Characterization and microwave dielectric properties of Mg2YVO6 ceramic

Journal of Alloys and Compounds 641 (2015) 93–98

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Characterization and microwave dielectric properties of Mg2YVO6 ceramic Chia-Hui Su, Yi-Sheng Wang, Cheng-Liang Huang ⇑ Department of Electrical Engineering, National Cheng Kung University, 1 University Road, Tainan 70101, Taiwan

a r t i c l e

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Article history: Received 26 November 2014 Received in revised form 15 March 2015 Accepted 29 March 2015 Available online 1 April 2015 Keywords: Mg2YVO6 ceramics Microwave dielectric properties Crystal structure

a b s t r a c t Tetragonal-structured Mg2YVO6 ceramics were prepared by conventional solid-state method, and their physical and microwave dielectric properties were investigated for the first time. The forming of Mg2YVO6 main phase was confirmed by XRD diffraction pattern. XPS and Raman spectrum were recorded to clarify the chemical states of elements and vibration and rotation modes of the specimen, respectively. In addition, the relationships between sintering temperature, packing fraction, and microwave dielectric properties in Mg2YVO6 ceramics were also studied. The new microwave dielectric material Mg2YVO6 ceramics sintered at 1290 °C for 4 h has a dielectric constant (er) of 10.88, a Q  f of 68,300 GHz (f = 10.389 GHz), and a sf  53.9 ppm/°C, demonstrating a candidate for microwave application. Ó 2015 Published by Elsevier B.V.

1. Introduction Due to the fast development in the wireless communication technology, the demand for high performance microwave dielectric materials has never been stopped [1–4]. For years, researchers have been continuously investigating on numbers of dielectric materials, such as perovskites ABO3, complex perovskites AA0 BB0 O3, spinel AB2O4, and wurtzite A2BO4 structures [5–8]. In addition, more researchers have been focusing on ABO4 (A = Ca, Pb, Ba, Ni, Mg, Zn and B = Mo, W) compositions recently because of their diversity for the substitution of different cations to obtain variant microwave dielectric properties [9]. The composition-related microwave dielectric properties of ABO4 were reported to have a low dielectric constant (7.05–16.44) with a high Q  f (34,000–57,000 GHz), which is significant in decreasing signal propagation delay and crosstalk for ultra high frequency applications. The ABO4 compositional family includes not only ternary structures, such as barite, scheelite, and zircon type, but also binary structures, rutile and fluorite. The microwave dielectric properties of zircon type tetragonal (Bi1–xCex)VO4, for instance, were investigated by D. Zhou et al. and reported to possess a er of 30, a Q  f of 23,900 GHz, and a sf of 200 ppm/°C [10,11]. Mg2YVO6 ceramic is also zircon type in the ABO4 family and belongs to the tetragonal crystal (a = b – c, a = b = c) system with a space group I41/amd. However, its microwave dielectric

⇑ Corresponding author. E-mail address: [email protected] (C.-L. Huang). http://dx.doi.org/10.1016/j.jallcom.2015.03.204 0925-8388/Ó 2015 Published by Elsevier B.V.

properties have yet been studied. Therefore, this paper focused on the investigation of physical and microwave dielectric properties of Mg2YVO6 ceramic. And the results were analyzed based on the densification, X-ray diffraction, XPS, Raman, scanning electron microscopy and packing fraction of the specimen. 2. Experiment The samples were synthesized by a conventional solid-state route with starting materials of high-purity oxide powders: MgO, Y2O3, and V2O5. Prepared powders were mixed by ball milling with agate media in distilled water for 24 h, and the mixtures were dried and calcined at 1000 °C for 4 h. The calcined powders were added with 5 wt% of a 10% solution of PVA as a binder, granulated by sieving through a 200 mesh, and pressed into pellets with 11 mm in diameter and 5 mm in thickness. All samples were prepared using an automatic uniaxial hydraulic press at 2000 kg/cm2. These pellets were sintered at 1230 °C–1350 °C for 4 h in air. The crystal phase of the sintered ceramics were identified by XRD using Cu Ka (k = 0.15406 nm) radiation with a Siemens D5000 diffractometer (Munich, Germany) operated at 40 kV and 40 mA. The microstructures were evaluated by scanning electron microscopy (UHR–SEM; Hitachi SU-8000). X-ray photoelectron spectroscopy (XPS) measurements were performed by a PHI 5000 Versa Probe using Al Ka radiation to obtain information on the chemical binding energy of the samples. All spectra were fitted using computer software (Origin 8.0). Raman spectra were excited with the 532 nm light source of a Microscopes Raman Spectrometer (Jobin Yvon/Labram HR). The relative densities of the sintered pellets were measured by the Archimedes method. The dielectric constant (er) and the quality factor values (Q  f) at microwave frequencies were measured using the Hakki–Coleman dielectric resonator method [12,13]. A system combining an HP8757D network analyzer (HP, Palo Alto, CA) and an HP83630 sweep oscillator (HP, Palo Alto, CA) was used in the measurement. For temperature coefficient of resonant frequency (sf), the technique is the same as that of quality factor measurement. The test cavity was placed over a thermostat in the temperature range of 25–80 °C. The sf value (ppm/°C) was calculated by noting the change in resonant frequency (Df)

94

sf ¼

C.-H. Su et al. / Journal of Alloys and Compounds 641 (2015) 93–98

f2  f1 f 1 ðT 2  T 1 Þ

ð1Þ

where f1 and f2 represent the resonant frequencies at T1 and T2, respectively.

3. Results and discussion Fig. 1 illustrates the room temperature XRD patterns recorded from the Mg2YVO6 ceramics sintered at different temperatures for 4 h. The patterns for the present ceramics can be indexed as a tetragonal Mg2YVO6 (ICDD-PDF#00-057-0663) main phase in space group I41/amd with a MgO (ICDD-PDF#45-0946) second phase detected within the detectable level of XRD. Moreover, the content of MgO in Mg2YVO6 ceramics (Table 1, calculated from XRD patterns) at different temperature is similar. Evolution of lattice parameters and cell volume of Mg2YVO6 ceramics sintered at different temperatures are presented in Table 2. The lattice parameters slightly decreased with increasing sintering temperature to a maximum and increased thereafter. However, the variation is small and <0.1%. Accordingly, it shows a minimum cell volume of 317.667 Å3 at 1290 °C. The SEM surface morphological of Mg2YVO6 ceramics sintered at different temperatures for 4 h are demonstrated in Fig. 2. At 1230 °C, the specimen seemed to be already densified with an average grain size <2 lm. It appears that average grain size increases slightly and systematically with the sintering temperature. However, degradation in grain uniformity becomes significant at 1350 °C, which would damage its dielectric properties. In order to clarify the chemical states of elements, the XPS spectra of the Mg2YVO6 ceramic sintered at 1290 °C for 4 h is shown in Fig. 3, with Fig. 3(a) displaying the whole scanning spectrum which indicates the existence of Mg, Y, V, and O elements. The Mg 2p peak, as appeared in Fig. 3(c), had a constituent peak at 49.6 eV corresponds to Mg in the metallic state, which has been attributed

Fig. 1. X-ray diffraction patterns of Mg2YVO6 ceramics sintered at different temperatures for 4 h.

Table 1 The content of MgO in Mg2YVO6 ceramics sintered at different temperatures for 4 h. Sintering temperatures (°C)

Proportion (%)

1230 1260 1290 1320 1350

11.302 11.409 11.623 11.314 12.937

Table 2 Lattice parameters and cell volume of the Mg2YVO6 ceramics sintered at different temperatures for 4 h. Sintered temperature (°C)

a = b (Å)

c (Å)

Cell volume (Å3)

Reference

Standard

7.107

6.284

317.41

1230 1260 1290 1320 1350

7.1118 ± 0.0022 7.1100 ± 0.0021 7.1093 ± 0.0011 7.1138 ± 0.0042 7.1167 ± 0.0032

6.2879 ± 0.0028 6.2861 ± 0.0026 6.2852 ± 0.0018 6.2849 ± 0.0053 6.2919 ± 0.0040

318.028 317.776 317.667 318.055 318.668

ICDDPDF#00057-0663 This work This work This work This work This work

to the formation of Mg(OH)2. The Y 3d spectrum shown in Fig. 3(c) contains two peaks originate from the spin–orbit split between Y3d5/2 and Y3d3/2, whereas the reference peak positions for Y3d5/2 and Y3d3/2 for Y2O3 are 156.8 and 158.8 eV, respectively [14]. For the two peaks in Fig. 3(d), the stronger count peak corresponds to V2p3/2, and the other peak corresponds to V2p1/2, and the spin–orbit V2p1/2–V2p3/2 peak splitting is observed to be 7.4 eV, which are consistent with the literature value for vanadium oxide [15]. In Fig. 3(e), the O 1s line is made of at least three components located at 529.7 eV and 531.3 eV are assigned to the chemisorbed oxygen in the forms of O2 and O. Typical values of oxide ions are 531.2 eV for MgO, 529.9 eV for V2O5, and 529.5 eV for Y2O3 [16–18]. The room temperature Raman spectrum of Mg2YVO6 ceramics sintered at different temperatures for 4 h (Fig. 4) shows five main modes at 446, 488, 719, 750, and 894 cm1. The 446 cm1 is assigned to O–V–O stretching vibrations, the mode at 488 is VO3 4 ions vibration is B2g mode, in the region between 700 and 900 cm1 are assigned to V–O stretching vibrations. Among them, the MgO stretching vibrations in the range from 600 to 800 cm1 is a part overlapping with V–O stretching vibrations, and the main peak of MgO strength vibration is near 719 cm1. Several weaker features are either second-order features. Mg2YVO6 ceramics with different sintering temperatures have similar vibration and rotation modes, which indicated that Mg2YVO6 ceramics are steady state with different sintering temperature [19–21]. Fig. 5 shows the relative densities and dielectric constants (er) of Mg2YVO6 ceramics sintered at different temperatures for 4 h. The theoretical density of Mg2YVO6 ceramic is 2.976 g/cm3. The relative density increased with increasing sintering temperature to the maximum of 96.58% at 1290 °C and decreased thereafter. The variation of er was consistent with that of density and a maximum er value of 10.88 was obtained at 1290 °C. Moreover, the dielectric constant only shows a slight variance (10.8–10.88) over a wide sintering temperature range (1230–1350 °C) indicating a wide process window, which would benefit to practical applications. The Q  f and packing fraction of the Mg2YVO6 ceramics at different sintering temperatures are illustrated in Fig. 6. The measured microwave dielectric loss represents the overall loss, including not only intrinsic loss related to the lattice vibration modes, but also extrinsic contributions related to density, second phases, impurities, surface morphology, and lattice defect. Since

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(a)

95

(b)

(c)

(d)

(e) Fig. 2. SEM micrographs of Mg2YVO6 ceramics sintered at (a) 1230 °C, (b) 1260 °C, (c) 1290 °C, (d) 1320 °C, and (e) 1350 °C.

the variation of Q  f was consistent with that of density, it suggested the dielectric loss of the specimen was controlled by the density and the maximum Q  f of 68,000 GHz was obtained at 1290 °C. According to Kim et al. [22], the Q  f value also can be largely dependent on the packing fraction. Based upon crystal structural considerations, the packing fraction, defined by summing the volume of packed ions over the volume of a primitive unit cell, can be expressed as [22,23]:

volume of packed ions volume of primitive unit cell volume of packed ions Z ¼ volume of unit cell

Packing fraction ð%Þ ¼

ð2Þ

where Z is the number of formula units per unit cell. Accordingly, the results confirmed the dependence of the Q  f on the packing

fraction of the ceramics and their variations are similar. It was because the increase of packing fraction leading to the decrease of lattice vibrations resulted in a decrease of intrinsic loss and therefore the Q  f increased. Fig. 7 shows the sf value of Mg2YVO6 ceramics sintered at different temperatures for 4 h. Since there is no composition or structure change, the sf was not affected by the sintering temperature as expected and it remained in the range from 52.5 to 58.3 ppm/ °C for Mg2YVO6 ceramics sintered at 1230–1350 °C. Table 3 summarizes the microwave dielectric properties of Mg2YVO6 ceramics. The typical values of er = 10.88, Q  f = 68,300 GHz (at 10.389 GHz), sf = 53.9 ppm/°C were obtained for specimen sintered at 1290 °C for 4 h. In addition, the influence of MgO (er = 7.3–7.9, Q  f = 30,000–113,000 GHz, and sf = 50 ppm/°C) [24] on the microwave dielectric properties of

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Fig. 3. XPS spectra from the surface of the Mg2YVO6 ceramic sintered at 1290 °C for 4 h: (a) survey spectrum of Mg2YVO6 ceramic; (b) Mg 2p spectrum; (c) Y 3d spectrum; (d) V 2p spectrum; and, (e) O 1s spectrum.

Mg2YVO6 should be limited because they have a similar dielectric properties and the portion of MgO is small as shown in Table 1. Noticed that the Q  f value of specimen decreased to 22,000 GHz at 1350 °C, which is even lower than that of MgO, suggesting the actual Q  f of Mg2YVO6 ceramics at 1350 °C should be lower than 22,000 GHz.

4. Conclusion Mg2YVO6 ceramics were prepared using a conventional solidstate route and their dielectric properties were investigated in the microwave frequency region. Phase formation was confirmed by the XRD and Raman results. The forming of MgO second phase

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Fig. 7. sf of Mg2YVO6 ceramics sintered at different temperatures for 4 h.

Fig. 4. Raman Spectra of Mg2YVO6 ceramics sintered at 1290 °C for 4 h.

Table 3 Microwave dielectric properties of Mg2YVO6 ceramics sintered at different temperatures for 4 h. Sintering temperature (°C)

Relative density (%)

er

Qf

sf

1230 1260 1290 1320 1350

96.31 96.29 96.58 95.65 95.03

10.80 10.83 10.88 10.86 10.85

58,000 62,000 68,000 32,000 22,000

52.5 53.8 53.9 55.7 58.3

at 1290 °C for 4 h has a good combination of microwave dielectric properties: er 10.88, Q  f 68,300 GHz, and sf  53.9 ppm/°C, which makes it a suitable candidate for applications as microwave passive components.

Acknowledgement

Fig. 5. Relative density and dielectric constant of Mg2YVO6 ceramics sintered at different temperatures for 4 h.

This work was financially supported by the National Science Council of Taiwan under grant NSC103-2221-E-006-071.

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Fig. 6. Q  f and packing fraction of Mg2YVO6 ceramics sintered at different temperatures for 4 h.

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