effect of silica on the reactive sintering of polycrystalline nd:yag ceramics
TRANSCRIPT
Effect of Silica on the Reactive Sintering of Polycrystalline Nd:YAGCeramics
A. Maıtre,*,w C. Salle, R. Boulesteix, and J.-F. Baumard*Q2
Laboratoire Science des Procedes Ceramiques et Traitements de Surface, Universite de Limoges, UMR CNRS n06638,Limoges, Cedex, France
Y. Rabinovitch
Compagnie Industrielle des Lasers, Batiment ESTER, Limoges, Cedex, France
In the present work, the role of silica as sintering aids in thedensification and the grain growth of yttrium aluminum garnetdoped by neodymium (Nd:YAG) ceramics has been investigated.The samples were prepared by ball milling of pure oxides(Al2O3, Y2O3, Nd2O3, SiO2), shaped by cold uniaxial pressing,and sintered in vacuum between 1473 and 1973 K. After cooling,the specimens were annealed under air or vacuum. Their micro-structure and the chemical composition of the secondary phaseswere examined using electron probe microanalysis, scanningelectron microscopy, or transmission electron microscopy tech-niques. From these results, silica addition proved to be efficienton the densification kinetics between 1673 and 1873 K,especially when SiO2 content exceeds 0.05 wt%. Indeed, thesolid-state reaction between SiO2 and Nd:YAG particles inthe vicinity of 1660 K leads to a liquid phase that adopts aeutectic composition in the Al2O3–Y2O3–SiO2 system. Thisphase enhances the densification by improving rearrangementof particles and mass transport at the grain boundaries. At ahigh temperature, liquid phase and silica were partially removedand some intergranular and intragranular inclusions of residualsilica remained after cooling. The most promising thermal treat-ment consists of sintering Nd:YAG ceramics under vacuum at ahigh temperature (T � 1973 K) to reach fully dense pieces andto decrease the volume fraction of secondary phases (i.e., liquidphase or silica-based phases). Finally, these conditionswould allow to produce transparent YAG pieces due to theirgood homogeneity with regard to their microstructural proper-ties (grain size, volume fraction of secondary phases).
I. Introduction
FOR laser applications, single crystals of yttrium aluminumgarnet doped by neodymium (Nd:YAG) that are commer-
cially produced by the Czochralski method use the luminescenceof a small amount of Nd31 ions introduced as active species.Nd:YAG ceramics have been suggested as a suitable compoundfor power laser applications thanks to their good thermome-chanical properties and their good thermal conductivity. How-ever, the Czochralski method induces some limitations in theprocess:
(a) First, the single-crystal production remains expensive,because several weeks are usually required to form a smallquantity of it, and because it is necessary to reach the YAGhigh melting point.
(b) Second, the Nd concentration is limited to 1.4 at.%, as aresult of fluctuations in the axial direction for the grown crystal.1
and(c) Third, expensive Ir crucibles are required for the growth
of single crystals and contamination is hard to avoid.2
Under these conditions, the possibility offered by YAG trans-parent polycrystalline ceramics is attractive because of potentialadvantages in comparison with monocrystals: low durations andlow cost of manufacture, and no limitation in size and geometryfor the polycrystal design.3 Ikesue et al.4 were the first to dem-onstrate the possibility of elaborating transparent Nd:YAGceramics with the required optical properties. Many recentworks have shown that transparent polycrystalline Nd:YAGceramics are equivalent to or even better than a single crystalgrown by the Czochralski method.5,6
Nevertheless, numerous parameters of the fabrication processof transparent Nd:YAG ceramics using the reactive sinteringroute5,6 must be controlled such as: the grain size of the startingmaterials, the secondary phases (nature and volume fraction),and the residual porosity. Transparent ceramics can be pro-duced using phase-pure YAG powders or by the reactive sinte-ring approach in which a mixture of metal oxides is sinteredunder vacuum at a high temperature (T �1973 K).
Previous works that dealt with a reactive mixture of Al2O3,Y2O3, and Nd2O3
7,8 have shown that sintering additives (e.g.,MgO or SiO2) are required to achieve suitable optical propertiesin view of laser application. In the case of silica, secondaryphases such as silicate glass could be detected at the grainboundaries,8 in particular at low cooling rates after sintering(r10 K/min). According to the same authors, relationships existbetween the loss of transparency and the presence of residualintergranular phases.
Most studies9–11 have attempted to explain the beneficial roleof silica in sintering kinetics by an increase of the grain bound-ary (GB) diffusion coefficient or by the decrease of GB surfaceenergy in the presence of secondary phases. It is also generallyaccepted that the presence of a silica-based amorphous phase atthe YAG grain boundaries would favor the grain growth.12,13
Nevertheless, no study has reported detailed information on theformation mechanisms of the secondary phases.
Without any secondary phase formation, silica would inducedefect formation in the cationic lattice by substitution of Al31
into the tetrahedral site as follows14:
3SiO2 �!2Al2O3
3Si�Al þ V000Al þ 6Ox (1)
The formation of aluminum vacancies could enhance the lat-tice diffusion and could improve the YAG densification rate.Nevertheless, because of elastic distortions in the lattice due tothe large size mismatch between Si41 and Al31 (rSi
41 5 0.026 nm;rAl31 5 0.039 nm), silica dissolution remains small: only silicaquantities below 0.28 wt% dissolve in the YAG matrix at1973 K.14
J A C E 0 2 1 6 8 B Dispatch: 6.11.07 Journal: JACE CE: Vimala
Journal Name Manuscript No. Author Received: No. of pages: 8 PE: Rathna/nvs
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J. Ballato—contributing editor
*Member, the American Ceramic Society.wAuthor to whom correspondence should be addressed. e-mail: alexandre.maitre@un
ilim.fr
Manuscript No. 23412. Received July 3, 2007; approved October 2, 2007.
Journal
J. Am. Ceram. Soc., ]] []]] 1–8 (2007)
DOI: 10.1111/j.1551-2916.2007.02168.x
r 2007 The American Ceramic Society
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The purpose of the present study is to establish the effect ofsilica addition on the reactive sintering of Nd:YAG ceramics.From quantitative measurements of microstructural and chem-ical evolution of Nd:YAG ceramics during thermal treatment,conclusions can be drawn about the role of secondary phases inthe densification and the grain growth under different atmo-spheres.
II. Experimental Procedure
The manufacturing process used during this work is similar tothat reported by Rabinovitch et al.15
(1) Preparation of Mixtures
Submicrometer a-Al2O3 (purity 499.99%, Baıkowsky, France)and Y2O3 powders (purity 499.99%, Alpha Aesar, GermanyQ3 )were weighted with Nd2O3 (purity 499.99%, Alpha Aesar)to reach, after thermal treatment, the composition Y2.94
Nd0.06Al5O12. The respective proportions correspond to 42.7,55.6, and 1.7 wt% of a-Al2O3, Y2O3, and Nd2O3.
Nanosized silica particles (purity 499.8%, Alpha Aesar)were added with contents ranging from 0.05 to 0.3 wt%. Afterball milling in water including an organic dispersant, the powderwas shaped by cold uniaxial pressing. Calcination under air wascarried out to remove organic wastes.
(2) Sintering Conditions
Sintering was conducted in a tungsten mesh-heated furnace un-der vacuum (PT r10�2 Pa) for 15 min at temperatures rangingbetween 1473 and 1973 K. The specimens were placed in an al-umina crucible. Firstly, the sintering temperature was reachedaccording to a 10 K/min heating rate. Then the plateautemperature was maintained for 15 min before cooling at a17 K/min rate.
(3) Characterization of Sintered Samples
The thermal behavior of the green pellets was examined witha differential thermal analysis equipment (DTA, Labsys 1600type, Setaramt, FranceQ4 ), by heating up to 1723 K at 15 K/minunder air. The DTA crucibles (capacity of 20 mL) are made ofalumina.
In the same way, the size variations due to sinteringwere measured with a thermal mechanical analyzer (TMA 92,Setaramt) operated using a 5 K/min heating rate up to 1873 Kunder air or vacuum.
To characterize their microstructures, sintered samples werepreviously polished and thermally etched under air or undervacuum at temperatures lower than its sintering temperature by100 K.
After cooling, microstructural investigations were performedusing different techniques combining scanning electron micros-copy (SEM, Philipst XL30 and FEG-SEM, JEOLt 7400)Q5 andtransmission electron microscopy (TEM, JEOLt 2010 andPhilipst CM20 STEM). For TEM characterizations, specimenswere prepared by cutting samples into 3 mm diameter thin disks.These disks were ground, dimpled, and finally thinned toperforation by Ar milling from the substrate side towardthe coating in a Gatan Precision Polishing System (PIPS 692Q6 )operated at 4 kV.
The chemical compositions of secondary phases were deter-mined by electron probe microanalysis (EPMA, CamecatSX100Q7 ).
The crystalline phases present after sintering were indexed byX-ray diffraction (XRD) using a D5000 SiemenstQ8 apparatusequipped with a copper anticathode (lCu5 0.1541 nm). Thephases were indexed using the Diffract1software and more spe-cifically the PDFmaint database.
III. Results and Discussion
(1) Determination of the Reaction Sequence
(A) ‘‘Pure’’ YAG: According to the results obtained onAl2O3/Y2O3/Nd2O3 mixtures prepared to the stoichiometricproportions reported in Section II(1), the sequence leading toYAG includes three successive transformations under vacuum.This sequence has already been reported in previous studies.1,16
The first of them can be written as
2Y2O3 þAl2O3 ! Y4Al2O9 (2)
where Y4Al2O9 is denoted by YAM due to the monoclinic crys-talline symmetry. This transformation occurs in the temperaturerange 1173–1373 K according to the literature.17 The secondtransformation, which appears from 1373 up to 1523 K, corre-sponds to the YAP formation (YAlO3—perovskite structure):
Al2O3 þY4Al2O9 ! 4YAlO3 (3)
This transformation is generally accompanied by a significantdensification as shown in Fig. 1(a) (denoted as ‘‘A’’). For sinte-ring temperatures higher than 1523 K, the rest of Al2O3 reactswith the YAP phase to form YAG (denoted as ‘‘B’’ in Fig. 1(a))according to
Al2O3 þ 3YAlO3 ! Y3Al5O12 (4)
This transformation starts in the vicinity of 1573 K whereasbeyond 1673 K, no other phase is detected (Fig. 2). The shrink-age zone (denoted as ‘‘C’’) reported in Fig. 1(a) corresponds tothe densification of YAG.
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Fig. 1. Thermal mechanical analyzer thermogram under air (a) andunder vacuum (b) of a yttrium aluminum garnet doped by neodymiumsample without silica (heating rate 5 K/min).
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A swelling appears in the vicinity of 1623 K undervacuum and air (Figs. 1(a) and (b)). This phenomenoncould be explained by the presence of two competitive processessuch as:
(1) the YAP-YAG transformation, which is accompaniedby a significant density decrease between the perovskite (5.35 g/cm3), and the garnet phases (4.56 g/cm3); and
(2) the sintering of YAG phase, which is usually character-ized by a shrinkage effect.
From theoretical considerations that take into account theresidual alumina quantities given by Eq. (3), it becomes possibleto calculate the sample volume variations due to the YAP-YAG reaction. Indeed, XRD patterns were registered for sam-ples sintered at 1573 and 1673 K, respectively. Then, accuratemeasurements of the relative abundance of alumina, YAP andYAG phases, present in the sintered samples, were carried outusing Rietveld’s method implemented in TOPAS 3 program.18
From both the mass fractions thus obtained and the densityvalues of each phase, the volume variation associated with theYAP-YAG transformation (i.e., between 1573 and 1673 K)can be computed. Under these conditions, the volume variationdue to the structure change between YAP and YAG may beassociated with a swelling effect that would reach 0.5%. Thislatter value is higher than the experimental data (0.25%) givenby TMA and reported in Fig. 1(a), confirming the presence ofcompetitive processes.
Moreover, it is interesting to notice the effect of the oxygenpartial pressure (i.e., the sintering atmosphere) on the swellingamplitude. Indeed, it appears that the swelling effect is smaller(0.05%) during sintering under vacuum (Fig. 1(b)) in compar-ison with similar treatments under air (Fig. 1(a)). The decreaseof the oxygen partial pressure would promote the lattice diffu-sion of the oxygen anion, which is usually considered as the rate-limiting process of the YAG densification.19,20
After thermal treatment up to 1873 K, porosity is not fullyeliminated because the linear shrinkage of samples does not ex-ceed 15%. Specimens treated in air exhibit a similar shrinkagecurve (see Fig. 1(b)): no change and no shift are detected incomparison with the size variations reported previously forNd:YAG specimens sintered under vacuum.
(B) Nd:YAG Containing SiO2: The DTA thermograms(Fig. 3) obtained on silica–free or silica–containing samples ex-hibit some differences. Except for the wide exothermic peak(denoted ‘‘A’’), characteristic of the organic binder removal (be-tween 400 and 900 K), the thermogram of Nd:YAG doped with0.3 wt% silica differs by the presence of a weak endothermicpeak (denoted ‘‘C’’) at 1663 K. The exothermic peak recorded at1575 K (denoted ‘‘B’’) could be attributed to the YAP-YAGtransformation by comparison with the previous dilatometricanalysis (Fig. 1).
Moreover, the shrinkage curves of Nd:YAG samples con-taining several amounts of silica are reported in Fig. 4. Thetemperature at which shrinkage starts, determined from these
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C: α-Al2O3 P: YAPG: YAG Y: Y2O3M: YAM
2 theta (°)
Fig. 2. Examples of X-ray diffraction patterns for samples sintered be-tween 1473 and 1673 K. The phases identified are: C (a-Al2O3), G(YAG: Y3Al5O12), M (YAM: Y4AlO9), P (YAP: YAlO3), and Y (Y2O3).
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Fig. 4. Thermal mechanical analyzer thermogram of yttrium aluminumgarnet doped by neodymium samples performed under vacuum and sil-ica content (heating rate 5 K/min).
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Fig. 5. Relative density of yttrium aluminum garnet doped by neo-dymium samples sintered under vacuum versus sintering temperature asa function of silica content.
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curves (about 1713 K), does not seem to vary in the presence ofsilica. Nevertheless, the shrinkage rate appears to be affected bythe silica: when Nd:YAG ceramics contains 0.3 wt% SiO2, itincreases drastically. For samples containing silica, especially atthe highest content (i.e., 0.3 wt%), swelling occurs in the vicinityof 1660 K. This phenomenon can be correlated to the endo-thermic effect detected by DTA at 1663 K (Fig. 3).
The presence of both thermal and dilatometric effects at 1660
K can be interpreted based on the ternary phase diagram Al2O3–Y2O3–SiO2. Indeed, this temperature range corresponds to theeutectic valley in the phase diagram,21–23 which exhibits a eu-tectic composition that melts at around 1658 K.
The dilatometric analyses performed at 1623 K for samplescontaining silica (Fig. 4) do not show a swelling effect similar tothat recorded for free-silica samples (see Section III(1)(A)).Previous studies24 indicated that the YAP-YAG transforma-tion should be shifted to a lower temperature in the presence ofsilica during sintering under vacuum. Consequently, this lattertransformation would not interfere with the YAG densificationand no swelling effect should be detected.
The variation of density for YAG samples, once sintered atseveral temperatures under vacuum, has been reported in Fig. 5versus soaking temperature according to the silica content. Asimilar behavior for the whole set of samples is observed, withinexperimental errors, except for the largest silica content (i.e., 0.3wt%). For this latter sample, the shrinkage is somewhat favoredfor sintering temperatures in the range 1673 and 1873 K. Nev-ertheless, the final relative density remains smaller than 88%recorded for all the samples. This result is probably due to in-sufficient soaking times (r15 min) even if temperatures as highas 1973 K were used.
From these preliminary results, it appears that silica additionplays a role in Nd:YAG sintering between 1673 and 1873 Kimplying a microstructural study. This latter work is required toobtain details concerning the formation of secondary phasesthat could lead to a liquid phase.
(2) Microstructural Evolution
The microstructural evolution of samples has been examined bytaking into account the effects of silica content, temperature,and sintering atmosphere.
(A) Heat Treatment in Air: After annealing in air, chem-ical compositions were determined by EPMA. Figure 6(a)presents some characteristic microstructures of Nd:YAG sam-ples after treatment at several temperatures (1673, 1773, and1973 K) and for the same silica content. Hence, at a lower tem-perature (i.e., 1673 K), three phases were identified when 0.3wt% of SiO2 was added. The first zone contains a silica–basedphase (denoted (1a)), the second one contains a silica–alumina–yttria mixed phase (denoted (2a)), and the third zone is a YAGmatrix (denoted (3a)). The chemical compositions are reportedin Table I. When the sintering temperature increases (1773 K), itis observed that only the composition of the mixed phase evolvesto the silica–rich corner in the Al2O3–SiO2–Y2O3 system(Fig. 6(b) and Table I). This evolution could be explained con-sidering the progressive dissolution of the residual silica into theprevious eutectic liquid phase while heat treatment is main-tained. SEM observations of samples treated at the highestsintering temperature (i.e., 1973 K) have been reported inFig. 6(c) and Table I. It is important to note both the drasticdecrease of the secondary phase volume fraction and the pre-cipitation of some alumina particles (Fig. 6(c)). Globally, thesecondary phases, especially the starting liquid phase, tendto disappear at a high temperature while alumina-rich phasesappear.
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1a
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Fig. 6. Scanning electron microscopic observations of yttrium alumi-num garnet doped by neodymium samples with 0.3 wt% SiO2, thermallyetched under air after sintering at: 1673 K (a); 1773 K (b); 1973 K (c).The corresponding chemical analyses have been reported for differentzones in Table I.
Table I. Chemical Compositions of Secondary Phases Deter-mined by EPMA and Observed in Figs. 6(a–c)
Temperature (K) Zones (see Fig. 6) Chemical composition
1673 (1a) 20 SiO21Al2O3
(2a) 8 SiO215 Y2O312 Al2O3
(3a) Nd0.06Y2.94Al5O12
1773 (1b) 20 SiO21Al2O3
(2b) 18 SiO219 Y2O31Al2O3
(3b) Nd0.06Y2.94Al5O12
1973 (1c) Nd0.06Y2.94Al5O12
(2c) Al2O3
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(B) Sintering Under Vacuum: To compare the micro-structural evolutions after sintering in different atmospheres,several samples were annealed under vacuum (same pressure asfor sintering) at the same heating rate as that used previously.
The SEM observations of Nd:YAG samples, treated at 1973K under vacuum, containing different silica contents, providevaluable information regarding the grain growth process. Inparticular, it is possible to attain normal grain growth when ahigh silica content ( � 0.1 wt%) is introduced previously (seeFigs. 7(a) and (b)). Indeed, the YAG grain size (�3 mm) remainshomogeneous whatever the zone examined. Consequently, thesamples sintered after addition of a small amount of silica (i.e.,0.05 wt%) (Fig. 7(c)) are characterized by the appearance ofsome large grains sporadically dispersed in the microstructure.This abnormal grain growth could be attributed to the low vol-ume fraction of liquid phase that would be formed during sinte-ring.
The use of small silica amounts yields some limitations inobtaining samples with well-dispersed silica. Under these con-ditions, the abnormal grain growth would be located especiallyin silica-poor zones as confirmed by the observation of theNd:YAG heterogeneous microstructures obtained without silica(Fig. 7(d)).
Figure 8 depicts the microstructure of samples sintered at1673 K (Fig. 8(a)) and 1773 K (Fig. 8(b)) and containing 0.3wt% of SiO2. Several representative zones can be observed: thefirst one (denoted (1a) and (1b)) is composed mainly of aNd:YAG phase, the second one (denoted (2a)) is made of al-umina, and the last one is mainly composed of silica (denoted(3a) and (2b)).
EPMA investigations were difficult to perform due to thesmall size of the secondary phases. Nevertheless, several com-positions determined for the larger and annotated zones couldbe obtained and are reported in Table II. No significant fluctu-ations concerning the chemical compositions of the main phaseswere detected under vacuum and under air. From these results,it should be reasonable to link the disappearance of the AlxYy-
SizOt ternary phases to the precipitation of alumina particles atthe higher temperature, i.e. to their thermal decomposition.
TEM characterizations of samples treated up to the higher
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10 µm
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10 µm 10 µm
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Fig. 7. Scanning electron microscopic observations of yttrium aluminum garnet doped by neodymium samples thermally etched under vacuum aftersintering at 1973 K, containing: 0.3 wt% SiO2 (a); 0.1 wt% SiO2 (b); 0.05 wt% SiO2 (c); 0% SiO2 (d).
1a
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Fig. 8. Scanning electron microscopic observations of yttrium alumi-num garnet doped by neodymium samples with 0.3 wt% SiO2, thermallyetched under vacuum after sintering at 1673 K (a), 1773 K (b).
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sintering temperature (i.e., 1973 K) and containing 0.3 wt%silica reveal the following:
(1) The absence of an intergranular vitreous and continuousfilm as suggested by high-magnification TEM micrographs(Fig. 9(a)).
(2) Some intragranular spherical inclusions (Fig. 9(b)) forwhich no quantitative analysis could be performed due to thesmall size (o1 mm).
(3) The presence of residual pockets of secondary phases attriple points (Fig. 9(c)) whose chemical composition corre-
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Table II. Chemical Compositions of Secondary Phases Ob-served in Fig. 8
Temperature (K) Zones (see Fig. 7) Chemical composition
1673 (1a) Nd0.06Y2.94Al5O12
(2a) Al2O3
(3a) SiO215 Y2O319 Al2O3
1773 (1b) Nd0.06Y2.94Al5O12
(2b) 4 SiO212 Y2O31Al2O3
(d)(a)
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28
46
3
51
22O Kα
Al Kα
Si Kα
Y Kα
Nd Kα
5 nm
G1
G2
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1 µm
Fig. 9. Transmission electron microscopic observations of a yttrium aluminum garnet doped by neodymium sample containing 0.3 wt% SiO2, sinteredat 1973 K under vacuum: analysis of the grain boundary (a); micrograph of an intragranular phase (b); micrograph of an intergranular phase (c); and thecorresponding EDXS analyses of this phase at triple point (d).
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sponds to silica as indicated by EDXS analyses (Fig. 9(d)).These inclusions are also characterized by nanosized crystalsemerging in the vitreous phase.
Further investigations carried out at lower silica contents(0.05 and 0.1 wt%) led to similar conclusions concerning thedistribution of the secondary phases, their composition, andmorphology.
IV. Discussion
The present results pointed out the complex mechanism of theNd:YAG sintering in the presence of silica aids. On an oxidizingatmosphere, a liquid phase appears at about 1660 K. This phaseprobably results from the direct reaction between YAG and sil-ica. To check this hypothesis, we used some starting mixtureswith a commercial Nd:YAG powder (purity 499.8%, MarionTechnologies, FranceQ9 ) in which the experimental conditions(ball milling, heating, cooling, nature of binder) all remainedsimilar to those used for reactive oxide mixtures. The corre-sponding DTA pattern (Fig. 10) showed that the silica effect isposterior to the YAG formation: the comparison between the-rmograms obtained from either Nd:YAG/SiO2 or Al2O3/Y2O3/Nd2O3/SiO2 reactive mixtures revealed similar thermal effects,except that detected between 1200 and 1600 K due to YAM,YAP, or YAG successive formation from oxide mixtures.
The composition and melting point of secondary phases seemto be in good agreement with predictions made from the Al2O3/Y2O3/SiO2 phase diagram.22,23,25–27 The compositions of thesecondary phases detected surround the quasi-ternary eutectic,which melts at 1660 K (Fig. 11). The corresponding invariantequilibrium can be written as follows:
Y2Si2O7 þ SiO2ðtridymiteÞ þ ð3Al2O3; 2SiO2Þ� ðmulliteÞ¼ liquid (5)
In the presence of a liquid phase, the particle rearrangementbecomes easier and mass transport by grain boundary diffusiontakes place at a lower temperature, leading to enhanced dens-ification kinetics.
During cooling, the residual liquid phase yields glasses withgood thermal stability, especially when their compositions aretaken from the eutectic (Eq. (5)). This fact was well demonstrat-ed in the literature from mixing enthalpies calculations fordifferent glass compositions in an Al2O3–Y2O3–SiO2 system.27
Finally, at room temperature, the glass devitrification yields two
recrystallized phases: alumina and mixed phases, which belongto the Y2Si2O7–Al2O3 isoplethal sections.
The sintering atmosphere (air or vacuum) does not play amajor role in the sequence of reactions leading to fully denseNd:YAG samples. The intermediate phases display the samechemical compositions after firing under vacuum as those firedunder air. The difficulties encountered in carrying out quantita-tive analyses of secondary phases after sintering under vacuumwere linked to the decrease of the vitreous phase volume frac-tion. These difficulties probably implied several reasons:
(1) First, a dynamic vacuum must favor the kinetic of theliquid phase volatilization at a high temperature.
(2) Second, the crystallization of glass seems to be slowerwhen cooling is operated under vacuum,28 which makes the an-alyses hazardous.
V. Conclusion and Outlooks
This work highlights the sintering mechanism of Nd:YAG ce-ramics that is sensitive to the amount of silica added and to at-mosphere (air or vacuum). After formation of the garnet phaseby successive reactions leading successively to YAM, YAP, andYAG, the silica effect is accompanied by the liquid-phase ap-pearance whose composition obeys the eutectic in an Al2O3–Y2O3–SiO2 system. The sequence of reactions seems to be sim-ilar whatever the sintering atmosphere.
The densification rate is significantly enhanced by both in-creasing the sintering temperature (above 1700 K) and the silicacontent. The silica addition (for low contents, i.e., 0.05 wt%)plays a benefic role in the grain growth: indeed, SiO2 inhibitsexaggerated coalescence, which is well known to alter the trans-parency of Nd:YAG samples even at a high sintering tempera-ture.1
The role of the atmosphere remains significant during coolingbecause the stability of glasses would depend on the oxygenpartial pressure. Indeed, the devitrification transformation be-comes more difficult under vacuum.
Further fine microstructural characterizations and dilatomet-ric investigations under isothermal conditions for Nd:YAG ce-ramics, in the presence of silica, are required for sinteringtemperatures ranging between 1600 and 1800 K. These data al-low to correlate the nature, and the distribution of the secondaryphase at the grain boundary to the mechanism of grain growthand densification.
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−3
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−1
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1
2
3
4
5200 400 600 800 1000 1200 1400 1600 1800
Al2O3 / Y2O3 / Nd2O3 / SiO2YAG:Nd / SiO2
Temperature (K)
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ic
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1663
1573
B
C
Weak endothermic
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Fig. 10. Differential thermal analysis equipment experiments for stoichio-metric reactive Al2O3/Y2O3/Nd2O3/SiO2 and Nd:YAG/SiO2 mixtures. Thesilica content is equal to 0.3 wt% for the two tests performed under air at aheating rate of 15 K/min.
Fig. 11. Liquidus surface for the ternary system Y2O3–SiO2–Al2O3
from Cock et al.21 The reported compositions correspond to the sam-ples sintered between 1473 and 1873 K and then thermally etched in air(see Fig. 6): (8 SiO215 Y2O312 Al2O3) (1673 K) �; (18 SiO219Y2O31Al2O3) (1773 K) m; (4 SiO212 Y2O315 Al2O3) (1873 K) ~.
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Acknowledgments
The authors thank Mr. Johan Ravaux and Dr. J. Ghanbaja (Service Communde Microscopie, UMR CNRS 7555, Nancy, France) for the help during theEPMA experiments and for TEM analyses, respectively.
The authors are also grateful to Dr. Daniel Tetard and Dr. Bernard Soulestin(SPCTS, UMR CNRS 6638, University of Limoges, France) for their technicalassistance during sintering treatment and for the preparation of TEM samples,respectively.
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