Microstructure characteristics and microwave dielectric properties of calcium apatite ceramics as microwave substrates

Journal of Alloys and Compounds 731 (2018) 264e270

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Microstructure characteristics and microwave dielectric properties of calcium apatite ceramics as microwave substrates Jianbing Song a, Kaixin Song a, *, Jinsheng Wei a, Huixin Lin b, **, Junming Xu a, Jun Wu a, Weitao Su c a b c

College of Electronic Information and Engineering, Hangzhou Dianzi University, Hangzhou 310018, China Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China College of Materials Sciences and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 25 May 2017 Received in revised form 3 October 2017 Accepted 5 October 2017 Available online 6 October 2017

Apatite structure of CaLa4Si3O13 ceramics was prepared by the conventional solid-state reaction route. The Rietveld refinement of powder X-ray diffraction (XRD) and analysis of scanning electron microscope images demonstrated that CaLa4Si3O13 belonged to a hexagonal structure with space group of P63/m (No. 176) and grew in a shape of hexagonal prism. The effects of crystallinity, porosity, grain morphology, grain size distribution, and oxygen vacancy on the microwave dielectric properties were investigated in detail as a function of sintering temperature. The existence of Oxygen vacancies and the stretching and bending modes of SiO4 tetrahedral units were discussed by Raman spectra. Excellent microwave dielectric properties were obtained with εr ¼ 14.5, Q  f ¼ 31,100 GHz (at 9.05 GHz), tf ¼ 22 ppm/ C, indicating possible potential applications for microwave substrate applications. © 2017 Elsevier B.V. All rights reserved.

Keywords: Apatite Ceramics Microstructures Microwave dielectric properties

1. Introduction Recent years, the Information Communications Technology (ICT) has developed rapidly in an explosive way. The progresses in Internet of Things (IoT) technology, microwave telecommunications, Direct-broadcast satellite television (DBS TV), satellite broadcasting, Intelligent Transport Systems (ITS) and Industry 4.0 have brought enormous changes in our lives and affected the scientific research of new materials [1e3]. Microwave dielectric materials applied in dielectric resonators, filters, substrates, etc. play a key role in communication system [4,5]. In order to promote the development of information technology, scientists are searching for novel microwave materials with high-quality factor (Q  f values) to depress energy loss, low dielectric constant (εr) to reduce the delaytime of electronic signal transmission, and near zero temperature coefficient of resonant frequency (tf) for frequency stability [6,7]. Many novel microwave ceramic materials have been reported, such as MgZrNb2O8 [8], Li2Mg3TiO6 [9], ZnTiTa2O8 [10], Li2ZnGe3O8 [11], BaMg2V2O8 [12], Li2ZnTi5O12 [13], etc. In addition

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (K. Song), [email protected] (H. Lin). https://doi.org/10.1016/j.jallcom.2017.10.028 0925-8388/© 2017 Elsevier B.V. All rights reserved.

to these novel dielectrics materials, there are many kinds of low εr silicate compounds reported and the microwave dielectric properties of which are summarized in Table 1. Due to the strong effects of covalent bond in silicate basic units of [SiO4] tetrahedrons, silicates usually have low dielectric constants which indicate potential application as microwave subtracts [14]. Felsche reported the rare earth silicates (Mg, Ca, Sr, Ba)2RE8 [SiO4]6O2 with the apatite structure for the first time in 1972 [24]. Apatite is named for a large family of isomorphous compounds with general formula A10(MO4)6O2, where A represents a divalent cation, MO4 represents a trivalent or tetravalent anion. In apatite structure, A2þ cations are located in two different sites: 4f with nine-fold coordination and 6h with seven-fold coordination. The cation in 6h site is coordinated to O(4) oxygen ion presented in the channel, resulting in larger average A-O covalence than that of the 4f site. Over decades, considerable attention was focused on substitutions at the A and M lattice sites [25e28]. Boyer et al. presented the dependence of site occupation on luminescent properties in apatite phosphor [25]. Sebastian et al. addressed the microwave dielectric properties of SrRE4Si3O13 and CaRE4Si3O13 ceramics (RE ¼ La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Tm, Yb, and Y) [29,30]. In this paper, CaLa4Si3O13 ceramics were prepared via a conventional solid-state reaction route. The evolutions of crystallinity, porosity, grain morphology, distribution of grain length size, and

J. Song et al. / Journal of Alloys and Compounds 731 (2018) 264e270 Table 1 The microwave dielectric properties of some silicate ceramics reported in references. Silicate Ceramics

Q  f (GHz)

εr

tf (ppm/ C)

Ref

Mg2SiO4 Zn2SiO4 LiAlSiO4 Al2SiO5 Sr2Al2SiO7 Ba2ZnSi2O7 Y3MgAl3SiO12 Ba9Y2Si6O24 Mg2Al4Si5O18

240,000 21,900 36,000 41,800 33,000 26,600 57,340 22,400 39,000

6.8 6.6 4.8 4.4 7.2 8.1 10.1 14.9 6.3

70 65 8.6 17 37 51 32 36 32

[15] [16] [17] [18] [19] [20] [21] [22] [23]

oxygen vacancy were systematically analyzed along with their effects on the microwave dielectric properties. The presence of oxygen vacancies were confirmed and investigated by Raman spectra tests along with the analysis of vibrational modes of SiO4. 2. Experimental The dense samples were prepared by a conventional solid-state sintering method using high-purity oxide powders CaCO3(Sinopharm Chemical Reagent Co., Ltd, China, 99.9%), La2O3(Sinopharm Chemical Reagent Co., Ltd, 99.9%), SiO2(Aladdin Industrial Corporation, 99.99%) as raw materials. Stoichiometric proportions of the above powders according to the chemical formula of CaLa4Si3O13 were mixed in ethanol for 24 h with zirconia balls as milling media. The resulting slurries were dried in an oven at 90  C. After sieving, the powders were calcined at 1250  C for 4 h at heating rate of 4  C/ min. After ball re-milled, the calcined powders were then mixed with 5 wt % solution of polyvinyl alcohol solution and pressed into pellets at the pressure of 100 MPa using a stainless die. The diameter of pellets is 12 mm and the height is about 6 mm for microwave dielectric property tests. The pellets were sintered at 1300 C-1450  C in air for 4 h to yield the dense ceramics. After sintering, the samples were cooled to 1000  C at a rate of 1  C/min, and then shut down the power, further cooled inside the furnace. The bulk densities of as sintered pellets were measured by the Archimedes method using distilled water as medium. The roomtemperature crystalline phase constituents were identified by

265

powder X-ray diffraction (XRD) (RIGAKU D/max 2550/PC, Rigaku Co., Tokyo, Japan) analysis with CuKa (l ¼ 1.54,056 Å) radiation at the voltage of 40 kV and current of 30 mA.The scanning rate was 10 min1 in the 2q range from 10 to 90 . The data for the Rietveld analysis were collected in a step-scanning mode with a step size of 0.02 and 5 s counting time per step over a 2q range from 5 to 120 [31]. The Rietveld refinement was performed with the general structure analysis system (GSAS) software. The Raman spectra were collected at room temperature using a Raman spectrometer (Renishaw in Via Raman in the Key Laboratory of Submarine Geosciences, State Oceanic Administration) with a CCD detector. The 514 nm line of an Arþ ion laser was used as excitation source. Microstructures of the polished and thermal etched surface of the sintered samples were observed using Field Emission Scanning Electron Microscopy (Ultra55, Germany). The thermal etching was carried out at a temperature 50  C lower than the sintering temperature for 30 min. The grain diameters and lengths were scaled in the SEM patterns using the software of Image J. The dielectric constant εr and the temperature coefficient of resonant frequency tf were measured by the paralleling plate method [32] using a vector network analyzer (E8363B, Agilent Technologies Inc., Santa Clara, CA). tf was measured in the temperature range of 20 Ce80  C. The quality factor Q was evaluated by the resonant-cavity method [33] using a silver-coated cavity connected to the network analyzer. To ensure the accuracy of the data, at least two samples were measured for each composition. As the Q-factor generally varied inversely with frequency (f) in the microwave range, the product Q  f, rather than Q alone, was used to evaluate the dielectric loss. 3. Results and discussion Fig. 1(a) shows the XRD pattern of CaLa4Si3O13 powders sintering at different temperature for 4 h. The X-ray diffraction patterns of all samples sintered at different temperatures show the same profile and can be indexed as CaLa4Si3O13 (JCPDS No.71e1368), with no secondary crystal phase being detected. The XRD analysis indicates the CaLa4Si3O13 belongs to a hexagonal crystal system with space group of P63/m (176). As shown (b) of Fig. 1, the crystallinity of CaLa4Si3O13 ceramic samples increases with the augment of sintered temperature from 1300  C to 1375  C, and decreases with

Fig. 1. (a) XRD pattern of CaLa4Si3O13 ceramic samples sintered at different temperature in air for 4 h. (b) The crystallinity of CaLa4Si3O13 ceramic samples sintered at different temperature.

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Fig. 2. Rietveld refinement pattern and crystal structure diagram of CaLa4Si3O13ceramics sintered at 1400  C for 4 h.

further increasing sintering temperature, which indicates the variety of amorphous phase in all samples as a function of sintering temperature. The X-ray diffraction data of CaLa4Si3O13 samples was further refined by the Rietveld method based on GSAS software. The refined profile of the sample sintered at 1400  C for 4 h was given in Fig. 2. The values of Rwp, Rp, and c2 were 10.5%, 7.6% and 4.619 respectively, which confirming that the sample is a single phase and isostructural to the known crystal structure of CaLa4Si3O13(PDF No.71-1368) with unit cell parameters a ¼ b ¼ 9.66 Å,c ¼ 7.15 Å and V ¼ 579.09 Å3. As seen from the insert figure of Fig. 2, Ca2þand La3þ ions occupy two sites. The Ca(1) and La(1) occupy 4f sites with 9-fold coordination, connecting three O(1), three O(2) and three O(3). Ca(2) and La(2) occupy 6h sites with 7-fold coordination, connecting one O(1), one O(2), four O(3), and one O(4). Each Si4þ ion is coordinated to four oxygen ions, which forms a tetrahedron with two oxygen ions occupied O(3) sites and the others at O(1) and O(2). The refined coordinates of each atom sites and occupancies are shown in Table 2. The polyhedron with 9-fold coordination is connected to SieO tetrahedron and the others polyhedrons with 7-fold coordination. Table 3 lists the average bond lengths for Ca/La(1)eO, Ca/ La(2)eO, and SieO tetrahedron. Moreover, the (4f) position with the average Ca/La(1)-O distance of 2.608 Å, which offers larger space for divalent and trivalent cation substitution as compared to the 7-fold coordinated (6h) lattice site with the average Ca/La(2)-O distance of 2.523 Å [25]. Fig. 3 displays the Raman spectra of CaLa4Si3O13 ceramic samples sintered at various temperatures. All spectra indicate similar

Table 2 Atomic positions of CaLa4Si3O13 ceramics refined by Rietveld refinement. Atom

Wyckoff position

x

y

z

Occupation

La1 Ca1 La2 Ca2 Si1 O1 O2 O3

4f 4f 6h 6h 6h 6h 6h 12i

0.3333000 0.3333000 0.2458000 0.2458000 0.3729000 0.4692000 0.4907000 0.2553000

0.6667000 0.6667000 0.0147000 0.0147000 0.4031000 0.5965000 0.3281000 0.3422000

0.0035000 0.0035000 0.2500000 0.2500000 0.2500000 0.2500000 0.2500000 0.0697000

0.520 0.480 0.986 0.014 1.000 1.000 1.000 1.000

peak positions and half-widths. Meanwhile, the spectra can be divided into two parts at the boundary of 350 cm1 [34,35]. The peaks above 350 cm1 can be assigned to internal modes of SiO4 tetrahedral units. The symmetric stretching mode y1 of SiO4 tetrahedral is assigned to the strongest peak at about 850 cm1, while the asymmetric mode y3 is responsible for the weak bands at about 930 cm1. The peak at about 525 cm1 is attributed to the asymmetric bending y4 modes and the second stronger peak at about 395 cm1 belongs to the symmetric bending y2 mode. The external modes involving translations and rotational oscillations of the SiO4, LaO6 and SrO6 units are expected to contribute to the Raman peaks at below 350 cm1. Importantly, as reported by Peng [36] and McBride [37], the weak Raman spectrum peaks at around 570 cm1 could be attributed to the presence of O vacancies. Fig. 4 exhibits the SEM micrographs of CaLa4Si3O13 ceramic surfaces sintered at different temperatures. As shown in Fig. 4(a), the grain sizes of particles are small and the grain boundaries are not distinct. However, with the augment of sintering temperature from 1350  C to 1400  C, seen in Fig. 4(b)e(d), the particle grain boundaries gradually become clear and distinct. As indexed by the purple hexagon, hexagonal crystallized morphology of grains is clearly observed at Fig. 4(d), which is coincident with crystal growth habit of hexagonal crystal system. The sample sintered at 1400  C shows a dense microstructure where the grain sizes are uniformly distributed. When the sintering temperature is above 1400  C, some grain particles become abnormally larger, and the grain edges and corners happen to appear smooth and ambiguous, the phenomenon of which is also supported by the crystallinity calculation of XRD data.

Table 3 Selected bond length of CaLa4Si3O13 ceramics refined by Rietveld refinement. Bond

length (Å)

Bond

length (Å)

Ca(1)/La(1)-O(1)  3 Ca(1)/La(1)-O(2)  3 Ca(1)/La(1)-O(3)  3 SieO(1) SieO(2) SieO(3) SieO(3)

2.485(5) 2.468(5) 2.871(9) 1.622(2) 1.622(2) 1.622(2) 1.622(2)

Ca(2)/La(2)-O(1) Ca(2)/La(2)-O(2) Ca(2)/La(2)-O(3) Ca(2)/La(2)-O(3) Ca(2)/La(2)-O(3) Ca(2)/La(2)-O(3) Ca(2)/La(2)-O(4)

2.507(1) 2.755(4) 2.610(4) 2.436(5) 2.610(4) 2.436(5) 2.305(5)

J. Song et al. / Journal of Alloys and Compounds 731 (2018) 264e270

Fig. 3. Raman spectra for CaLa4Si3O13 samples sintered at 1325  C, 1350  C, 1375  C, 1400  C and 1425  C.

267

Using Image J software, Fig. 5(a) shows the statistical distribution maps of grain diameter sizes with different sintering temperatures. The grain diameter sizes follow the normal distribution (color curve) with average value from 0.2 mm to 1.2 mm. The centering distribution of grain diameter sizes is about 0.4 mm at 1350  C, 0.57 mm at 1375  C, 0.62 mm at 1400  C and 0.75 mm at 1425  C, respectively. As shown in Fig. 5(b), the average diameters increase with increasing sintering temperature. Fig. 5(c) displays the approximate distribution of grain length sizes from the SEM images. The grain lengths distribute mostly around 1.3 mm at 1350  C, 1.9 mm at 1375  C, 2.7 mm at 1400  C and 4.0 mm at 1425  C, respectively. It is easy to find out that the average length sizes follow the similar trend of distribution of average diameter sizes. It indicates that the grain size grows larger with the increasing sintering temperature. The increasing trend of grain diameter and length sizes is proportional to the evolution of SEM images, which is closely associated with its microwave properties. As shown in the insert figure of Fig. 6, the relative densities of CaLa4Si3O13 ceramic samples increase with increasing sintering temperature from 1300  C to 1400  C, and reach the maximum value of relative density (98.9%) at 1400  C, then decrease with further increasing sintering temperature. It matches well with the trend of grain growth in SEM pictures and crystallinity calculation of XRD data. Herein, it can be deduced that when sintering

Fig. 4. The SEM micrographs of CaLa4Si3O13 ceramics sintered at various temperatures: (a) 1325  C, (b) 1350  C, (c) 1375  C, (d) 1400  C, (e) 1425  C and (f) 1450  C.

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Fig. 5. (a), (c) The distribution profiles of grain diameters and length sizes sintered at 1350  C, 1375  C, 1400  C and 1425  C, respectively. (b), (d) the average diameter and length sizes of CaLa4Si3O13 ceramic samples as a function of sintering temperatures.

temperature is below 1400  C, the grains are not fully grown and there is some porosity which lead to the low εr and Q  f value, when beyond 1400  C, the grain secondary growth takes on. As shown in Fig. 6, the Q  f values of as sintered CaLa4Si3O13 ceramics increase from 17,200 GHz to 31,100 GHz as the sintering temperature increases from 1300  C to 1400  C, and then decreases dramatically to 19,500 GHz at the sintering temperature of 1450  C. The variation in Q  f value as a function of sintering temperature mainly attributes to the presence of extrinsic defects, such as secondary phases, grain size, porosity, oxygen vacancy, etc. Associated with curves of relative density, crystallinity calculation, and the distribution of grain diameter and length sizes, the increase of Q  f values of CaLa4Si3O13 ceramic samples should benefit from the improvement of these extrinsic factors. As grains further grow abnormally, some defects increase like amorphous phase, ununiform distribution of grain sizes which cause the deterioration of Q  f values with further increasing sintering temperature. Meanwhile, it is also a factor that oxygen vacancy remains within super large grains. Some similar phenomena on the effect of oxygen vacancies on microwave properties were also reported in as followed

microwave ceramics such as Sr2TiO4, Sr2LaAlTiO7, Sm2Si2O7, Ba9Y2Si6O24 [38, 39, 40, 22]. The dielectric constant (εr) varies as a function of sintering temperature. It increases from 13.1 to 14.5 with the sintering temperature increasing from 1300  C to 1400  C and then decreases to 13.7 at 1475  C. The variation in εr presents a similar trend with the variation of relative density and crystallinity, indicating that porosity and amorphous phase play important roles in εr. Meanwhile, the influence of the porosity on the dielectric constant could be eliminated by applying Bosman and Havinga's correction [41] as shown in Eq. (1)

 εr ¼ εm ð1 þ 1:5PÞP ¼

1  r=r

 th

(1)

where, εr and εm are the corrected and measured values of permittivity, respectively. P andrare the fractional porosity and density of the samples. The corrected εr value is about 14.8 which is close to the measured value of 14.5 for CaLa4Si3O13 ceramic samples sintered at 1400  C. It also indicates that the porosity of the ceramics is very low at 1400  C. It is clearly displayed that the

J. Song et al. / Journal of Alloys and Compounds 731 (2018) 264e270

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12] Fig. 6. Microwave dielectric properties of CaLa4Si3O13 ceramics sintered at different temperature. The inset profile is relative densities as a function of sintering temperature.

[13] [14]

variation in εr and Q  f is closely relevant to sintering temperatures. The negative temperature coefficient of resonant frequency (tf) varies from 23 ppm/ C to 35 ppm/ C when sintered in the temperature range of 1300 Ce1450  C, which is still necessary to be modified to zero to meet the requirements of commercial application [42]. In the further work, we will concentrate on the improvement of its temperature coefficient of resonant frequency. 4. Conclusions Calcium-based apatite CaLa4Si3O13 ceramics were prepared by the conventional solid-state reaction method. The analysis of XRD patterns indicates the as-sintered CaLa4Si3O13 samples belong to the hexagonal crystal system with a space group of P63/m (No. 176). Scanning electron microscope images indicate that the grain particles of CaLa4Si3O13 gradually grow up with a shape of hexagonal prism, and porosities decrease with the increasing sintering temperature. Week Raman spectra peaks (~570 cm1) are detected due to the presence of O vacancy. The crystallinity, porosity, grain morphology, grain size distribution and oxygen vacancy have critical effects on microwave dielectric properties. The best combination of microwave dielectric properties are obtained in CaLa4Si3O13 ceramic sintered at 1400  C for 4 h (εr ¼ 14.5, Q  f ¼ 31,100 GHz (at 9.05 GHz), tf ¼ 22 ppm/ C). Acknowledgements This work was supported by the National Natural Science Foundation of China under grant number 51672063 and 51202051 and Science and Technology Program of Zhejiang Province under grant number 2016C31110. References [1] M.T. Sebastian, R. Ubic, H. Jantunen, Low-loss dielectric ceramic materials and their properties, Int. Mater. Rev. 60 (2015) 392e412. [2] J.X. Bi, C.F. Xing, Y.H. Zhang, C.H. Yang, H.T. Wu, Correlation of crystal structure and microwave dielectric properties of Zn1xNixZrNb2O8(0x0.1) ceramics, J. Alloys Compd. 727 (2017) 123e134. [3] J.E.F.S. Rodrigues, P.J. Castro, P.S. Pizani, W.R. Correr, A.C. Hernandes, Structural ordering and dielectric properties of Ba3CaNb2O9-based microwave ceramics, Ceram. Int. 42 (2016) 18087e18093. [4] S. Lei, H.Q. Fan, X.H. Ren, J.W. Fang, L.T. Ma, Z.Y. Liu, Novel sintering and band

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