Systematic Optimization of Spray Drying for YAG Transparent Ceramics 2015 Journal of the European Ceramic Society

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Journal of the European Ceramic Society 35 (2015) 23912401

Systematic optimization of spray drying forLe Zhang a,b,, Hao Yang a,b, Xuebin Qiao a,b, Tianyuan Zho

a,b

mal Uangsulogicauary 2y 201

Abstract

Spray dryin hiomceramics. T in semeasuremen ized and 60 C, a balanbinder PVB 000 performance sizehomogeneou qual 2015 Elsevier Ltd. All rights reserved.

Keywords: Spray drying method; Transparent ceramics; YAG; Forming performance

1. Introdu

Since higain mediutransparent[4], Cr:ZnShave been cal measurwell establ(or gradienand large sent ceramicEspecially,have used table laser ocomplicate

CorresponJiangsu Normfax: +86 516

E-mail ad

http://dx.doi.o0955-2219/ction

gh efficiency Nd:YAG transparent ceramics as laserm were reported by A. Ikesue in 1995 [12], various

laser materials such as Yb/Nd:YSAG [3], Nd:Y2O3 [5], have been developed. Their optical qualitiesgreatly improved and their applications in physi-

ements and medical uses as laser materials are nowished [6]. Transparent ceramics offer homogeneoust) and large dopant concentration, design flexibility,cale required for high power laser. YAG transpar-s have been proved to be the best laser host material.

other rare earth ions (Yb, Ho, etc.) [78] doped YAGo generate different wavelengths, pulse width or tun-utputs [910]. Furthermore, composite lasers with

d structure of YAG ceramics to improve the thermal

ding author at: School of Physics and Electronic Engineering,al University, Xuzhou 221116, China. Tel.: +86 516 83403242;

83403242.dress: [email protected] (L. Zhang).

management and increase the pump efficiency have been widelyreported [1112].

In general, the residual pores, grain boundary impurities anddopant segregation as optical scattering centers still exist in theceramic materials [13]. For laser materials, the technology toremove those scattering sources becomes extremely important.Super full-dense ceramics with pore-free structure and cleangrain boundary are needed to be fabricated to lower their opti-cal loss [14]. However, the agglomeration of raw powders forceramics results in major microstructure defects before ceramicforming [15]. The homogeneous distribution of raw powdersbecomes more difficult during shaping process. Those unex-pected accidents will produce the residual pores or secondaryphases after sintering, severely lowering the optical quality. Inaddition, the strong friction force between raw powders withsmall size (

2392 L. Zhang et al. / Journal of the European Ceramic Society 35 (2015) 23912401

Spray drying method is a powder processing techniquewidely used in the manufacturing of dried food, oxide ceramicsand pharminto dry pahot mediumders withinpowerful teand great h

As knowand procesrate of dryious produabove manbe found [2granulate aproductionposition anand the resto verify thcrushing bepension coperformancics have alsreinspect th

2. Materia

2.1. Suspe

High puJapan), Y2OChina) powders were Y3Al5O12TEOS (0.5(0.25, 0.50dispersant,M.W. 40,00tent was 40

2.2. Spray

The sus6HOP, PREoptimized hot air (inlrespectivelfeeding spedardized bwas 8000 r

2.3. Ceram

The gran22 disk acompacts wing rate (0.

component and then sintered at 1780 C for 8 h under high vac-uum (1.0 106 Pa), and finally annealed at 1450 C for 10 h.

hara

mo

nd ccopy

dispution

andrds oore rcur

achros me

Elms we

room

ults

ptim

prit to pion. e sizetersng ange of 78

pro etc. er, thtlyn ou. Th

tivelfact,

inletpiratthe rquirate ten t), theard pt, whping

terin. As fter,y whanulple oes toaceuticals, by which the suspension is transformedrticles by spraying thousands of small droplets into a

[17]. The solvent dries quickly and the small pow- each droplet form a solid granule [1819]. It is achnique to granulate powders with good flowabilityomogeneity of components.n, the suspension state (such as density and viscosity)

s conditions (such as temperature, flux and aspirationing medium) play important roles in producing var-ct shapes and sizes. The spray process is a result ofy interwoven complex factors and a balance must0]. In this paper, spray drying method was used to

stoichiometric mixture of commercial oxides for the of YAG transparent ceramics. The suspension com-d processing parameters were changed in sequence,ulting granulated particles were analyzed by SEMeir shapes, sizes and distributions as well as theirhaviors. The aim was to achieve the optimized sus-

mposition and process parameters for better forminge of powders. Furthermore, YAG transparent ceram-o been fabricated by solid-state reaction sintering toe optimized experimental parameters.

ls and experimental details

nsion preparation

rity Al2O3 (99.99%, TM-DAR, Taimei Chemicals,3 (99.999%, Jiahua Advanced Materials Resources,ders were used as starting materials. These pow-

blended together according to the stoichiometric ofand ball-milled for 15 h in anhydrous alcohol with

wt.%) and MgO (0.1 wt.%) as sintering aids, DS005, 0.75, 1.0 wt.%, Polymer Innovations, Vista, CA) as

and polyvinyl butyral, PVB, (0.0, 1.0, 2.0, 3.0 wt.%,070,000, Aladdin) as binder. The starting solid con-

wt.%, binder 1.0 wt.%, dispersant 0.25 wt.%.

drying

pensions were dried using a spray dryer (TR120AT-CI, Japan). Spray drying parameters needed to be

including: temperature, flux and aspiration rate ofet/outlet, full capacity (100%) are 50 Hz and 60 Hz,y), atomizer speed (8000, 11,000, 14,000 r/min). Theed of suspension was fixed at about 30 ml/min stan-

y pure ethanol solvent. The starting atomizer speed/min.

ic fabrication

ulated particles were dry-pressed under 40 MPa intond then cold isostatically pressed 200 MPa. Greenere firstly calcined at 900 C for 10 h with slow heat-

5 C/min) to completely remove the residual organic

2.4. C

Thecles amicrosenergydistribdensitystandaThe pby meQuantics waPerkinsampleout at

3. Res

3.1. O

Thedroplecollectparticlparamof dryiin a raature odryingspeed,of bindbe sligPVB. I61.5 Crespec

In flux (FThe asmines also reevapor

Wh(100%very hsolvening shaby sinenergyand sophologThe grstill apgranulcterizations

rphology and microstructure of granulated parti-eramics were examined using scanning electron

(SEM, JSM-6510, JEOL, Japan) coupled with anersive X-ray spectrometer (EDS). The particle size

was recorded by Image J software. The apparent tap density were measured according to nationalf China, GB5061-85 and GB5162-85, respectively.

size distribution of green compact was evaluatedy intrusion porosimetry (MIP, Poremaster GT-60,me, USA). Optical transmittance of YAG ceram-asured using a UV/VIS spectrometer (Lambda 950,er, USA). Before the measurements, both surfaces ofre polished to 3 mm. All measurements were carried

temperature.

and discussion

ization of processing parameters

mary steps of spray drying include atomization,article conversion (solvent evaporation) and particleThe atomization is a key parameter in determininge. Before studying this step, the other processing

such as inlet (Tinlet) and outlet (Toutlet) temperaturesir should be determined firstly. Generally, Tinlet variesf 10 C with respect to the ethanol boiling temper-C [21]. Toutlet indicates the energy consumed duringcess and depends on Tinlet, outlet pressure and feedMeanwhile, in order to obtain a better bond behaviorhe mean temperature in the drying chamber should

higher than the glass transition temperature (Tg) ofr experiments, the softening temperature of PVB wasus, Tinlet and Toutlet were selected at 75 C and 60 C,

y.for an efficient drying, Tinlet and drying medium

) decide the total energy transferred to the droplet.ion rate (the capacity of outlet blower, Foutlet) deter-esidence time of granules in the chamber, which is

ed to ensure sufficient drying. They work together tohe solvent completely from the droplet.he inlet blower worked at full capacity of 50 Hz

excessive Finlet would lead to poorly granulated andarticles (not shown here) due to rapid evaporation ofich would lead to the formation of large pores dur-

by pressing. These pores could be hardly eliminatedg. Thus, Finlet needed to be decreased to reduce theFinlet decreasing, the particle became more spherical

and the granulated particles showed the best mor-en the inlet blower worked at half capacity of 25 Hz.

es size was about 2050 m although its shape wasr doughnut. Meanwhile, the mass collection rate of

theoretical yield increased with the decrease of Finlet

L. Zhang et al. / Journal of the European Ceramic Society 35 (2015) 23912401 2393

Table 1Collection rates of granules to theoretical yield with different capacities ofblowers.

Capacity (Fin

100% (50, 6070% (35, 4250% (25, 30

(Table 1), aafter 100-mand 30 Hz, Foutlet loweobstruction25 Hz and

3.2. Optim

The atosize. In thisatomizationequation o[22]:

Dd = C(

C is a consf are densiof the liquisuspensionvarious proAnd Dd issurface tenters of suspdispersant

3.2.1. SoliAs the p

atomized fr(g) and pag

d= D

3d

D3g The inc

pension visand large sparticles atof 50 wt.%shape with(Fig. 1(b))and increasFig. 1(c anmined by a

As knowtion of solv

to outsize of the droplet and the solid powders are also carriedalong. At a lower solid content 40 wt.%, the faster and more

ovem

pletse thatur

to eved in

shorrs beded tddit

gh dtra-lad to hre dcaus

, beist red to

Binderal

thaves tes. Ht.. 2 shes wes at(a anith

der le shpartim w

nulets wd fro% tolly bder

essivutionrmalt lowstrib

ancordilet, Foutlet) Mass collection rateWithout sieving (%) 100-Mesh sieving (%)

Hz) 35.7 Hz) 78.7 65.9 Hz) 94.1 85.6

nd it was higher than 94% without sieving and 85%esh sieving when Finlet and Foutlet decreased to 25 Hz

respectively. However, a further decrease of Finlet andr than 50% would cause the incomplete drying and

of nozzle. Therefore, Finlet and Foutlet were selected30 Hz, respectively.

ization of suspension composition

mization is a key parameter in determining particle study, the centrifugal force was employed to assist

process. After selecting an atomizer, the empiricalf control droplet diameter (Dd) could be expressed

0.25L 0.06L

0.375

0.375A

)(fL

fLvL + fAvA

)(1)

tant, and it depends on nozzle design. , , , v andty, viscosity, surface tension, velocity and flow rated (L) or hot air (A). This equation indicates that the

composition plays more important role in producingduct shapes and sizes than processing parameters.

mostly proportional to the density, viscosity andsion of suspension. However, these three parame-ension are effectively changed by its solid content,

content and binder content.

d content of suspensionrimary factor, the solid content (Cd) of droplet (d)om suspension has a strong influence on the densityrticle size (Dg) of granules (g) [23]:

Cd

Cg(2)

rease of solid content results in an increase of sus-cosity and leads to the formation of dense granulesize. Fig. 1(a) presents the morphology of granulatedomized from suspension with increased solid content

free mthe drodecreatemperbegin producwouldpowdeit avoi

In aand hithe exseeme

structucould dryinga modeselecte

3.2.2. Gen

a meshand giparticlconten

Figparticlparticl(Fig. 3gates wthe binor appmean

60 the gracontenjumpe1.0 wt.graduaing binan exc

distribfor nocontenmal diperform

Acc

. All granules exhibited better spherical and solidout any pinhole and their mean size was 48.9 m. There were great improvement in particle shapee in size compared to that of solid content 40 wt.%,d d), under the same processing parameters deter-bove results.n, the droplet shrinks because of the fast evapora-ent during drying. The solvent moves from inside

viscosity onificant incgranules siform a flexat the startdecreases tto evaporatral upliftsent of solid powders would form a soft shell around and produce the hollow granules. The shell woulde evaporation rate through the droplet surface. As thee of the droplet inside increasing, the solvent wouldaporate directly from the inside and a void would beevitably in Fig. 1(c). However, a higher solid content

ten the drying process because the movement of solidcame more difficult due to the low moisture. Thus,he formation of hollow and hard particles [24].ion, the granules was formed with spherical shapeensity when solid content was 50 wt.% in Fig. 1(a),rge granules were also formed. High solid contentave a negative effect on the formation of well packeduring shaping. Moreover, such high solid contente the obstruction of nozzle in the mid-stage of sprayng difficult to continue work for atomizer. Therefore,duction of solid content was needed and 45 wt.% wascontinue the optimization of suspension composition.

er content of PVBly, binder has to be added in the suspension to createt keeps the solid particles combined with each otherhe outstanding increase in strengths for granulatedere, PVB was selected as the binder to optimize its

ows SEM images and size distribution of granulatedith different contents of binder. All the granulatedomized from suspension of PVB content 1.0 wt.%d b)) exhibited monodispersed and fully dense aggre-good spherical shape and smooth surface. However,content increased to more than 2.0 wt.%, a doughnutape formed again in most granules (Fig. 3(c)). Thecles size gradually became larger from 30 m toith increasing binder content (Fig. 3(d)). Especially,s exhibited similar mean size 40 m when binderere 0.5 wt.% and 1.0 wt.%. But the maximum sizem 87 m to 101 m when PVB content was from

2.0 wt.% (Fig. 3(d)). In addition, the size distributionecame wider from = 7.0 to = 23.5 with increas-content and the maximum increase occurred whene addition of PVB 3.0 wt.% (, scale parameter of, was used to describe the dispersion degree of data

distribution). The granulated particles with binderer than 1.0 wt.% had similar and more standard nor-

ution. This was in favor of imparting better forminge of granulated particles for green compact.ng to Eq. (1), the granules size is proportional to thef suspension, and the binder could result in a sig-rease in viscosity, leading to the gradual increase ofze. However, the excessive binder would also easilyible shell with low permeability around the dropletsing stage of droplet-to-particles conversion [25]. Ithe evaporation rate of solvent, and the solvent beginste directly inside, leading to the formation of cen-

of droplet. This situation can severely destroy the


Fig. 1. SEM images and size distributions of granulated particles with different solid contents. (a and b) 50 wt.%, (c and d) 40 wt.%.

Fig. 2. SEM images of granulated particles with different binder contents (a) 0 wt.%, (b) 1.0 wt.%, (c) 3.0 wt.% and their size distributions (d).


01.3

1.4

1.5

1.6

1.7

01.3

1.4

1.5

1.6

1.7

Den

sity

/ g c

m

-3 D

ensi

ty/ g

cm

-3

AD

TD

TD

AD

Fig. 3. Apparcontents of bi

sphericity and decreatap density

AD andshown in Fhad the hig1.65 g/cm3density of Yand narrowger size an3.0 wt.%) w

As welland filling interactingthe increaseforces betwfilling densis in favoralso increaswould brindecreases tthe granuleand exhibit

The grecontents wTheir pore ited only compacts w

0.0

0.2

0.4

0.6

0.8 (a)

Acc

umul

atio

n/ %

4

5

5

6

9

9

10

Den

sity/

gcm

-3

a) Poferent.0 1.0 2.0 3.0

.25 0.50 0.75 1.00

(b) Di spers ant

Content/ wt. %

(a) Binder

Content/ wt. %

ent density and tap density of granulated particles with differentnder PVB (a) and dispersant DS005 (b).

degree and produces that doughnut or apple shape,

Fig. 4. (with difse the flowability and the apparent density (AD) and (TD) of granulated particles.

TD of granules with different binder contents areig. 3(a). Obviously, the particles with 1.0 wt.% PVBhest AD and TD values and they were about 1.50 and, respectively, reached 33% and 36% of theoretical

AG crystal (4.55 g/cm3). Both smaller particle sizeer distribution (0.0, 0.5 wt.%), as well as very big-d wider distribution with non-spherical shape (2.0,ould decrease AD and TD.

known, the value of AD represents the flowabilityproperty of granules, which is determined by the

force between granules. More binder would result in of particle size that would decrease the Vander Walseen particles, imparting better flowability and higherity [26]. Meanwhile, the increased binder content

of the formation of fully dense particles that cane AD value. However, the bigger granulated particlesg bigger voids content during natural packing thathe filling density. Under the confluence of two factors,s with 1.0 wt.% PVB have the highest AD and TDed the best forming performance.en compacts from granules with different binderere cold isostatically pressed (CIP) under 200 MPa.size distributions are shown in Fig. 4(a). They exhib-one peak in 20200 nm excepting that the greenith excessive binder, 2.0 and 3.0 wt.%, had another

distributionthem. The 1est pore sizsmaller anddensificatiodensity chaThe green lower densHowever, tsintering athe highesture 1700 is consisteand MIP aceramic wimittance. T1.0 wt.%.

3.2.3. DispSolid pa

erate due tdispersant the powdestatic repulstronger thin suspensiare functio80120400.10.01

20 m20 nm c

b

de

a

(a) 0.0 wt.%(b) 0.5 wt.%(c) 1.0 wt.%(d) 2.0 wt.%(e) 3.0 wt.%

35 m

Pore size/ m

3.02.52.01.51.00.50.05

0

5

0

6

8

0(b)

Sinter ing 1700 C*8 h CIP 200 Mpa Dry pres sing 40 Mpa

Binder/ wt.%%

re size distributions of green compacts from granulated particles binder contents and (b) the followed density change of ceramics. peak at about 35 m, representing larger pores in.0 wt.% binder formed green compact had the small-

e distribution centered at about 60 nm. This relatively more uniform pore size would greatly promote then of ceramics during sintering. Fig. 4(b) shows thenge of fabricated ceramics under different stages.compacts with 2.0 and 3.0 wt.% PVB showed theity, although they had little difference after CIP.he difference was acutely magnified after vacuumnd the fabricated ceramic with 1.0 wt.% binder hadt density 99.3% under a lower sintering tempera-C (they all finished densification >1750 C). Thisnt with the results of SEM, AD/TD of granulesnalysis of green compacts. In fact, the fabricatedth 1.0 wt.% binder also had the highest optical trans-herefore, the optimized binder content of PVB is

ersant content of DS005 and speed of atomizerrticles in the suspension have a tendency to agglom-o the attractive Van der Waals force. Appropriateis needed to eliminate that tendency by changingr surface properties. Through either the electro-sion or steric hindrance, the repulsive force becomean the attractive force to remain themselves separateon [20]. DS005 is a strong polymeric dispersant thatning via both electrostatic and steric mechanism due


Fig. 5. wt.%,

to a hydrocMeanwhilecosity and particle sizprocessingdramaticallshear forcewere system

Fig. 5 slated particwith 0.25 wand fully dand b)). Ha apple or ules (Fig. from 55 0.25 wt.% occur withfor 1.0 wt.%ules had thsimilar anddistributionin AD and performancbest flowabsize, the Asant contenthe poor an

dditSEM images of granulated particles with different dispersant contents (a) 0.25

arbon chain and a polar ionic part (COOH, SO3). In a

, dispersant addition can dramatically reduce the vis-surface tension of suspension, and further change thee according to Eq. (1). In addition, as well known, the

parameter, the speed of the centrifugal atomizer cany change the size of granules due to the change of

[27]. Here, dispersant DS005 for stable suspensionatically investigated.

hows SEM images and size distributions of granu-les with different dispersant contents. All granulest.% and 0.50 wt.% DS005 exhibited monodispersed

ense aggregates with good spherical shape (Fig. 5(aowever, the binder content increased to 1.0 wt.%,even long wax gourd shape formed in most gran-5(c)). The mean particles size firstly decreasedm to 40 m with increasing the dispersant from

to 0.50 wt.% (Fig. 5(d)). But a slight increase would further increasing the dispersant and reached 45 m

DS005 sample. The 0.50 wt.% DS005 added gran-e smallest particle size. Their size distributions were

all values of were about 10, closing to the normal. Meanwhile, the granules showed some differenceTD in Fig. 3(b) that indicated their different forminge. And the granules with 0.50 wt.% DS005 had theility and filling property. Like the decrease of particleD and TD had a decrease with increasing the disper-t, especially the sample of 1.0 wt.% DS005 due tod not spherical shape.

distributionter formingfailed wheBecause cowall of chato the lowepoor morphregularity 8000 r/minobviously content gra14,000 r/mwas very sithe reductimore domitent. In faccontents wapersant on of binder. HDS005 add

In fact, sion viscosit could noface. The the lower cBut the Vacosity. The (b) 0.50 wt.%, (c) 1.0 wt.% and their size distributions (d).

ion, in order to obtain different particles sizes and

s, the atomizer speed was tuned to finally obtain bet-

performance. Unfortunately, these attempts weren the atomizer speed was lower than 8000 r/min.nsiderable amount suspension was adhered on thember and lots of larger aggregates were formed duer rotating speed, leading to a low collection rate andology of misshapen particles. However, an obvious

was observed in Fig. 6(a) when it was higher than. With increasing the atomizer speed, the mean sizedecreased and it was less sensitivity for dispersantdually. Especially, when the atomizer speed reachedin, the mean size from different dispersant contentsmilar with each other. This is easily understood thaton effect due to the increased atomizer speed playsnant role than the effect of different dispersant con-t, the change of AD/TD with different dispersants not big, comparing to Fig. 3(a), and the effect of dis-

the granules properties were less important than thatowever, still the regularity existed that the 0.50 wt.%ed granules had the smallest size.the addition of dispersant can change the suspen-ity. When the dispersant content is lower (0.25 wt.%),t reach the saturation adsorption on particle sur-

repulsive force between particles is weaker due toharge density and thinner double electrical layers.

n der Waals force is stronger due to the higher vis- suspension is only partly in decondensation state


20

30

40

50

60M

ean

Size

/ m

0

0

0

0

0

Acc

umul

atio

n/ %

Fig. 6. (A) Mand (B) the pcontents.

and has poit graduallforce becodecondensaity (0.50 wof multi-comore than persant moconcentratirepulsive fowould formThey togethstability of

Fig. 6(bwith differsome samponly one pdistributionDS005 formexhibited thproved by a

Therefocontent 45content of suitable for

rushing behavior of granulated particles crushing behavior of granulated particles during form-a ve

as an co

nsityh of he co. 7, ahomo80 Mreaspos

essu 02es an, thee andoros

the 0 MP. 8(Bnderne pas l

nm11.000.750.500.25.0

.0

.0

.0

.0(A)

(b)

(a)

(c)

Dispersant/ wt.%

Atomize r spe ed: r/ min(a) 800 0

(b) 11000

(c) 140 00

0.10.01

.0

.2

.4

.6

.8 (B)

(a) 0.25 wt.%(b) 0.50 wt.%(c) 0.75 wt.%(d) 1.0 wt.%

c

b

d

a

Pore size/ m

ean sizes of granulated particles with different atomizer speeds

3.3. C

Theing is and it hof greethe destrengtter of tIn Figbut a under ual incare com

CIP prvoid inparticlFinallyarrangtotal pdue toafter 8

Figpacts uonly osure w

of 30

ore size distributions of green compacts with different dispersant

or stability. With increasing the dispersant content,y reaches the saturation adsorption. The repulsivemes stronger. The suspension has been completelyted and has the smallest viscosity and best stabil-

t.%), which assuring the homogeneous distributionmponents. However, when the dispersant content isthat of the saturation state needed, the excessive dis-lecules enter into the solvent and increase the ionicon and decrease the Zeta potential, weakening therce. In addition, the excessive dispersant molecules

a network by the action of bridge connection [28].er lead to the lower viscosity, poorer flowability and

suspension (1.0 wt.%).) is the pore size distributions of the green compactsent dispersant contents. Unlike the double peaks ofles in Fig. 4(a), all green compacts here exhibitedeak in the range of 30100 nm and had similar size

centered at about 60 nm, although the 0.50 wt.%ed green compact had the smallest pore size and it

e best flowability and filling property already beingbove AD/TD results in Fig. 3(b).

re, the optimized suspension composition are: solid wt.%, binder content of PVB 1.0 wt.%, dispersantDS005 0.50 wt.% and the atomizer rotating speed

this suspension 8000 r/min.

distributionchange of it decreasewidth of digeneous poof pressurethe pore siwas larger total decre70 nm). Th4080 MPaished the cr40 MPa.

In additgradually injump was fafter 60 MPoretical tranand smalleThey are insintering pr

These aexcellent fpacts withoalmost comhomogeneoabout 55.9%performancis possible quality.ry important characteristic to evaluate their quality significant impact on the density and microstructurempact. Here, SEM images of fracture surfaces and

changes of green compact are used to evaluate thegranules. The observation point for SEM was the cen-mpact under different CIP pressures of 10100 MPa.

carcass of spherical shape remained until 60 MPa,geneous surface structure was completely formedPa. The densities of green compacts showed a grad-e in Fig. 8(A) and two turning points. The granulesed of multi-levels of particles sizes. With increasingre, the granules firstly begin to arrange and fill the0 MPa. Then, the granules begin to crush into secondd filled interspace between granules in 2080 MPa.

second particles were crushed into raw particles to fill under above 80 MPa. Fig. 8(A) curve (b) is the

ity change measured by MIP. It decreased graduallyexpulsion of air and had already reached about 40%a.

) shows the pore size distributions of green com- different CIP pressures. Obviously, they all exhibitedeak indicating the better homogeneity. When pres-ow at 10 MPa, the pore size was in a wide range

m. With increasing the pressure, the pore size gradually shifted to the smaller size. The detailedmean pore size is shown in Fig. 8(B, inset) andd from 120 nm to 70 nm with gradually narrowerstribution, representing the smaller and more homo-re in compact. In addition, an obvious turning point

was observed at 40 MPa. From 10 MPa to 40 MPa,ze had a great decrease. In these compacts, therepore (>200 nm). But from 40 MPa to 80 MPa, thease of pore size was only 10 nm (from 80 nm tois indicated the green compacts pressured under

were in the same stage. Therefore, the granules fin-ush to form dense compact only pressured higher than

ion, the transmittance of sintered YAG ceramics alsocreased with increasing CIP pressure and the biggestound before 40 MPa and followed a slight increasea. It reached 84.20% at 100 MPa, very near to the the-smittance. Generally, a higher pressure, less porosity

r pore size shorten the distance between the particles. favor of the diffusion and the densification duringocess.bove results indicate the granulated particles haveorming performance. They could form green com-ut inter-granules void only under 40 MPa and arepletely crushed under 80 MPa to form the dense andus structure. The packing density under 100 MPa is. Using the granulated particles with good forming

e and suitable strength, even at low CIP pressure, itto fabricate transparent ceramics with higher optical


Fig. 7. SEM images of fracture surfaces of green compacts u

0 20 40 60 80 100

1.6

1.8

2.0

2.2

2.4

2.6

Density Total poro sity

38

40

42

44

46

48

50(A)

Total porosity/ %

Pressure/ MPa

Den

sity

/ gcm

-3

0.01 0.1 1 10

0.0

0.2

0.4

0.6

0.8

20 40 60 80 100

80

100

120(B)

Mea

n si

ze/

m

Press ure/ MPa

f

(a) 10 MPa

(b) 20 MPa

(c) 40 MPa

(d) 60 MPa

(e) 80 MPa

(f) 100 MPa

Acc

umul

atio

n/ %

Pore size / m

70 nm

cb

d

e

a

Fig. 8. (A) Densities and total porosities of green compacts under different CIPpressure, (B) their pore size distributions and the change of mean pore size(inset).

3.4. Micro

Fig. 9(Aof one grangranulated face, Fig. 9was Fig. 9small pore,different kiparticle andysis is showdistributionY/Al = 3/5 greatly shothe densifical composbinder in Fthe better ssmaller sizbigger sheethat in that deviated fr

In fact, tdue to the bDiffusion pand particle(Dc) affect

Dc = k K6nder different CIP pressures.

structure and optical transmittance

) shows the typical sphere (A1) and its surface (A2)ulated particle with the optimized suspension. Everyparticle had very smooth and entirely spherical sur-(A1). Its magnification (20,000 times) of local surface(A2), it showed the dense structure with little and and different particle morphologies corresponded tonds of raw material, i.e., Al2O3 and Y2O3 for small

big sheet, respectively. The micro-region EDS anal-n in Fig. 9(A3). The surface showed homogenous

of Y/Al = 0.593, very closing to theoretical value= 0.60. The homogeneous mixing of raw materialsrtens the diffusion distance and rapidly complete

cation during sintering process. However, the typi-ition deviation was observed in the granules withoutig. 9(B). Although the granulated particle still had

pherical shape, Fig. 9(B1), more Al2O3 particles withe were on its surface and they seemed to cover thet Y2O3 particles in Fig. 9(B2). EDS analysis showedgranulated particle without binder, Y/Al = 0.562 wasom the theoretical value 0.60.his is known as the microencapsulation phenomenonig difference in particle sizes of multi-components.lays an important role in the solvent evaporation

self-assembly stages. Different diffusion coefficients the particle movement in the droplet [29]:

BT

cRc(3)


Fig. 9. SEM without binde

where k, KBmanns con

respectivelcomponentcomponentand Y2O3 wWhen the seffect betwsuitable adfaster undethe self-asssible for a result in thewas not biging processceramics. Timportant t

The typface (A2) are shown ture and nosize was aAnd a predlittle intergimages of one granulated particle (1) and its surface (2) and the EDS analysis (3) r (B).

, T, c and Rc are diffusion correction factor, Boltz-stant, temperature, viscosity and component size,

y. Thus, Dc has a negative correlation to the size of. The component moves quickly with a decrease of

size. In our suspension, the particle sizes of Al2O3ere 100200 nm and 1 m, respectively, in Fig. 9.

uspension has no binder, Fig. 9(B), the combinationeen particles would be weakened, comparing to thedition of binder. Smaller Al2O3 particles could mover the capillary force in the droplet, and then completeembly of particles into close-packed arrays. It is pos-smaller component to coat a larger component and

composition deviation [30]. Although this deviation and could be compensated by the followed sinter-, it actually decreases the transmittance of sinteredherefore, a suitable addition of PVB binder is very

o form homogeneous and dense granules.ical microstructures of surface (A1) and fracture sur-of sintered ceramic with the optimized suspensionin Fig. 10(A). They had a homogeneous microstruc-

pores or other defects could be observed. The grainbout 10 m with clear and clean grain boundary.ominant transgranular fracture was observed withranular fracture. This homogeneous microstructure

assured theintra- or inceramics weters. Fig. (B2) of sinsimultaneosurface becticles, and of dispersaand then de

Finally, our optimi1064 nm antively, whiactive ionsnon-optimidecrease oor apple shthe transmithe maximther increaswith the abpore distribof granulatwith the optimized suspension (A, binder content 1.0 wt.%) and

high optical transmittance. However, the defects liketer-crystalline pores were generally observed in theith non-optimized suspension or processing param-10(B) shows the surface (B1) and fracture surfacetered ceramic with 1.0 wt.% DS005. The pores wereusly trapped into the grains on the surface and fractureause of the poor spherical shape of granulated par-lower AD/TD values. Therefore, a suitable additionnt is also important to obtain good spherical granulesfect-free ceramics.the highest optical transmittance was observed underzed technology in Fig. 11. The transmittance atd 400 nm reached up to 84.74% and 80.17%, respec-ch were high enough for laser application if the

were doped in pure YAG host. In addition, thezed composition of suspension would lead to thef transmittance, accompanying with the doughnutape of granules. Under different contents of PVB,ttance of YAG ceramic firstly increased and reachedum value at 1.0 wt.% and then decreased with fur-ing PVB content in Fig. 11(inset). This is consistent

ove results of morphology, size distribution, AD/TD,ution, etc. This indicates that the quality controls

ed particles, including particle size, size distribution,


Fig. 10. SEM images of surface (1) and fracture surface (2) of ceramics with the d

30002500200015001000500

0

20

40

60

80

100

3.02.52.01.51.00.50.00

20

40

60

80

80.17 %

(a) @ 1 064 nm(b) @ 40 0 nm

84.74 %

Wav eleng th/ nm

Tra

nsm

ittan

ce/ %

(b)

(a)

Tran

smitt

ance

/ %

Binder/ wt.%

Fig. 11. Transmittance of YAG ceramic from the optimized technology and itschange with different binder contents (inset).

morphologmicrostructransmittan

In additfor YAG waing flow w45 wt.% an1.0 kg/h. other unexpcles is showproductionprovided min large scaphotographin Fig. 12

Fig. 12. (a) SEM image of granulated particles using the optimized technology for scaled-up prodproducts.ispersant content (A, 0.5 wt.%) and 1.0 wt.% (B).

y and elements distribution, contribute to refine theture of sintered compact, and then improve the opticalce of YAG ceramics.ion, the scaled-up production of granulated particless operated under the optimized technology. The feed-as 40 g/min for suspension. The solid content wasd thus the mass yield of granulated particles reached

In a steady work state without nozzle obstruction andected accidents, the SEM image of granulated parti-n in Fig. 12(a) for scaled-up production 10 kg. High

rate with well mono-dispersed spherical granulesore opportunities for their applications, especiallyle or thickness, composite structure of YAG. Theirs of various transparent YAG products are shown(b). They were all transparent and suitable for

uction of 10 kg; (b) photographs of their various transparent YAG


different applications with excellent forming performance andhigh optical quality.

4. Conclusions

In this paper, spherical YAG granulated particles with goodforming performance and transparent ceramics with high opti-cal transmittance were successfully obtained by a systematicoptimizatioparametersselected atat 25 Hz aatomizer rocompositioPVB 1.0 wgranulated aggregatestheir mean density of g(3) The cointer-granuhigh opticareached upand suspenbasic princthe semi-inceramics.

Acknowled

The authPriority AcEducation Foundation

Reference

[1]. Ikesue A,[2]. Ikesue A,

erties of hlasers. J A

[3]. Feng T, Claser outp2008;23:1

[4]. KaygorodOsipov VcompactioCeram So

[5]. Sorokina et al. Bro2002;27:1

[6]. Wang SF,ceramics:2013;41:2

[7]. Yang H, Zhang J, Qin XP, Luo DW, Ma J, Tang DY, et al. PolycrystallineHo:YAG transparent ceramics for eye-safe solid state laser applications. JAm Ceram Soc 2012;95:525.

[8]. Ikesue A, Yoshida K, Kamata K. Transparent Cr4+-doped YAG ceramics fortunable lasers. J Am Ceram Soc 1996;79:5079.

[9]. Sang YH, Qin HM, Liu H, Zhao LL, Wang YN, Jiang HD, et al. Partial wetroute for YAG powders synthesis leading to transparent ceramic: a coreshellsolid-state reaction process. J Eur Ceram Soc 2013;33:261723.

[10].Hostasa J, Esposito L, Piancastelli A. Influence of Yb and Si content on thesintering and phase changes of Yb:YAG laser ceramics. J Eur Ceram Soc

2;32:2, Li J/Nd:er ExXW, sparen3;42:2st W,

in-lin4;34:2rez Mtatic pessin

rez Mnd pr

4;40:4YF, Qders am Son X, Zng. D

XQ,ng of39.nte Jation

tsekouerties-castiWB, ders l012;3bvre asiew9;72:6ker Wacterilouti a aggrsson

cts ofgriz Nnger;trand Gacteriam Sog S, Lray d

ta ALerocoats. J En of spray drying. (1) The optimized processing were: inlet and outlet temperatures of drying air

75 C and 60 C, its flux and aspiration rate setnd 30 Hz (at half capacity), respectively, and thetating speed 8000 r/min. The balanced suspension

n were: solid content 45 wt.%, binder content oft.%, dispersant content of DS005 0.50 wt.%. (2) Theparticles exhibited mono-dispersed and fully dense

with good spherical shape and smooth surfaces, andsize was about 40 m. The apparent density and tapranules were about 1.50 and 1.65 g/cm3, respectively.mpacts pressured only at 40 MPa could eliminateles void, indicating suitable strength of granules. Thel transmittance of sintered ceramics that at 1064 nm

to 84.74%. (4) The optimized processing parameterssion composition as well as the study methods andiples provided by this paper, could directly serve fordustrialized productions of optical or other functional

gements

ors acknowledge the generous financial support fromademic Program Development of Jiangsu HigherInstitutions (PAPD), and National Natural Science

of China (51402133, 51302115, 61405081).

s

Aung YL. Ceramic laser materials. Nat Photonics 2008;2:7217. Kinoshita T, Kamata K, Yoshida K. Fabrication and optical prop-igh-performance polycrystalline Nd: YAG ceramics for solid-statem Ceram Soc 1995;78:103340.ai H, Qi YF, Jiang DY, Zhou J, Lou QL, et al. Optical property andut of Nd: Y3Sc1.5Al3.5O1.2 transparent ceramics. J Inorg Mater0414.ov AS, Ivanov VV, Khrustov VR, Kotov YA, Medvedev AI,V, et al. Fabrication of Nd:Y2O3 transparent ceramics by pulsedn and sintering of weakly agglomerated nanopowders. J Eurc 2007;27:11659.IT, Sorokin E, Mirov S, Fedorov V, Badikov V, Panyutin V,adly tunable compact continuous-wave Cr2+:ZnS laser. Opt Lett0402.

Zhang J, Luo DW, Gu F, Tang DY, Dong ZL, et al. Transparent processing, materials and applications. Prog Solid State Chem054.

201[11].Ge L

YAGMat

[12].Ba tran201

[13].Pabreal201

[14].Suaisosproc

[15].Suasis a201

[16].Liu powCer

[17].Chedryi

[18].Caodryi243

[19].Viceform

[20].TsepropSlip

[21].Liu powInt 2

[22].Lefe[23].Luk

198[24].Wal

char[25].Bak

min[26].Elve

effe[27].Ash

Spri[28].Ber

charCer

[29].Wanin sp

[30].CosHetmen94956., Zhou ZW, Qu HY, Dong MJ, Zhu Y, et al. Fabrication of compositeYAG/YAG transparent ceramics for planar waveguide laser. Optpress 2014;4:10429.Li J, Pan YB, Liu J, Guo JK. Preparation of composite YAGt ceramics by aqueous tape casting route. Rare Metal Mater Eng347.Hostasa J, Esposito L. Porosity and pore size dependence of thee transmission of YAG and alumina ceramics. J Eur Ceram Soc74556.

, Fernandez A, Menendez JL, Nygren M, Torrecillas R, Zhao Z. Hotressing of optically active Nd:YAG powders doped by a colloidalg route. J Eur Ceram Soc 2010;30:148994., Rocha V, Fernandez A, Menendez JL, Torrecillas R. Synthe-ocessing of spinel powders for transparent ceramics. Ceram Int0659.in XY, Xin HX, Song CJ. Synthesis of nanostructured Nd:Y2O3

by carbonate-precipitation process for Nd:YAG ceramics. J Eurc 2013;33:262531.heng BC, Shen J. Morphologies of polymer grains during sprayry Technol 2013;31:4338.

Vassen R, Schwartz S, Jungen W, Tietz F, Stoever D. Spray- ceramics for plasma-spray coating. J Eur Ceram Soc 2000;20:

, Pinto J, Menezes J, Gaspar F. Fundamental analysis of particle in spray drying. Powder Technol 2013;247:17.

A, Agrafiotis C, Milias A. Optimization of the rheological of alumina slurries for ceramic processing applications. Part I.ng. J Eur Ceram Soc 2001;21:36373.Zhang WX, Li J, Zhang D, Pan YB. Preparation of spray-driedeading to Nd:YAG ceramics: the effect of PVB adhesive. Ceram8:25964.

A. Atomization and sprays. New York: CRC Press; 1988.icz SJ. Spray-drying ceramic powders. J Am Ceram Soc1724.J, Reed JS, Verma SK. Influence of slurry parameters on the

stics of spray-dried granules. J Am Ceram Soc 1999;82:17119.S, Chartier T, Baumard JF. Binder distribution in spray-dried alu-lomerates. J Eur Ceram Soc 1998;18:211721.

J, Millqvist-Fureby A. Particle size and density in spray drying carbohydrate properties. J Pharm Sci 2005;94:204960.. Handbook of atomization and sprays: theory and applications.

2011., Filiatre C, Mahdjoub H, Foissy A, Coddet C. Influence of slurry

stics on the morphology of spray-dried alumina powders. J Eurc 2003;23:26371.angrish T. A review of process simulations and the use of additivesrying. Food Res Int 2009;42:1325., Matteucci F, Dondi M, Zama I, Albonetti S, Baldi G.gulation-spray drying process for the inclusion of ceramic pig-ur Ceram Soc 2008;28:16976.

Systematic optimization of spray drying for YAG transparent ceramics1 Introduction2 Materials and experimental details2.1 Suspension preparation2.2 Spray drying2.3 Ceramic fabrication2.4 Characterizations

3 Results and discussion3.1 Optimization of processing parameters3.2 Optimization of suspension composition3.2.1 Solid content of suspension3.2.2 Binder content of PVB3.2.3 Dispersant content of DS005 and speed of atomizer

3.3 Crushing behavior of granulated particles3.4 Microstructure and optical transmittance

4 ConclusionsAcknowledgementsReferences

Documents

Systematic Optimization of Spray Drying for YAG Transparent Ceramics 2015 Journal of the European Ceramic Society