Journal of the European Ceramic Society 34 (2014) 1001–1008
Comparison of spray freeze dried nanozirconia granules usingultrasonication and twin-fluid atomisation
Yifei Zhang a,∗, Jon Binner a, Chris Rielly b, Bala Vaidhyanathan a
a Department of Materials, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdomb Chemical Engineering Department, Loughborough University, Loughborough, Leicestershire LE11 3TU, United Kingdom
Received 12 June 2013; received in revised form 22 October 2013; accepted 26 October 2013Available online 28 November 2013
bstract
ranulation of nanostructured 3 mol% yttria stabilised zirconia using spray freeze drying was investigated to achieve flowable and crushableranules for subsequent die pressing. Commercial nanosuspension consisting of ∼16 nm particles was concentrated to ∼55 wt% solids contentia a patented technique, followed by spraying into liquid nitrogen using either a vibrating ultrasonic probe or a twin-fluid atomizer and freezeried to yield spherical granules. Control of the granule size fractions was investigated by changing the amplitude and the feeding rate of the
anosuspension during ultrasonication, whilst the flow rate of compressed air used for spraying was varied during twin-fluid atomisation. Granulesetaining good crushability for pressing were in a size range of 125–250 �m, which were achieved with ∼60 wt% yields using the atomisationoute, whilst a maximum of 35 wt% of granules in this size range were produced in previous research using ultrasonication.
Nanocrystalline ceramics, i.e. those with mean grain sizes ofess than 100 nm, have become of interest in recent years dueo their novel mechanical, thermal and optical properties, whichllow them to access a wide range of industrial applications.1
he research presented here focuses on zirconia ceramics and,n particular, 3 mol% yttria stabilised zirconia (3YSZ). Conven-ional 3YSZ is a common material for engineering components,ince it displays high strength and toughness. Furthermore, zir-onia has the potential to be used in the bioceramics industriesor bone replacement and dental applications due to its intrin-ic bioinertness.2 However, conventional 3YSZ suffers fromydrothermal ageing, which is a degradation process caused
y a tetragonal to monoclinic phase transformation in the pres-nce of water or water vapour, usually at elevated temperatures.3
ecent results have suggested that the mechanical performance
f nanostructured zirconia is at least as good as that of conven-ional submicron zirconia, whilst the nanostructured material isotally immune to hydrothermal ageing.4
The processing of nanoceramic components is difficult how-ver, since ultrafine particles possess very high surface areas andence have extremely high free energy; they stick together dueo van der Waals forces.5 This hinders the manufacture of highensity green bodies and can lead to excessive grain growth afterintering.6 Dry forming is the industrially preferred processingoute for technical ceramics because of its high manufacturingfficiency and low cost. However, dry nanopowders are not free-owing due to the random aggregation of nanosized primaryarticles.7 Hence it is essential to granulate them into sphereso enhance their flowability before dry forming. Spray dryings a widely used technique to produce ceramic powders withood flowability; however, spray dried nanogranulates retainery high strength and hence are difficult to crush completelyt typical industrial pressing pressures of up to 200 MPa.8 Theesidual uncrushed granules will affect the homogeneity of greenodies formed and in a sintered component, they act as flaw
rigins and thus reduce the strength of the component severely.9
At Loughborough University, nano3YSZ granules have beenroduced by using a spray freeze drying (SFD) technique; this
itnauUaaoLrtst(Fato enable spraying. The concentrated nanosuspension was sta-bilised at low viscosity because of the use of electrostatic
002 Y. Zhang et al. / Journal of the Europe
rocess has also been used by many other researchers for pro-ucing soft ceramic granules consisting of submicron10,11 andanosized particles.7,12 However, the granules produced wereot always reported to be satisfactory for dry forming. Adolfssonnd Shen10 reported that some sintered components formed fromFD granules demonstrated low density and/or poor strength;
he former was formed by granules with inhomogeneous struc-ure and large internal defects, whilst the latter was due to these of granules with high densities, which were difficult torush completely during pressing. Vicent et al.12 observed poorowability from spray freeze granulated nanopowders; this wasainly due to the less spherical granules produced and their
ncontrolled size distribution. The research presented here hasolved these issues; the SFD granules within a size fraction of25–250 �m have been used for dry forming and they displayxcellent flow and crush properties.1,8,9 Homogeneous, flaw-ree compacts with high green densities (∼55% of theoretical)ave been die pressed using these 125–250 �m granules.9 How-ver, the ultrasonic spraying process that has been used to datenly yields a maximum of 35 wt% granules in the desired sizeange,1 mainly due to the lack of control over the sprayingrocess.
A vibrating ultrasonic probe has been used to disperse theanosuspensions into fine droplets, which are frozen in liquiditrogen and subsequently freeze-dried to sublime off the ice.1
reviously the nanosuspensions were supplied to the ultrasonicrobe manually using a pipette; however, it was very difficulto control the feeding rate during the process. In this paper, aeristaltic pump and a pressure bottle have been used to feed theanosuspension at controlled flow rates; nevertheless, it is diffi-ult to use the ultrasonic spraying process for medium to largecale production of SFD granules. Stuer et al.11 demonstratedhe use of an ultrasonic spraying device that generated spherical,early monosized SFD granules of ∼250 �m, yet this equip-ent can only process ∼0.06 L h−1 suspension. An alternative
pproach is introduced in this paper, a twin-fluid atomizer, whichields a significantly higher amount of suitably sized granulest controllable spraying rates.
. Experimental
Aqueous 3YSZ nanosuspensions with an initial solids con-ent of ∼24 wt% (5 vol%) and consisting of well dispersedanoparticles of ∼16 nm in size were used as the precursor.hey were supplied by MEL Chemicals Ltd., Manches-
er, UK. The as-received suspensions were concentrated to55 wt% (∼17 vol%) using a patented route described in detail
lsewhere.13 In brief, it involved changing the pH from ∼3.5 to10.5 by adding tetra-methyl ammonium hydroxide (TMAH,ldrich Chemicals Ltd., Dorset, UK), followed by adding an
mount of the dispersant tri-ammonium citrate (TAC, Fishercientific UK Ltd., Loughborough, UK) equivalent to 3 wt%f the solids in the nanosuspension. The latter was then heated
◦
ently at 58 C using a water bath, with continuous mechanicaltirring to avoid agglomeration. The pH of the nanosuspen-ion was regularly checked to ensure it remained above ∼9.0,hilst multistage ultrasonic treatments were applied at regular F
ig. 1. Particle size distributions for as-received and concentrated nano3YSZuspensions.
ntervals using a Soniprep 150-MSE ultrasonicator (MSE Scien-ific Instruments, Manchester, UK) to avoid agglomeration of theanoparticles. The particle size distributions of the as-received,s well as the concentrated nanosuspensions were measuredsing a Mastersizer 2000 (Malvern Instruments Ltd., Malvern,K); see Fig. 1. The particle size distribution is unchanged
fter concentration, indicating that the particles remain un-gglomerated. A volume fraction of 2% of Freon 11 (a solutionf fluorotrichloromethane in methanol, Fisher Scientific UKtd., Loughborough, UK), used as an additive to weaken the
esulting granules, was thoroughly mixed with the concen-rated nanosuspensions in a sealed container for ∼30 min beforepraying.14 The viscosities of the as-received and the concen-rated nanosuspensions were determined using a Rheolab QCAnton Paar GmbH, Graz, Austria) viscometer, as shown inig. 2. The viscosity for the latter remained less than ∼20 mPa st shear rates in the range 200–800 s−1, which was suitable
ig. 2. Viscosity data for as-received and concentrated nano3YSZ suspensions.
Y. Zhang et al. / Journal of the European C
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ig. 3. (a) Ultrasonication spray freeze granulation using a peristaltic pump. (b)ltrasonication spray freeze granulation using a pressure bottle.
ispersant TAC during concentration, accompanied by TMAHsed as a pH modifier.
Spray freeze drying was performed by spraying the con-entrated nanosuspensions into liquid nitrogen using either anltrasonication or an atomisation process. Figs. 3 and 4 showhe setup of the equipment for both of these routes. A vibratingltrasonic probe (MSE Scientific Instruments, Manchester, UK)ith a flat tip surface of 9.5 mm diameter operating at a fixed
requency of 23 kHz was used in the former method. The sizeistributions of the resulting granules were varied by changinghe amplitudes applied, as well as the feed rate of the nanosus-ensions; the latter was controlled by using either a Masterflex/L peristaltic pump (Cole-Parmer, London, UK) or a pressureottle, as shown in Fig. 3(a) and (b), respectively. For the atom-sation route, a twin-fluid atomizer (BUCHI Labortechnik AG,lawil, Switzerland) was employed to spray the nanosuspensiont various flow rates by varying the flow rates of the compressedir used for spraying; this was measured using a flow metre.rozen granules were collected from the liquid nitrogen andubsequently underwent freeze drying using a VirTis Benchtop
TK-2 K Freeze Drier (SP Scientific, Stone Ridge, NY, USA)
or ∼48 h. The condenser temperature was set at −62 ◦Cnd the vacuum was maintained at ∼7 Pa (50 mTorr). After
Fig. 4. Spray freeze granulation using a twin-fluid atomizer.
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eramic Society 34 (2014) 1001–1008 1003
reeze-drying, the granules were sieved to yield granulesithin the targeted size range of 125–250 �m for die pressing.o binder or plasticiser was used in the granulation processescribed here.
To characterise the flowability of the as-produced SFD gran-les using the two different methods, calibrated conical flowunnels with standard dimensions and discharge orifices of.5 mm, 5.0 mm and 7.5 mm were used to measure the mass andolume flow rates (see British Standard BS EN ISO 4490:2008).n industry standard spray-dried submicron 3YSZ powder, TZ-YSB (Tosoh Corporation, Yamaguchi, Japan) was used asenchmark and its flowability data was also measured for com-arison. The compaction performance was studied by pressinghe SFD granules into cylindrical pellets of ∼10 mm in diameternd ∼2.4 mm thick using a hardened steel die at 200 MPa; theensities of the pressed materials were measured geometricallyy taking the mass and dimensions of each pellet.
The strengths of individual granules were tested using a man-al granulate strength testing system (etewe GmbH, Karlsruhe,ermany) located at the Fraunhofer Institut fuer Keramischeechnologien und Systeme (IKTS) in Dresden, Germany. This
nvolved the crushing of single granules, which were placed on platform. During the test, the platform moved towards the tipurface of a micrometre at a controlled speed of 10 �m s−1. Theracture was recorded by video and the granule size was mea-ured. A total of 200 granules with sizes between 50 �m and50 �m were measured individually in each batch of powder.he force used to crush the granule and the displacements of thelatform were used to calculate the granule fracture strength.
The surface and internal structure of the granules, as well ashe fracture surfaces of the pressed green pellets, were charac-erised using a Leo 1530VP high resolution field emission guncanning electron microscope (FEGSEM, Leo ElektronenskopiembH, Oberkochen, Germany).
. Results and discussion
.1. Granule size distributions
Fig. 5 shows the effect of the feeding rate on the resultantranule size distributions for the ultrasonic spraying process;he amplitude applied was 12 �m. At a very low feeding ratef 0.14 L h−1, ∼60 wt% of the granules were produced withizes of less than 125 �m, whilst the yield of the granules in theame size range was only ∼26 wt% when the nanosuspensionas supplied at a much higher flow rate of 0.85 L h−1. During
he spraying process, many small sized cavities were createdithin the exposed nanosuspension through the vibration of theltrasonic probe, which then collapsed and the local shockwaveenerated provided a strong hydrodynamic shear force to tearhe nanosuspension into fine droplets.15 It is worth noting thathe ultrasonic waves were generated locally only at the tip of therobe, where the actual spraying process took place. When the
eeding rate was low, there was a longer duration for the nanosus-ension to stay at the probe tip; this could generate more cavitiesnd hence more small-sized granules. However, when the feed-ng rate was high, there was insufficient residence time at the
1004 Y. Zhang et al. / Journal of the European Ceramic Society 34 (2014) 1001–1008
Fu
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yraFwt∼it
Fu
59
35
6
53
42
5
52
40
8
23
31
46
0
10
20
30
40
50
60
70
<125 μm 125-250 μm >250 μm
Perc
enta
ge /
wt%
Granule size distribu�ons / µm
0.15 l/h0.30 l/h0.46 l/h1.80 l/h
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ig. 5. Size distributions of the SFD granules produced at different feeding ratessing the ultrasonication method with peristaltic pump.
robe tip, resulting in the generation of fewer cavities within theupplied nanosuspension before it fell from the probe tip; hencehe granules generated were larger. In addition to the cavitationheory, fine droplets could also be formed from the breakage ofhe crests of liquid capillary waves generated via the implosionf cavities; the size of the droplets produced is proportional tohe wavelength of ultrasound applied.16 In this paper, the ultra-ound used for spraying had a fixed frequency of 23 kHz; hencehe wavelength remained the same among different experimentsnd did not affect the size of the resultant granules.
Results showed that the use of a feeding rate of 0.35 L h−1
ielded the most granules, ∼45 wt%, within the desired sizeange, however, spraying at this feeding rate but using differentmplitudes did not affect the yields significantly, as shown inig. 6. The yield of the granules within the 125–250 �m rangeas increased by ∼2 wt% to 47 wt% when the maximum ampli-
ude of 16 �m was applied, whilst reducing it to 7 �m resulted in
1% loss by weight. Since the resultant granules were separated
nto each size fraction by sieving, considering the resolution ofhe experiments, it can be concluded that there was no significant
ig. 6. Size distributions of the SFD granules produced at different amplitudessing the ultrasonication method with peristaltic pump.
soafitdlo
sdwoerttnsith
ig. 7. Size distributions of the SFD granules produced at different feeding ratessing the ultrasonication method with pressure bottle.
ffect within the 125–250 �m size range by using differentmplitudes. However, a clear trend showed that more granulesith sizes above 125 �m, especially those above 250 �m, wereroduced by increasing the amplitude from 7 to 16 �m. Whilstt is expected that higher amplitudes, hence greater ultrasoundntensities, should yield smaller droplet sizes, it was observedhat when a high amplitude, e.g. 16 �m, was used for spraying,he nanosuspension supplied was shaken off from one side ofhe probe tip without being atomised fully to form fine droplets;he granules produced were hence coarser compared to thosebtained using a lower amplitude, e.g. 12 �m. This can bexplained as there being insufficient interface energy betweenhe nanosuspension and the ultrasonic probe; if the momentumas too high the suspension was shaken off from the probe with-ut being atomised. The same effect was observed by Rajan andandit16 and they proposed that there is a range of ultrasoundmplitudes that can be used to generate fine droplets with smallerizes; below or above this range coarse droplets are formed with-ut being atomised fully. In this paper, the lowest amplitudepplied was 7 �m; below this the ultrasound intensity was insuf-cient to atomise the nanosuspension into fine droplets; yet at
his amplitude the granules produced had ∼44 wt% within theesired size range, whilst much smaller granules with their sizesess than 125 �m were produced, indicating a good spray wasbtained.
Since it was difficult to achieve a stable flow of nanosuspen-ion using a peristaltic pump, a pressure bottle was employed toeliver constant flow rate and the amplitude used for sprayingas kept at 12 �m. However, this did not improve the yieldsf granules within the desired size range further (see Fig. 7)ven when the nanosuspension was supplied at similar flowates to those delivered by the peristaltic pump. This indicateshat the size of granules was affected by the configuration ofhe ultrasonic probe, rather than the method for dispensing theanosuspension during ultrasonication. It should be noted thaturface tension of the nanosuspension plays an important role
n droplet formation during spraying. This was not studied inhe current research, since to change the surface tension wouldave required the use of additives and these were not desired
Y. Zhang et al. / Journal of the European Ceramic Society 34 (2014) 1001–1008 1005
12
35
53
25
60
15
92
71
0102030405060708090
100
<125 μm 125-250 μm >250 μm
Perc
enta
ge /
wt%
Granule size distribu�ons / μm
3.2 l/min a ir (0 .95 l/h s uspension )3.8 l/min air (1.2 l/h suspension)4.1 l/min air (1.3 l/h suspension)
Fu
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Table 1Volume flow rate of the feeding material during twin-fluid atomisation.
Air flow rate (L min−1) 3.2 3.8 4.1Volume flow rate of the concentrated 0.95 1.2 1.3
4srsa1nhmfwfitsuua
3
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3
ig. 8. Size distributions of the SFD granules produced at different air flow ratessing the twin-fluid atomisation process.
n the final granules. Any excess organics would be retained inhe granules, producing gas bubbles during debinding and yield-ng undesired porosity in the die-pressed components. Granulesith excessive organics also retained high strength and henceere difficult to crush completely during die pressing.In the previous research mentioned above,1 the maximum
ield of granules within the desired size range, 125–250 �m,as up to 35 wt%. For the current research presented here, whilstsing the peristaltic pump to control the feeding rate of theanosuspension improved the yield of the granules producedithin the desired size fraction by more than 10 wt% to a 47 wt%aximum, another issue was the difficulty of scaling up the lab-
ratory scale ultrasonic spraying process. The major limitationf this method was the low volume handling capacity, due to themall area of the vibrating surface. The best conditions achievedo date involved spraying the nanosuspension at the fairly lowow rate of 0.35 L h−1; this may not be acceptable for industrycale processing of some ceramic powders. Whilst larger prober the use of multiple transducers could be envisaged, in thisork, a twin-fluid atomizer was investigated. The principal was
o use high velocity compressed air to tear the supplied nanosus-ensions into fine droplets. The distance between the nozzleip and liquid nitrogen surface was kept at ∼15 cm, which washe same for the ultrasonication method to minimise the differ-nce between two approaches. Whilst the ‘time of flight’ for theroplets formed before they were frozen in liquid nitrogen canffect the shape of the granules produced, it has been proved thathe granules obtained using the current condition were mostlypherical and displayed good flowability. Increasing the distanceetween the nozzle tip and the liquid nitrogen surface did notmprove the shape or the flowability of the granules obtained fur-her; instead, a liquid nitrogen bath with a much bigger surfacerea, and hence more liquid nitrogen was needed to capture allroplets formed. The depth of liquid nitrogen was maintainedt ∼15 cm throughout the experiment to keep all the dropletsrozen and to ensure no further agglomeration can occur.
The resultant granule size distributions were controlled bysing different amounts of air for spraying, as shown in Fig. 8.t can be seen that the use of a higher air flow rate, i.e.
f1
nanosuspension (L h−1)
.1 L min−1 enabled higher flow rates of suspensions to beprayed; see Table 1. However, spraying under these conditionsesulted in more than 90 wt% of the granules produced havingizes below 125 �m. Reducing the air flow rate to 3.8 L min−1
chieved a maximum yield of 60 wt% granules within the desired25–250 �m size range; the corresponding feed rate of theanosuspension was measured at ∼1.2 L h−1, which was muchigher than could be achieved using the ultrasonic sprayingethod. When the supply of the compressed air was reduced
urther to 3.2 L min−1, ∼53 wt% of the granules were producedith their sizes above 250 �m due to insufficient formation ofne droplets. It is also noted that the resultant granules from
he twin-fluid atomisation route demonstrated overall narrowerize distributions compared to the granules produced via theltrasonication method; this enabled higher yields of the gran-les produced within the desired size range using the twin-fluidtomisation route.
.2. Flowability
Fig. 9 shows the mass and volume flow rates, through theonical flow funnels, of the benchmark powder and the SFDranules produced within the size range of 125–250 �m usingoth the atomisation methods. The benchmark Tosoh powderemonstrated significantly higher mass flow rates, but similarolumetric flow rates, compared to the SFD granules; the Tosohowders are spray dried and are much denser than the granulesroduced via spray freeze drying. For the SFD process, mostf the granules yielded by the twin-fluid atomisation route werepherical, and hence these flowing powders had higher mass flowates compared to the granules prepared using the ultrasonicationethod; see Fig. 10. The irregularly shaped clusters produced
y the latter method, were formed as agglomerates of two orore smaller granules. This indicates collision and coalescence
f the fine droplets during freezing, since the cone angle of thepray produced by the ultrasonic probe was not as large as thator the twin-fluid atomizer. In terms of the volume flow rates, theFD granules produced via the twin-fluid atomisation processffered the best flowability compared to the others investigated,lthough the differences were not very large. Whilst the sphericalhape of the resulting granules ensured their good flowability, its worth noting that the SFD granules were sieved into a specificize fraction, which also contributed towards improving theirowability.
.3. Crushability
Homogeneous, flaw-free fracture surfaces were obtainedrom the green compacts die pressed at 200 MPa using the25–250 �m SFD granules produced by both methods; see
1006 Y. Zhang et al. / Journal of the European Ceramic Society 34 (2014) 1001–1008
Fig. 9. Mass (left) and volume (right) flow rates of the SFD granules (125–250 �m) and the benchmark powder.
aying
Fossg
uvmsd
tttrtsoi
Fm
Fig. 10. SFD granules obtained via the ultrasonic spr
ig. 11. High green densities up to 55% of theoretical werebtained after removing all the residual organics at 700 ◦C. Thisuggests that the granules produced using either the ultrasonicpraying, or the twin-fluid atomisation route, exhibited veryood crushability for die pressing.
Fig. 12 shows an example of an SFD granule producedsing the twin-fluid atomisation process; granules obtainedia ultrasonication showed similar surface morphology and
icrostructure. Whilst the atomisation method determined the
ize distribution of the granules, their internal structure wasetermined by the water content and the rate at which freezing
nno
ig. 11. Fracture surfaces of green compacts pressed at 200 MPa using SFD granulethods.
(left) and the twin-fluid atomisation (right) process.
ook place; this was broadly similar in both cases. By analysinghe internal microstructure of the granules, it was found thatheir good crushability originated from the ice crystal structureetained within the granules; this was formed by the sublima-ion of the ice dendrites during the freeze-drying process. Thepeed of the freezing process resulted in a high nucleation ratef ice crystals, giving the fine microstructure that can be seenn Fig. 12. The Freon addition was immiscible with the aqueous
anosuspension and the drops provided an increased density ofucleation sites during freezing, which resulted in the formationf a large number of ice dendrites. Hence, the resultant SFD
es produced using ultrasonic spraying (left) and twin-fluid atomisation (right)
Y. Zhang et al. / Journal of the European Ceramic Society 34 (2014) 1001–1008 1007
Fig. 12. SFD granule (left) and its in
gdc
sdttsc2awucfwSS
4
t
sufsthibofsgpflItaflw
A
pffGt
R
Fig. 13. Granule strength profile of the SFD granules.
ranules retained a porous structure with micron-sized holesistributed uniformly; this improved their crushability signifi-antly.
To investigate the crushability of the SFD granules further, thetrength of the individual granules in different size fractions pro-uced via the twin-fluid atomisation process was measured usinghe manual granulate strength testing system. Fig. 13 shows thathe first fracture points for all the SFD granules appeared attresses less than 2 MPa; this suggests that individual granulesrush at pressures far below the pressure used for die pressing of00 MPa. The latter is needed to overcome the friction stressest the die walls and between and within the granules. It is alsoorth noting that the stresses required to crush individual gran-les decreased with increasing granule size; the large granulesrushed more easily than the smaller granules. Within the sizeraction of 125–250 �m the granules showed similar strengths,hilst those smaller than 125 �m were significantly stronger.imilar observation was made in the previous research with theFD granules produced using the ultrasonication method.9
. Conclusions
In this research, a peristaltic pump has been used to con-rol the feeding rate of nanosuspensions during an ultrasonic
ternal microstructure (right).
pray freezing process; this has improved the yields of the gran-les produced within the desired size fraction, viz. 125–250 �m,rom 35 wt% to ∼47 wt% comparing to the previous research onpray freeze drying conducted at Loughborough University. Awin-fluid atomizer has also been used for spray formation andas allowed further increases in the yields to ∼60 wt% particlesn the size 125–250 �m, without compromising the good flowa-ility and crushing properties of the resulting granules. The usef this twin-fluid atomisation process also allowed a much highereed rate of the nanosuspensions and it is believed that it can becaled up more easily for industrial-scale processing of the SFDranules. The granules obtained from the twin-fluid atomisationrocess displayed the best flowability in terms of their volumeow rates due to the formation of individual spherical granules.n terms of crushability, the individual granules produced viahe atomisation process retained low strength and were as goods those produced via the ultrasonication method, resulting inaw-free green bodies with densities up to 55% of theoreticalhen die pressed at 200 MPa.
cknowledgements
The authors would like to acknowledge MEL Chemicals forroviding the precursor nano3YSZ suspensions and Prof. Man-red Fries and Ms. Susanna Eckhard from the Fraunhofer Institutuer Keramische Technologien und Systeme (IKTS) in Dresden,ermany for providing access to the manual granulate strength
esting equipment and assistance in using it.
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