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Sintering characteristics and microwave dielectric properties of low loss MgZrNb2O8 ceramics achieved by reaction sintering process
Journal of Alloys and Compounds 687 (2016) 274e279
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Sintering characteristics and microwave dielectric properties of low loss MgZrNb2O8 ceramics achieved by reaction sintering process H.L. Pan, C.F. Xing, J.X. Bi, X.S. Jiang, Y.X. Mao, Haitao Wu* School of Materials Science and Engineering, University of Jinan, Jinan, 250022, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2016 Received in revised form 31 May 2016 Accepted 6 June 2016 Available online 8 June 2016
Wolframite structure MgZrNb2O8 ceramics were prepared by a simple and effective reaction sintering method for the first time. The crystal structure, microstructure and phase composition were studied by the X-ray diffraction, scanning electron microscopy (SEM) techniques and energy disperse spectroscopy (EDS), respectively. Sintering characteristics and microwave dielectric properties were investigated under various sintering temperatures ranging from 1150 C to 1350 C. In addition, bond energy, bond valence and the distortion of oxygen octahedral were calculated to evaluate the structural characteristics. At the sintering temperature of 1250 C, the MgZrNb2O8 ceramics exhibited the excellent microwave properties of εr ¼ 26.54, Q·G ¼ 57,477 GHz and tf ¼ 17.69 ppm/ C. These results demonstrated that MgZrNb2O8 ceramics could be promising candidate materials for the application of highly selective microwave ceramic resonators and filters. © 2016 Elsevier B.V. All rights reserved.
Keywords: MgZrNb2O8 Reaction sintering Microwave dielectric properties Bond energy
1. Introduction With the rapid development of modern communication technologies such as global positioning system (GPS), satellite communication and wireless local area network (WLAN), microwave dielectric materials have been widely applied in microwave components including resonators, filters and antennas [1e3]. As the core materials, microwave dielectric ceramics should meet three kinds of critical characteristics, a high dielectric constant (εr) to reduce the size of the components, a high quality factor (Q·f) to increase frequency selectivity and a near zero temperature coefficient of resonant frequency (tf) to guarantee high temperature stability [4,5]. Recently, the researches of low dielectric loss microwave materials based on a new class of MOeZrO2eNb2O5 (M ¼ Mg, Zn, Co, Mn) system have begun [6e8]. The ternary system has a monoclinic wolframite structure belonging to the P2/c space group. Among such compounds, the MgZrNb2O8 (MZN) ceramics possessed the excellent microwave dielectric properties. Ramarao et al. [9] reported that MZN ceramics exhibited microwave dielectric properties of εr ¼ 9.6, Q·G ¼ 58,500 GHz and tf ¼ 31.5 ppm/ C at the sintering temperature of 1500 C by the conventional solid state
* Corresponding author. E-mail address: [email protected] (H. Wu). http://dx.doi.org/10.1016/j.jallcom.2016.06.029 0925-8388/© 2016 Elsevier B.V. All rights reserved.
method. The crystal cell parameters were a ¼ 4.80936 Å, b ¼ 5.63804 Å, c ¼ 5.07981 Å with the b angle of 91.626 . In our previous work, MZN ceramics sintered at 1225 C had the microwave dielectric properties of εr ¼ 27.08, Q·G ¼ 29,070 GHz and tf ¼ 24.31 ppm/ C by the conventional solid state method [10]. However, the high sintering temperature and lower dielectric properties limited the applications in microwave devices. The reaction sintering (RS) method was a simple and effective process and has been attracted more attention due to its high efficiency by avoiding calcination and second milling process. The major difference between RS process and conventional solid state method was that the particles in the compact pellets prior to sintering were not highly agglomerated clusters as those in the calcined or high-energy milled powders [11]. Therefore, the RS method could enhance the densification progress. Several materials, such as ZnZrNb2O8, ZnNb2O6, ZnTiNb2O8 ceramics [12e14], had been successfully prepared using the new method. Until now, the investigations on the preparation of low loss dielectric materials of the MZN ceramic by the RS method have not been reported. In the present study, firstly, MZN ceramics were prepared successfully through the RS process. Crystal structure and microstructure were also investigated. Subsequently, microwave dielectric properties as a function of sintering temperatures were investigated in detail. Moreover, the refinement based on X-ray technique was also used to analyze the structure of the crystalline phase. Finally, bond energy, distortion of the oxygen octahedral and
H.L. Pan et al. / Journal of Alloys and Compounds 687 (2016) 274e279
bond valence were calculated to evaluate the structural characteristics of MZN ceramics. 1.1. Experimental procedure Highly pure powder of MgO, ZrO2 and Nb2O5 (all from Sigma Aldrich Co, USA) was used to prepare the MZN ceramics by RS method as shown in Fig. 1. The mixed powders according to desired stoichiometry were milled for 6 h with distilled water in a nylon container. After drying, all the slurries were crushed and sieved with a 60 mesh screen, then with polyvinyl alcohol as a binder. Latter, the sieved powder was pressed into pellets with 10 mm in diameter and 5 mm in thickness at a pressure of 200 MPa. Finally, all these pellets were preheated at 500 C for 4 h to expel the binder and then sintered at selected temperatures for 4 h in air at a heating rate of 5 C/min. The apparent densities of the sintered pellets were measured using the Archimedes method. The theoretical density was obtained from the crystal structure and atomic weight [15]. The crystalline phases of the sintered samples were identified by the Xray diffraction (Model D/MAX-B, Rigaku Co., Japan) using Ni filtered CuKa radiation (l ¼ 0.1542 nm) at 40 kV and 40 mA settings. Based on XRD analysis, The network analyzer (N5234A, Agilent Co., America) was used for the measurement of microwave dielectric properties. Dielectric constants were measured by Hakki-Coleman post-resonator method using an electric probe as suggested by Hakki and Coleman [16]. Unloaded quality factors were measured using TE01d mode by the cavity method [17]. All measurements were conducted at the room temperature and in the frequency of 8e12 GHz. The temperature coefficient of resonant frequency (tG) was measured in the temperatures ranging from 25 C to 85 C and calculated by the following formula.
tf ¼
f2 f1 f1 ðT2 T1 Þ
(1)
where G1 and G2 were the resonant frequency at T1 and T2, respectively. 2. Results and discussion Curves of apparent densities and diametric shrinkage ratio of
275
MZN ceramics sintered at 1150e1350 C were plotted in Fig. 2. MZN ceramic had a theoretical density of 5.18 g/cm3 and its shrinkage tendency was characterized by the ratio of diametric size before and after the ceramics sintering. With the sintering temperatures increasing from 1150 C to 1350 C, the apparent densities increased from 3.38 to 4.87 g/cm3. At 1250 C a relative saturated value was found to be 4.87 g/cm3 and the relative density reached as high as 94.02%. The curve of diametric shrinkage ratio also showed the similar tendency with the maximum value of 14.55% at 1250 C. Based on the results of sintering characteristics, it was concluded that the MZN ceramics sintered at 1250 C for 4 h had nearly full density. The micro-structures of the MZN ceramics sintered at different sintering temperature using the RS method were presented in Fig. 3. It was easily found that many pores existed in MZN ceramics sintered at 1150e1200 C as shown in Fig. 3 (aeb) and all pores almost disappeared at 1250 C as shown in Fig. 3(c). With the sintering temperature increasing from 1250 C to 1350 C, abnormal grain growth could be found as shown in Fig. 3 (dee). Therefore, the MZN ceramics were successfully prepared through the RS process at 1250 C. In addition, EDS analysis about grains chosen randomly from the sample signed A was shown in Fig. 3(f) and the quantitative result about elementary composition was presented. The concentrations of Mg, Zr, Nb and O ions were analyzed to be 7.27, 7.76, 15.90 and 69.07 at%, respectively. The ratio of Mg/Zr/Nb/O was approximately corresponding to the formula of MZN phase. The XRD patterns of MZN ceramics using the RS method were shown in Fig. 4. The predominant phase was identified as the MZN phase. The X-ray diffraction patterns of MZN ceramics did not obvious change throughout the sintering temperatures ranging from 1150 C to 1350 C. The predominant phase formed at 1150 C to 1200 C and the XRD patterns became sharper with the sintering temperatures increasing from 1250 to 1350 C, especially for the peaks of (111) and (111) planes. Most of the peaks matched with the JCPDS number of 48-0329 with a unknown peak at the sintering temperature of 1350 C, which would have a harmful influence on the microwave dielectric properties. That the reflections were shifted to the lower angle was attributed to the increase of the unit cell volume as shown in Table 3. The MZN sample sintered at 1250 C was refined and the refinement parameters such as the crystal cell parameters, atomic positions and occupancies were given in Table 1.
High purity powders (MgO, ZrO2, Nb2O5)
Milled for 6 h and dried
Sieved through a 60 mesh with binder
Pressed into pellets
Sintered at different temperatures for 4 h Fig. 1. Charts for the preparation of MgZrNb2O8 ceramics by the reaction sintering process.
Fig. 2. Curves of apparent densities and diametric shrinkage ratio of MgZrNb2O8 ceramics sintered for 4 h depending on sintering temperatures from 1150 C to 1350 C.
276
H.L. Pan et al. / Journal of Alloys and Compounds 687 (2016) 274e279
Fig. 3. FE-SEM micrographs of MgZrNb2O8 ceramics sintered at different sintering temperatures for 4 h ((a)e(e) corresponding to 1150 C, 1200 C, 1250 C, 1300 C and 1350 C) and EDS analysis (f).
Fig. 4. X-ray diffraction patterns of MgZrNb2O8 ceramics sintered at different sintering temperatures from 1150 C to 1350 C for 4 h.
Changes of εr, Q·G and tf values as a function of sintering temperatures were shown in Fig. 5. The εr value initially increased and then decreased after reaching a maximum value at 1250 C. The εr value was dependent on the density, dielectric polarizabilities and structural characteristics such as the distortion, tilting and rattling spaces of oxygen octahedron in the unit cell [18,19,20]. Based on the results of sintering characteristic curves and micro-structure as shown in Figs. 2 and 3, The εr increased with the decrease of pores
and grain boundaries, while its degradation was due to the poor densification. The curve of εr values showed a similar tendency with those of apparent density and shrinkage ratio, which was sensitive to dense degree of ceramics significantly. The εr value obtained at 1250 C were compared to other reports [9,10]. For example, V.R.K. Murthy et al. [9] reported a lower dielectric constant of 9.6, J.D. Guo et al. [10] also reported that the MZN ceramics sintered at 1225 C exhibited the permittivity of εr ¼ 27.08. Therefore, the RS process showed the significant advantages than the conventional solid state method. As for the correlation between crystal structure and dielectric constant, the additive rule with ionic polarizability of composing ions or oxides [21] could be used for the calculation of theoretical dielectric polarizability (atheo.). The result was calculated to be 27.06 as formulated in Eq. (2). For the samples prepared by the RS process, the observed dielectric polarizability (aobs.) was calculated to be 26.54 based on the Clausius-Mossotti equation as formulated in Eq. (3) with measured dielectric constant at microwave frequencies [22]. By comparison values of atheo. and aobs. were in good agreement with each other for the samples and the minor deviation from the aobs. and atheo. could attribute to relative density because the aobs. value was easily affected by specimens and fabrication process.
aðMgZrNb2 O8 Þ ¼ a Mg2þ þ a Zr 4þ þ 2a Nb5þ þ 8a O2
(2) 1 b
aobs: ¼ Vm
ε1 εþ2
(3)
where a (Mg2þ) ¼ 1.32 Å3, a (Nb5þ) ¼ 3.97 Å3, a (O2) ¼ 2.01 Å3, and
H.L. Pan et al. / Journal of Alloys and Compounds 687 (2016) 274e279
277
Table 1 The refinement parameters of MgZrNb2O8 ceramic sintered at 1250 C. Lattice parameters ( )
Lattice parameters (Å) a MgZrNb2O8
4.8265
b
c
5.6626
5.1099
Vol (Å3)
a
b
g
90.0000
91.7943
90.0000
139.5900
Atom
x
y
z
Occupation
Mg Nb O1 O2 Zr
0.5000 0.0000 0.2241 0.2737 0.5000
0.6912 0.1852 0.1082 0.3882 0.6912
0.2500 0.2500 0.9385 0.4163 0.2500
0.5000 1.0000 2.0000 2.0000 0.5000
obtained by the chemical bond and the electronegativity [25e27]. The bond energy were calculated to evaluate the structural characteristics and then used to establish the relationship with the microwave dielectric properties. Zhang et al. [28e31] reported that the bond energy had a relationship with the tf values and the change of |tf| values showed a opposite tendency with the bond energy values in the NdNbO4 ceramics, ZnZrNb2O8 ceramics and the Li2MgTi3O8 ceramics. Based on the electro-negativity and bond energy theory, the bond energy E of a complex crystal could be written as the following Eqs. (4)e(7).
E¼
X m Eb
(4)
m
m
Fig. 5. Curves of εr, Q·G and tf values as a function of sintering temperatures for MgZrNb2O8 ceramics in the different temperature regions of 1150e1350 C.
where Eb was bond energy for the type m bond, which was m composed of non-polar covalence energy Ec and complete ionicity m energy Ei parts as follows. m
m
m
Eb ¼ tc Ec þ ti Ei
a (Zr4þ) ¼ 3.25 Å3 reported by Shannon [22]. Moreover, Vm, ε and b indicated the molar volume of samples, dielectric constant and constant value (4p/3), respectively. It had been reported that Q·f depended on the extrinsic factors such as density, impurity, secondary phase and grain size, and intrinsic factors such as crystal structure and lattice defects [23,24]. In the present study, with the increasing of sintering temperatures, the Q·G values increased to a maximum value and thereafter slightly decreased as shown in Fig. 5. The remarkable increase of the Q·G values ranging from 1150 C to 1250 C was explained by the reduction of porosity and the growth of grain according to the results of SEM microstructures shown in Fig. 4 (aec). Additionally, it was found that the Q·G value slightly decreased to be 46,133 GHz at the temperature of 1350 C. According to the SEM results as shown in Fig. 4 (dee), the abnormal grain growth and the second phase would have a harmful influence on the Q·G value when adopting excessively sintering process. By comparison, Murthy et al. [9] reported a higher Q·G value of 58,500 GHz by the solid-state technique at the sintering temperature of 1500 C, Guo et al. [10] also reported that the excellent result of Q·G ¼ 29,070 GHz by the solidstate method. Therefore, the RS process showed the significant advantages than the conventional solid state method on microwave dielectric properties. In addition, It was well known that the shorter bond length correlated with higher bond energy, and higher bond energy indicated that the system would be more stable. Because the bond length was shorter, the overlap degree of the electron cloud was bigger. More energies were demanded to fracture the bond and the bond was more stronger, which indicated that the system would be more stable. Sanderson reported that the bond energy could be
(5) m
The energy of the ionic form Ei was the unit charge product divided by the bond length dm, adjusted to kcal/mol by the factor 33,200 when the bond length was pm. m
Ei ¼
33200 dm
(6)
For any binary crystal AmBn type compounds, the non-polar m covalence energy Ec parts could be calculated as following: m
Ec ¼
ðrcA þ rcB Þ ðEAA EBB Þ1=2 dm
(7)
where rcA and rcB were the covalent radii, EA-A and EB-B were the bond energy. In this paper, EZreZr ¼ 298.2 kJ/mol, ENbeNb ¼ 513.0 kJ/ mol, EMgeMg ¼ 11.3 kJ/mol, and EOeO ¼ 498.4 kJ/mol could be obtained from the handbook of bond energies [32]. The final results were obtained as shown in Table 2. Moreover, the tf values of MZN ceramics fluctuated around 16 ppm/ C with the increasing of sintering temperatures, which was similar with the results reported by others. For example, V.R.K. Murthy et al. [9] reported a slightly worse tf value of 31.5 ppm/ C, J.D. Guo et al. [10] also reported a similar value of tf ¼ 24.31 ppm/ C. The samples sintered at 1250 C had the t value of 17.69 ppm/ f C, which showed the advantage over the conventional sintering process. Based on the refinement, the crystal parameters, Nb-site bond length, bond valence (VNbeO) and distortion of the oxygen octahedral with different sintering temperatures were obtained as shown in Table 3. The bond valence of the [BO6] oxygen octahedral was calculated by Eqs. (8)e(9) [33].
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Table 2 The bond energy of MgZrNb2O8 ceramics was calculated at the sintering temperature of 1250 C. The 1 and 2 were used to distinguish the oxygen atoms with different bond lengths. Bond type
Bond length
A site bond (kJ/mol)
B site bond (kJ/mol)
Ec (kJ/mol)
Ei (kJ/mol)
E (kJ/mol)
MgeO(1) MgeO(2)1 MgeO(2)2 ZreO(1)1 ZreO(2)2 ZreO(2) NbeO(1)1 NbeO(1)2 NbeO(2) Total
2.0366 2.0897 2.2168 2.0366 2.0897 2.2168 1.8867 1.9968 2.1761 e
11.3000 11.3000 11.3000 298.2000 298.2000 298.2000 513.0000 513.0000 513.0000
498.3600 498.3600 498.3600 498.3600 498.3600 498.3600 498.3600 498.3600 498.3600
74.4314 72.5401 68.3810 410.7517 400.3144 377.3624 562.7904 531.7592 487.9448 2986.2756
681.7362 664.4131 626.3190 681.7362 664.4131 626.3190 735.9008 695.3245 638.0332 6014.1951
290.0246 282.6550 266.4490 506.0479 493.1891 464.9121 616.1661 582.1918 534.2221 4035.8579
Table 3 The crystal parameters, Nb-site bond length, bond valence (VNbeO) and distortion of the oxygen octahedral of MgZrNb2O8 ceramics were calculated with different sintering temperatures. The 1 and 2 were used to distinguish the oxygen atoms with different bond lengths. Sintering temperature ( C)
1150
1200
1250
1300
1350
a (Å) b (Å) c (Å) a( ) b( ) g( ) V (Å3) NbeO(1)12(Å) NbeO(1)22(Å) NbeO(2)2(Å) RNbeO (Å) VNbeO
4.8054 5.6511 5.0747 90.0000 91.3680 90.0000 137.7700 1.8866 1.9816 2.1771 1.9110 8.3984 14.4162
4.8172 5.6503 5.0854 90.0000 91.3797 90.0000 138.3800 1.8859 1.9830 2.1739 1.9110 8.3686 14.2980
4.8265 5.6626 5.1099 90.0000 91.7943 90.0000 139.5900 1.8867 1.9968 2.1761 1.9110 8.4893 14.2677
4.8526 5.6694 5.1081 90.0000 91.4089 90.0000 140.4900 1.8957 1.9939 2.1835 1.9110 8.5984 14.2168
4.8424 5.6727 5.1139 90.0000 91.4335 90.0000 140.4300 1.8945 1.9945 2.1834 1.9110 8.5951 14.2228
Doctahedroal distortion (%)
Vij ¼
X
vij
(8)
Rij dij vij ¼ exp b0
(9)
where the Vij represented the sum of all of valences from a given atom i, dij indicated the length of a bond between atoms i and j, Rij indicated the bond valence parameter, and b0 indicated a universal constant equal to 0.37. Curves of Q·G values and bond valence values as a function of sintering temperatures for MZN ceramics were
shown in Fig. 6. The Q·G values of the samples had a close relation with the B-site bond valence and moved toward to the positive direction with the increasing of B-site bond valence, which agreed with the findings of Kim et al. [34]. Kim et al. [35] reported that tf had a closely relation with the distortion of the oxygen octahedral. In addition, the distortion of the oxygen octahedral was formulated by Eq. (10) [36].
Doctahedron ¼
B O distancemax B O distancemin B O distanceaverage
(10)
The correlation among the oxygen octahedral distortion values and the tf values were shown in Fig. 6. The distortion values of the oxygen octahedral showed a similar tendency with the tf values, which agreed with the results of Kim et al. [37].
3. Conclusions
Fig. 6. Curves of Q·G values and bond valence values and the correlations among the tf values and oxygen octahedral distortion values as a function of sintering temperatures from 1150 C to 1350 C.
The wolframite structure MZN ceramics were obtained successfully by the RS process, which showed obvious advantages than the conventional solid-state method. Subsequently, microwave properties depending on sintering temperatures were investigated systematically. Refinement was used to analyze the crystal structure of MZN ceramics sintered at 1250 C and obtained the parameters. Moreover, for the first time, bond energy, distortion of the oxygen octahedral and bond valence were calculated to evaluate the structural characteristics. MZN samples with nearly full density were obtained at 1250 C and had excellent microwave dielectric properties of εr ¼ 26.54, Q·G ¼ 57,477 GHz and tf ¼ 17.69 ppm/ C, which would make these ceramics promising for applications in microwave components.
H.L. Pan et al. / Journal of Alloys and Compounds 687 (2016) 274e279
Acknowledgments This work was supported by the National Training Plan Innovation Project of college students (No.201510427001), National Natural Science Foundation (No. 51472108). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Sintering characteristics and microwave dielectric properties of low loss MgZrNb2O8 ceramics achieved by reaction sintering process H.L. Pan, C.F. Xing, J.X. Bi, X.S. Jiang, Y.X. Mao, Haitao Wu* School of Materials Science and Engineering, University of Jinan, Jinan, 250022, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 April 2016 Received in revised form 31 May 2016 Accepted 6 June 2016 Available online 8 June 2016
Wolframite structure MgZrNb2O8 ceramics were prepared by a simple and effective reaction sintering method for the first time. The crystal structure, microstructure and phase composition were studied by the X-ray diffraction, scanning electron microscopy (SEM) techniques and energy disperse spectroscopy (EDS), respectively. Sintering characteristics and microwave dielectric properties were investigated under various sintering temperatures ranging from 1150 C to 1350 C. In addition, bond energy, bond valence and the distortion of oxygen octahedral were calculated to evaluate the structural characteristics. At the sintering temperature of 1250 C, the MgZrNb2O8 ceramics exhibited the excellent microwave properties of εr ¼ 26.54, Q·G ¼ 57,477 GHz and tf ¼ 17.69 ppm/ C. These results demonstrated that MgZrNb2O8 ceramics could be promising candidate materials for the application of highly selective microwave ceramic resonators and filters. © 2016 Elsevier B.V. All rights reserved.
Keywords: MgZrNb2O8 Reaction sintering Microwave dielectric properties Bond energy
1. Introduction With the rapid development of modern communication technologies such as global positioning system (GPS), satellite communication and wireless local area network (WLAN), microwave dielectric materials have been widely applied in microwave components including resonators, filters and antennas [1e3]. As the core materials, microwave dielectric ceramics should meet three kinds of critical characteristics, a high dielectric constant (εr) to reduce the size of the components, a high quality factor (Q·f) to increase frequency selectivity and a near zero temperature coefficient of resonant frequency (tf) to guarantee high temperature stability [4,5]. Recently, the researches of low dielectric loss microwave materials based on a new class of MOeZrO2eNb2O5 (M ¼ Mg, Zn, Co, Mn) system have begun [6e8]. The ternary system has a monoclinic wolframite structure belonging to the P2/c space group. Among such compounds, the MgZrNb2O8 (MZN) ceramics possessed the excellent microwave dielectric properties. Ramarao et al. [9] reported that MZN ceramics exhibited microwave dielectric properties of εr ¼ 9.6, Q·G ¼ 58,500 GHz and tf ¼ 31.5 ppm/ C at the sintering temperature of 1500 C by the conventional solid state
* Corresponding author. E-mail address: [email protected] (H. Wu). http://dx.doi.org/10.1016/j.jallcom.2016.06.029 0925-8388/© 2016 Elsevier B.V. All rights reserved.
method. The crystal cell parameters were a ¼ 4.80936 Å, b ¼ 5.63804 Å, c ¼ 5.07981 Å with the b angle of 91.626 . In our previous work, MZN ceramics sintered at 1225 C had the microwave dielectric properties of εr ¼ 27.08, Q·G ¼ 29,070 GHz and tf ¼ 24.31 ppm/ C by the conventional solid state method [10]. However, the high sintering temperature and lower dielectric properties limited the applications in microwave devices. The reaction sintering (RS) method was a simple and effective process and has been attracted more attention due to its high efficiency by avoiding calcination and second milling process. The major difference between RS process and conventional solid state method was that the particles in the compact pellets prior to sintering were not highly agglomerated clusters as those in the calcined or high-energy milled powders [11]. Therefore, the RS method could enhance the densification progress. Several materials, such as ZnZrNb2O8, ZnNb2O6, ZnTiNb2O8 ceramics [12e14], had been successfully prepared using the new method. Until now, the investigations on the preparation of low loss dielectric materials of the MZN ceramic by the RS method have not been reported. In the present study, firstly, MZN ceramics were prepared successfully through the RS process. Crystal structure and microstructure were also investigated. Subsequently, microwave dielectric properties as a function of sintering temperatures were investigated in detail. Moreover, the refinement based on X-ray technique was also used to analyze the structure of the crystalline phase. Finally, bond energy, distortion of the oxygen octahedral and
H.L. Pan et al. / Journal of Alloys and Compounds 687 (2016) 274e279
bond valence were calculated to evaluate the structural characteristics of MZN ceramics. 1.1. Experimental procedure Highly pure powder of MgO, ZrO2 and Nb2O5 (all from Sigma Aldrich Co, USA) was used to prepare the MZN ceramics by RS method as shown in Fig. 1. The mixed powders according to desired stoichiometry were milled for 6 h with distilled water in a nylon container. After drying, all the slurries were crushed and sieved with a 60 mesh screen, then with polyvinyl alcohol as a binder. Latter, the sieved powder was pressed into pellets with 10 mm in diameter and 5 mm in thickness at a pressure of 200 MPa. Finally, all these pellets were preheated at 500 C for 4 h to expel the binder and then sintered at selected temperatures for 4 h in air at a heating rate of 5 C/min. The apparent densities of the sintered pellets were measured using the Archimedes method. The theoretical density was obtained from the crystal structure and atomic weight [15]. The crystalline phases of the sintered samples were identified by the Xray diffraction (Model D/MAX-B, Rigaku Co., Japan) using Ni filtered CuKa radiation (l ¼ 0.1542 nm) at 40 kV and 40 mA settings. Based on XRD analysis, The network analyzer (N5234A, Agilent Co., America) was used for the measurement of microwave dielectric properties. Dielectric constants were measured by Hakki-Coleman post-resonator method using an electric probe as suggested by Hakki and Coleman [16]. Unloaded quality factors were measured using TE01d mode by the cavity method [17]. All measurements were conducted at the room temperature and in the frequency of 8e12 GHz. The temperature coefficient of resonant frequency (tG) was measured in the temperatures ranging from 25 C to 85 C and calculated by the following formula.
tf ¼
f2 f1 f1 ðT2 T1 Þ
(1)
where G1 and G2 were the resonant frequency at T1 and T2, respectively. 2. Results and discussion Curves of apparent densities and diametric shrinkage ratio of
275
MZN ceramics sintered at 1150e1350 C were plotted in Fig. 2. MZN ceramic had a theoretical density of 5.18 g/cm3 and its shrinkage tendency was characterized by the ratio of diametric size before and after the ceramics sintering. With the sintering temperatures increasing from 1150 C to 1350 C, the apparent densities increased from 3.38 to 4.87 g/cm3. At 1250 C a relative saturated value was found to be 4.87 g/cm3 and the relative density reached as high as 94.02%. The curve of diametric shrinkage ratio also showed the similar tendency with the maximum value of 14.55% at 1250 C. Based on the results of sintering characteristics, it was concluded that the MZN ceramics sintered at 1250 C for 4 h had nearly full density. The micro-structures of the MZN ceramics sintered at different sintering temperature using the RS method were presented in Fig. 3. It was easily found that many pores existed in MZN ceramics sintered at 1150e1200 C as shown in Fig. 3 (aeb) and all pores almost disappeared at 1250 C as shown in Fig. 3(c). With the sintering temperature increasing from 1250 C to 1350 C, abnormal grain growth could be found as shown in Fig. 3 (dee). Therefore, the MZN ceramics were successfully prepared through the RS process at 1250 C. In addition, EDS analysis about grains chosen randomly from the sample signed A was shown in Fig. 3(f) and the quantitative result about elementary composition was presented. The concentrations of Mg, Zr, Nb and O ions were analyzed to be 7.27, 7.76, 15.90 and 69.07 at%, respectively. The ratio of Mg/Zr/Nb/O was approximately corresponding to the formula of MZN phase. The XRD patterns of MZN ceramics using the RS method were shown in Fig. 4. The predominant phase was identified as the MZN phase. The X-ray diffraction patterns of MZN ceramics did not obvious change throughout the sintering temperatures ranging from 1150 C to 1350 C. The predominant phase formed at 1150 C to 1200 C and the XRD patterns became sharper with the sintering temperatures increasing from 1250 to 1350 C, especially for the peaks of (111) and (111) planes. Most of the peaks matched with the JCPDS number of 48-0329 with a unknown peak at the sintering temperature of 1350 C, which would have a harmful influence on the microwave dielectric properties. That the reflections were shifted to the lower angle was attributed to the increase of the unit cell volume as shown in Table 3. The MZN sample sintered at 1250 C was refined and the refinement parameters such as the crystal cell parameters, atomic positions and occupancies were given in Table 1.
High purity powders (MgO, ZrO2, Nb2O5)
Milled for 6 h and dried
Sieved through a 60 mesh with binder
Pressed into pellets
Sintered at different temperatures for 4 h Fig. 1. Charts for the preparation of MgZrNb2O8 ceramics by the reaction sintering process.
Fig. 2. Curves of apparent densities and diametric shrinkage ratio of MgZrNb2O8 ceramics sintered for 4 h depending on sintering temperatures from 1150 C to 1350 C.
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Fig. 3. FE-SEM micrographs of MgZrNb2O8 ceramics sintered at different sintering temperatures for 4 h ((a)e(e) corresponding to 1150 C, 1200 C, 1250 C, 1300 C and 1350 C) and EDS analysis (f).
Fig. 4. X-ray diffraction patterns of MgZrNb2O8 ceramics sintered at different sintering temperatures from 1150 C to 1350 C for 4 h.
Changes of εr, Q·G and tf values as a function of sintering temperatures were shown in Fig. 5. The εr value initially increased and then decreased after reaching a maximum value at 1250 C. The εr value was dependent on the density, dielectric polarizabilities and structural characteristics such as the distortion, tilting and rattling spaces of oxygen octahedron in the unit cell [18,19,20]. Based on the results of sintering characteristic curves and micro-structure as shown in Figs. 2 and 3, The εr increased with the decrease of pores
and grain boundaries, while its degradation was due to the poor densification. The curve of εr values showed a similar tendency with those of apparent density and shrinkage ratio, which was sensitive to dense degree of ceramics significantly. The εr value obtained at 1250 C were compared to other reports [9,10]. For example, V.R.K. Murthy et al. [9] reported a lower dielectric constant of 9.6, J.D. Guo et al. [10] also reported that the MZN ceramics sintered at 1225 C exhibited the permittivity of εr ¼ 27.08. Therefore, the RS process showed the significant advantages than the conventional solid state method. As for the correlation between crystal structure and dielectric constant, the additive rule with ionic polarizability of composing ions or oxides [21] could be used for the calculation of theoretical dielectric polarizability (atheo.). The result was calculated to be 27.06 as formulated in Eq. (2). For the samples prepared by the RS process, the observed dielectric polarizability (aobs.) was calculated to be 26.54 based on the Clausius-Mossotti equation as formulated in Eq. (3) with measured dielectric constant at microwave frequencies [22]. By comparison values of atheo. and aobs. were in good agreement with each other for the samples and the minor deviation from the aobs. and atheo. could attribute to relative density because the aobs. value was easily affected by specimens and fabrication process.
aðMgZrNb2 O8 Þ ¼ a Mg2þ þ a Zr 4þ þ 2a Nb5þ þ 8a O2
(2) 1 b
aobs: ¼ Vm
ε1 εþ2
(3)
where a (Mg2þ) ¼ 1.32 Å3, a (Nb5þ) ¼ 3.97 Å3, a (O2) ¼ 2.01 Å3, and
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277
Table 1 The refinement parameters of MgZrNb2O8 ceramic sintered at 1250 C. Lattice parameters ( )
Lattice parameters (Å) a MgZrNb2O8
4.8265
b
c
5.6626
5.1099
Vol (Å3)
a
b
g
90.0000
91.7943
90.0000
139.5900
Atom
x
y
z
Occupation
Mg Nb O1 O2 Zr
0.5000 0.0000 0.2241 0.2737 0.5000
0.6912 0.1852 0.1082 0.3882 0.6912
0.2500 0.2500 0.9385 0.4163 0.2500
0.5000 1.0000 2.0000 2.0000 0.5000
obtained by the chemical bond and the electronegativity [25e27]. The bond energy were calculated to evaluate the structural characteristics and then used to establish the relationship with the microwave dielectric properties. Zhang et al. [28e31] reported that the bond energy had a relationship with the tf values and the change of |tf| values showed a opposite tendency with the bond energy values in the NdNbO4 ceramics, ZnZrNb2O8 ceramics and the Li2MgTi3O8 ceramics. Based on the electro-negativity and bond energy theory, the bond energy E of a complex crystal could be written as the following Eqs. (4)e(7).
E¼
X m Eb
(4)
m
m
Fig. 5. Curves of εr, Q·G and tf values as a function of sintering temperatures for MgZrNb2O8 ceramics in the different temperature regions of 1150e1350 C.
where Eb was bond energy for the type m bond, which was m composed of non-polar covalence energy Ec and complete ionicity m energy Ei parts as follows. m
m
m
Eb ¼ tc Ec þ ti Ei
a (Zr4þ) ¼ 3.25 Å3 reported by Shannon [22]. Moreover, Vm, ε and b indicated the molar volume of samples, dielectric constant and constant value (4p/3), respectively. It had been reported that Q·f depended on the extrinsic factors such as density, impurity, secondary phase and grain size, and intrinsic factors such as crystal structure and lattice defects [23,24]. In the present study, with the increasing of sintering temperatures, the Q·G values increased to a maximum value and thereafter slightly decreased as shown in Fig. 5. The remarkable increase of the Q·G values ranging from 1150 C to 1250 C was explained by the reduction of porosity and the growth of grain according to the results of SEM microstructures shown in Fig. 4 (aec). Additionally, it was found that the Q·G value slightly decreased to be 46,133 GHz at the temperature of 1350 C. According to the SEM results as shown in Fig. 4 (dee), the abnormal grain growth and the second phase would have a harmful influence on the Q·G value when adopting excessively sintering process. By comparison, Murthy et al. [9] reported a higher Q·G value of 58,500 GHz by the solid-state technique at the sintering temperature of 1500 C, Guo et al. [10] also reported that the excellent result of Q·G ¼ 29,070 GHz by the solidstate method. Therefore, the RS process showed the significant advantages than the conventional solid state method on microwave dielectric properties. In addition, It was well known that the shorter bond length correlated with higher bond energy, and higher bond energy indicated that the system would be more stable. Because the bond length was shorter, the overlap degree of the electron cloud was bigger. More energies were demanded to fracture the bond and the bond was more stronger, which indicated that the system would be more stable. Sanderson reported that the bond energy could be
(5) m
The energy of the ionic form Ei was the unit charge product divided by the bond length dm, adjusted to kcal/mol by the factor 33,200 when the bond length was pm. m
Ei ¼
33200 dm
(6)
For any binary crystal AmBn type compounds, the non-polar m covalence energy Ec parts could be calculated as following: m
Ec ¼
ðrcA þ rcB Þ ðEAA EBB Þ1=2 dm
(7)
where rcA and rcB were the covalent radii, EA-A and EB-B were the bond energy. In this paper, EZreZr ¼ 298.2 kJ/mol, ENbeNb ¼ 513.0 kJ/ mol, EMgeMg ¼ 11.3 kJ/mol, and EOeO ¼ 498.4 kJ/mol could be obtained from the handbook of bond energies [32]. The final results were obtained as shown in Table 2. Moreover, the tf values of MZN ceramics fluctuated around 16 ppm/ C with the increasing of sintering temperatures, which was similar with the results reported by others. For example, V.R.K. Murthy et al. [9] reported a slightly worse tf value of 31.5 ppm/ C, J.D. Guo et al. [10] also reported a similar value of tf ¼ 24.31 ppm/ C. The samples sintered at 1250 C had the t value of 17.69 ppm/ f C, which showed the advantage over the conventional sintering process. Based on the refinement, the crystal parameters, Nb-site bond length, bond valence (VNbeO) and distortion of the oxygen octahedral with different sintering temperatures were obtained as shown in Table 3. The bond valence of the [BO6] oxygen octahedral was calculated by Eqs. (8)e(9) [33].
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Table 2 The bond energy of MgZrNb2O8 ceramics was calculated at the sintering temperature of 1250 C. The 1 and 2 were used to distinguish the oxygen atoms with different bond lengths. Bond type
Bond length
A site bond (kJ/mol)
B site bond (kJ/mol)
Ec (kJ/mol)
Ei (kJ/mol)
E (kJ/mol)
MgeO(1) MgeO(2)1 MgeO(2)2 ZreO(1)1 ZreO(2)2 ZreO(2) NbeO(1)1 NbeO(1)2 NbeO(2) Total
2.0366 2.0897 2.2168 2.0366 2.0897 2.2168 1.8867 1.9968 2.1761 e
11.3000 11.3000 11.3000 298.2000 298.2000 298.2000 513.0000 513.0000 513.0000
498.3600 498.3600 498.3600 498.3600 498.3600 498.3600 498.3600 498.3600 498.3600
74.4314 72.5401 68.3810 410.7517 400.3144 377.3624 562.7904 531.7592 487.9448 2986.2756
681.7362 664.4131 626.3190 681.7362 664.4131 626.3190 735.9008 695.3245 638.0332 6014.1951
290.0246 282.6550 266.4490 506.0479 493.1891 464.9121 616.1661 582.1918 534.2221 4035.8579
Table 3 The crystal parameters, Nb-site bond length, bond valence (VNbeO) and distortion of the oxygen octahedral of MgZrNb2O8 ceramics were calculated with different sintering temperatures. The 1 and 2 were used to distinguish the oxygen atoms with different bond lengths. Sintering temperature ( C)
1150
1200
1250
1300
1350
a (Å) b (Å) c (Å) a( ) b( ) g( ) V (Å3) NbeO(1)12(Å) NbeO(1)22(Å) NbeO(2)2(Å) RNbeO (Å) VNbeO
4.8054 5.6511 5.0747 90.0000 91.3680 90.0000 137.7700 1.8866 1.9816 2.1771 1.9110 8.3984 14.4162
4.8172 5.6503 5.0854 90.0000 91.3797 90.0000 138.3800 1.8859 1.9830 2.1739 1.9110 8.3686 14.2980
4.8265 5.6626 5.1099 90.0000 91.7943 90.0000 139.5900 1.8867 1.9968 2.1761 1.9110 8.4893 14.2677
4.8526 5.6694 5.1081 90.0000 91.4089 90.0000 140.4900 1.8957 1.9939 2.1835 1.9110 8.5984 14.2168
4.8424 5.6727 5.1139 90.0000 91.4335 90.0000 140.4300 1.8945 1.9945 2.1834 1.9110 8.5951 14.2228
Doctahedroal distortion (%)
Vij ¼
X
vij
(8)
Rij dij vij ¼ exp b0
(9)
where the Vij represented the sum of all of valences from a given atom i, dij indicated the length of a bond between atoms i and j, Rij indicated the bond valence parameter, and b0 indicated a universal constant equal to 0.37. Curves of Q·G values and bond valence values as a function of sintering temperatures for MZN ceramics were
shown in Fig. 6. The Q·G values of the samples had a close relation with the B-site bond valence and moved toward to the positive direction with the increasing of B-site bond valence, which agreed with the findings of Kim et al. [34]. Kim et al. [35] reported that tf had a closely relation with the distortion of the oxygen octahedral. In addition, the distortion of the oxygen octahedral was formulated by Eq. (10) [36].
Doctahedron ¼
B O distancemax B O distancemin B O distanceaverage
(10)
The correlation among the oxygen octahedral distortion values and the tf values were shown in Fig. 6. The distortion values of the oxygen octahedral showed a similar tendency with the tf values, which agreed with the results of Kim et al. [37].
3. Conclusions
Fig. 6. Curves of Q·G values and bond valence values and the correlations among the tf values and oxygen octahedral distortion values as a function of sintering temperatures from 1150 C to 1350 C.
The wolframite structure MZN ceramics were obtained successfully by the RS process, which showed obvious advantages than the conventional solid-state method. Subsequently, microwave properties depending on sintering temperatures were investigated systematically. Refinement was used to analyze the crystal structure of MZN ceramics sintered at 1250 C and obtained the parameters. Moreover, for the first time, bond energy, distortion of the oxygen octahedral and bond valence were calculated to evaluate the structural characteristics. MZN samples with nearly full density were obtained at 1250 C and had excellent microwave dielectric properties of εr ¼ 26.54, Q·G ¼ 57,477 GHz and tf ¼ 17.69 ppm/ C, which would make these ceramics promising for applications in microwave components.
H.L. Pan et al. / Journal of Alloys and Compounds 687 (2016) 274e279
Acknowledgments This work was supported by the National Training Plan Innovation Project of college students (No.201510427001), National Natural Science Foundation (No. 51472108). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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