 O19 ↓ þ 4H2 O ð3Þ K4 H4 Nb6 O19 þ 15H þ þ 11OH -6NbðOHÞ5 ↓ þ 4K þ
ð4Þ
In order to analyze the evolution of NdNbO4 phase, the asformed NdNbO4 xerogel was characterized using thermogravimetry (TG) and differential thermal analysis (DTA) (Model STA
Fig. 1. TG–DTA curves of Nd–Nb xerogel in air atmosphere.
Q.J. Mei et al. / Ceramics International 41 (2015) 907–912
orthorhombic-structure of NdNb5O14 took place at 700 1C as the intermediate phase, which was in agreement with the XRD pattern of JCPDS no. 22-0737. The XRD pattern of the xerogel fired at 800 1C still consisted of predominant peaks of NdNb5O14 with sharper peaks, while at 900 1C predominant peaks of fergusonite-type NdNbO4 were found matching with JCPDS no. 32-0680. According to the XRD result it was indicated that the calcination temperature for synthesizing NdNbO4 phase was remarkably decreased to 900 1C, which was lower than the conventional mixed oxide route reported earlier [4–6].The TEM micrograph of NdNbO4 nanopowders calcined at 900 1C is illustrated in Fig. 3. It was worth noting that the particles were well-distributed with high uniformity and are basically regular in shape. The particle sizes were measured by the liner intercept method [22] and the range of particle sizes was about 20–40 nm. Curves of apparent densities and diametric shrinkage ratio of NdNbO4 ceramics sintered for 2 h at sintering temperatures ranging from 950 1C to 1300 1C are plotted in Fig. 4, through
Fig. 2. X-ray diffraction patterns of Nd–Nb xerogel calcined at 700–900 1C for 30 min.
Fig. 3. TEM micrograph of raw NdNbO4 nanopowders calcined at 900 1C for 30 min.
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which the optimized sintering temperature could be determined. Here NdNbO4 ceramic had a theoretical density of 6.335 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 3.9 to 5.9 g/cm3 with sintering temperatures increasing from 950 to 1150 1C. At 1150 1C a saturated value of apparent densities was found to be nearly 5.9 g/cm3 and the curve of diametric shrinkage ratio also showed the similar tendency. Based on the result of sintering characteristics, it was concluded that the ceramics of NdNbO4 sintered at 1150 1C for 2 h had full density. In order to characterize the microstructure of NdNbO4 ceramics sintered at different temperatures, SEM micrographs obtained from 1000 to 1250 1C are illustrated in Fig. 5(a–e). It was easily found that many more pores existed in samples sintered at 1000 1C and 1100 1C shown in Fig. 5(a) and (b). Also the apparent porosity decreased as sintering temperature increased from 1000 1C to 1100 1C and all pores almost disappeared at 1150 1C on the surface of NdNbO4 samples. With increase of sintering temperature from 1150 1C to 1250 1C shown in Fig. 5(c–e), the grain size increased rapidly and average grain sizes measured was less than 1 μm at 1150 1C as shown in Fig. 5(c). Therefore, it was obvious that NdNbO4 ceramics were successfully prepared with full density via the sol–gel process at 1150 1C for 4 h and the sintering temperature was reduced significantly compared to the solid-state reaction methods [4–6]. Kim et al. [4] reported that the ceramics were sintered at 1250 1C for 2 h by the solid-state method. In addition, EDS analysis about grains chosen randomly from the samples sintered at 1150 1C are shown in Fig. 5(f). The inset in Fig. 5(f) presents a quantitative result about elementary composition. The concentrations of Nd, Nb and O ions in the grain were analyzed to be 15.35, 17.61 and 67.04 at%, respectively. The ratio of Nd/Nb/O was approximately corresponding to the formula of NdNbO4 phase. The XRD patterns of NdNbO4 ceramics sintered at 1000–1250 1C are shown in Fig. 6. The predominant phases were identified as the fergusonite-type with the space group of 12/a (15) and no secondary phases of the orthorhombic-structure of NdNb5O14 existed. The X-ray diffraction patterns of NdNbO4 ceramics did not have significant change throughout the sintering temperature range from 1000 1C to 1250 1C.
Fig. 4. Apparent densities and shrinkage ratio of NdNbO4 ceramics as a function of sintering temperatures.
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Fig. 5. FE-SEM micrographs of NdNbO4 ceramics sintered at different sintering temperatures for 2 h ((a)–(e) corresponding to 1000 1C, 1100 1C, 1150 1C, 1200 1C, 1250 1C) and EDS analysis shown in (f).
Changes of εr, Qƒ and τƒ values as a function of sintering temperatures were shown in Fig. 7. It was found that εr values of NdNbO4 ceramics steadily increased from 9.1 to 29.8 with the increase of sintering temperature from 950 to 1150 1C, and then
saturated at E30 during the temperature the 1150–1300 1C. Based on results of sintering characteristic curves and the microstructure shown in Figs. 4 and 5, it was obvious that low εr values were caused by pores (εr E1) at sintering temperatures
Q.J. Mei et al. / Ceramics International 41 (2015) 907–912
Fig. 6. X-ray diffraction patterns of NdNbO4 ceramics sintered at different sintering temperatures for 2 h.
Fig. 7. Curves of εr, Qƒ and τƒ values as a function of sintering temperatures for NdNbO4 ceramics in the temperature region of 950–1300 1C.
lower than 1150 1C. The curve of εr values showed a similar tendency to those of apparent density and shrinkage ratio, which were sensitive to dense degree of ceramics significantly. The εr value obtained at 1150 1C was 29.8, which was significantly higher than the results reported by the solid-state method [4]. To clarify the effects of crystal structure on dielectric constant, the theoretical dielectric polarizability (αtheo) was calculated to be 17.02, according to the additive rule with ionic polarizability of composing ions or oxides [23] as formulated in Eq. (5), while the observed dielectric polarizability (αobs) was calculated to be 17.15 according to the Clausius–Mossotti equation as formulated in Eq. (6) with measured dielectric constant at microwave frequencies [24]. By comparison the values of αtheo and αobs were in good agreement with each other, and the minor deviation between the αobs and αtheo values could be attributed to relative density because the αobs value depended on specimens and the fabrication process: αtheo ¼ αðNdNbO4 Þ ¼ αðNd3 þ Þþ αðNb5 þ Þþ 4αðO2 Þ αobs ¼
1 ε1 Vm b εþ2
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where α (MgO) and α (TiO2) represented oxide polarizabilities reported by Shannon [23]. Moreover, Vm, ε and b indicated the molar volume of samples, the dielectric constant and a constant (4π/3), respectively. With the increase of sintering temperatures from 950 to 1150 1C, Qƒ values increased from 31,600 to 49,010 GHz with saturated Qƒ values of E 50,000 GHz in the sintering temperature region of 1150–1300 1C. The remarkable increase in Qƒ values ranging from 950 to 1150 1C was also related to the reduction of porosity according to results of SEM microstructures shown in Fig. 5(a–c). As for Qƒ 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 Qƒ values [25]. Among these factors, the porosity was suggested to affect Qƒ values obviously below 1150 1C. It was found that relative density was one of the most important factors in controlling dielectric loss, which was demonstrated in many other microwave dielectric materials. NdNbO4 ceramics sintered at 1150 1C with increased density had a Qƒ value of 49,010 GHz, which was significantly higher than the results reported by the solid-state method [4]. For example, Kim et al. [4] reported a Qƒ value of 33,000 GHz by the solid-state technique. Once the as-prepared samples were of nearly full density, Qƒ values were mainly affected by intrinsic factors, such as crystal structure and lattice vibrations, and especially covalence of cation–oxygen bonds. Thus, covalence of cation–oxygen bonds of compounds was also calculated by using refined bond length and the correlation of covalence and bond length was further discussed in detail in the next work. Moreover, remarkable changes in τƒ values of NdNbO4 ceramics fluctuated around 50 ppm/1C with the increase of sintering temperatures from 1000 1C to 1300 1C and these values ranged from 25.7 to 91.5 ppm/1C, which was not similar to the results by other reports [4–6]. Thus, it was considered that additional improvement in τƒ value was required for dielectric resonator applications at high frequency. 4. Conclusions Fergusonite-type NdNbO4 powders with particle sizes of 20–40 nm were obtained successfully by the aqueous sol–gel synthetic route, which showed major advantages over other reported methods. A considerable decrease in synthesis temperature (at 900 1C) was obtained in air atmosphere for the formation of NdNbO4 nanopowders. Moreover, the sintering ability and microwave properties of NdNbO4 ceramics depending on sintering temperatures were systematically investigated. NdNbO4 samples with nearly full densities were obtained at 1150 1C and had excellent microwave dielectric properties of εr ¼ 29.8, Qƒ¼ 49,010 GHz and τƒ ¼ 53.8 ppm/1C.
ð5Þ
Acknowledgments
ð6Þ
This work was supported by the project development plan of Science and Technology of Ji'nan City (no. 201303061), Ji'nan
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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 of China (no. 51472108). References [1] A. Vanderah. Terrell, Talking ceramics, Science 298 (2002) 1182–1184. [2] H. Ohsato, Functional advances of microwave dielectrics for next generation, Ceram. Int. 38 (2012) S141–S146. [3] M.T. Sebastian, in: Dielectric Materials for Wireless Communications, 1st ed., Elsevier Publishers, Oxford, 2008. [4] D.W. Kim, D.K. Kwon, S.H. Yoon, K.S. Hong, Microwave dielectric properties of rare-earth ortho-niobates with ferroelasticity, J. Am. Ceram. Soc. 89 (2006) 3861–3864. [5] P. Zhang, T. Wang, W.S. Xia, L.X. Li, Microwave dielectric properties of a new ceramic system NdNbO4 with CaF2 addition, J. Alloys Compd. 535 (2012) 1–4. [6] P. Zhang, Z.K. Song, Y. Wang, Y.M. Han, H.L. Dong, L.X. Li, Effect of CaTiO3 addition on microwave dielectric properties of NdNbO4 ceramics as multi-function material, J. Alloys Compd. 581 (2013) 741–746. [7] Z.K. Song, P. Zhang, Y. Wang, L.X. Li, Improved quality factor of NdNbO4 microwave dielectric ceramic by Mn2 þ substitution, J. Alloys Compd. 583 (2014) 546–549. [8] P. Zhang, Z.K. Song, B.L. Gao, Y. Wang, T. Wang, Y.M. Han, W.S. Xia, L.X. Li, Improved sintering characteristics and microwave dielectric properties of NdNbO4 ceramic system, Mater. Res. Innov. 18 (2014) 284–288. [9] H.K. Li, W.Z. Lu, W. Lei, Microwave dielectric properties of Li2ZnTi3O8 ceramics doped with ZnO–B2O3 frit, Mater. Lett. 71 (2012) 148–150. [10] J. Zhang, Y.Y. Zhou, Z.X. Yue., Low-temperature sintering and microwave dielectric properties of LiF-doped CaMg1 xZnxSi2O6 ceramics, Ceram. Int. 39 (2013) 2051–2058. [11] Y.C. Zhang, B.J. Fu, Y.H. Liu, Y.H. Liu, X. Wang, T. Li, Z.X. Yue, Synthesis of MgTa2O6 nano-powders by citrate sol–gel method, J. Alloys Compd. 505 (2010) 750–753. [12] S.C. Navale, V. Samuel, V. Ravi, Low temperature synthesis of MgTa2O6 powders, Mater. Lett. 59 (2005) 3926–3928.
[13] X.Y. Yang, X.H. Wang, M. Huang, S.P. Zhang, L.T. Li, Synthesis and characterization of CaTiO3–(Sm,Nd)AlO3 microwave ceramics via sol– gel method, J. Sol–Gel Sci. Technol. 69 (2014) 61–66. [14] H.T. Wu, L.X. Li, Q. Zou, Q.W. Liao, P.F. Ning, P. Zhang, Synthesis, characterization, and microwave dielectric properties of Mg4Nb2O9 ceramics produced through the aqueous sol–gel process, J. Alloys Compd. 509 (2011) 2232–2237. [15] C.X. Hu, P. Liu, Preparation and microwave dielectric properties of SiO2 ceramics by aqueous sol–gel technique, J. Alloys Compd. 559 (2013) 129–133. [16] M.Z. Dong, Z.X. Yue, H. Zhuang, S.Q. Meng, L.T. Li, Microstructure and microwave dielectric properties of TiO2-doped Zn2SiO4 ceramics synthesized through the sol–gel process, J. Am. Ceram. Soc. 91 (2008) 3981–3985. [17] B.W. Hakki, P.D. Coleman, A dielectric resonator method of measuring inductive capacities in the millimeter range, IEEE Trans. 8 (1960) 402–410. [18] W.E. Courtney, Analysis and evaluation of a method of measuring the complex permittivity and permeability of microwave insulators, IEEE Trans. 18 (1970) 476–485. [19] H.T. Wu, Y.S. Jiang, Y.L. Yue, Low-temperature synthesis and microwave dielectric properties of trirutile-structure MgTa2O6 ceramics by aqueous sol–gel process, Ceram. Int. 38 (2012) 5151–5156. [20] H.T. Wu, Y.S. Jiang, Y.J. Cui, X.H. Zhang, X. Jia, Y.L. Yue, Improvements in the sintering behavior and microwave dielectric properties of Geikielite-type MgTiO3 ceramics, J. Electron. Mater. 42 (2013) 445–451. [21] H.T. Wu, Y.S. Jiang, W.B. Wu, F. Yang, Y.L. Yue, Synthesis and microwave dielectric properties of columbite-structure MgNb2O6 ceramics by aqueous sol–gel technique, J. Electroceram. 28 (2012) 191–196. [22] A. Thorvaldsen, The intercept method-2. Determination of spatial grain size, Acta Mater. 45 (1997) 595–600. [23] R.D. Shannon, G.R. Rossman, Dielectric constants of silicate garnets and the oxide additivity rule, Am. Miner. 77 (1992) 94–100. [24] R.D. Shannon, Dielectric polarizabilities of ions in oxides and fluorides, J. Appl. Phys. 73 (1993) 348–366. [25] S.J. Penn, M.N. Alford, A. Templeton, X. Wang, M. Xu, M. Reece, K. Schrapel, Effect of porosity and grain size on the microwave dielectric properties of sintered alumina, J. Am. Ceram. Soc. 80 (1997) 1885–1888.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 907–912 www.elsevier.com/locate/ceramint
Improvements in the sintering behavior and microwave dielectric properties of fergusonite-type NdNbO4 ceramics Q.J. Mei, C.Y. Li, J.D. Guo, L.P. Zhao, H.T. Wun Shandong Provincial Key Laboratory of Preparation and Measurement of Building Materials, University of Jinan, Jinan 250022, China Received 8 August 2014; received in revised form 29 August 2014; accepted 1 September 2014 Available online 6 September 2014
Abstract Microwave dielectric ceramics based on fergusonite-type NdNbO4 were prepared by an aqueous sol–gel process. The precursor powders and dielectric ceramics were characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and microwave methods. Highly reactive nano-sized magnesium titanate powders with particle sizes of 20–40 nm were successfully obtained at 900 1C as precursors. The sintering characteristics and microwave dielectric properties of NdNbO4 ceramics were studied depending on sintering temperatures ranging from 950 1C to 1300 1C. With the increase of sintering temperature, density, εr and Qƒ values increased and saturated at 1150 1C with excellent microwave properties of εr ¼ 29.8, Qƒ ¼49,010 GHz and τƒ ¼53 ppm/1C. The correlations of microstructure and dielectric properties for NdNbO4 ceramics were also investigated. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: NdNbO4; Nanopowder synthesis; Sol–gel; Microwave dielectric properties
1. Introduction Recently microwave and millimeterwave dielectric materials have been developed for wide applications in mobile and satellite communication systems such as miniaturization for mobile phones, transmitters and receivers with high performance [1–3]. The family of ortho-niobates materials has similar fergusonitetype structure (monoclinic, C2/C) and properties with RENbO4, where RE refers to lanthanoid atoms, from La to Lu, as well as Y. These materials have many exceptional characteristics making them promising in applications as multi-functional materials. Among them, the NdNbO4 ceramic with high quality factors, and an appropriate dielectric constant, became a type of well-known dielectric ceramic promisingly used as resonators, filters and antennas for communication operating at millimeter wave frequencies [4] and more recently, much attention has paid to research on its microwave dielectric properties [5–8]. For example, Kim reported that it possessed the quality factor Qf of 33,000 GHz, dielectric constant εr of 19.6, and temperature n
Corresponding author. Tel.: þ86 531 82765473; fax: þ 86 531 87974453. E-mail address: [email protected] (H.T. Wu).
http://dx.doi.org/10.1016/j.ceramint.2014.09.008 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
coefficients of the resonant frequency τƒ of 24 ppm/1C [4]. At present the synthesis of NdNbO4 phase was performed by traditional solid-state reaction methods. The mixed powders were usually calcined at 1100 1C for 2 h and then sintered at temperature over 1250 1C. Obviously high sintering temperature of the ceramics would limit their applications for practical cases. Usually it was believed that lowering the sintering temperature could be achieved by many methods such as chemical processing, adding glass flux, and using starting materials with smaller particle sizes. As we know, adding glass flux usually caused a detrimental effect on the microwave properties of ceramics [9,10]. In order to reduce the sintering temperature and improve the sintering ability there were many other investigations of the chemical processing or other special methods [11,12], which had been developed as alternatives to the conventional solid-state reaction of mixed oxides for producing ceramics by using starting materials with smaller particle sizes. Among all these methods, the sol–gel was undoubtedly one of a useful process for producing powders with good control over stoichiometry and homogeneity, yielding nano-sized particles and widely used in many other ceramics systems [13–16]. However, few researches about the preparation of NdNbO4 ceramics by the aqueous sol–gel process are reported in the present literature.
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Q.J. Mei et al. / Ceramics International 41 (2015) 907–912
The primary objectives of present research are to synthesize pure and nano-sized NdNbO4 powders as precursors for the preparation of ceramics by the simple and unexpensive aqueous sol–gel method. The whole process involved complexation of aqueous metal ions with 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, NdNbO4 nano-particles were synthesized at relatively low temperatures. Additionally microwave dielectric properties of NdNbO4 ceramics as a function of sintering temperatures were investigated in detail. The results showed that the preparation of NdNbO4 ceramics with excellent microwave properties could be obtained at low sintering temperatures by the aqueous sol–gel process. 2. Experimental Analytical-grade Nb2O5, K2CO3, Nd(NO3)3, HNO3, citric acid (CA) and ethylene glycol (EG) were used as raw materials to synthesize the NdNbO4 nanopowders. Firstly, the mixture of Nb2O5 and K2CO3 was co-melted at 900 1C in order to obtain the K3NbO4 phase according to the phase diagram. Secondly, the K3NbO4 phase was dissolved in distilled water; then, the solution was set at pH 2 to form a precipitate of Nb(OH)5. The whole formation process of the Nb(OH)5 phase could be formulated from Eqs. (1) to (4). Thirdly, the Nb(OH)5 precipitate was filtered off and washed with distilled water six times to remove the K þ ions and then dissolved completely in citric acid water solution by continuous magnetic stirring at 300 rpm for 15 min.. Meanwhile, a stoichiometric amount of Nd(NO3)3 was added to the above solution and then the solution was stirred for another 30 min. Finally, ethylalcohol (20–40 ml) was added to the as-prepared mixed solution in drops and stirred for 1 h to form a transparent and stable sol. The pH of the solution was maintained in the range of 3.5–5 by adding buffering agents. The sol was heated at 80–90 1C for 1 h to obtain a xerogel. The xerogel was decomposed at various temperatures ranging from 700 1C to 900 1C 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 1C for 4 h to expel the binder and then sintered at selected temperatures for 2 h in air at a heating rate of 5 1C/min. 3K2 CO3 þ Nb2 O5 -2K3 NbO4 þ CO2
ð1Þ
6K3 NbO4 þ 5H2 O-18K þ þ Nb6 O819 þ 10OH
ð2Þ
449 F3, NETZSCH Co., Germany). Phase analysis of NdNbO4 powder was conducted with the help of a Rigaku diffractometer (Model D/MAX-B, Rigaku Co., Japan) using Ni filtered CuKα radiation (λ=0.1542 nm) at 40 kV and 40 mA settings. Based on XRD analysis, the morphology and particle sizes were examined using a transmission electron microscope (Model JEOL JEM2010, FEI Co., Japan). A network analyzer (Model N5234A, Agilent Co., America) was used for the measurement of microwave dielectric properties. Dielectric constants were measured using the Hakki–Coleman post-resonator method by exciting the TE011 resonant mode of dielectric resonator using an electric probe as suggested by Hakki and Coleman [17]. Unloaded quality factors were measured using the TE01d mode by the cavity method [18]. All measurements were made at room temperature and in the frequency of 8–10 GHz. Temperature coefficients of resonant frequency were measured in the temperature range of 25–85 1C. 3. Results and discussion TG–DTA curves of the NdNbO4 xerogel at a heating rate of 10 1C/min are shown in Fig. 1. It was indicated that all chemical reactions involving weight loss, such as decomposition of the organic polymeric network, finished completely below 500 1C. According to TG curves, the total weight loss was about 90% and occurred in two steps. Firstly obvious weight loss began at 200 1C and initial weight loss (about 50%) was below 400 1C, resulting from the evaporation of residual solvent and the decomposition of the organic polymeric network with evolution of CO2 and H2O. Secondly weight loss (about 20–40%) occurred in TG curves, combined with a significantly exothermal peak in the temperature region of 400–550 1C, which was attributed to the oxidation of metalorganic groups. TG results of the NdNbO4 xerogel were also similar to those of other ceramic xerogels synthesized by sol– gel methods [19–21]. It was obvious that no further significant weight loss and thermometric peaks were observed above 600 1C in TG-DTA curves, which indicated the minimum firing temperature to synthesize the magnesium titanate phases. The XRD patterns of NdNbO4 xerogel calcined at temperatures ranging from 700 to 900 1C for 60 min in air atmosphere are shown in Fig. 2. It was found that the crystallization of the
4K þ þ Nb6 O819 þ 4OH þ 8H þ -K4 H4 Nb6 O19 ↓ þ 4H2 O ð3Þ K4 H4 Nb6 O19 þ 15H þ þ 11OH -6NbðOHÞ5 ↓ þ 4K þ
ð4Þ
In order to analyze the evolution of NdNbO4 phase, the asformed NdNbO4 xerogel was characterized using thermogravimetry (TG) and differential thermal analysis (DTA) (Model STA
Fig. 1. TG–DTA curves of Nd–Nb xerogel in air atmosphere.
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orthorhombic-structure of NdNb5O14 took place at 700 1C as the intermediate phase, which was in agreement with the XRD pattern of JCPDS no. 22-0737. The XRD pattern of the xerogel fired at 800 1C still consisted of predominant peaks of NdNb5O14 with sharper peaks, while at 900 1C predominant peaks of fergusonite-type NdNbO4 were found matching with JCPDS no. 32-0680. According to the XRD result it was indicated that the calcination temperature for synthesizing NdNbO4 phase was remarkably decreased to 900 1C, which was lower than the conventional mixed oxide route reported earlier [4–6].The TEM micrograph of NdNbO4 nanopowders calcined at 900 1C is illustrated in Fig. 3. It was worth noting that the particles were well-distributed with high uniformity and are basically regular in shape. The particle sizes were measured by the liner intercept method [22] and the range of particle sizes was about 20–40 nm. Curves of apparent densities and diametric shrinkage ratio of NdNbO4 ceramics sintered for 2 h at sintering temperatures ranging from 950 1C to 1300 1C are plotted in Fig. 4, through
Fig. 2. X-ray diffraction patterns of Nd–Nb xerogel calcined at 700–900 1C for 30 min.
Fig. 3. TEM micrograph of raw NdNbO4 nanopowders calcined at 900 1C for 30 min.
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which the optimized sintering temperature could be determined. Here NdNbO4 ceramic had a theoretical density of 6.335 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 3.9 to 5.9 g/cm3 with sintering temperatures increasing from 950 to 1150 1C. At 1150 1C a saturated value of apparent densities was found to be nearly 5.9 g/cm3 and the curve of diametric shrinkage ratio also showed the similar tendency. Based on the result of sintering characteristics, it was concluded that the ceramics of NdNbO4 sintered at 1150 1C for 2 h had full density. In order to characterize the microstructure of NdNbO4 ceramics sintered at different temperatures, SEM micrographs obtained from 1000 to 1250 1C are illustrated in Fig. 5(a–e). It was easily found that many more pores existed in samples sintered at 1000 1C and 1100 1C shown in Fig. 5(a) and (b). Also the apparent porosity decreased as sintering temperature increased from 1000 1C to 1100 1C and all pores almost disappeared at 1150 1C on the surface of NdNbO4 samples. With increase of sintering temperature from 1150 1C to 1250 1C shown in Fig. 5(c–e), the grain size increased rapidly and average grain sizes measured was less than 1 μm at 1150 1C as shown in Fig. 5(c). Therefore, it was obvious that NdNbO4 ceramics were successfully prepared with full density via the sol–gel process at 1150 1C for 4 h and the sintering temperature was reduced significantly compared to the solid-state reaction methods [4–6]. Kim et al. [4] reported that the ceramics were sintered at 1250 1C for 2 h by the solid-state method. In addition, EDS analysis about grains chosen randomly from the samples sintered at 1150 1C are shown in Fig. 5(f). The inset in Fig. 5(f) presents a quantitative result about elementary composition. The concentrations of Nd, Nb and O ions in the grain were analyzed to be 15.35, 17.61 and 67.04 at%, respectively. The ratio of Nd/Nb/O was approximately corresponding to the formula of NdNbO4 phase. The XRD patterns of NdNbO4 ceramics sintered at 1000–1250 1C are shown in Fig. 6. The predominant phases were identified as the fergusonite-type with the space group of 12/a (15) and no secondary phases of the orthorhombic-structure of NdNb5O14 existed. The X-ray diffraction patterns of NdNbO4 ceramics did not have significant change throughout the sintering temperature range from 1000 1C to 1250 1C.
Fig. 4. Apparent densities and shrinkage ratio of NdNbO4 ceramics as a function of sintering temperatures.
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Fig. 5. FE-SEM micrographs of NdNbO4 ceramics sintered at different sintering temperatures for 2 h ((a)–(e) corresponding to 1000 1C, 1100 1C, 1150 1C, 1200 1C, 1250 1C) and EDS analysis shown in (f).
Changes of εr, Qƒ and τƒ values as a function of sintering temperatures were shown in Fig. 7. It was found that εr values of NdNbO4 ceramics steadily increased from 9.1 to 29.8 with the increase of sintering temperature from 950 to 1150 1C, and then
saturated at E30 during the temperature the 1150–1300 1C. Based on results of sintering characteristic curves and the microstructure shown in Figs. 4 and 5, it was obvious that low εr values were caused by pores (εr E1) at sintering temperatures
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Fig. 6. X-ray diffraction patterns of NdNbO4 ceramics sintered at different sintering temperatures for 2 h.
Fig. 7. Curves of εr, Qƒ and τƒ values as a function of sintering temperatures for NdNbO4 ceramics in the temperature region of 950–1300 1C.
lower than 1150 1C. The curve of εr values showed a similar tendency to those of apparent density and shrinkage ratio, which were sensitive to dense degree of ceramics significantly. The εr value obtained at 1150 1C was 29.8, which was significantly higher than the results reported by the solid-state method [4]. To clarify the effects of crystal structure on dielectric constant, the theoretical dielectric polarizability (αtheo) was calculated to be 17.02, according to the additive rule with ionic polarizability of composing ions or oxides [23] as formulated in Eq. (5), while the observed dielectric polarizability (αobs) was calculated to be 17.15 according to the Clausius–Mossotti equation as formulated in Eq. (6) with measured dielectric constant at microwave frequencies [24]. By comparison the values of αtheo and αobs were in good agreement with each other, and the minor deviation between the αobs and αtheo values could be attributed to relative density because the αobs value depended on specimens and the fabrication process: αtheo ¼ αðNdNbO4 Þ ¼ αðNd3 þ Þþ αðNb5 þ Þþ 4αðO2 Þ αobs ¼
1 ε1 Vm b εþ2
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where α (MgO) and α (TiO2) represented oxide polarizabilities reported by Shannon [23]. Moreover, Vm, ε and b indicated the molar volume of samples, the dielectric constant and a constant (4π/3), respectively. With the increase of sintering temperatures from 950 to 1150 1C, Qƒ values increased from 31,600 to 49,010 GHz with saturated Qƒ values of E 50,000 GHz in the sintering temperature region of 1150–1300 1C. The remarkable increase in Qƒ values ranging from 950 to 1150 1C was also related to the reduction of porosity according to results of SEM microstructures shown in Fig. 5(a–c). As for Qƒ 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 Qƒ values [25]. Among these factors, the porosity was suggested to affect Qƒ values obviously below 1150 1C. It was found that relative density was one of the most important factors in controlling dielectric loss, which was demonstrated in many other microwave dielectric materials. NdNbO4 ceramics sintered at 1150 1C with increased density had a Qƒ value of 49,010 GHz, which was significantly higher than the results reported by the solid-state method [4]. For example, Kim et al. [4] reported a Qƒ value of 33,000 GHz by the solid-state technique. Once the as-prepared samples were of nearly full density, Qƒ values were mainly affected by intrinsic factors, such as crystal structure and lattice vibrations, and especially covalence of cation–oxygen bonds. Thus, covalence of cation–oxygen bonds of compounds was also calculated by using refined bond length and the correlation of covalence and bond length was further discussed in detail in the next work. Moreover, remarkable changes in τƒ values of NdNbO4 ceramics fluctuated around 50 ppm/1C with the increase of sintering temperatures from 1000 1C to 1300 1C and these values ranged from 25.7 to 91.5 ppm/1C, which was not similar to the results by other reports [4–6]. Thus, it was considered that additional improvement in τƒ value was required for dielectric resonator applications at high frequency. 4. Conclusions Fergusonite-type NdNbO4 powders with particle sizes of 20–40 nm were obtained successfully by the aqueous sol–gel synthetic route, which showed major advantages over other reported methods. A considerable decrease in synthesis temperature (at 900 1C) was obtained in air atmosphere for the formation of NdNbO4 nanopowders. Moreover, the sintering ability and microwave properties of NdNbO4 ceramics depending on sintering temperatures were systematically investigated. NdNbO4 samples with nearly full densities were obtained at 1150 1C and had excellent microwave dielectric properties of εr ¼ 29.8, Qƒ¼ 49,010 GHz and τƒ ¼ 53.8 ppm/1C.
ð5Þ
Acknowledgments
ð6Þ
This work was supported by the project development plan of Science and Technology of Ji'nan City (no. 201303061), Ji'nan
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