Effects of P2O5 on crystallization, sinterability and microwave dielectric properties of MgO-Al2O3-SiO2-TiO2 glass-ceramics

Journal of Non-Crystalline Solids 459 (2017) 123–129

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Effects of P2O5 on crystallization, sinterability and microwave dielectric properties of MgO-Al2O3-SiO2-TiO2 glass-ceramics Xianpei Huang a, Changlai Yuan a,b,⁎, Xinyu Liu a,b,⁎, Fei Liu c, Qin Feng a,b, Jiwen Xu a,b, Changrong Zhou a,b, Guohua Chen a,b a b c

College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, PR China School of Mechanical and Electrical Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China

a r t i c l e

i n f o

Article history: Received 6 September 2016 Received in revised form 29 December 2016 Accepted 3 January 2017 Available online xxxx Keywords: Glass-ceramics Two-step crystallization Microwave dielectric properties LTCC technology

a b s t r a c t Glass-ceramics of MgO-Al2O3-SiO2-TiO2-P2O5 were prepared by two-step heat-treatment at selected nucleation temperature and followed by crystallization temperatures. After the addition of 2.5 wt.% P2O5, the MgO-Al2O3SiO2-TiO2-P2O5 glass-ceramics (abbreviated as P2.5) can be fully precipitated at ≤950 °C. The predominant phases in the glass-ceramics are found to be Mg2SiO4, Mg2Al4Si5O18, Ti4Si2P6O25 and SiO2 in the crystallization temperature range of 930– 970 °C. The results showed that P5+ could promote the phase separation of MgO-Al2O3-SiO2TiO2 glass and that the glass was divided into two areas, such as Mg2SiO4 and the containing P5+ area at ≤950 °C. Furthermore, P5+ inhibited the presence of Mg2Al4Si5O18 when the heat treatment temperature was ≥950 °C. The 19.0MgO-23.0Al2O3-53.0SiO2-4.0TiO2-2.5P2O5 (wt.%) glass-ceramics (abbreviated as P2.5) exhibits the ultra-low dielectric constant εr ~ 3.9– 4.0 ± 0.1, a high quality factor Q × f ~ 13,785 ± 50 GHz at 13– 14 GHz and a near-zero temperature coefficient of resonant frequency τf ~ − 7.8 to − 6.4 ± 0.1 ppm/°C, suggesting that it will be a promising material in LTCC technologies. With the further increase of the P2O5 content up to 3.5 wt.%, the linear shrinkage, the density and dielectric properties inversely deteriorate. © 2017 Elsevier B.V. All rights reserved.

1. Introduction In the past 20 years, the microwave telecommunication and satellite broadcasting have become one of the fastest-developing segments in the communications and electronics industry. Meanwhile, the widespread usage of several wireless systems has required dielectric components that can provide low-cost, lightweight, small, multifunctional and highly reliable to reduce the device size. Furthermore, in recent years, low-temperature cofired ceramics (LTCC) have been developed to increase the volume efficiency by integrating passive components that are interconnected with 3D stripline circuitry, such as capacitors, inductors, resistors, resonators and filters [1,2]. The most important parameter for LTCC substrate materials are the sintering temperature at b1000 °C in order to be co-fired with low melting point metals e.g. Cu, Ag and Au [3,4]. In addition to these basic requirements, the LTCC substrate materials can possess a low dielectric constant (εr b 7) and/or low dielectric losses (tanδ) to reduce the signal propagation delay [5]. Currently, the achievement of LTCC technology mainly depends on ⁎ Corresponding authors at: College of Material Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, PR China. E-mail addresses: [email protected] (C. Yuan), [email protected] (X. Liu).

http://dx.doi.org/10.1016/j.jnoncrysol.2017.01.005 0022-3093/© 2017 Elsevier B.V. All rights reserved.

three types of materials: glass-ceramics, glass/ceramic and singlephase ceramics. Amongst these, the glass-ceramic system is the most frequently used. The majority of low-permittivity materials have been reported, including the production of glass-ceramic (crystallization glass) and glass/ceramic composites (multiphase ceramics) [6,7]. Glass-ceramics formed by controlled crystallization of glass are materials of high crystalline grade. For the glass/ceramic composites, low-softening glass is mixed with the ceramics where the low-softening glass reduces the sintering temperature. However, in fabrication of desirable LTCC substrates, a complete densification and sufficient crystallization are generally necessary for the required mechanical properties and dielectric properties. Therefore, based on the comprehensive properties of the glass-ceramic, it seems to be more potential for applications in LTCC technology than glass/ceramic composites. Glass-ceramics with cordierite as major crystal phase by crystallization of glass in the system of MgO-Al2O3-SiO2 have many advantages, such as high mechanical strength, excellent dielectric properties, good thermal stability and thermal shock resistance [8]. The well-studied cordierite glass-ceramic from MgO-Al2O3-SiO2 system is one of the excellent microwave dielectric materials with high Q × f value. However, there have been few reports on MgO-Al2O3-SiO2 cordierite based glass

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ceramics derived at b1050 °C by volume crystallization. And the high viscosity and narrow sintering temperature range (1050– 1460 °C) of MgO-Al2O3-SiO2 limit its applications [9,10]. In this paper, we introduce MgO-Al2O3-SiO2-TiO2-based glass-ceramics with the composition 19MgO-23Al2O3-53SiO2-4TiO2 (wt.%) as the project. Also, the aim of the present study is to investigate the effect of the addition of P2O5 on the sintering, nucleation and crystallization behavior of MgO-Al2O3-SiO2-TiO2 glass-ceramics. At the same time, we found that the addition of P2O5 (mass percent b2.5 wt.%) into MgO-Al2O3-SiO2-TiO2 systems would not be lead to the crystallization of the glasses under 950 °C. After the addition of 2.5 wt.% P2O5, the MgO-Al2O3-SiO2-TiO2-P2O5 glass-ceramics can be fully precipitated at ≤950 °C. With the further increase of the P2O5 content up to 3.5 wt.%, the linear shrinkage, the density and dielectric properties inversely deteriorate. Thus, in present work, it is main to introduce MgO-Al2O3-SiO2TiO2-P2O5 glass-ceramics with the addition of 2.5 wt.% P2O5 as the project, will provide a promising candidate for LTCC applications.

2. Experimental procedures The starting materials were analytical grade reagents SiO2 (purity N99%), Al2O3 (purity N99%), MgO (purity N99.5%), TiO2 (purity N99%), and NH4H2PO4 (purity N 99.99%). The chemical compositions of 19.0MgO-23.0Al2O3-53.0SiO2-4.0TiO2-x P2O5 [x = 2.5, 3.0, 3.5 (wt.%)] are presented in Table 1 [According to the content of P2O5 (wt.%), names as P2.5, P3 and P3.5, respectively]. A glass batch of homogeneous mixture was prepared by ball milling and then melted in alumina crucibles at 1500 °C for 2 h in a muffle furnace and then quenched in deionized water to form frits, which were crushed and wet-milled for 50 h. To prepare the bulk samples, the obtained glass powders were granulated with 5% PVA solution as a binder and then were pressed into pellets with dimensions of 12 ± 0.01 mm in diameter and 5– 6 ± 0.01 mm in thickness under a pressure of 150 MPa. All of the samples are crystallized by two-stage heat treatment (820– 880 °C for 2 h and 20 h; 930– 970 °C for 2 h and 20 h). The glass crystallization behavior was characterized by differential scanning calorimeter (DSC, Model STA-449-F3-Jupiter, Netzsch). Approximately 10 mg of ground glass was used for DSC measurement at a rate of 10 K/min up to 1200 °C. the MgO-Al2O3-SiO2-TiO2-P2O5 glassceramic samples were examined by an X-ray powder diffraction (XRD, D8-Advance, Bruker, Germany) using Ka radiation at room temperature to investigate the phase evolution. Bulk density of the glass-ceramics was measured by the Archimedean method using deionized water as medium. Surface topologies of the glass-ceramics were observed by scanning electron microscopy (SEM) (JSM-6460, JEOL, Tokyo, Japan). Meanwhile, the composition of different phases was studied by energy-dispersive X-ray spectroscopy (EDS). Dielectric constant (εr) and dielectric loss tangent (tanδ) were measured by the TE01δ shielded cavity method with a network analyzer (8720ES, Agilent, Palo Alto, CA). In order to determine the random errors and systematic errors in the data, at least 3 specimens were used for each composition during dielectric properties testing. The resonant peak frequency f ranges from 13 to 14 GHz, and the quality factor Q × f is equal to f/tanδ. The temperature coefficient of resonant frequency τf was measured in the range from 25 °C to 75 °C by following the drift in f, as the temperature is slowly

varied and calculated from the equation below: τf ¼

f 75 − f 25  10−6 f 25 ð75−25Þ

ð1Þ

Here, f25 and f75 are the resonant frequencies measured at 25 °C and 75 °C, respectively, when the temperature is steady [11]. The measurement errors are around 0.1% for εr, 0.5% for tanδ, 0.1% for τf and 0.1% for relative density and linear shrinkage. Errors were assumed from previous calibrations of the equipment. 3. Results and discussion 3.1. Crystallization and phase transformation Fig. 1 demonstrates the DSC curves of the glass samples P2.5, P3.0 and P3.5. From the DSC patterns, it is observed that the samples P3.0 and P3.5 exhibit four exothermic peaks (Tp). Further, the exothermic peak for the glass P3.0 (at 639.5 °C and 1152 °C) and for the glass P3.5 (at 645.5 °C and 1150 °C) are weak, and the height and sharpness of other exothermic peaks for the glass P3.0 (at 965 °C and 1067 °C) and for the glass P3.5 (at 969.5 °C and 1065.5 °C) increase. Above all, the sample P2.5 only exhibits two exothermic peaks (at 965 °C and 1067 °C). And it is clear that the glass transition temperature (Tg) gets to broaden with an increase of P2O5 content. As P2O5 content up to 3.5 wt.%, the glass transition temperature (Tg) also declines to about 500 °C. The DSC curves indicate that the addition of P2O5 influences the crystallization process of the glasses. It is obvious that a small quantity of P2O5 (2.5 ≤ x b 3.5 wt.%) addition can decrease glass transition temperature, retard the glass crystallization, widen the range between Tg and Tp and improve the sintering densification of glass-ceramics. Whereas, over-addition of P2O5 (≥3.5 wt.%) will accelerate the crystallization of glasses and then increase the glass viscosity and finally restrain the sintering densification of glass-ceramics. As reported, a two-stage sintering approach was applied to increase the densification of the glass ceramics [12]. In the two-stage sintering process, the pressed pellet of the glass was initially heated to a temperature slightly above Tg, and annealed for 2 h and 20 h respectively, to enhance the densification and then heated to the final temperature, where it was annealed for an additional 2 h and 20 h, respectively. From Fig. 1, it is found that the reasonable nucleation temperature should be selected at 760– 880 °C and the crystallization temperature is in the range of 850– 970 °C. Then, the P2.5 glass samples and P3.0 are crystallized by two-step crystallization heat-treatment at selected nucleation temperature (at 820– 880 °C

Table 1 Composition of the MgO-Al2O3-SiO2-TiO2 glass-ceramics with P2O5 addition (wt.%). Glass MgO (wt.%), ±0.01

Al2O3 (wt.%), ±0.01

SiO2 (wt.%), ±0.01

TiO2 (wt.%), ±0.01

P2O5 (wt.%), ±0.01

Total (wt.%), ±0.01

P2.5 P3.0 P3.5

23.00 23.00 23.00

53.00 53.00 53.00

4.00 4.00 4.00

2.5.00 3.00 3.5.00

100.00 100.00 100.00

19.00 19.00 19.00

Fig. 1. Differential scanning calorimeter curves of the parent glasses P2.5, P3 and P3.5 heated at a rate of 10 K/min.

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for 2 h and 20 h) and followed by crystallization temperatures (at 930– 970 °C for 2 h and 20 h). And the P3.5 sample is heat-treated by the same procedure (nucleation at 760– 820 °C for 2 h and 20 h and followed by crystallization temperatures at 850– 900 °C for 2 h and 20 h). The XRD patterns of the glass-ceramics with different P2O5 contents are shown in Fig. 2. It is found that the XRD curves exhibit a similar profile, which indicates that the crystalline phases can be grouped into the same crystal structures for all samples as shown in Fig. 2. As can be seen from the Fig. 2(a) and (b), P2.5 can be observed that the difference between the phase compositions is mainly attributed to heat treatment processes. Further, after two-stage heat-treated, the P2.5 sample shows two strong SiO2 and Mg2Al4Si5O18 diffraction peaks at ~30° and 50° respectively, which indicates that P2O5 can promote the presence of SiO2 and Mg2Al4Si5O18 at ≤950 °C and reduce the activation energy of the sample. Some weak diffraction peaks, which can be attributed to the presence of (Al5Mg3)(Al4Si2)O20, Al2O3, Mg2SiO4 and MgSiO3, can be also clearly observed. Additionally, a small number of AlPO4 and MgAl2O4 diffraction peak is also detected in the glass P2.5 after two-stage heat-treated. Crucially, after two-stage heat-treated at 850 °C/2 h + 970 °C/20 h for the P2.5, double diffraction peaks at ~ 24° of the SiO2 and Ti4Si2P6O25 are not detected, but a weak Mg3(PO4)2 diffraction peaks is found. Fig. 2(c) depicts the XRD patterns for the glasses P3.0 and P3.5 after two-stage heat-treated. As a result, there are some main crystalline phases Ti4Si2P6O25, SiO2 and Mg2Al4Si5O18 also present in the glasses P3.0 and P3.5. From Fig. 2(c), the SiO2 and Ti4Si2P6O25 diffraction peaks at the angles of 22–25° are observed for the specimens P3.0

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after two-stage heat-treated, but their become very weak relative to the P2.5. At the same time, the SiO2 diffraction peaks at ~ 26° become strong, relative to the P3.5. For the specimens P3.5, however, double diffraction peaks at the angles of 22– 25° are not observed, instead become a single Ti4Si2P6O25 diffraction peak. While the intensity of MgSiO3 diffraction peaks at ~34° dramatically increases with the further addition of P2O5 content. In addition, the P3.0 and P3.5 after two-stage heat treatment, the SiO2 content slightly decrease relative to the P2.5 because an excessive P5+ ion is repelled into the remaining glass. This is the reason that P5+ ion is difficult to incorporate into the glass crystallization due to the high charge density of ion. Consequently, an excess of P5+ ion inhibits the further formation of crystalline phase during sintering process. 3.2. Sintering behavior Fig. 3 shows the SEM micrographs of the glass-ceramics P2.5 after two-stage heat-treatment. As seen in Fig. 3, only two crystalline phases, the highlight white grains and the grey strip-like phase, and liquid phase are found from the contrast of the SEM. From Fig. 3(a–c), for the compositions P2.5 (nucleation temperature ~ 850 °C/2 h), the degree of densification and crystallization increases with the increasing sintering temperature from 930 °C to 950 °C for 20 h, and then decreases gradually as the sintering temperature is raised around 970 °C. Additionally, in order to investigate the effect of nucleation temperature on grain size, Fig. 3(d–f) shows the SEM micrographs of the sample P2.5 after heat treatment at different nucleation temperature. From the

Fig. 2. X-ray diffractometer patterns of glass after two-stage heat-treated at various nucleation temperature and crystallization temperature with (a) P2.5; (b) P2.5; (c) P3 and P3.5.

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Fig. 3. SEM of glass P2.5 after two-stage heat-treated at various nucleation temperature and followed by various crystallization temperature with (a) 850 °C/2 h + 930 °C/20 h; (b) 850 °C/ 2 h + 950 °C/20 h; (c) 850 °C/2 h + 970 °C/20 h; (d) 820 °C/2 h + 950 °C/20 h; (e) 850 °C/2 h + 950 °C/20 h and (f) 880 °C/2 h + 950 °C/20 h.

SEM image in Fig. 3(e), a large number of micro-grains are distributed in the sample after two-stage treated at 850 °C/2 h + 950 °C/20 h. And it can be found that the grain size of sample after two-stage treated at 850 °C/2 h + 950 °C/20 h is obvious higher than two-stage treated at 820 °C/2 h + 950 °C/20 h and 880 °C/2 h + 950 °C/20 h. Fig. 4 shows SEM images of the sample P3 after two-stage heattreated at 850 °C/2 h + 950 °C/20 h and the sample P3.5 after twostage heat-treated at 780 °C/2 h + 880 °C/2 h. Compared with Figs. 4(a–b) and 3(e), the samples P3 [see Fig. 4(a)] and P3.5 [see Fig. 4(b)] have more pores and lesser crystallization than P2.5 [see Fig. 3(e)].

This phenomenon implies that the content of the precipitated crystalline phases in sample P2.5 (850 °C/2 h + 950 °C/20 h) is higher than that of other samples, which corresponds to the XRD patterns of sample P2.5 as shown in Fig. 2. At the same time, it is observed that a few crystal grains buried in large amount of glass phases appear in P3.0 [see Fig. 4(a)] and P3.5 [see Fig. 4(b)] except samples P2.5 [see Fig. 3(e)]. To obtain more detailed information about the crystalline phases, sample P2.5 heat-treated at 850 °C/2 h + 950 °C/20 h are investigated by EDS and element distribution mapping (EDM) in Fig. 5(a) and (b). According to the combination of XRD patterns in Fig. 2 and EDS data

Fig. 4. SEM of glass after two-stage heat-treated at nucleation temperature and followed by crystallization temperature with (a) P3, 850 °C/2 h + 950 °C/20 h; (b) P3.5, 780 °C/ 2 h + 880 °C/20 h.

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Fig. 5. The results of quantitative surface analysis for sample P2.5 and the element distribution mapping heat-treated at 850 °C/2 h + 950 °C/20 h.

in Fig. 5, it is considered that the white grains in the glass-ceramics is close to the compositions SiO2, Mg2Al4Si5O18 and phosphorus has a more complicated formula AlSi2P3O12. It is observed that the P5+ concentration in the white grains increases considerably, whereas a small number of P5+ is detected in the dark-grey region. In addition, some dark-grey region is dispersed in the glass-ceramics, which is due to that the etching rate of Mg2SiO4 and MgSiO3 is faster than other phases. On the other hand, the dark-grey region is mainly composed of P, Si, Al, Mg and O, as shown in Fig. 5(a), there is only a small difference in stoichiometry between observed value and the theoretical value for MgO-

Al2O3-SiO2-TiO2-P2O5 glass-ceramics. Thus, the dark-grey region should be the glass phase.

3.3. Microwave dielectric properties The changes in dielectric constant (εr), quality factor (Q × f) and temperature coefficient of resonant frequency (τf) were investigated and shown in Fig. 6 and Table 2. As seen in Fig. 6, it is clear that the sample P2.5 after two-stage heat-treated at 850 °C/2 h + 950 °C/20 h has

Fig. 6. Changes in εr, Q × f, and τf with glass-ceramics P2.5 two-stage heat-treated at different temperature.

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Table 2 Changes in εr, Q × f, and τf with the glass-ceramics P3 and P3.5 after two-stage heat-treated at different temperature. Crystallization temperature (°C/h + °C/h)

εr, ±0.1

Q × f (GHZ), ±50

f (GHZ), ±0.005

τf (ppm/°C), ±0.1

850/2 + 950/2 (P3) 850/2 + 950/20 (P3) 850/20 + 950/20 (P3) 780/2 + 880/2 (P3.5) 780/2 + 880/20 (P3.5) 780/20 + 880/20 (P3.5)

3.85 3.96 3.81 3.86 3.77 3.66

6431.5 5439.5 7774.7 11,241.5 2856.0 4470.4

13.560 13.404 13.702 13.569 13.408 13.729

−11.2 −8.2 −8.7 −15.6 −14.8 −13.8

the optimal dielectric properties, and is preferred as the material for LTCC. Firstly, it possesses the highest Q × f ~ 13,786 GHz at 13.784 GHz. Secondly, its dielectric constant (3.9– 4.1) is lower than that of Al2O3 (~9), and is absolutely necessary to reduce the propagation delay and parasitic capacitance (Cp) between internal conductor patterns of LTCC and to achieve the requirements of high signal propagation speed. Thirdly, it possesses a low τf (− 7.9 to − 6.5 ppm/°C) and higher flexural strength. Lastly, the low sintering temperature (≤950 °C) helps to achieve the LTCC with Ag and Cu electrodes. As seen in Fig. 6 and Table 2, with increasing P2O3 addition, the εr value shows a little change tendency (3.66–4.1), wherein the maximum ~ 4.1 can be obtained for the sample P2.5 two-stage heat-treated at 850 °C/2 h + 950 °C/20 h. As well known, the dielectric properties of a material are mainly determined by the summation of each phase presented in the material [13,14]. In the MgO-Al2O3-SiO2-TiO2-P2O5 glass-

ceramics, the εr of MgSiO3, Mg2Al4Si5O18, Mg2SiO4 and SiO2 are about 6, 6.7, 6.8 and 3.85, respectively [15,16,17,18]. As mentioned above, the MgO-Al2O3-SiO2-TiO2 glass with 2.5 wt.% P2O5 addition heat-treated at 850 °C/2 h + 950 °C/20 h promotes the crystallization of SiO2, the relative lower εr could be attributed to the increase SiO2 phases with a lower εr than that of MgSiO3, Mg2Al4Si5O18 and Mg2SiO4. On the other hand, the Q × f value reaches the maximum value at about 13,785.4 GHz (at 13.784 GHz), and then decreases with increasing P2O5 content. Also, the changed trends of Q × f value of specimens agree with the trends of their linear shrinking percentage in Fig. 7. Many factors are believed to affect the Q × f value, and these factors can be divided into two categories: the intrinsic loss and the extrinsic loss. In this study, Q × f value is primarily dependent on the basic silicon oxide network of the remnant glass phase and densification of glass-ceramics [19,20]. Additionally, roles of the P2O5 in the glass are to facilitate the devitrification, and it is noted that the residual glass decreases with the increase in the total amount of crystalline phases, which means that a higher Q × f value will be obtained. Therefore, the Q value of the MgOAl2O3-SiO2-TiO2-P2O5 glass added with 2.5 wt.% P2O5 content heattreated at 850 °C/2 h + 950 °C/20 h is higher than that of other glass-ceramics samples. The dependence of the relative density and linear shrinkage rate of the MgO-Al2O3-SiO2-TiO2-P2O5 samples with different amount of P2O5 additions is shown in Fig. 7. The optimal density of the P2.5 glass is achieved at 850 °C/2 h + 950 °C/20 h. This indicates that the sample P2.5 is denser than the sample P3.0 and P3.5, which corresponds to the relative density of sample as shown in Fig. 7. The densities obviously

Fig. 7. Changes in relative density and linear shrinkage with glass-ceramics P2.5, P3 and P3.5 two-stage heat treatment procedure.

X. Huang et al. / Journal of Non-Crystalline Solids 459 (2017) 123–129

decrease with the P2O5 additions and only the P2.5 samples show high relative densities of ~84.2%, which reveals that an excess of P5+ ions exists in MgO-Al2O3-SiO2-TiO2-P2O5 system, the linear shrinkage, the density and dielectric properties inversely deteriorate. Therefore, a small quantity of the high charge density of P5+ ion in the remaining glass favors to promote the phase separation of the sample P2.5, which leads to the precipitation of SiO2 at ≤950 °C. It can be seen from Fig. 7, the linear shrinkage rate and relative density decreases with P2O5 content, reaching a minimum value of 1.95% with 3.5 wt.% P2O5. In addition, the MgO-Al2O3-SiO2-TiO2-P2O5 glass-ceramic has a high linear shrinkage rate of 13.9%. The larger linear shrinking rate means the higher relative density, and the less crystal imperfection and porosity in the samples' interior structure, which means that a higher Q × f value will be obtained. 4. Conclusions Glass-ceramics of MgO-Al2O3-SiO2-TiO2-P2O5 were prepared by two-step crystallization after heat-treatment at selected nucleation temperature 820 °C, 850 °C, 880 °C for 2 h and 20 h and followed by crystallization temperatures 930 °C, 950 °C, 970 °C for 2 h and 20 h, respectively. The addition of P2O5 could lower the melting temperature and crystallization temperature of the glass-ceramics. And the glass transition temperature as well decreased with the increase of P2O5 content. Moreover, for an appropriate addition of P2O5 (2.5 wt.%), a high-Q glass-ceramic heat-treated at 850 °C for 2 h + 970 °C for 20 h was obtained with εr ≈ 4.0, Q × f ≈ 13,785.4 GHz, and τf ≈ −6.4 ppm/°C. Superior and reliable properties such as low temperature sinterability, thermal expansion coefficient and dielectric properties at 13– 14 GHz were successfully derived in the dense specimen prepared from MgO-

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Al2O3-SiO2-TiO2-P2O5 glass-ceramic powder containing 2.5 wt.% P2O5 content.

Acknowledgements Financial supports of the National Natural Science Foundation of China (Grants No. 11464006) and the Middle-aged and Young Teachers in Colleges and/or Universities in Guangxi Basic Ability Promotion Project of China (Grants No. KY2016YB534) are gratefully acknowledged by the authors. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

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