The sintering and dielectric properties modification of Li2MgSiO4 ceramic with Ni2+-ion doping based on calculation and experiment

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Original Article

The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment Rui Peng a , Hua Su a , Di An b , Yongcheng Lu a , Zhihua Tao a , Daming Chen c , Liang Shi a , Yuanxun Li a,∗ a

State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China b School of Materials Science and Engineering, Hainan University, Haikou 570228, China c Institute of Electronic and Information Engineering of UESTC in Guangdong, Dongguan 523000, China

a r t i c l e

i n f o

Article history:

a b s t r a c t The microwave dielectric properties and sintering behavior of Li2 (Mg1-x Nix )SiO4

Received 22 October 2019

(x = 0.00–0.10) ceramics were researched with the help of first principle calculation

Accepted 21 November 2019

and solid-state reaction experiment. The crystal structure, electron density, and formation

Available online xxx

energy were obtained through the density functional theory. The X-ray diffraction, scanning

Keywords:

used in this study. The substitution of Ni2+ -ion to Mg2+ -ion could lower the densification

electron microscopy, energy-dispersive X-ray spectroscopy, and Network Analyzer were Low dielectric constant

temperature from 1250 ◦ C to 1150 ◦ C and improve the microwave dielectric properties of

LTCC

composite ceramics, demonstrated by the result of lattice parameters, bond population,

Microwave dielectric properties

electron density, and microstructure. A peak dielectric property of Li2 (Mg1-x Nix )SiO4

DFT

(x = 0.00-0.10) ceramics was achieved when x = 0.04 (εr = 5.69, Q × f = 28,448 GHz at 16 GHz, f = −15.3 ppm/ ◦ C) sintered at 1150 ◦ C. Besides, LBBS glass was used as the sintering aid to lower the densification temperature from 1150◦ down to 900◦ . The Li2 (Mg0.96 Ni0.04 )SiO4 ceramic with 2 wt% LBBS sintered at 900

◦

C obtained excellent microwave dielectric

properties, εr = 5.89, Q × f = 29,320 GHz (at 16 GHz), f = −13.8 ppm/◦ C, and there was no chemical reaction between the composite ceramics and Ag. It’s a promising material for the applications of the millimeter-wave devices in the field of low temperature co-fired ceramics. © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. ∗

Introduction

With the arrival of the 5 G era, the research and development

Corresponding author. of microwave devices are facing great challenges. For the sake E-mail: [email protected] (Y. Li). https://doi.org/10.1016/j.jmrt.2019.11.061 2238-7854/© 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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of reducing the delay of signal transmission and the distortion of signal, these materials used in the high frequency devices need to be with extremely low dielectric constant (εr ), high quality factor ( Q × f ) and nearly zero temperature coefficient of the resonant frequency (f ), which are essential in the field of wireless communication nowadays [1–4]. Low temperature co-fired ceramics (LTCC) technology, a packaging method with high cost performance, uses Ag as the conductor usually, because it has outstanding conductivity and low loss [5,6]. In order to realize this packaging technology, the sintering temperature of substrate materials needs to be lower than 961 ◦ C, which is the melting point of Ag[7,8]. Lately, a large number of researches have been conducted on microwave dielectric ceramic, such as ZnAl2 O4 , Ag2 MoO4 , LiZnPO4 , Eu2 Zr3 (MoO4 )9 , La2 Zr3 (MoO4 )9 and so on, and these materials have potential application values in the field of LTCC[9–13]. Silicate is known as low dielectric constant and high quality factor, thus, it’s a promising material for the microwave devices. Li2 MgSiO4 (LMS), with low dielectric constant and low dielectric loss, attracts the interests of researchers recently. Georage et al. have reported that the εr value of LMS ceramic is 5.1, Q × f value is 1.54 × 104 and relative density is 92% when pre-sintered at 850 ◦ C and resintered at 1250 ◦ C [14]. However, the low level of densification would deteriorate the quality factor and dielectric constant of ceramic, because there is a tight relationship between the dielectric properties and the pores of ceramic. Zhang et al. have investigated the effects of Mg/Ni ratio on the microwave dielectric and sintering properties of (Mg1-x Nix )2 SiO4 ceramics, demonstrating that the Ni2+ -ion substitution could not only significantly improve the densification behavior but also boost the microwave dielectric properties [15]. Dong et al. found that appropriate substitution of Ni2+ -ion to Mg2+ -ion could improve the microwave dielectric properties and lower the densification temperature with the decrease of cell volume of Li(Mg1-x Nix )PO4 ceramics [16]. Hence, we assume that the lattice distortion that Ni2+ -ion substitution has led to could lower the densification temperature, and the low temperature sintering could happen with less sintering aid. Considering that the εr shows a tendency to increase with the increase of dielectric polarizability, the substitution of Ni2+ -ion (1.23 Å3 ) to Mg2+ -ion (1.32 Å3 ) would decrease the εr value to some extent [17,18]. Except that, the substitution of ions with smaller ionic radius (0.55 Å for Ni2+ ) to which with larger ionic radius (0.57 Å for Mg2+ ) would increase the f value slightly [15,19]. What’s more, the first principle calculation based on the density functional theory (DFT) is an effective method to study the variation of mechanism in detail [20,21]. Therefore, in this present study, the first principle calculation was performed to investigate the influence of Ni2+ -ion doping on the structure of ceramics on the atomic scale. The Li2 (Mg1-x Nix )SiO4 (LMNS) (x = 0.00–0.10) ceramics were synthesized through the solid-state reaction method, the microstructure, microwave dielectric properties, and sintering properties of these ceramics have been investigated. Besides, different amount of LBBS glass was added into the ceramic to lower the densification temperature. The compatibility between Ag and the LMNS ceramic has been researched too.

2.

Experimental procedures

LMNS (x = 0.00–0.10) ceramics were prepared by the conventional solid-state reaction route with high-purity (>99%) powders of Li2 CO3 , MgO, NiO, and SiO2 (Chron Chemicals Co. Ltd., Chengdu, China). According to their molar ratio, these powders were milled in a nylon pot filled with zirconia balls for 24 h. After that, the milled slurry was dried, and the precalcination was performed at 830 ◦ C for 4 h. The pre-sintered powders were re-milled in distilled water for 24 h and dried then. After thorough grounding, the powders were pressed into disks (12 mm in diameter and 6 mm in thickness) with 5 wt% of polyvinyl alcohol (PVA) binder under a pressure of 9 MPa. These disks were sintered at 1100, 1150 and 1200 ◦ C for 4 h, respectively. Furthermore, Li2 (Mg0.96 Ni0.04 )SiO4 ceramic mixed with x wt% (x = 0.5–4.0) of LBBS glass were sintered at 875, 900 and 925 ◦ C for 4 h, respectively. In order to investigate the chemical compatibility of the ceramic with Ag, the Li2 (Mg0.96 Ni0.04 )SiO4 composite and Ag were co-fired at 900 ◦ C for 4 h. LBBS glass was prepared using Li2 CO3 , B2 O3 , Bi2 O3 , and SiO2 (>99%, Chron Chemicals Co. Ltd., Chengdu, China) by quenching method. All the DFT calculation is realized with the help of the Cambridge Serial Total Energy Package (CASTEP). The exchangecorrelation interaction was treated with the generalized gradient approximation (GGA) based on the Perdew-BurkeErnzerhoff (PBE) function. The electron interaction of ions is approximated by the Vanderbilt Ultrasoft Pseudopotential method. The valence electronic configurations for the ultrasoft potential are 1s2 2s1 for Li, 2s2 2p4 for O, 3s2 3p2 for Si, 2p6 3s2 for Mg, and 3d8 4s2 for Ni. After a series of tests, the energy cutoff was set as 350 eV, and the Monkhorst-Pack kpoint mesh was set as 3 × 2×1. The optimization of geometry was realized using Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm. More detailed parameters were 1.0 × 10−6 eV/atom for self-consistence tolerance, 5.0 × 10−6 eV/atom for energy, 0.01 eV/Å for maximum force, 0.02 GPa for maximum stress, and 5.0 × 10−4 Å for the maximum displacement. For the atomic concentration of impurities comparable to experiments, a supercell was constructed (2 × 2×1) with relaxed primitive cells. Eq. 1 was used to calculate the formation energy, [22] E = EDo − EUnDo −



ki pi

(1)

i

where EDo is the energy of the doped system, EUnDo is the energy of the un-doped system, pi is the chemical potentials of exchanged atoms, and ki = 1 (−1) for added (removed) atoms. The crystalline phase structure of these materials were analyzed by X-ray diffraction (XRD: DX-2700, Haoyuan Co.) using Cu K␣ radiation. The results were analyzed by the Rietveld profile refinement method using Fullprof program. The microstructure was investigated by scanning electron microscopy (SEM: JEOL, JSM-6490LV), and the microwave dielectric properties were acquired by the Hakki–Coleman method and Agilent N5230A Network Analyzer (300 MHz–20 GHz). [23] The quality factor was obtained through the transmission cavity method. The temperature

Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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Fig. 1 – The primitive cell of LMS ceramic (a); the schematic diagram of four kinds of MgO4 tetrahedron before and after Ni2+ -ions substitution (b).

coefficient of resonant frequency (f ) was measured by the following formula using the resonant frequencies at 80 ◦ C (fT ) and 20 ◦ C (f0 ) [15], f =

fT − f0 × 106 f0 (T − T0 )

(2)

Archimedes method is the way of getting the bulk density of the samples, the relative densities came from the ratio of bulk densities and the theoretical densities.

3.

Results and discussion

The crystal structure of LMS ceramic (P 21 /n, No. 14) is presented in Fig. 1(a), and the parameters are a = 4.99 Å, o b = 10.66 Å, c = 6.32 Å, and ␣ = ␤ = ␥ = 90 , respectively. There are four kinds of Mg2+ -ion surrounded by four oxygen ions in the system. The structural variation of MgO4 tetrahedron with Ni2+ -ion substitution could be observed in the schematic diagram obtained from calculation (Fig. 1b). The geometric characteristic, including position and bond length, of tetrahedron have changed due to the different ionic radius and polarizability of Ni2+ -ion compared to the one of Mg2+ -ion. Table 1 presents the bond length and the bond population of cation-oxygen belong to MgO4 tetrahedron with or without Ni2+ -ion substitution (L-B marked for the bond length before substitution, L-A marked for the bond length after substitution, P-B marked for the bond population before substitution, and P-A marked for the bond population after substitution). As to the bond length of the tetrahedron, half of it shows a decreasing trend, and the other part shows an increasing trend. However, all the value of the bond population varies from negative to positive as the Mg2+ -ion has been replaced by Ni2+ -ion in MgO4 tetrahedron, manifesting that the covalency of cation-oxygen has been strengthened. The lower ionic polarizability and unpaired 3d8 of Ni2+ -ion compared to the one of Mg2+ -ion should be responsible for such variation [18,19]. Table 2 exhibits the formation energy of the doped system. The chemical potential of a magnesium atom is −973.5126 eV, and the one of nickel atom is −1354.5839 eV. Actually, the total energy and formation energy of four kinds of

Table 1 – The bond length and bond population of MgO4 tetrahedron before and after Ni2+ -ions substitution. Cation

Anion

L-B (Å)

L-A (Å)

P-B

Mg1

O23 O18 O27 O30 O22 O26 O31 O19 O25 O20 O16 O29 O24 O17 O21 O28

2.02185 1.98055 1.97023 1.97703 2.02185 1.97023 1.97703 1.98055 1.97023 1.97702 1.98055 2.02184 1.97023 1.98055 1.97703 2.02184

2.03167 1.92484 2.06457 1.94999 2.03285 2.06413 1.94942 1.92389 2.06629 1.94870 1.92314 2.03206 2.06555 1.92465 1.94974 2.03123

−0.89 −1.08 −1.18 −1.13 −0.89 −1.18 −1.13 −1.08 −1.18 −1.13 −1.08 −0.89 −1.18 −1.08 −1.13 −0.89

Mg2

Mg3

Mg4

P-A 0.36 0.44 0.35 0.39 0.36 0.35 0.39 0.44 0.35 0.39 0.44 0.36 0.35 0.44 0.39 0.36

Table 2 – The total energy and formation energy of four kinds of doped system. Occupying site

Total energy (eV)

Formation energy (eV)

Non-Ni Mg1 Mg2 Mg3 Mg4

−51528.2967 −51903.1224 −51903.1225 −51903.1225 −51903.1218

\ 6.2456 6.2455 6.2455 6.2462

doped systems is similar. Hence, there is no preferable position for Ni2+ -ion to occupy within four kinds of tetrahedron. What’s more, the electron density map around four kinds of MgO4 tetrahedron has been drawn to investigate the variation of bonding characteristics (Fig. 2), (a–d) are the clear system, and (e–h) are the doped system. The electron interaction between cation and oxygen ion was enhanced due to the substitution of Ni2+ -ions in each kind of MgO4 tetrahedron, corresponding to the variation of the bond population value. Besides, the symmetry of electron density distribution has been changed slightly.

Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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Fig. 2 – The electron density map of the plane with four kinds of Mg2+ -ions before (a–d) and after (e–h) Ni2+ -ions substitution.

Fig. 3(a) and (b) depict the XRD patterns of the LMNS (x = 0.00–0.10) ceramics sintered at 1150 ◦ C for 4 h. All the main diffraction peaks can be well indexed in terms of the standard patterns of Li2 MgSiO4 (JCPDS # 24-0636). No other peaks were detected, indicating that no second phase in the aimed materials was formed. Actually, with the increase of x value, the diffraction peaks move to a high angle region slightly,

which phenomenon is weak because the substitution of Ni2+ ion is too seldom. One reason for this phenomenon should be that the radius of Ni2+ -ion (0.55 Å) is smaller than Mg2+ ion’s (0.57 Å). Hence, the substitution of Ni2+ -ion to Mg2+ -ion would lead to the lattice distortion happen, and the cell volume decreases gradually with the increase of x value. Another reason may be that the Ni2+ -ion substitution causes the hap-

Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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Fig. 3 – XRD patterns of LMNS (x = 0.00–0.10) ceramics sintered at 1150 ◦ C for 4 h (a); locally magnified peak profiles (b); the refined result of LMNS ceramic as x = 0.04.

pening of the preferred grain orientation. [15] To study the crystal structure details of the doped ceramics, the refinement was performed using Fullprof software. The observed and calculated XRD patterns of the sample as x = 0.04 are shown in Fig. 3(c), and all the fitted curves consistent with the experimental one. Besides, the Bragg-positions match well with the indexed peaks. The refined lattice parameters are a = 4.9901 Å, b = 10.6699 Å, c = 6.3147 Å and ␤ = 90.1109◦ with acceptable Rp (7.1), Rwp (8.2), Rexp (5.8), and ␹2 (1.9). Fig. 4 exhibits the variation of micro-parameters of LMNS (x = 0.000.10) ceramics with the increase of x value. It can be found that with the increase of x value, b shows an increasing trend, while

a and c show a decreasing trend. Except that, ␤ has manifested a trend same with a, leading to a decrease of cell volume. The lattice distortion that the substitution of an ion with small radius and ionic polarizability (0.69 Å and 1.23 Å3 for Ni2+ -ion) to the one with large radius and ionic polarizability (0.72 Å and 1.32 Å3 for Mg2+ -ion) has led should be responsible for the variation of micro-parameters, and it’s consistent with the result regarding XRD discussed above. The variation of bond length values related to four kinds of MgO4 tetrahedron could be found in Fig. 5, and variation could be observed in each value. It should be noted that the variation tendency that Fig. 5 presents is consistent with the one in Table 1, but

Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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Fig. 4 – The lattice parameters of LMNS (x = 0.00–0.10) ceramics sintered at 1150 ◦ C for 4 h.

the amplitude of variation is much smaller than the one from calculation. Thus, considering the formation energy and the experimental result, the substitution of Ni2+ -ions to Mg2+ -ions could happen simultaneously in all kinds of MgO4 tetrahedron. In order to investigate the microstructure of the ceramics with different x values as well as different sintering temperatures, the SEM of it has been made (Fig. 6). From micrograph

a–c, it should be noted that with the increase of x value, the pores (pink circles) have disappeared, and the grains have grown demonstrated by the grain size distribution statistics (Fig. 7). A closely packed microstructure was obtained as x = 0.04 with uniformly distributed grains, which is in good agreement with the relative density value (93.8%). However, the grain boundary seems to melt (green circles) with the following increase of x value, and, accordingly, grains grow

Fig. 5 – The bond length values of four kinds of MgO4 tetrahedron (x = 0.00–0.10) sintered at 1150 ◦ C for 4 h. Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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Fig. 6 – SEM micrographs of LMNS ceramics as x = 0.00 (a), x = 0.02 (b), x = 0.04 (c), x = 0.06 (d), x = 0.08 (e), and x = 0.10 (f), sintered at 1150 ◦ C for 4 h; LMNS (x = 0.04) ceramic with 2 wt% LBBS sintered at 900 ◦ C for 4 h (g); the cross-section of LMNS (x = 0.04) ceramic adding 2 wt% LBBS with Ag sintered at 900 ◦ C for 4 h (h).

abnormally. There were some trapped pores as x > = 0.08. The reason for this phenomenon should be that a large number of Ni2+ -ions adding leads to over-sintering happen, indicating that a small amount of Ni2+ -ions substitution could improve

the densification level of the LMS ceramics significantly. The extraction of pores from green bodies was inhibited at the high sintering temperature, which leads to the emerge of trapped pores. Thus, relative high sintering temperature may hinder

Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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Fig. 7 – The grain size distribution statistics of the LMNS ceramics as x = 0.00 (a), x = 0.02 (b), and x = 0.04 (c) sintered at 1150 ◦ C for 4 h.

the densification of the ceramics and consequently deteriorate the microwave dielectric properties. Tests were conducted on the relative density, Q × f , εr , and f values of LMNS ceramics with different x values (Fig. 8). The relative density of composite ceramics, related to the microstructure of grain crystal, increases with the increase of x value and peaked at x = 0.04 (about 93.8% sintered at 1150 ◦ C ). Thus, in this work, the addition of nickel could help to decrease the densification temperature of LMS ceramic from 1250 ◦ C to 1150 ◦ C. Furthermore, when x > 0.04, a slight decrease in relative density of all temperature was observed, which is connected with the level of porous discussed in SEM micrographs. The microwave dielectric loss, involving the intrinsic loss and the extrinsic loss, is crucial to Q × f value [24]. The crystal structure of ceramics is a dominant factor of the intrinsic loss, and the lattice vibration modes have great impact on it too [25]. However, there are so many driven factors, such as the second phase, grain size, liquid-phase of crystal boundaries, cracks, impurities and porosity, can be the reason for extrinsic loss [26]. The Q × f value of samples as x = 0.00 sintered at 1100–1200 ◦ C was quite low, no more than 15,500 GHz. The further increasing of x value leads to the increase of Q × f value, and the highest point is about 28,448 GHz when x = 0.04 sintered at 1150 ◦ C. After that, this value decreases dramatically. It’s obvious that the variation tendency that Q × f value shows is similar to the relative density value. The ceramic with high compact and uniform crystal grains would present high Q × f value. Besides, a more covalency variation of the bond population in the MgO4 tetrahedron should be another attribution for

the increasing Q × f value. The following dramatic decrease in Q × f value may be attributed to abnormal grain growth and trapped pores that can be seen in SEM, and it would deteriorate dielectric properties. Except that, the value of packing fraction has been obtained from Eq. (3) (Fig. 9a), because it is another characterization parameters of Q × f value [27], packing fraction (%) =

volume of packed ions volume of primitive unit cell

(3)

where the ionic radius is 0.59 Å (Li+ ), 0.57 Å (Mg2+ ), 0.55 Å (Ni2+ ), 0.26 Å (Si4+ ), and 1.38 Å (O2− ), respectively [28]. The variation trend of these two values is similar as x < = 0.04. However, an opposite trend has emerged with further increasing x value. Therefore, the improvement of Q × f value is contributed by the enhancement of relative density, packing fraction, and bond covalency. The follower decreasing of Q × f value is mainly attributed to the worsening of the densification level. As to the εr value, it increases in the first place with the increase of x value, the maximum εr value (about 5.79 sintered at 1150 ◦ C) was achieved as x = 0.06. Then, it turns to decrease to some extent. The variation of relative density and dielectric constant show the same tendency in general, which means that there is a tight relationship between relative density and εr . However, there is some difference between them, relative density peaked at x = 0.04, but εr starts to decline until x = 0.06. What’s more, the theoretical ionic polarizability of Mg2+ -ions is larger than Ni2+ -ions, and εr should show a tendency to decrease with the increase of x value, which,

Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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Fig. 8 – The relative density, Q × f , εr , and  f values of LMNS (x = 0.00–0.10) ceramics sintered at 1100, 1150, and 1200 ◦ C for 4 h.

however, is different from the experiment result. Shannon’s additive rule, demonstrating that molecular polarizabilities of materials could be estimated by summing the polarizabilities of each ion, could explain this abnormal phenomenon [18,29]. The polarizabilities (˛) of ceramics could be obtained using the following equation, ˛Li2 (Mg1−x Nix )SiO4 = 2˛Li+ + (1 − x) ˛Mg2+ + x˛Ni2+ + ˛Si4+ + 4˛O2−

(4)

where the ionic polarization was chosen to be 1.2 Å3 for Li+ , 1.32 Å3 for Mg2+ , 1.23 Å3 for Ni2+ , 0.87 Å3 for Si4+ , and 2.01 Å3 for O2− , respectively [18]. Besides, the Clausius–Mosotti relation should be used to calculate dielectric properties, ε=

3V + 8˛ 3V − 4˛

(5)

where the V is the cell volume of LMNS (x = 0.00-0.10) ceramics. Fig. 9(b) presents the polarizabilities ( ˛ ), measured and calculated εr values of composite ceramics. The value of ˛ decreases with the increase of Ni2+ substitution because of the smaller polarizabilities of Ni2+ -ion. Considering the variation of cell volume, the calculated εr value shows an increasing tendency, which is similar to the measured one. Hence, the increasing εr value should be attributed to the improved densification, and the additional dielectric part coming from unpaired 3d electrons of Ni2+ -ion (3d8 ) also has an impact on the εr value. The trapped porous, abnormal grains, increasing bond population, and decreased polarizability should be responsible for the decreasing trends. Hence, the conclusion that relative density and cell volume play major role in the variation of εr rather

than the theoretical ionic polarizability could be made. In addition, the sintering window of LMS ceramics has been widened slightly. The f increase with the increasing amount of Ni2+ ions. It’s consistent with the conclusion regarding ionic radius and f value, and it’s the common result of densification and ionic radius variation; f = −15.3 ppm/◦ C as x = 0.04 sintered at 1150 ◦ C particularly. The EDS test of LMNS ceramic as x = 0.04 sintered at 1150 ◦ C was carried out to identify the composition (Fig. 10, Table 3). The marked grain consists of O, Mg, Si and Ni elements in a molar ratio of O:Mg:Si:Ni = 65.23:17.97:16.21:0.59. The lithium was not detected in the grain, because the relative atomic mass of it is too light to be checked. Considering the influence of detection error, the ratio of the ions O:Mg:Si:Ni is almost 4:0.96:1:0.04. Therefore the composition of the matrix ceramic could consider to be Li2 (Mg0.96 Ni0.04 )SiO4 . In order to realize low-temperature sintering at about 900 ◦ C, different amount of LBBS glass were added into the Li2 (Mg0.96 Ni0.04 )SiO4 ceramic. The relative density and dielectric properties of it are exhibited in Fig. 11. The relative density shows an increasing tendency with the increasing amount of LBBS glass, a saturated relative density (95.9%) was obtained with 2 wt% LBBS glass sintered at 900◦ . The further added glass results in the emergence of uncompact microstructure, manifesting that 2 wt% LBBS glass is enough to achieve the peak value of relative density when the specimen sintered at 875-925 ◦ C. The effect of heat transferring through the liquid phase (LBBS glass) should be responsible for the further densification, and the following decrease could be attributed to the trapped pores and abnormal grain grows led by oversintering. In addition, the dielectric permittivity and Q × f values of composite ceramics show a similar trend with relative density, and all of it peaked (εr = 5.89, Q × f = 29,320 GHz

Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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Fig. 9 – The Q × f values and packing fraction (a), polarizabilities (˛), measured and calculated εr values (b) of the LMNS (x = 0.00–0.10) ceramics.

Table 3 – The EDS result (weight and atomic fraction) of LMNS ceramic as x = 0.04. Element

Ratio

Mg K

Ni K

Si K

OK

Wt (%)

At (%)

Wt (%)

At (%)

Wt (%)

At (%)

Wt (%)

At (%)

22.18

17.97

1.76

0.59

23.10

16.21

52.96

65.23

at 16 GHz, f = −13.8 ppm/ ◦ C) as 2 wt% LBBS glass adding sintered at 900 ◦ C. The SEM image of it can be seen from Fig. 6(g). The liquid phase as indicated by arrows between grain boundary has emerged, which could not be observed from the pure samples sintered at high temperature. Therefore, considering the macroscopic and microscopic properties of the composite materials, the densification temperature could be lowered

with the help of LBBS glass. Fig. 6(h) presents the micrograph of the cross-section of Ag and composite ceramic. An obvious heterogeneous interface has generated, indicating that there is no chemical reaction happened between silver and the ceramic-glass composite. Therefore, this material could be applied in the field of LTCC.

Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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Fig. 10 – The EDS of LMNS ceramic as x = 0.04 sintered at 1150 ◦ C for 4 h (energy spectrum, and test area).

Fig. 11 – The relative density, εr and Q × f values of Li2 (Mg0.96 Ni0.04 )SiO4 ceramic with x wt% LBBS sintered at 875–925 ◦ C for 4 h.

Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061

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Conclusion

Based on the method of first principle calculation and experimental verification, the microwave dielectric properties and sintering behavior of Li2 (Mg1-x Nix )SiO4 (x = 0.00–0.10) ceramics were investigated. The discrepancy of ionic polarizability and ionic radius between Ni2+ -ion and Mg2+ -ion results in the covalencify of the cation-oxygen bonds belong to four kinds of MgO4 tetrahedron. The bond length and the symmetry of electron density around Mg2+ -ion have been modified. The possibilities of occupation for four kinds of MgO4 tetrahedron are equal. The modification of lattice parameters led by Ni2+ -ion substitution could improve the densification level and microwave dielectric properties of the composite ceramics. The densification temperature has been lowered from 1250 ◦ C to 1150◦ ◦ C. The microwave dielectric properties of Li2 (Mg1-x Nix )SiO4 (x = 0.00-0.10) were peaked as x = 0.04 (εr = 5.69, Q × f = 28,448 GHz at 16 GHz, f = −15.3 ppm/ ◦ C) sintered at 1150 ◦ C. Besides, LBBS glass was used as the sintering aid to lower the densification temperature from 1150 ◦ C down to 900 ◦ C, and the microwave dielectric properties have been improved. The Li2 (Mg0.96 Ni0.04 )SiO4 ceramic with 2 wt% LBBS sintered at 900 ◦ C for 4 h has achieved excellent microwave dielectric properties with εr = 5.89, Q × f = 29,320 GHz (at 16 GHz), f = −13.8 ppm/ ◦ C, and the composite ceramic and Ag are compatible. Therefore, it is a possible material for millimeter-wave device applications in the field of LTCC.

Conflict of interest The authors declare no conflicts of interest.

Acknowledgements This work was supported by the Sichuan Science and Technology Program (Grant Nos. 2019YFG0280); Natural Science Foundation of Guangdong Province (Grant Nos. 2016A03031006); Guizhou science and technology major projects (Grant Nos. 20163011); and Dongguan entrepreneurial talent program.

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Please cite this article in press as: Peng R, et al. The sintering and dielectric properties modification of Li2 MgSiO4 ceramic with Ni2+ -ion doping based on calculation and experiment. J Mater Res Technol. 2019. https://doi.org/10.1016/j.jmrt.2019.11.061