Journal of Alloys and Compounds 644 (2015) 242–248

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Journal of Alloys and Compounds

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Synthesis and microwave dielectric properties of 2ZnO�3B2O3-dopedZnAl2O4 low-permittivity ceramics

http://dx.doi.org/10.1016/j.jallcom.2015.04.1510925-8388/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel./fax: +86 20 87112897.E-mail address: chwsp@scut.edu.cn (S. Wu).

Xin Ouyang, Songping Wu ⇑, Zhuolin Wang, Yunhao LiuSchool of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China

a r t i c l e i n f o

Article history:Received 3 February 2015Received in revised form 11 April 2015Accepted 22 April 2015Available online 28 April 2015

Keywords:Dielectric propertiesZnAl2O4 ceramicsZinc borateSintering aid

a b s t r a c t

2ZnO�3B2O3-doped ZnAl2O4 ceramics were synthesized via solid-state method. Influence of different2ZnO�3B2O3 additions on the microstructure, sintering behavior and microwave dielectric propertieshas been investigated. The results showed that zinc borate could effectively improve the densificationand expand the sintering temperature range. Microwave dielectric properties of 2ZnO�3B2O3-dopedZnAl2O4 ceramics were exhibited as follow: er = 8.32, Q � f = 93,600 GHz (at 14.0 GHz) andsf = �68.5 ppm/�C. Spinel-structured zinc borate-doped ZnAl2O4 ceramics possessed comparatively lowsintering temperatures, wide temperature regions (about 100 �C), simultaneously, high Q � f and stablesf values. These outstanding performance make them promising candidate materials formillimeter-wave devices at high frequency.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Microwave technology is recently growing in the direction ofmillimeter and submillimeter wave, with the rapid developmentof modern communication technologies, including satellite com-munication, global positioning system (GPS) and wireless localarea network (WLAN) [1,2]. Microwave ceramic materials, whichhas been widely applied in resonators, filters and antennas in wire-less communication systems nowadays [3,4], are also being soughtfor more breakthroughs to keep up with the pace. Particularly,researches for those ceramics with a low dielectric constant (usu-ally under 10) and high quality factor value have gained muchattention, because of their extensive usage as advanced materialsin microwave circuits (MIC) [5] and passive component.

Common low-K microwave ceramic materials include but notlimited to: Al2O3 [6], MgO–SiO2 [7], RE2SiO5 (RE = Sm, Nd) [8,9]and spinel MAl2O4 (M = Mg, Ni) [10,11] etc. In our previous studies,the spinel MgGa2O4 [12], ZnGa2O4 [13] and Mg(Al, Ga)2O4 [14] solidsolution have been investigated for gaining more insights to the spi-nels. Among them, spinel MAl2O4 (M = Zn, Mg) ceramics have beenspecially considered due to the simple process and low cost.

It is noted that ZnAl2O4 posses a low dielectric constant (er) anda high quality factor (Q � f). In 2007, Chen et al. [10] reportedthat spinel-structured ZnAl2O4 by heat treatment for 4 hat 1700 �C offered following dielectric properties: er = 8.5,

Q � f = 106,000 GHz, sf = �63 ppm/�C. Analogous results forMgAl2O4 were gathered by Surendran et al. [15], i.e. microwavedielectric properties of er = 8.8, Q � f = 68,900 GHz, andsf = �75 ppm/�C. Obviously, MAl2O4 (M = Zn, Mg) could be consid-ered as good microwave substrate materials. However, some inher-ent disadvantages in the spinels, such as considerable negative sf

values, especially high sintering temperature, have limited the prac-tical application of ZnAl2O4 ceramics to date. Therefore, severalstrategies have been developed to overcome these issues.

Converse spinels, i.e. 2MO–TiO2 (M = Co, Mg, and Mn), were uti-lized to adjust the microwave dielectric properties of ZnAl2O4

ceramics as an aiding agent. As reported by Lei et al. [16], the com-posite, comprising the ZnAl2O4 and 2MO–TiO2 (M = Co, Mg, andMn) by solid-state method, possessed comparable microwavedielectric properties, for example er = 9.6, Q � f = 160,800 GHz,sf = �65.3 ppm/�C for the 0.79ZnAl2O4–0.21Mg2TiO4 compound[16]. Of special note is that the formation of solid solution is verydifficult due to the various structures between converse spineland normal spinel for ZnAl2O4. Another frequently-used agentis TiO2 for the modification of ceramics. A fine combination ofdielectric properties (er = 12.67, Q � f = 100,000 GHz, andsf = 0.74 ppm/�C) could be achieved in the 0.83ZnAl2O4–0.17TiO2

ceramics [17]. But, for all of them, the sintering temperature wasstill as high as �1600 �C, accompanying with complicated process,obvious impurity and crystal distorts.

Even though a variety of studies were conducted onspinel-structured ZnAl2O4 ceramics, ZnAl2O4-based microwavedielectric ceramics still face huge challenges and some stubborn

Fig. 1. (a) XRD patterns of ZnAl2O4 ceramics with 3.0 wt.% 2ZnO�3B2O3 sintered atvarious temperatures for 4 h; (b) XRD patterns of ZnAl2O4 ceramics with differentamount of 2ZnO�3B2O3 sintered at 1500 �C for 4 h.

X. Ouyang et al. / Journal of Alloys and Compounds 644 (2015) 242–248 243

problems need to be dealt with. Herein, we concentrated our atten-tion on the how to lower the sintering temperature, which isbecoming a very severe issue to hinder the practical applicationof ZnAl2O4-based ceramics. In the previous work, Belous et al., con-firmed that ZnB2O4 were effective sintering aids for the certainspinel-based ceramic materials [18]. Similarly, B2O3 [19] and ZnO[2] were also the frequently-used additives. So it’s no wonder thatwe choose the above-mentioned zinc borate-doped ZnAl2O4 as ouraim to develop a new type microwave/millimeterwave ceramicmaterial with relatively low sintering temperature and gooddielectric properties. Actually, synthesis and microwave propertiesof 2ZnO�3B2O3-doped ZnAl2O4 were seldom researched.

In this paper, effects of different amounts of 2ZnO�3B2O3 on thesintering behavior and microwave dielectric properties of ZnAl2O4

ceramics have been investigated based on densification, crystallinephase and microstructure analysis. As a result, the excellent micro-wave dielectric properties for 2ZnO�3B2O3-doped ZnAl2O4 ceramicswere gathered as follow: er = 8.32, Q � f = 93,600 GHz (at 14.0 GHz)and sf = �68.5 ppm/�C.

2. Experimental

2.1. Sample preparations

Conventional solid-state method was utilized to synthesize spinel ZnAl2O4

ceramics. The starting materials were ZnO (AR, Shantou, Guangdong), Al2O3

(99.9%, Aladdin, Shanghai, China) powders. Both oxides were weighed accordingto the molar ratio in ZnAl2O4, then mixed and milled for 6 h in polyethylene jarswith zirconia balls in a planetary milling machine (QM-3SP4, Zhenguang,Nanjing, China). The mixtures were dried at 100 �C and thoroughly milled beforeit was calcined at 1000 �C for 2 h.

The obtained powders were pressed into green bodies (10 mm in diameter and2 mm in thickness) with a manually tablet machine (769YP, Tianjin, China) underabout 8 MPa pressure, then sintered to yield bulk ceramics with ahigh-temperature electric furnace (SSJ-1600, Shenjia kiln, Luoyang, China)(1235–1535 �C) for X-ray diffraction (XRD) research. Subsequently, the calcinedpowders with different amount of 2ZnO�3B2O3 (AR, Guangzhou, Guangdong)additive were re-milled for another 6 h. The mixtures were dried, then, granulatedwith 5 wt.% PVA as an organic binder, finally, the mixed powders were pressed intodisks of 10 mm diameter and 7 mm thickness. The sample disks were sintered atvarious temperatures (1300–1500 �C) for 4 h to obtain ceramic specimens.

2.2. Characterization

The crystalline phases of samples were identified by X-ray diffraction (XRD)(Bruker, D8 ADVANCE, Germany) using Cu Ka (k = 0.15406 nm) radiation with agraphite monochromator in the 2h range of 10–80� operated at 30 kV and 30 mA.The cell parameters were refined with software Jade 6.0. Transmission electronmicroscopy (TEM) (JEOL, JEM 2100F, Tokyo, Japan) was utilized to observe the mor-phology of particles. Micro structural observations and analysis of sintered pelletswere performed by scanning electron microscopy (SEM)(Zeiss, LEO 1530 VP,Germany). An energy dispersive spectrometer (EDS) (EPMA1600, Shimadzu,Kyoto, Japan) was adopted to investigate the element composition in specimens.Fourier-transform infrared spectroscopy (FT-IR) (Bruker, VERTEX 70, Germany)was further employed to explore the internal structure of samples. The bulk densi-ties of the sintered ceramics were evaluated with the Archimedes method. Hakki–Coleman dielectric resonator method was used to acquire the microwave dielectricconstants (er) and quality factor values (Q � f) at microwave frequencies withNetwork Analyzer (N5230 PNA-L; Agilent, Santa Clara, U.S.A). The technique formeasuring temperature coefficients of resonant frequency (sf) was the same as thatused to measure the dielectric constant, and then the sf value was calculatedaccording to the following Eq. (1) with temperature increasing from 25 �C to 75 �C

sf ¼f 75 � f 25

f 25 � 50ð1Þ

where f75 and f25 represent the resonant frequency at 75 �C and 25 �C, respectively.

3. Results and discussion

3.1. XRD analysis

Fig. 1(a) depicted the XRD profile of ZnAl2O4 ceramics contain-ing 3 wt.% 2ZnO�3B2O3 sintered at different temperatures ranging

from 1300 �C to 1500 �C. Apparently, cubic spinel ZnAl2O4 (JCPDSNo. 05-0669, with the space group Fd-3m) was the only crystallinephase, regardless of the sintering temperature, meaning that the3 wt.% 2ZnO�3B2O3-doped ZnAl2O4 ceramics have a relatively widesintering temperature range and can exist stably in the above tem-perature region. The refined lattice parameters of specimen, i.e.3 wt.% zinc borate-doped ZnAl2O4 sintered at 1500 �C, were calcu-lated as a = b = c = 0.8099 nm with the Jade 6.0 software. Fig. 1(b)exhibited the XRD patterns of ZnAl2O4 ceramics with differentamounts of 2ZnO�3B2O3 sintered at 1500 �C. Notably, all the reflec-tions could be indexed satisfactorily to cubic spinel ZnAl2O4 whenthe amount of additive was less than 3 wt.%. As a comparison, thediffraction peaks of cubic Zn4O(BO2)6 (JCPDS No. 72-1316) werebelieved to appear while the content of zinc borate was more than3 wt.%. It could be inferred that B3+ got into the octahedral struc-ture of ZnAl2O4 particles, combining with Zn2+, O2� to formZn4O(BO2)6 [20], which would be expounded by infrared spectrain the following section. Besides, generation of secondary phasedirectly resulted in distorting of grain and further decreasingmicrowave dielectric properties [17,19].

3.2. TEM and HRTEM analysis

In order to gain further insight into the nature of doped ZnAl2O4

particles, typical TEM images of 3 wt.% 2ZnO�3B2O3-doped ZnAl2O4

Fig. 2. (a) TEM; (b) HRTEM; (c) SEAD; (d) FFT corresponded to the white frame area in (a) for ZnAl2O4 ceramics with 3.0 wt.% 2ZnO�3B2O3 sintered at 1450 �C for 4 h.

Fig. 3. SEM images of the specimens sintered at different temperatures: (a) 1300 �C, (b) 1350 �C, (c) 1400 �C, (d) 1450 �C, (e) 1500 �C for 3.0 wt.% 2ZnO�3B2O3-doped ZnAl2O4

ceramics; (f) a and b curves corresponded to EDS patterns of white snowflake micro areas in (c) and (e), respectively.

244 X. Ouyang et al. / Journal of Alloys and Compounds 644 (2015) 242–248

particles (calcined at 1450 �C for 4 h) were exhibited in Fig. 2(a),which are indicative of highly crystallized micro-sizedbox-shaped particles. HRTEM images (Fig. 2(b)), recorded fromthe white frame in Fig. 2(a), clearly demonstrated that the spacingsbetween any two adjacent lattice fringes is 0.27 nm, corresponding

to that of the (311) lattice plane of the cubic ZnAl2O4. The exis-tence of sharp diffraction spots was explicitly observed in theselective area electron diffraction (SAED) pattern (Fig. 2(c)), indi-cating that the formation of well-formulated, single-crystallinecubic spinel. Fast Fourier Transformation (FFT) pattern (Fig. 2(d))

Fig. 4. SEM photographs of the ZnAl2O4 ceramics with different amount of supplement sintered at 1500 �C for 4 h: (a) pure ZnAl2O4 ceramics, (b) 1 wt.%, (c) 2 wt.%, (d) 3 wt.%,(e) 4 wt.%, (f) 5 wt.%, (g) 6 wt.% addition of 2ZnO�3B2O3; (h) a and b curves corresponded to EDS patterns of black snowflake micro areas in (b) and (g), respectively.

X. Ouyang et al. / Journal of Alloys and Compounds 644 (2015) 242–248 245

taken from a corresponding area can be indexed as cubic ZnAl2O4

single-crystal. SAED and FFT strongly proved that each nanocrystalwas comprised of a single crystalline domain.

3.3. SEM and EDS analysis

It is generally known that the microstructure can exert a signif-icant influence in the dielectric performance of functional ceramics.To reveal the grain size variation and porosity distribution, SEMmicrographs of 3 wt.% 2ZnO�3B2O3-doped ZnAl2O4 ceramics sin-tered at 1300 �C–1500 �C for 4 h were illustrated in Fig. 3(a–e).From Fig. 3(a), it demonstrated that the grain sizes of 3 wt.%2ZnO�3B2O3-doped ZnAl2O4 ceramics sintered at low temperature(1300 �C) were about 1.5–2.0 lm, accompanied with lots of pores.Subsequently, the grain size reduced to 1.0–1.5 lm and the porousmicrostructure receded when the specimen was sintered at 1350 �C(Fig. 3(b)). Under high sintering temperature, i.e. more than1400 �C, the microstructure became more dense with the sinteringtemperature increasing. For example, step-like microstructure wasallowed to appear on the surface of sample sintered at 1400 �C(Fig. 3(c)); Moreover, as sintering temperature raised to 1450 �C,the globular ZnAl2O4 grain disappeared basically, and thestep-like morphology further developed in grains (Fig. 3(d)). Asexpectedly, dense microstructures were established and there onlyexisted step-like ZnAl2O4 grain for the specimen forged at 1500 �C(see Fig. 3(e)), suggesting that high sintering temperature wasadvantageous to the growth of grains, which brought about theimprovement of microwave dielectric properties in a great extent[21]. EDS patterns (Fig. 3(f)), gathered at two positions marked with

the white snowflake in Fig. 3(c) and (e), respectively. By means ofEDS, the atomic ratio of Zn, Al, O was 1:2.3:4.3 (very close to thevalue of 1:2:4), separately, for the ZnAl2O4 ceramic in Fig. 3(e),showing that element compositions agreed with stoichiometricratio. The results were indicative of the excellent thermal stabilityof ZnAl2O4 ceramics, i.e. no loss of element, under high sinteringtemperatures.

Of particular interest is that a dramatic change in microstruc-tures of specimens (sintered at 1500 �C) occurred with increasing2ZnO�3B2O3 contents (Fig. 4(a–g)). A porous microstructureemerged in pure ZnAl2O4 ceramics, where the grain sizes weresmall to 0.5 lm (Fig. 4(a)). Subsequently, the spherical ZnAl2O4

grain reduced gradually and step-like morphology became moreobvious, as the amount of 2ZnO�3B2O3 improves from 1 wt.% to3 wt.% (Fig. 4(b–d)). As a representative example, the ceramicsbecame impressively dense and possessed explicitly uniformstep-like grain structure when the dosage of zinc borate was3 wt.% (Fig. 4(d)), greatly differing from the flower-shaped structurein zinc borate doped-cordierite/Al2O3 ceramic [22] and plate-likestructure in ZnO/B2O3-zinc lanthanum borates composite [23].Since then, along with the amount of zinc borate increasing, densemicrostructure and larger grains with more obvious step-like char-acteristics occurred (Fig. 4(e–g)), probably due to strong wettingeffect from Zn4O(BO2)6 confirmed by XRD synthesis [20]. Whilethe content of zinc borate reached a maximum (6 wt.%), it was sim-ple for us to observe the quadrangular pyramids ZnAl2O4 grains(Fig. 4(g)), implying that Zn4O(BO2)6 enable the orientation of graingrowth. Taken together, we could conclude that 3 wt.% may be theoptimum level of zinc borate for ZnAl2O4 ceramics, completely

Fig. 5. (a) FT-IR patterns of ZnAl2O4 ceramics with different amount of 2ZnO�3B2O3

sintered at 1500 �C for 4 h; (b) the enlarged infrared spectra of low wave number forFig. 5(a).

246 X. Ouyang et al. / Journal of Alloys and Compounds 644 (2015) 242–248

consistent with the analysis of microwave dielectric propertiesbelow, because 3 wt.% 2ZnO�3B2O3 has allowed ceramics to densify,directionally grow, and simultaneously, dispel the impurity con-tamination. Fig. 4(h) represented the EDS patterns of black snow-flake micro areas in Fig. 4(b) and (g), respectively. On the basis ofEDS, the atomic ratio of Zn, Al in Fig. 4(g) was 1:1.8, close to thevalue of 1:2. A handful of small globular ZnAl2O4 grains existed,possibly owing to heterogeneous nucleation on the ceramic surface.

3.4. FT-IR spectrum analysis

FT-IR spectra of ZnAl2O4 ceramics with different amount of2ZnO�3B2O3 sintered at 1500�C were exhibited in Fig. 5(a) andthe enlarged infrared spectra of low wave number for Fig. 5(a)were illustrated in Fig. 5(b). In the spectra, the absorption bandsat 505, 563 and 676 cm�1 were assigned to ZnAl2O4 particles ofthe regular spinel structure, with only octahedrally coordinatedaluminum centers [24]. Two well-defined peaks, located at 1640and 3450 cm�1, corresponded to Zn–O–B stretching and Zn–O–B

vibration in the crystallites [25]. Of special note is that, the smallabsorption bands at �2085 cm�1, attributed to the bending modefor O–B [20], appeared and strengthened gradually with the con-tents of 2ZnO�3B2O3 increasing (above 3 wt.%), reasonablyexplained by the enhancement of O–B bending vibration derivedfrom the raising percent content of boron and oxygen [20,25].

3.5. Microwave dielectric properties

Microwave dielectric properties of ZnAl2O4 ceramics with dif-ferent amounts of 2ZnO�3B2O3 sintered at various temperaturesfor 4 h were presented in Fig. 6. Generally speaking, with the sin-tering temperatures going up, the relative density would increaserapidly and get to the maximum value, meaning that temperaturemade a great difference to it. For the pure ZnAl2O4 ceramics, therelative density was extremely low, just only 73%, even thoughthe sintering temperature was high to 1500 �C (Fig. 6(b)).Nevertheless, the relative density increased markedly for adding2ZnO�3B2O3. In particular, it reached to the maximum value of92% when the specimen with 3 wt.% zinc borate was sintered at1500 �C (Fig. 6(a)), associated with the compact and uniformmicrostructure of ceramics. The results could be ascribed to the liq-uid phase sintering from doped zinc borate glass, which couldimprove the densification during sintering [26].

To date, the denfication of ZnAl2O4 has been confirmed to bedifficult; however, when sintering aid, i.e. 2ZnO�3B2O3, was utilizedand Zn4O(BO2)6 was formed during sintering, the situation hasgreat changed. The resulting Zn4O(BO2)6, as a glass liquid phase,gradually melted into the ZnAl2O4 particles and then acceleratedthe particle movement and rearrangement [27,28], leading to thedense microstructure.

The dielectric constants (er) of 2ZnO�3B2O3-doped ZnAl2O4

ceramics sintered at various temperatures for 4 h were exhibitedin Fig. 6(c and d). When the amount of sintering aid was less than3 wt.%, the shift tendency of er was similar to that of relative den-sity. That was to say, in the case of adding little amount of zincborate, relative density was the key factor to affect the er ofZnAl2O4 ceramics. A high relative density tended to result in a largedielectric constant, as proved by several related researches [8–10].As a sharp contrast, while the content of additive was above 4 wt.%,the er increased at 1350 �C, then declined form 1400 �C to 1500 �C.We assumed that the formation of Zn4O(BO2)6 may exert effect onthe dielectric constant of 2ZnO�3B2O3-doped ZnAl2O4 ceramics.Such a variation could be explained by the well-known mixing ruleEq. (2):

Lner ¼ c1Lner1 þ c2Lner2 ð2Þ

According to the mixing theory, the rule links the resultingdielectric constant (er) of a composite versus the er of the com-pounds belonging to the material and characterized by their vol-ume fractions (respectively er1, c1, and er2, c2) [29,30]. Forabove-mentioned ZnAl2O4 ceramics, the er value of ZnAl2O4

(er = 8.56) [24] and Zn4O(BO2)6 (er = 7.24) [31] could be obtainedin the literature, and the volume fraction of both could be estab-lished by XRD. The final results were showed in Table 1. It explic-itly suggested that the experimental dielectric constant valueswere a slightly smaller than the theoretical values, indicating thatthe formation of Zn4O(BO2)6 definitely decreased er value for2ZnO�3B2O3-doped ZnAl2O4 ceramics. And it is noteworthy thatthe er value is not more than 8.5 for all examples sintered in therange from 1300 �C to 1500 �C.

Fig. 6(e) showed the quality factors (Q � f) of2ZnO�3B2O3-doped ZnAl2O4 ceramics sintered at different temper-atures for 4 h. For pure ZnAl2O4 ceramic, the Q � f values werequite low, less than 5,000 GHz, even if sintered at 1500 �C, due tothe low density and porous morphology. However, they sharply

Fig. 6. (a) and (b) relative density, (c) and (d) dielectric constant, (e) Q � f, (f) sf of the ZnAl2O4 ceramics with various amounts of 2ZnO�3B2O3 sintered at differenttemperatures for 4 h.

Table 1Theoretical and experimental dielectric constant of the ZnAl2O4 ceramics with variousamounts of 2ZnO�3B2O3 sintered at 1500�C for 4 h.

Dielectric constant Mass fraction of 2ZnO�3B2O3

4 wt.% 5 wt.% 6 wt.%

Theoretical 8.42 8.40 8.45Experimental 8.34 8.27 8.3

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increased with addition of sintering aid and enhancement of sin-tering temperature, to a maximum value of 93, 600 GHz (at14.0 GHz) for specimen with 3 wt.% 2ZnO�3B2O3 sintered at1500 �C. Of Particular note is that 3 wt.% zinc borate-dopedZnAl2O4 ceramics still possessed a comparable Q � f value(80,000 GHz) under a relatively low sintering temperature of1450 �C. The relative density, grain growth and microstructureplayed vital roles in affecting the Q � f value. In general, high den-sification, large grain size and uniform dense grain microstructureproduce a tangible contribution to a high Q � f value [32]. In thework, the rapid improvement of Q � f value in 2ZnO�3B2O3-dopedZnAl2O4 ceramics, for example�200% Q � f value (at 1500 �C) com-pared with samples (at 1400 �C) of 3 wt.% 2ZnO�3B2O3, could beascribed to the consequence of the synergistical effect betweenhighly crystallized microstructure and high relative densities.

The temperature coefficient of resonant frequencies (sf) of the2ZnO�3B2O3-doped ZnAl2O4 ceramics sintered at various tempera-tures was illustrated in Fig. 6(f). There was no significant change in

sf values, despites the amounts of sintering aid increased from1 wt.% to 6 wt.%. What’s more, they were comparably stable andwaved between �68 ppm/�C and �79 ppm/�C in the sintering tem-perature region of 1300–1500 �C. It seemed that zinc borate shift sf

to a little more negative value compared with pure ZnAl2O4 ceramic.The influential factors of sf values were extremely complicated con-sidering the joint function of composition, additives and presence ofsecondary phases in a ceramic system. Lee reported that the temper-ature coefficient of the resonant frequency largened with an increas-ing unit-cell volume of MNb2O6 [33]. Meanwhile, a correlationbetween the value of sf and the occurrence of O-octahedra tilts hasbeen established, for Sr(Zn1/3Nb2/3)O3-Ba(Zn1/3Nb2/3)O3 compositesystem [34]. In the work, sf was insensitive to the sintering proce-dure in 2ZnO�3B2O3-doped ZnAl2O4 ceramics.

In brief, Q � f values of the 2ZnO�3B2O3-doped ZnAl2O4 ceramicswere comparable with the Q � f values of (1�x)ZnAl2O4–xCo2TiO4,(1�x)ZnAl2O4–xMg2TiO4 [16], or (1�x)ZnAl2O4–xTiO2 [17] ceramicsand higher than those of (1�x)ZnAl2O4–xCaTiO3 [35],(1�x)(0.75ZnAl2O4–0.25TiO2)–xSrAl2Si2O8 [36] or (1�x)ZnAl2O4–xMn2TiO4 [16] ceramics. Herein, we revealed a 2ZnO�3B2O3-dopedZnAl2O4 spinel ceramic material with impressive microwave dielec-tric properties (with 3 wt.% zinc borate, sintered at 1500 �C): i.e.er = 8.32, Q � f = 93,600 GHz (at 14.0 GHz), and sf = �68.5 ppm/�C.Compared with conventional spinel-structured ZnAl2O4 ceramics,mentioned-above ZnAl2O4 ceramics have distinguished advantagesand features: i.e. the comparatively low sintering temperature range(1400–1500 �C), simple and low-cost process, single crystal phase,

248 X. Ouyang et al. / Journal of Alloys and Compounds 644 (2015) 242–248

non-defective microstructure and environmentally friendly nature[10]. Above excellent properties enable ZnAl2O4 ceramics to bepromising candidate materials for millimeter-wave devices.

4. Conclusions

ZnAl2O4 ceramics were synthesized by conventional solid statemethod. The single pure ZnAl2O4 phase existed when the amountof 2ZnO�3B2O3 was not more than 3 wt.%. FT-IR and Raman spectrawere used to further elaborate the internal structure of2ZnO�3B2O3-doped ZnAl2O4 ceramics. Zinc borate was effectivesintering aid for ZnAl2O4 ceramics. It was able to not only improvethe relative density markedly, but also enhance the quality factorsignificantly, while still maintain a low dielectric constant andstable temperature coefficient value. Specimens containing 3 wt.%2ZnO�3B2O3 exhibited excellent microwave dielectric properties:er = 8.32, Q � f = 93,600 GHz (at 14.0 GHz) and sf = �68.5 ppm/�C.They are promising candidates for microwave/millimeter-waveapplications at high frequency.

Acknowledgement

S.P. Wu gratefully acknowledges South China University ofTechnology.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jallcom.2015.04.151.

References

[1] C. Jiang, S.P. Wu, W.P. Tu, L. Jiao, Z.O. Zeng, Mater. Chem. Phys. 124 (2010) 347–352.

[2] S.P. Wu, J. Ni, X.H. Ding, J.H. Luo, L. Jiao, Mater. Chem. Phys. 117 (2009) 307–312.

[3] Y.S. Lin, C.C. Liu, K.M. Li, C.H. Chen, IEEE Trans. Microw. Theory 12 (2004)2718–2725.

[4] S.J. Fiedziuszko, I.C. Hunter, T. Itoh, Y. Kobayashi, T. Nishikawa, S.N. Stitzer, K.Wakino, IEEE Trans. Microw. Theory 3 (2002) 706–721.

[5] H.T. Wu, Y.S. Jiang, Y.L. Yue, Ceram. Inter. 38 (2012) 5151–5156.[6] N.M. Alford, S.J. Penn, J. Appl. Phys. 80 (1996) 5895–5898.[7] T. Sugiyama, T. Tsunooka, K.I. Kakimoto, H. Ohsato, J. Eur. Ceram. Soc. 26

(2006) 2097–2100.[8] S.P. Wu, C. Jiang, Y.X. Mei, W.P. Tu, J. Am. Ceram. Soc. 95 (2012) 37–40.[9] C. Jiang, S.P. Wu, Q. Ma, Y.X. Mei, J. Alloys Comp. 544 (2012) 141–144.

[10] C.W. Zheng, S.Y. Wu, X.M. Chen, K.X. Song, J. Am. Ceram. Soc. 90 (2007) 1483–1486.

[11] K.P. Surendran, N. Santha, P. Mohanan, M.T. Sebastian, Eur. Phys. J. B 41 (2004)301–306.

[12] S. Wu, J. Xue, R. Wang, J. Li, J. Alloys Comp. 585 (2014) 542–548.[13] J. Xue, S. Wu, J. Li, J. Am. Ceram. Soc. 96 (2013) 2481–2485.[14] S. Wu, J. Xue, Y. Fan, J. Am. Ceram. Soc. 97 (2014) 3555–3560.[15] K.P. Surendran, P.V. Bijumon, P. Mohanan, M.T. Sebastian, Appl. Phys. A 81

(2005) 823–826.[16] W. Lei, W.Z. Lu, J.H. Zhu, F. Liang, D. Liu, J. Am. Ceram. Soc. 91 (2008) 1958–

1961.[17] W. Lei, W.Z. Lv, J.H. Zhu, X.H. Wang, Mater. Lett. 61 (2007) 4066–4069.[18] A. Belous, O. Ovchar, D. Durilin, J. Am. Ceram. Soc. 89 (2006) 3441–3445.[19] Q. Zeng, W. Li, J.L. Shi, J. Eur. Ceram. Soc. 27 (2007) 261–265.[20] Maurizio Casarin, Chiara Maccato, Andrea Vittadini, J. Phys. Chem. B (2002)

2569–2573.[21] Yeon Hwa Jo, Min Soo Kang, Mater. Res. Bull. 43 (2008) 361–369.[22] Sinae Jo, Seunggu Kang, J. Nanosci. Nanotechnol. 13 (2013) 7675–7679.[23] S.H.N. Doo, Y.H. Jo, J.S. Lee, J. Am. Ceram. Soc. 93 (2010) 334–337.[24] Sanjay Mathur, Michael Veith, Michel Haas, J. Am. Ceram. Soc. 84 (2001) 1921–

1928.[25] Yih Chien Chen, Shi Li Yao, J. Phys. Chem. Solids. 73 (2012) 1240–1244.[26] Yin Lai Chai, Yee Shin Chang, Yu Jen Hsiao, Yu Chan Lian, Mater. Res. Bull. 43

(2008) 257–263.[27] Pei Feng, Youwen Deng, Songlin Duan, Int. J. Mol. Sci. 15 (2014) 14574–14590.[28] Jyothi Suri, Leon L. Shaw, Ceram. Inter. 40 (2014) 9179–9187.[29] M.T. Sebastian, N. Santha, P.V. Bijumon, A.K. Axelsson, N.M. Alford, J. Eur.

Ceram. Soc. 24 (2004) 2583–2589.[30] M. Hu, Y. Fu, C. Luo, H. Gu, W. Lei, Q. Shen, J. Am. Ceram. Soc. 93 (2010) 3354–

3359.[31] R.D. Shannon, J. Appl. Phys. 73 (1993) 348–366.[32] M.Z. Dong, Z.X. Yue, Hao Zhuang, S.Q. Meng, L.T. Li, J. Am. Ceram. Soc. 91 (2008)

3981–3985.[33] H.J. Lee, K.S. Hong, S.J. Kim, I.T. Kim, Mater. Res. Bull. 32 (1997) 847–855.[34] E.L. Colla, I.M. Reaney, N. Setter, J. Appl. Phys. 74 (1993) 3414–3425.[35] Ching Fang Tsengn, Ping Sung Tsai, Ceram. Inter. 39 (2013) 75–79.[36] Xiao Chuan Wang, Wen Lei, Ran Ang, Wen Zhong Lu, Ceram. Inter. 39 (2013)

1707–1710.