fast combustion synthesis and characterization of yag:ce+3 garnet nanopowders

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Fast combustion synthesis andcharacterization of YAG:Ce3þ garnet nanopowders

Andrzej Huczko*,1, Magdalena Kurcz**,1, Piotr Baranowski1, Michał Bystrzejewski1, Ajaya Bhattarai1,Sławomir Dyjak2, Rita Bhatta3, Balram Pokhrel3, and Bhim Prasad Kafle3

1 Department of Chemistry, Warsaw University, ul. Pasteura 1, 02-093 Warsaw, Poland2 Institute of Chemistry, Military University of Technology, ul. gen. Sylwestra Kaliskiego 2, 00-908 Warsaw, Poland3 School of Science, Kathmandu University, 6250 Dhulikhel, Nepal

Received 15 April 2013, revised 25 July 2013, accepted 10 September 2013Published online 10 October 2013

Keywords combustion synthesis, nanopowders, YAG:Ce3þ garnet

* Corresponding author: e-mail [email protected], Phone: þ48 22 822 02 11, Fax: þ48 22 822 59 96** e-mail [email protected]

Cerium-doped yttrium aluminum garnet can be used in whitelight emitting diodes and lasers. The YAG powders aretraditionally formed by the solid state method, but the productparticles are large, irregular, and non-homogeneous, which isdetrimental for its luminescence properties. We present here asimple and fast method for synthesizing YAG:Ce3þ nano-powder based on solution combustion synthesis from metal

nitrates (Ce content between 0.7 and 35wt%) and fuel (urea,starch, or glucose) water mixtures. The calcination of the rawproduct at 900 8C for 2 h yielded crystalline garnet nanopowderwith grain size well below 100 nm. The properties of the finalproduct were characterized by XRD, SEM, ATR, TG/DTA, andspectrofluorimetric measurements.

� 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Garnets are cubic crystals with acomplicated structure of three different cationic sites [1].They have been used for many years as laser crystals(neodymium:yttrium aluminum garnets, YAG:Nd), foroptical communications (yttrium indium garnets, YIG),high-power laser machinery (terbium gallium garnets,TGG), etc. Rare earth doped YAG single crystal phosphorsexhibit an intense and broad luminescence in the visibleregion (yellow) when excited with blue light. Cerium-dopedgarnets YAG:Ce (embedded in the resins covering bluelight emitting diodes (LEDs) are commercially used forthe production of white LEDs [1]. The YAG powder istraditionally formed by the solid state method, but theproduct particles are large, irregular, and non-homogeneous,which is detrimental for its luminescence properties.

We propose here the combustion synthesis self-propagating high-temperature synthesis (SHS) of YAG:Ce3þ garnet nanopowders. SHS is an elegant route for thefast and efficient production of various materials [2]. Thistechnique has been also widely used to produce novelnanomaterials [3]. Lima et al. synthesized Fe–Mo/Mgcatalysts used in single-walled carbon nanotube productionby solution combustion synthesis, using both batch [4] and

continuous [5] modes of operation. The efficient SHSproduction of silicon carbide nanowires (SiCNWs) has beenalso reported by Huczko et al. [6, 7]. Dyjak et al. [8] obtainednanostructured powders of neodymium-doped yttrium alumi-num garnet (YAG:Nd) while heating an aqueous solutions ofaluminum, yttrium, and neodymium nitrates with glucose,dextrin, and starch. Luminescent properties of YAG:Cephosphor, formed by combustion from mixed metal nitratereactants and urea, were investigated by Yang et al. [9]. AlsoBiswas et al. [10] obtained ceria-doped YAG by urea–formaldehyde polymer gel auto-combustion process.

Here, we propose the solution combustion synthesis,during which salts of metallic components are dissolved inwater, simultaneously with an organic fuel (combustibleagent) [11]. Usually, the nitrates are used to oxidize the organiccompound when water is evaporated and the temperature ofignition the combustion is reached. In this article, theformation and the characterization of Ce-doped YAG garnetnanopowders prepared by the solution combustion synthesisis reported. Four types of simple combustible agents for acombustion were compared, with two different Ce concen-trations in the final product. In the starting aqueous solutionof reactants, carbohydrates trap the metal ions. Thus, the

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homogeneity of their space distribution and reduction ofdiffusion barrier (as in a case of solid state reaction) areguaranteed. Amorphous Y-Al oxides containing metalcarboxylates and carbonates, along with un-reacted startingcomponents, are the raw products of the reactions. Theyundergo a transformation into monophase YAG-Ce crystal-lites during the following high-temperature calcination. Thefinal garnets were characterized using different techniquesand their properties were evaluated.

2 Experimental procedure The following metalcompounds, Y(NO3)3 · 6H2O and Al(NO3)3 · 9H2O, wereused as the sources of Y3þ and Al3þ, respectively.Ce(NO3)3 · 6H2O was used as the source of Ce, which isthe excitant of YAG:Ce. To prepare Y(NO3)3 · 6H2O, ittriumoxide was hot-plate boiled in conc. HNO3 (70%) till thesolution became transparent. Ittrium nitrate crystallitesappeared upon cooling and were dried in oven at 75 8Cover night.

2.1 Synthesis of YAG:Ce by solution combustionof nitrates with reductants The metal nitrates with thedopant were mixed with different reductants (urea, glucose,and starch) and weighted in an appropriate stoichiometricratio to get the desired molar proportion. The startingmixtures were dissolved in the crucible with 30ml of water.Regarding cerium addition in the final garnet, two different,low and high content, compositions for combustion wereprepared: 0.7wt% Ce3þ and about 35wt% Ce3þ. In order toverify the effect of the fuel, four different combustible agentswere chosen: urea, glucose, soluble, and insoluble starch.The amount of the fuel added to this final mixture (Table 1)was chosen based on the criteria of the stoichiometriccombustion represented (for glucose, as an example) as:

5AlðNO3Þ3 þ 2:97YðNO3Þ3 þ 0:03CeðNO3Þ3þ 5C6H12O6 ! 2:5Al2O3 þ 1:485Y2O3

þ 0:015Ce2O3 þ 30CO2 þ 30H2Oþ 12N2 þ 6O2:

The initiation of the combustion was carried out in anelectric muffle oven in air atmosphere. A 400ml quartzevaporating dish was used as a mixture container. The waterunder heating is rapidly evaporated, and the vigorouschemical reaction proceeds. The raw product was collectedfor analyses. In order to further eliminate un-reacted startingcomponents and any organic residue (by-products andamorphous carbon), the raw product was calcined at 900 8Cfor 2 h in air to produce final garnet nanopowders.

2.2 Characterization The products were character-ized by using powder X-ray diffraction collected by BrukerD8 Discover using Cu Ka radiation at 40 kV and 40mA anda linear detector VANTEC. SEM images were observed withSEM Model Ultra Plus. The ATR technique (Nicolet iS10)was used to measure the sample powder spectrum andidentify chemicals present in products. The TG/DTA

analysis was carried out using a LabSys SETARAM.Spectrofluorimetric measurements were run on Perkin ElmerSL-55 instrument with an excitation line 350 nm.

3 Results and discussion The combustion reaction,characterized by swelling of the reacting mixture, wasclearly observed for all of the starting compositions attemperatures between 350 and 500 8C. The process, asshown in Fig. 1, was accompanied by the release of hugeamount of gases (see equation above). As a result, the bulkloose powder of precursors, i.e., aerogel was formed. Anapparent density of the products was strikingly low. It couldbe related to the large amount of gaseous products formedduring the combustion, which generated porous structureclearly observed in the samples. The raw product wassubjected to the thermal treatment in a muffle furnace at900 8C. As a result, we obtained the finely dispersed powderof cerium ions-doped YAG of yellow color, which wasfurther analyzed following the listed above protocol.

3.1 X-ray diffraction The solid products werestructurally characterized by powder XRD. Figures 2and 3 show the examples of the XRD spectra patterns forthe as-synthesized precursors after combustion and the final

Table 1 Composition of starting reactants.

reagents for 0.7wt% Ce for 35wt% Ce

urea as a fuelAl(NO3)3 · 9H2O 3.002 g 2.996 gUrea 0.479 g 0.607 gCe(NO3)3 · 6H2O 0.019 g 1.56 gY(NO3)3 · 6H2O 1.833 g 1.823 g

weight of raw product before calcination: 1.06 g (for 0.7wt% Ce)and 1.683 g (for 35wt% Ce)

glucose as a fuelAl(NO3)3 · 9H2O 3.00 g 3.00 gGlucose 1.4412 g 1.8304 gCe(NO3)3 · 6H2O 0.02 g 1.54 gY(NO3)3 · 6H2O 1.82 g 1.82 g

weight of raw product before calcination: 1.186 g (for 0.7wt% Ce)and 1.971g (for 35wt% Ce)

soluble starch as a fuelAl(NO3)3 · 9H2O 3.00 g 3.00 gsoluble starch 1.301 g 1.64 gCe(NO3)3 · 6H2O 0.022 g 1.546 gY(NO3)3 · 6H2O 1.83 g 1.82 g

weight of raw product, i.e., before calcination: 1.214 g (for 0.7wt%Ce) and 1.999 g (for 35wt% Ce)

insoluble soluble starch as a fuelAl(NO3)3 · 9H2O 3.00 g 3.00 gInsoluble starch 1.3 g 1.64 gCe(NO3)3 · 6H2O 0.02 g 1.54 gY(NO3)3 · 6H2O 1.82 g 1.808 g

weight of raw product before calcination: 1.74 g (for 0.7wt% Ce)and 1.957 g (for 35wt% Ce)

2 A. Huczko et al.: Fast combustion synthesis of YAG:Ce3þ garnet nanopowders

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garnets after calcination, respectively. Table 2 presents thelisted diffraction patterns for all compositions.

The crystallinity of the raw products was generally verylow (Fig. 2). Occasionally, the diffraction patterns ofcrystalline CeO2 (as a by-product [12]) were spotted in a caseof higher dopant content while the majority of raw productswas amorphous. From the data of the XRD analysis, it wasfound that the basic phase of the powder obtained aftercalcination (Fig. 3) is the sought, cubic garnet with chemicalcomposition Y3Al5O12 (JCPDS card No. 33-40 [13]). Thisindicates a direct transition from the amorphous phase to the

crystalline phase. Clearly, the crystal state of Y3Al5O12 hasbeen completely formed at 900 8C. Occasionally, a secondphase, hexagonal YAlO3, appeared in the products of YAGsynthesis, because of the non-homogeneous composition ofthe YAG precursor [14]. No differences for the formation ofcrystalline YAG phase could be observed in the samplesdoped and un-doped with Ce3þ, indicating that some sites ofthe Y3þ could be replaced by the Ce3þ, based on theiridentical valence and similar radius [15]. Neither a kind ofthe combustible agent nor the dopant content seems to havea remarkable effect over the final garnet composition.Apparently, 2 h calcination at 900 8C allows for theformation of well crystallized Ce-doped Y3Al5O12 garnet.

3.2 Scanning electron microscopy SEM micro-graphs of selected samples show distinct differences betweenraw products (Fig. 4) and final calcined garnets (Fig. 5).

The raw product in non-homogeneous, foamy, partiallymelted, and porous agglomeration is observed. Theformation of the porous morphology of the matter is causedby the gases produced during the combustion of a reducer.Melted and further solidified phases of by-products are alsoclearly visible.

From the electron-microscopic photos (Fig. 5) the averagesizes of primary particles of final garnets are determined. Itwas found that the final powders were homogeneous, weaklyagglomerated, and easily dispersed to a state of nanoparticles

Figure 1 Solution combustion synthesis of YAG:Ceþ 3 garnet.

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Figure 2 XRD patterns of selected raw products: Me-nitratesþurea, 0.7wt% Ce; Me-nitratesþ urea, 35wt% Ce; Me-nitratesþglucose, 0.7wt% Ce.

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Figure 3 XRD patterns of selected garnets after calcination: Me-nitratesþ urea, 0.7wt% Ce; Me-nitratesþ urea, 35wt% Ce; Me-nitratesþ glucose, 0.7wt% Ce.

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size of ca 30-60 nm in diameter. The crystallites arediscrete, fairly uniform and slightly elongated particles.The crystallite size corresponds to the estimation obtainedfrom the XRD half-peak width. Thus, the size of nano-crystallites is 2–3 orders of magnitude lower comparing togarnets produced via other routes [16, 17]. This type ofnanostructure should make it easier to obtain dense ceramics.

3.3 Infrared spectroscopy The solid products werealso characterized with ATR technique. Figures 6 and 7present typical spectra of as-synthesized precursors aftercombustion and of the final garnets after calcination,respectively.

Spectra of raw products show a broad band at about 500–1000 cm�1, which indicates inorganic, amorphous structures.Two peaks around 1370 and 1550 cm�1 represent asymmetric and an anti-symmetric vibrational modes ofO–N bonds. A wide band in a range 2600–3700 cm�1

corresponds to O–H bonds of adsorbed water. Aftercalcination neither O–H band nor O–N peaks were visibleand inorganic structures have become much more pro-nounced. Inorganic region can be assigned to Y–O and Al–Obonds. The peaks at about 560 cm�1 and 716 cm�1 reflect Y–O vibrations, while the peaks at about 680 and 780 cm�1

reflect Al–O vibrations. A peak visible at about 520 cm�1 isprobably due to Ce–O vibrations, as it is more intense withan increasing addition of Ce to reactants, it is also visiblein a raw product, what was confirmed by XRD (CeO2).According to IR spectra, the calcination removes allimpurities (H2O, NO groups, which comes from un-reactedstarting materials) and final product is pure garnet, whichmight contain some excess CeO2 as well.

3.4 Thermo gravimetric analyses A thermal oxi-dation behavior of the raw products was examined using the

Table 2 XRD patterns for raw products and powders after calcination.

sampleno.

starting reactants analysed sample XRD patterns

1 Me-nitratesþ urea (0.7wt% Ce) raw product amorphous2 Me-nitratesþ urea (0.7wt% Ce) after calcination Y3Al5O12 cubic

a

3 Me-nitratesþ urea (35wt% Ce) raw product CeO2 cubicb

4 Me-nitratesþ urea (35wt% Ce) after calcination Y3Al5O12 cubicb; CeO2 cubic

b

5 Me-nitratesþ glucose (0.7wt% Ce) raw product amorphous6 Me-nitratesþ glucose (0.7wt% Ce) after calcination Y3Al5O12 cubic

a; YAlO3 cubicc; CeO2 cubic

c

7 Me-nitratesþ glucose (35wt% Ce) raw product amorphous8 Me-nitratesþ glucose (35wt% Ce) after calcination Y3Al5O12 cubic

b; YAlO3 hexagonalc; CeO2 cubic

a

9 Me-nitratesþ soluble starch (0.7wt% Ce) raw product amorphous10 Me-nitratesþ soluble starch (0.7wt% Ce) after calcination Y3Al5O12 cubic

a; CeO2 cubicc

11 Me-nitratesþ soluble starch (35wt% Ce) raw product amorphous12 Me-nitratesþ soluble starch (35wt% Ce) after calcination YAlO3 hexagonal

a; CeO2 cubica

13 Me-nitratesþ insoluble starch (0.7wt% Ce) raw product amorphous14 Me-nitratesþ insoluble starch (0.7wt% Ce) after calcination Y3Al5O12 cubic

a

15 Me-nitratesþ insoluble starch (35wt% Ce) raw product amorphous16 Me-nitratesþ insoluble starch (35wt% Ce) after calcination YAlO3 hexagonal

b; CeO2 cubica

aStrong; bMedium; cVery weak.

Figure 4 SEM images of the raw product: soluble starch, 0.7wt%Ce (left); glucose, 0.7wt% Ce (right).

Figure 5 SEM images of the nanostructured YAG:Ce powder(Tcalc¼ 900 8C): soluble starch, 0.7wt% Ce (upper left); urea,0.7wt% Ce (upper right); urea, 35wt% Ce (lower left); glucose,0.7wt% Ce (lower right).



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simultaneous TG/DTA technique to monitor both the weightand temperature changes of the samples. Approximately15mg of sample was heated in an open Al2O3 crucible underan air flow rate of 50mlmin�1 and a heating rate of10 8Cmin�1 from room temperature up to 1000 8C.

The TG curves (Fig. 8) show the weight loses of eachsample in the function of the temperature. The powdersprepared by using urea shows the least weight loss (Table 3)comparing with powders prepared with starch or glucose,which is characterized by one step decomposition (blackline). The samples with starch and glucose show two steps

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Figure 6 ATR spectrum of selected raw products: Me-nitratesþurea, 0.7wt% Ce; Me-nitratesþ urea, 35wt% Ce; Me-nitratesþglucose, 0.7wt% Ce.

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Figure 7 ATR spectrum of selected products after calcination:Me-nitratesþ urea, 0.7wt% Ce; Me-nitratesþ urea, 35wt% Ce;Me-nitratesþ glucose, 0.7wt% Ce.

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Figure 8 Thermogravimetric curves of raw products heated inthe air flow, obtained in reaction of Al, Y, and Ce nitrates withdifferent organic fuels.

Table 3 The total weight loss during calcination of samples up to1000 8C in the air.

the weight loss (%)

urea starch sol. starch insol. glucose

35% Ce �12.0 �20.9 �18.2 �16.80.7% Ce �13.9 �23.6 �17.9 �17.1

Figure 9 DTA profiles of raw products heating in the air flow.



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weight losses. The first mass loss is probably associated witha decomposition of un-reacted fuel (dehydratation). In thesecond step, a major weight loss (�10%) takes place in thetemperature range 500� 1000 8C. This is probably accom-panied by an oxidation process of residual carbon present inthe powder. It can be confirmed by presence of a small,exothermic, broad peak in the temperature range of 500–1000 8C (Fig. 9). The weight of the powders remainsconstant after reaching a temperature around 1000 8C.

During the reaction with nitrates, glucose, and starchpartially decompose with a formation of amorphous carbon,which can be further oxidized at temperature above 500 8C.The major weight loss is due to the burnout of fineamorphous carbon in the form of CO2.

3.5 Luminescence measurements The emissionspectra of raw and calcined products are shown in Figs. 10and 11 for excitation wavelengths of 350 and 475 nm,

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Figure 10 Emission spectra of as-produced and calcined YAGs,excitation wavelength 350 nm.

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Figure 11 Emission spectra of as-produced and calcined YAGs,excitation wavelength 475 nm.



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respectively. There are no significant differences betweenspectra of different reactants. The raw product and thematerial after calcination show almost the same spectra,the difference is in width of the bands. After calcinationthe spectrum is wider (450–600 nm) (excitation wavelength is 350 nm). However, a wavelength shift (red-shift) can also be responsible for the observed feature.The Ce concentration influences the intensity of thespectrum. The most prominent band with 350 nmexcitation wave length is around 450–600 nm while for475 nm excitation wave (blue light) it is 600–650 nm.It provides a perspective of use of this material in GaNLEDs (it matches the blue emission of GaN LED, thewide band 400–500 nm provides a possibility to usethis material for a generation of a white light in GaNLEDs [9, 15]).

As we are aware of the significance of the photo-luminescence properties of YAG, the more-in-depth studyof optical properties is still in progress, relying also oncathodoluminescence measurements.

4 Conclusions A new method for a fast and largescale production of optically active, pure YAG:Ce3þ

garnet was developed. The solution combustion synthesisis a simple and inexpensive technique that producesnanometric garnet powders with high crystallinity. Thissynthesis route provides a potentially new approach to aformation of doped YAG nanopowders. The presentedmethod could be also used for the production of severalkinds of nanostructured oxides.

Acknowledgement The research has been supported by theNCN grant No. UMO-2011/03/B/ST5/03256.

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fast combustion synthesis and characterization of yag:ce+3 garnet nanopowders

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