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Journal of the European Ceramic Society 36 (2016) 2767–2772

Contents lists available at www.sciencedirect.com

Journal of the European Ceramic Society

jo ur nal home p ag e: www. elsev ier .com/ locate / jeurceramsoc

ffect of the spark plasma sintering parameters, LiF additive, and Ndopant on the microwave dielectric and optical properties ofransparent YAG ceramics

iaorong Zhanga,b, Guifen Fana,b, Wenzhong Lua,b,∗, Yihong Chenc, Xuefeng Ruand

School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, PR ChinaKey Lab of Functional Materials for Electronic Information, MOE, Huazhong University of Science and Technology, Wuhan 430074, PR ChinaWuhan Sintec Optronics Technology Co. Ltd., Wuhan 430074, PR ChinaNano Science and Technology Research Center, Wuhan University, Wuhan 430072, PR China

r t i c l e i n f o

rticle history:eceived 6 December 2015eceived in revised form 6 April 2016ccepted 19 April 2016vailable online 28 April 2016

a b s t r a c t

Not only the optical properties, but the microwave dielectric properties of Yttrium aluminium garnet(Y3Al5O12, YAG) ceramics prepared by co-precipitation method combined with spark-plasma sintering(SPS) were investigated by means of adjusting the factors of LiF additive, Nd dopant, and SPS processparameters. Results showed that the in-line transmittance of YAG ceramic with LiF sintered at 1360 ◦Cwas 81.8% in the infrared range. With increased sintering temperature, the quality factor (Q × f) and

eywords:ransparent YAG ceramicspark plasma sinteringptical propertiesicrowave dielectric properties

dielectric constant (�r) values of the pure YAG ceramics increased to the maximum and then decreased.The optimal microwave dielectric properties (Q × f = 89,810 GHz, �r = 10.63 and �f = −51.4 ppm/◦C) areachieved for transparent YAG ceramics sintered at 1360 ◦C. The Q × f values of Nd:YAG ceramics werelower than those of the pure YAG ceramics. The optimal microwave dielectric properties of 2 at.%Nd:YAGceramic cylinder can be obtained with the Q × f of 64,026 GHz, �r of 10.2 and �f of −64.8 ppm/ ◦C.

. Introduction

Yttrium aluminium garnet (Y3Al5O12, YAG) has outstandingunctional and mechanical properties, such as excellent chem-cal stability, and good optical and high temperature creepesistance. Compared with single crystals, the transparent YAGolycrystalline ceramics have the advantage of simple preparation,ass-production, and low cost, which can produce high-doping

nd large-sized crystals. Therefore, transparent YAG ceramics,ith their excellent physical and chemical properties, have wide

pplicability in laser host materials, heat-resistant and armoredindows, IR transparent missile domes and luminous pipes forigh-intensity discharge lamps [1–3]. The manufacture of trans-arent YAG ceramics has been developed for many years. Sinteringrocess is a crucial stage in the preparation of transparent ceram-

cs, and directly affects the microstructure. Through solid-state

eaction, Li et al. [3] synthesized YAG nanopowders and com-ined them with TEOS, MgO, and compound additives to obtainransparent YAG ceramics under vacuum sintering. In 2012, Liu

∗ Corresponding author at: School of Optical and Electronic Information,uazhong University of Science and Technology, Wuhan 430074, PR China.

E-mail address: lwz@mail.hust.edu.cn (W. Lu).

ttp://dx.doi.org/10.1016/j.jeurceramsoc.2016.04.029955-2219/© 2016 Elsevier Ltd. All rights reserved.

© 2016 Elsevier Ltd. All rights reserved.

et al. [4] successfully achieved the multilayered YAG/Nd:YAG/YAGceramic under vacuum sintering at 1750 ◦C for 50 h. Zych andBrecher [5] demonstrated that YAG:Ce ceramics were prepared byhot pressing (HP) at 1750 ◦C for 4 h under 300 atm. Lee et al. [6]adopted hot isostatic pressing (HIP, 1675 ◦C, 200 MP) with 0.02%SiO2 as sintering additive to obtain the transparent Nd:YAG ceram-ics. Over the last decade, spark-plasma sintering (SPS), a novel HPsintering method, has been used for rapid densification of ceramicwithin a short period of time at comparatively lower tempera-tures. Many types of transparent ceramics have been sintered bySPS, such as Al2O3 [7], Y2O3 [8], MgAl2O4 [9] and MgO [10] asreported by numerous researchers. In recent years, the SPS werealso applied to prepare YAG ceramics and the better results havebeen achieved. For example, Palmero et al. [11] synthesized YAGpowders using reverse-strike co-precipitation, and performed dis-persion by ball milling or ultrasonication. Then, powders weresintered at 1200–1400 ◦C by SPS to obtain a transparent material(transmittance of about 60% at 600 nm, for 1 mm thickness). Frageet al. [12] reported that transparent 1 at.% Nd:YAG ceramics werefabricated by SPS from nanometric Nd:YAG powders.

However, the above-mentioned reports only have paid muchattention for the optical performance of YAG ceramic. In fact,dense YAG ceramics are also potential candidates for preparingmicrowave dielectric ceramics with low permittivity which have

2768 X. Zhang et al. / Journal of the European Ceramic Society 36 (2016) 2767–2772

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ig. 1. XRD, SEM, TEM images and laser grain-size analysis of YAG powders obtainhe reader is referred to the web version of this article.)

ong been neglected [13]. For today’s society, the rapid develop-ent of communication technology and equipment has resulted

n an increasingly high demand for microwave dielectric materials,hich are the key parts for manufacturing communication devices.nd low permittivity maximises the signal propagation speed

hrough the substrate and minimises the cross coupling effect withonductors [14–16]. Until now, there are only a few reports abouthe microwave dielectric properties of the non-transparent YAGeramics. In our previous paper [14], the microwave dielectricroperty of pure YAG ceramics fabricated by pressureless sinteringere studied, indicating that the opaque YAG ceramic was a goodicrowave dielectric material. If the microwave dielectric proper-

ies of transparent YAG ceramics can be improved further, ceramicsith good optical performance and microwave dielectric propertiesight be a new material with optical-electronic integration.In the present paper, in order to obtain the YAG ceramics

ith satisfactory both optical properties and microwave properties,n advanced preparation technology, improved co-precipitationethod combined with SPS sintering technology, were adopted.

he effects of LiF additive and SPS process parameters on the opti-al properties of transparent YAG ceramics were analyzed. More

mportantly, the influence of the Nd dopant on the microwaveielectric properties was also discussed. This provides theoretic and

er calcination at 1100 ◦C. (For interpretation of the references to colour in the text,

experimental basis for the preparation of the transparent laser andmicrowave dielectric ceramics.

2. Material and methods

2.1. Preparation of the YAG nano-powder

The co-precipitation route was used to prepared nanosizedYAG powders, employing 99.9% pure Al(NO3)3·9H2O (SinopharmChemical Reagent Co., Ltd., China), and 99.99% pure Y(NO3)3·6H2O(Sinopharm Chemical Reagent Co., Ltd., China) as the starting mate-rials, respectively. The nitrate and the ammonium sulfate solutionwas added drop wise to ammonium hydroxide with the string atroom temperature. The mixture stirred for 3 h and then filtered,washed four times with deionized water and dried at 80 ◦C. Toefficiently reduce agglomeration of the powders, ethanol soak wasexecuted at the early stages of the drying process. Soaking is moreefficient than washing in reducing hard agglomerate to avoid ballmilling.

2.2. Sintering of the YAG ceramics

The precursor was sieved through a 200-mesh stainless steelscreen. Afterward, the powders with and without 0.25 wt.% LiF

X. Zhang et al. / Journal of the European Ceramic Society 36 (2016) 2767–2772 2769

F : (a) LiF-doped-YAG 1250 ◦C, (b) LiF-doped-YAG 1280 ◦C, (c) LiF-doped-YAG 1300 ◦C, (d)1 360 ◦C.

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Table 1The chemical compositions of YAG powders.

Peak Position BE (eV) FWHM (eV) Atomic Mass Atomic Conc% Mass Conc%

O 1s 530.600 1.415 15.999 60.72 33.03

ig. 2. Physical appearance of YAG −1 mm placed above the paper one centimeter330 ◦C, (e) LiF-doped-YAG–1360 ◦C, (f) LiF-doped–1390 ◦C, (g) LiF-undoped-YAG 1

ere pressed with 5 MPa pressure into a cylinder (� = 15 mm, ≈ 5 mm) and then cold isostatic pressing consolidated with80 MPa for 3 min. Subsequently, the YAG polycrystalline ceram-

cs were obtained through SPS sintering (SPS-3.20MK II, Sumitomooal Mining Co., Ltd., Tokyo, Japan). YAG cylinder was placed in

cylindrical graphite die (� = 15 mm). A piece of 0.2 mm thickraphite foil was used to wrap the sample. An optical pyrometeras used to measure the temperature of the graphite die sur-

ace. The sintering temperature and pressure were controlled via arogrammable interface. In this work, transparent YAG body wasrepared by using a two-step pressure combined with low heatingemperature. The densification process in SPS includes three stages17,18]: namely, packing of particles, diffusion process accompany-ng neck formation and grain sliding, and removal of pores mainlyhrough the grain boundary. Therefore, pressure is necessary to pre-are transparent ceramics. The temperature was first increased to100 ◦C at a rate of 100 ◦C/min within 5 min and kept 3 min, there-fter increased to the final sintering temperature of 1250–1390 ◦Cith the heating rate of 10 ◦C/min and held for 20 min. The uni-

xial pressure of 28 MPa was pre-loaded before reaching 1100 ◦Cn a vacuum. This provides the body enough time to fully diffuse.uring the dwelling time, the pressure was increased to 80 MPand kept constant until cooling. Then, the samples were cut intowo sections. One is disc with the thickness about 1 mm for opticalerformance tests and another one is cylinder with thickness about

mm for microwave dielectric property tests.

.3. Microstructure and properties characterization

Phase identification was performed by the X-ray diffractionethod on the XRD-7000 (Shimadzu, Kyoto, Japan) with CuK� radi-

tion at 40 kV and 30 mA. Microstructures were characterized onhe scanning electron microscopy (SEM, JSM-7600F, JOEL, Tokyo,apan) and transmission electron microscopy (TEM, JOEL JEM-2100,00 kV, Tokyo, Japan), respectively. The particle size was charac-erized by laser particle size analyzer (England, Mastersizer 3000).elative densities were determined by the Archimedes methodith distilled waters as the immersion medium. The optical trans-ittance spectra of the mirror-polished samples on both sides wereeasured with a double-beam spectrophotometer (Lambda 35,

erkinEler, Inc., California, USA). Dielectric behaviors in microwaverequency (approximately 11 GHz) were measured by the TE011

ode via the Hakki and Coleman method by using an Agilentetwork analyzer (Agilent Technologies, Inc., E8362B, Santa Clara,SA). The temperature coefficients of resonant frequency �f valuesere calculated by the formula.

f = f2 − f1f1 (T2 − T1)

(1)

here f1 and f2 represent the resonant frequencies at T1 (30 ◦C) and2 (80 ◦C), respectively.

Y 3d 157.700 1.176 88.906 14.69 44.41Al 2p 74.200 1.307 26.982 24.59 22.56

3. Results and discussion

3.1. Preparation of the YAG nano-powders

SEM, XRD, TEM images and laser grain-size analysis of YAG pow-ders obtained through the co-precipitation method are shown inFig. 1. Fig. 1(a) shows the XRD spectra of the precursor and YAGpowders calcinated at different temperatures (900 ◦C, 1000 ◦C and1100 ◦C). No evident diffraction peaks were observed in the XRDspectra of the precursor, indicating that the precursor is amor-phous. YAG powders sintered at 900 ◦C were observed to havea few impure peaks, however, upon sintering at 1000 ◦C, all thepeaks belonging to YAG phase agreed with the JCPDS Card (No. 88-2048). Further increasing the temperature to 1100 ◦C, the degreeof crystallinity was improved. Therefore, the XRD diffraction peaksgradually become stronger and sharper. From Fig. 1(b), the SEMimage of the YAG nanopowders shows that the sizes of samplescalcined at 1100 ◦C were almost in the range of 50 nm to 100 nm.The particles present similar spherical shape and good dispersionwith homogeneous sizes. The EDS of the area marked by red linesis showed in the inset of Fig. 1(b). The peaks of all constituent ele-ments of the selected part, including Y, Al and O were observed.Combining these observations with the XRD results, it can be fur-ther confirmed that the synthetic material is pure Y3Al5O12. Fig. 1(c)shows the TEM image of the precursor calcinated at 1100 ◦C. Themicrographs clearly show that the particles are slightly agglomer-ated with some sintering necks and the particles are homogeneouswith the primary sizes of about 80 nm, which is in accordance withSEM images in Fig. 1(b). Fig. 1(d) shows the laser grain-size analysisof YAG powders. Particle size analysis exhibits unimodal distribu-tion, and the value of D50 is 0.532 �m. Though the size of singleparticle is about 100 nm, the several particles get together intoclusters due to high surface area of the particles. The size of theclusters is about 500 nm, as the micrographs revealed by SEM andTEM, which is in agreement with result of particle size analysis.

The YAG powders obtained by precipitation method are sub-mitted to XPS analysis and the resulting quantification report ofchemical compositions is showed in Table 1. In the powders, onlyAl, Y and O were detected. The atomic percent contents of Al, Y andO are 24.59%, 14.69% and 60.72%, respectively. And the Y/Al ratio is0.597, which is very close to 0.6 and consistent with the Y3Al5O12.

3.2. Optical property of the YAG ceramics

The photographs of the mirror-polished specimens with or

without LiF sintered at different temperatures are displayed inFig. 2. The samples were placed above the paper one centimeter.In the Fig. 2(a) and (b), the words can hardly be seen under theLiF-doped-YAG ceramic sintered at 1250 ◦C and 1280 ◦C. With the

2770 X. Zhang et al. / Journal of the European Ceramic Society 36 (2016) 2767–2772

mperatures for 20 min: (a)1300 ◦C, (b)1330 ◦C, (c)1360 ◦C, (d)1390 ◦C.

spiiipcc(YitradpMat

YFob1cdj1Fwcthtabact

Fig. 4. Transmittance spectra of YAG–1 mm sintered at different temperatures for20 min:(a) LiF-doped-YAG 1250 ◦C, (b) LiF-doped-YAG 1280 ◦C, (c) LiF-doped-YAG

Fig. 3. SEM images of the LiF-doped-YAG sintered at different te

intering temperature increase from 1300 ◦C to 1360 ◦C, the textlaced under the sample was becoming clearer through the disc,

ndicating increasingly satisfactory transmittance of the samplesn the range of visible wavelengths. To further increase the sinter-ng temperature to 1390 ◦C, the transparence of the sample becameoor. It’s worth noting that the transmittance of LiF-undoped-YAGeramic sintered at 1360 ◦C is inferior to the LiF-undoped-YAGeramic sintered at same temperature, as shown in Fig. 2(e) andg). The reasons why the LiF can optimize the transmittance ofAG ceramics were attributed to that the addition of the LiF can

mprove the mass transport related effects during the densifica-ion of YAG ceramics and eliminate the carbon contamination, aseported by other authors [12]. The LiF additive acts as a cleansinggent and assists in eliminating the carbon contamination, thus,uring high temperature processing, the carbon-containing com-ound that generates volatile fluoro-carbon species are dislodged.oreover, the LiF additive plays an important role as a grain bound-

ry movement accelerator, thus, helping eliminate the pores duringhe movement of grain boundaries [19,20].

SEM images of the polished surface of the SPSed LiF-doped-AG ceramics at different temperatures for 20 min are shown inig. 3. Fig. 3(a) shows that the sample sintered at 1300 ◦C was obvi-usly porous with irregularly shaped pores distributed in the grainoundary and grain surface, and an average grain size of about

�m. With increasing in sintering temperature, the density of theeramics slightly increased and the pores in the grain surface almostisappeared. However, a few residual pores still remained at triple

unctions or along grain boundaries for the specimen sintered at330 ◦C, and an average grain size of about 1.8 �m, as shown inig. 3(b). Fig. 3(c) shows that the pores can be removed completelyhen the samples are sintered at 1360 ◦C, and the density of YAG

eramic sintered at 1360 ◦C would be about 99.9%. At the sameime, the average grain sizes of the sample grew to 3 �m withomogeneous distribution. Further increase sintering temperatureo 1390 ◦C, the grain appear the abnormal growth phenomenon,nd the abrupt appearance of exaggerated grains (over 6 �m) can

e found. In addition, some pores appeared again in grain bound-ries. The above phenomenon shows that excessive temperaturean lead abnormal grain growth and generate pore, indicating thathe density of YAG ceramic would fall to about 99.7%, which was

1300 ◦C, (d) LiF-doped-YAG 1330 ◦C, (e) LiF-doped-YAG 1360 ◦C, (f) LiF-doped-YAG1390 ◦C, (g) LiF-undoped-YAG 1360 ◦C.

detrimental for the optical properties and mechanical properties.The evolution of the microstructure can exactly explain why thetransmittance of the samples sintered at 1360 ◦C is superior to thanthat of samples sintered at 1390 ◦C, as shown in Fig. 2(e) and (g).According to the above analysis, the green body sintered at 1360 ◦Cis considered favorable for LiF-doped YAG ceramics.

The in-line transmittance spectra of the pure YAG ceramics(1 mm) with and without 0.25 wt.% LiF are presented in Fig. 4. Itis obvious that the optical transmittance of the LiF-doped samplessintered at 1250, 1280, 1300, 1330, 1360 and 1390 ◦C for 20 min firstsharply increases and then decreases. When the sintering temper-ature was 1250 ◦C, the transmittance of LiF-doped-YAG ceramic isless than 1%. And at 1280 ◦C, the transmittances of samples were17.3% at the wavelength of 600 nm (Tin,600) and 8.15% at the wave-length of 1064 nm (Tin,1064), respectively. The optical transmittance(Tin,600 = 24.6%, Tin,1064 = 46.8%) of the transparent YAG ceramic thatwas synthesized at 1300 ◦C is lower than that of the specimens

prepared at 1330 ◦C (Tin,600 = 36.2%, Tin,1064 = 60.5%), as shown inFig. 4(c) and (d). For the 1 mm-thick sample sintered at 1360 ◦Cfor 20 min, the optical transmittance of the LiF-doped samples

X. Zhang et al. / Journal of the European Ceramic Society 36 (2016) 2767–2772 2771

Fs

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Fig. 6. The Q × f and �f of LiF-doped-YAG ceramics sintered at different sinteringtemperatures.

ig. 5. The relative density and �r of LiF-doped-YAG ceramics sintered at differentintering temperatures.

chieves the maximum (Tin,600 = 72.3%, Tin,1064 = 81.8%), which islightly higher than those of the specimens sintered at 1390 ◦CTin,600 = 64.5%, Tin,1064 = 74.2%). The possible reasons should bettributed to the fine grain size and fully dense microstructure.he grain and pore growth is two main factors to deteriorate theransmission of the samples because the over-sized grain boundarynd the pores will lead to serious light scattering [9]. According toayleigh formula, for smaller wavelength (�), the scattering inten-ity is greater, which results in the decrease of transmittance. So,he values of transmittance at the wavelength of 600 nm are allower than that at the wavelength of 1064 nm.

In addition, for the LiF-undoped-YAG ceramics, the transmit-ance only reached 23.9% (Tin,600) and 50.1% (Tin,1064) at sinteringemperature of 1360 ◦C for 20 min. The result was consistent withhat of the physical picture (Fig. 2(g)). The possible reasons shoulde attributed to carbon contamination, which can lead to seri-us light scattering. The addition of the LiF can contribute to thelimination of carbon contamination, thereby improve the trans-ittance of YAG ceramicsr [12].

.3. Microwave dielectric property of the YAG ceramics

To test of the microwave dielectric properties, a sintered cylin-er with a diameter of 15 mm and a thickness of 6 mm wasbtained at different temperatures. The relative density and dielec-ric constant of LiF-doped-YAG ceramics with different sinteringemperatures are shown in Fig. 5. It should be noted that with thencrease of sintering temperature, the values of the relative den-ity also increased to the maximum value of 99.89% at 1360 ◦C andeduced thereafter. When the sintering temperature is too high,he grain sizes become too large to hinder the elimination of theesidual closed pores. The bulk density results agreed with the SEMbservations. The �r was basically identical with the reported num-ers (approximately 10) [14]. A maximum �r value of 10.63 wasbtained for the sample sintered at 1360 ◦C, as shown in Fig. 5. Theielectric constant �r changed in a manner similar to density.

The values of Q × f and �f for LiF-doped transparent YAG ceram-cs made at varying temperatures are illustrated in Fig. 6. Withncreasing sintering temperature, the values of Q × f showed annitial increase and then decrease. The maximum of 89,810 GHzan be realized at 1360 ◦C. Generally, it can be thought that therere two categories microwave dielectric losses in bulk ceramics

9,21]. One is called intrinsic loss which is caused by a harmonichonon decay processes in the pure crystal lattice. And another one

s called extrinsic loss which is mainly caused by relative density,rystallinity, inner stress and structure defect such as the pores,

Fig. 7. The microwave dielectric properties of the LiF-doped-Nd:YAG ceramics sin-tered at optimal temperature (1360 ◦C).

second phases, grain boundaries, and so on. This may explain whysintering temperature increasing in a certain range (1250–1360 ◦C)will be beneficial to relative density and crystallizability, so it is alsobenefit to the Q × f. The over-high sintering temperature will leadto appearance of abnormal grains and pores which are detrimentalto the Q × f. Given that no change in composition occurs, the param-eter of the �f is basically stable and independent, which is between−51 and −57 ppm/◦C. Therefore, the optimal microware dielectricproperties of the specimen is the Q × f values of 89,810 GHz, the �r

values of 10.63, and the �f values of −51.4 ppm/ ◦C. Jin et al. reportedthat the Q × f of YAG ceramics sintered at 1750 ◦C for 12 h in the vac-uum atmosphere ≤ 1 × 10−3 Pa was 220,100 GHz [22]. The reasonthat the peak value Q × f (89,810 GHz) of transparent YAG ceramicsintered by SPS processes in this paper is lower than the reportedvalue (220,100 GHz) may be due to the carbon contamination andmore oxygen vacancies.

It is well known that many applications of YAG ceramics requiredoping with luminescent ions, and Nd3+ ion is the most popularion for laser application. In order to exploit the dielectric materialswith laser character, the neodymium doped YAG ceramics with LiFas additive were studied in this section. The Q × f values, �r and�f of the Nd:YAG ceramics sintered at 1360 ◦C with different Nddoped contents are exhibited in Fig. 7. The Q × f values of Nd:YAGceramics sintered at 1360 ◦C with 0 at.%, 1 at.%, 2 at.% and 3 at.% were89,810 GHz, 65,644 GHz, 64,026 GHz and 53,172 GHz, respectively.The Q × f values of Nd:YAG ceramics were all lower than those ofthe pure YAG. And the Q × f value of the sample with 2 at.% Nd is

almost same to that of the sample with 1% Nd. However, excessiveamounts of Nd (3 at.%) degraded the quality factor.

Likewise, all �r values were approximately 10, which are slightlylower than the �r values of pure YAG ceramics. The ionic radius of

2 an Ce

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d3+ is larger than that of Y3+, so the unit cells were expanded.onsequently, the ionic polarizability in the unit cell resulted inecreased permittivity [23,24]. The �f values of Nd:YAG ceramicsanged between −51.4 and −67.7 ppm/ ◦C, which decreased as Ndncreased. Based on the above results, the 2 at.% Nd:YAG ceramic ishe best formula for optical, laser and microware dielectric prop-rties. The Q × f, �r and �f the 2 at.% Nd:YAG ceramic sintered at360 ◦C reach 64,026 GHz, 10.2 and −64.8 ppm/ ◦C, respectively.

. Conclusions

YAG and Nd:YAG ceramics with good optical properties andielectric properties have been synthesized by co-precipitationethod combined with spark-plasma sintering technique. The

ddition of the LiF can significantly improve the transparent ofhe YAG ceramics. The sintering temperature has a major effectn the optical properties and dielectric properties of the LiF-doped-AG ceramics. With the increase of temperature, the transmittance,

× f values, �r of the LiF-doped-YAG ceramics showed the trend ofncrease first and then decrease, but the �f is basically stable andndependent, which is between −51 and −57 ppm/ ◦C. The trans-

ittance at wavelengths of 1064 nm, the Q × f values, and �r of theiF-doped-YAG ceramics sintered at 1360 ◦C were 81%, 89,810 GHz,nd 10.63, respectively. With the addition of the Nd, the Q × f, �r

nd �f values of the Nd:YAG ceramics slightly decreased. The Q × f,r values and �f values of the 2 at.% Nd:YAG ceramics sintered at360 ◦C for 20 min can reach to 64,026 GHz, 10.2, and −64.8 ppm/ ◦Cespectively.

cknowledgments

This work was supported by the National Natural Science foun-ation of China through Grant NO. 61201051 and Hubei Sciencend Technology Innovation Plan (ZDG2014000019). The authorsre grateful to Analytical and Testing Center, Huazhong Universityf Science and Technology for XRD and SEM analysis.

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