007) 921–924www.elsevier.com/locate/matlet

Materials Letters 61 (2

Preparation and characterization of nanocrystalline Nd-YAG powder

Rashmi Singh ⁎, R.K. Khardekar, Arun Kumar, D.K Kohli

Target Laboratory, Raja Ramanna Centre for Advanced Technology, PO CAT, Indore, Madhya Pradesh, 452 013, India

Received 4 January 2006; accepted 7 June 2006+Available online 10 July 2006

Abstract

Nanocrystalline materials have assumed notable importance in a wide variety of fields owing to numerous possible applications offered bythem. These include transparent ceramics wherein they facilitate synthesis as well as sintering at significantly lower temperatures. We reportpreparation of nanocrystalline neodymium doped yttrium aluminum garnet (YAG) with an ultimate intent to make transparent Nd-YAG ceramic.The Liquid Mix method employed involves mixing of metal nitrates with excess amounts of citric acid followed by dissolution and polymerizationin ethylene glycol to form complex chelates. Amorphous powder obtained by firing of polymeric chelate compound at 400 °C permits formationof nanoparticles of Nd:YAG at as low a crystallization temperature as 920 °C as shown by the thermal analysis. Progressive evolution of wellcrystallized phase-pure YAG was studied by XRD of amorphous powders subjected to different calcination temperatures. Scanning electronmicroscopic (SEM) study of the crystalline material shows that particle size ranges between 50 and 100 nm.© 2006 Elsevier B.V. All rights reserved.

Keywords: Nanomaterials; Electron microscopy; Powder technology; X-ray techniques

1. Introduction

Neodymium doped yttrium aluminum garnet (Nd:YAG) singlecrystal made by Czochralski method [1] has been the most widelyused solid state laser material during the last four decades with awide variety of applications in medicine, materials processing,military and research. But its fabrication requires expensive equip-ment and crucible material. Moreover, it is extremely difficult tofabricate large diameter crystals or, dope themwith greater than 1%neodymium because of the segregation problem associated withthe latter. Interest in polycrystalline ceramics, suitable as a laserhost dates back to early seventies of the twentieth century whenlasing action was successfully demonstrated with these ceramics[2]. Nd:YAG transparent ceramic has received much attention aslaser host material because of its several advantages [3]. Thesematerials enable fabrication of large (150 mm) diametertransparent discs and high power (1.46 kW laser output) Nd:YAG lasers employing such discs have been reported [4].

Ever since lasing action was demonstrated in Nd:YAGtransparent ceramic in 1995 there has been a significant spurt in

⁎ Corresponding author. Tel.: +91 0731 2442394.E-mail address: rashmi@cat.ernet.in (R. Singh).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.06.013

activities relating to low temperature synthesis of fine powder ofNd:YAG and its sintering at the lowest possible temperatures toobtain transparent ceramic. The solid state synthesis of ceramicfrom their respective oxide powder usually requires extensivemechanical mixing and prolonged heat treatment above 1600 °C[5,6]. The processing conditions do not allow easy control overmicrostructure grain size in the resulting powder. On the otherhand sol-gel method based on molecular precursor allowschemical interactions amongst the initial mixture of precursorspecies favoring the evolution of solid state structure with atomiclevel mixing which results in as low as 700 °C crystallizationtemperatures [7], but the use of alkoxide approach is restricted dueto rather intricate synthesis procedures and limited commercialavailability of various metal alkoxides. Single phase crystalliza-tion temperatures for coprecipitationmethod [8], self-propagatingcombustion [9] and precipitation by urea method [10] are 900 °C,1250 °C and 1200 °C respectively.

Nanocrystalline materials have turned into a class of theirown in recent times with their properties being quite differentfrom those of the corresponding bulk crystalline materials [11].Transparent polycrystalline Nd:YAG is considered to be analternative to single crystal new routes for low-temperaturesynthesis of nano-sized, pure and homogeneous Nd:YAG pow-ders that continue to be explored by the researchers.

Fig. 1. TG/DTA curve of the precursor gel (crystallization peak at 920 °C can beclearly seen in the inset region).

Fig. 2. XRD patterns of the gel heated at 600, 800, and 950 °C.

922 R. Singh et al. / Materials Letters 61 (2007) 921–924

We report here a potential and inexpensivemethod of synthesisof nanocrystalline Nd:YAG powder by Liquid Mix method. Thischelate-polymerization method employing citric acid wasinvented by Pechini more than three decades ago and since thenhas been used to synthesize a wide variety of ferromagnetic,ferroelectric and other mixed-oxide materials [12]. It uses metalsalts or alkoxides and chelating agents to form water solublepolymeric complexes, which can be decomposed to obtain thedesired mixed-oxide phase. Here the cations of metal salts oralkoxides are made to react with certain weak acids to formpolybasic acid chelates. These chelates undergo polyesterificationwhen heated with a polyhydroxyalcohol to form polymeric gel-like compound which has cations uniformly mixed anddistributed. Subsequently the gel is charred at 400 °C and calcinedto obtain crystalline oxide powder. The most important feature ofthis method is formation of amixedmetal citric acid complex witha chosen stoichiometric ratio stabilized in a polyester based resin,which probably helps in the formation of phase-pure YAG.Advantages of Liquid Mix method are that nano-sized, homoge-neously doped phase-pure mixed-oxide powders can be preparedat relatively low crystallization temperatures.

2. Experimental

The starting materials were yttrium nitrate hexahydrate (Y(NO3)3·6H2O, 99.9%, Aldrich), aluminum nitrate nonahydrate(Al(NO3)3·9H2O, 98.5%, Merck), and neodymium nitratehexahydrate (Nd(NO3)3·6H2O, 99.9%, Aldrich). The constituentnitrates taken in stoichiometric proportions were independentlymixed and ground with excess quantities of citric acid and dis-solved in ethylene glycol. Transparent solutions were obtainedafter heating with constant stirring. The quantity of neodymiumnitrate was taken for 3 at.% doping in YAG. All the three trans-parent solutions weremixed and brought to boiling temperature at90 °C. Solution turned light yellow and then deep yellow andfinally brown black after 3 h. Evaporation of solution initiatedpoly-condensation (polyesterification reaction) resulting in res-inous gel which on further heating dried into sticky brown blacklumps. The sticky mass was fired in a resistance muffle furnace

hooked to Eurotherm 2604 programmer-temperature controllerup to 400 °C employing a heating rate of 5 °C/min for 6 h to burnout organics. At 400 °C the material got self-ignited to red-hotcondition (∼800 °C) within lumps because of highly exothermicreactions. Nd:YAG precursor obtained after initial firing wascalcined for 3 h each at temperatures 600°, 700°, 800° and 950 °Cemploying heating rate of 5 °C/min. After every step grinding inagate mortar was done.

The thermogravimetric–differential thermal analysis (TG–DTA) measurements were performed on dried polymeric gelsample employing SETARAM TGA92. The sample was heatedup to 1200 °C in oxygen:argon (1:2) atmosphere at a rate of 5 °C/min using a platinium–rhodium crucible. Powder X-ray diffrac-tion measurements were performed at room temperature on sam-ples subjected to 600, 700, 800 and 950 °C employing aRIGAKU(Geigerflex) diffractometer operating with Cu–Kα radiation. Theinfrared spectra of dried gel and that fired at 400 °Cwere recordedon a FTIR (Perkin Elmer PARAGON 1000).

The particle size of the crystalline powder was measuredwith a scanning electron microscope (Philips XL30CP). Specialcare was taken in preparing samples for SEM, as the calcinationat 950 °C resulted in agglomeration of the powder. Dispersionof the powder in acetone was made by ultrasonically agitatingthe medium for one hour. Graphite holder was dipped indispersion medium and then dried. DC sputtering technique wasused to coat the samples with a gold film of 50 Å to avoidelectrostatic charging during analysis.

3. Results and discussion

The mechanism of thermal decomposition and the crystallizationtemperature of the dried polymeric gel precursor as studied by TG–DTA are shown in the thermal analysis curve depicted in Fig. 1. Sample

Fig. 4. SEM photograph of Nd-YAG powder obtained after calcination at 950 °Cfor 6 h (bright spots are particles).

923R. Singh et al. / Materials Letters 61 (2007) 921–924

with starting weight of 80 mg was loaded in TG–DTA crucible. Thethermogravimetry curve shows that the mass decreased rapidly below600 °C due to removal of hydroxyl/ester/carboxylic and other organicgroups of the polymeric precursor. The thermal decompositionbehavior associated with a very large exothermic peak for the sampleas seen in the differential thermal analysis curve is suggestive of anauto-ignited combustion process, arising from highly exothermicreactions. The exothermic peak at 920 °C (blown up view shown in theinset) corresponds to the crystallization of YAG phase. The peak is verysmall because the mass left after decomposition and removal ofcarbonaceous matter was very small.

Fig. 2 shows the XRD pattern of the YAG powder calcined at 600,800 and 950 °C. The patterns at 600 °C and 800 °C show that thepowder was amorphous with a broad and emerging peak centeredaround the highest intensity peak of YAG phase. The powder calcined at950 °C revealed peaks of phase-pure crystalline YAG. The intensitypeaks are slightly broadened due to line broadening effect as report-ed for nano-sized particles [13]. The addition of 3% neodymium,understandably, did not cause any shift in the peak positions oradditional new peaks due to neodymium oxide. Identical XRD patternshave been reported elsewhere [14] for phase-pure crystalline YAGpowder.

The IR spectra of the dried gel and the powder fired at 400 °C arepresented in Fig. 3. The broad band centered around 3411 cm−1 is due tobonded hydroxyl group stretching frequency of polymeric intra- andintermolecular bonds. Since the starting materials contained significantamounts of hydroxyl groups their signature in the IR spectra wasexpected. The peak represented by 2926 cm−1 corresponds to C–Hstretching frequencies in hydrocarbon groups. The peak at 1734 cm−1 ischaracteristic of carbonyl stretching vibration of aldehydes. Whereas nostarting compounds contained aldehyde group their presence in thepolymerized gel could be attributed to oxidation of alcoholic groupby nitrate ions released by the metal nitrates. Presence of carboxylate

Fig. 3. FTIR spectra of the Nd-YAG precursor gel and powder fired at 400 °C.

anion –COO– group is confirmed by twin bands at 1586 and 1406 cm−1

which correspond to antisymmetrical and symmetrical vibrations of –COO– group. Peak at 1078 cm−1 is due to primary –OH bending. Weobserve a peak at 879 cm−1 representing peroxide linkage which can beattributed to the oxidative reactions of carboxylic acidic groups present. Abroad peak at 605 cm−1 represents the characteristic metal-oxygenvibrations. Slight shift in some of the characteristic frequencies is due topolymeric bonding with heavy metals [15].

Scanning Electron Micrograph of the powder derived aftercalcinations at 950 °C is seen in Fig. 4. Particles are seen as brightspots typically of sizes between 50 and 100 nm on graphite substrate.Particles are scattered and located far from each other with nearabsence of agglomerates due to special care taken during samplepreparation.

4. Conclusion

The present study demonstrates the potential of Liquid Mixmethod to yield phase-pure nanocrystalline Nd-YAG powder ata crystallization temperature <1000 °C which is low as com-pared to the temperature required for solid state and severalother methods of synthesis. The method requires inexpensivereagents. Nanocrystalline Nd-YAG powder synthesized by thismethod can be used for producing highly transparent YAGceramics at low sintering temperatures in view of the largesurface energy associated with nano-sized powder [16]. Theonly disadvantage of the method appears to be requirement ofhandling of very large quantities of reagent in comparison to thefinal mixed-oxide powder generated. The fine and fluffy natureof the powder formation of agglomerates is understandable.

Acknowledgement

We thank Dr. V.S. Tiwari, Dr. S.M. Gupta and Shri IndranilBhaumik of Laser Materials Division, CAT, for providing XRD,FTIR and TG–DTA data for our samples. Thanks are also due toDr. R.V. Nandedkar and Ms. Pragya Tripathi for SEM results.

924 R. Singh et al. / Materials Letters 61 (2007) 921–924

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