OCTOBER 19-20, 2012 - YMCA University of Science & Technology

Proceedings of the National Conference on
Trends and Advances in Mechanical Engineering,
YMCA University of Science & Technology, Faridabad, Haryana, Oct 19-20, 2012
rapid dehydration and foaming followed by decomposition, generating combustible gases. These volatile
combustible gases ignite and burn with a flame yielding voluminous solid. Carbohydrazide was oxidized by
nitrate ions and served as a fuel for propellant reaction. Combustion synthesized nanoparticles were annealed at
different temperatures from 700°C to 900°C in order to know the effect of annealing on the particle size/shape
and luminescence properties.
3. Characterization
The crystal phase of Ca 2 V 2 O 7 :Eu 3+ nanoparticles prepared was characterized by Rigaku Miniflex-II X-ray
powder diffraction with CuKα radiation at 30 kV tube voltage and 15 µA tube current. The particle size and
morphology were evaluated using Jeol JSM-6510 scanning electron microscope (SEM). The excitation and
emission spectra of nanoparticles in the ultraviolet-visible region were obtained by using a Hitachi F-7000
fluorescence spectrophotometer with Xe- lamp at room temperature.
4. Results and discussion
The XRD patterns of Ca 2(1-x) Eu 2x V 2 O 7 (Eu =4 mol %) particles, both as synthesized and annealed at
temperatures 700° and 900°C for 3h are shown in fig.1(a). Calcium pyrovanadate crystallizes in the triclinic
space group P1 and has a structure with two distinct V 5+ sites and a V-O-V bond angle of 124.0°. This resembles
the “dichromate” pyrovanadate structure. The V(1) site in Ca 2 V 2 O 7 includes four V-O bonds (dV-O) (1.70-1.74
Å) in a fairly symmetric tetrahedron, whereas the V(2) site may be considered pentacoordinated with five V-O
distances in the range 1.65- 2.05 Å [21]. All diffraction peaks are in good agreement with the JCPDS data (no.
36-0155) [5]. Ca 2 V 2 O 7 phase formation is initiated in the as-synthesized powder by combustion, which increases
with increasing annealing temperature. No peak corresponding to any of the source materials or allotropic forms
was found after annealing at 900°C (Fig.1) suggesting that a pure compound with the same structure as Ca 2 V 2 O 7
exists. Fig. 1(b) shows the XRD pattern of the sample Ca 2(1-x) Eu 2x V 2 O 7 nanoparticles (Eu =1, 2, 3, 4 mol %)
annealed at 900°C for 3h. From the analysis of XRD, it was revealed that the introduction of an activator (Eu 3+ )
did not influence the crystal structure of the phosphor matrix. The XRD pattern of the Ca 2 V 2 O 7 :Eu 3+ showed the
presence of broad peaks. The broad peaks either indicate particles of very small crystalline size or particles are
semicrystalline in nature.
Fig.2 depicts the typical red photoluminescence from Eu 3+ ions in the Ca 2 V 2 O 7 :Eu 3+ nanoparticles when rooting
the excitation wavelength at 394 nm. It is clear that the PL intensity of the as–synthesized nanomaterials at
500°C, increased rapidly with increase of annealing temperature. This is mainly due to the improvement in
doping and crystallinity. In particular, the most intense emission peak at 613 nm corresponds to 5 D 0 → 7 F 2 and
occurs through the forced electric dipole, while the 5 D 0 → 7 F 1 band at 588 nm is the magnetic dipole transition
[14].The emission spectrum is dominated by 5 D 0 → 7 F 2 hypersensitive transition (∆ J=2), which is because the
Eu 3+ is located at a low symmetry local site in the Ca 2 V 2 O 7 host lattice. Moreover, the splitting number of
5 D 0 → 7 F j transitions can provide information of the surroundings of the Eu 3+ ions and the site symmetry of Eu 3+
ion with a maximum number of lines “2J+1” for each lattice site [22]. Unique 5 D 0 → 7 F 0 (581 nm) transition
indicates that the Eu 3+ ions occupy single site in Ca 2 V 2 O 7 lattice.
Generally, the luminescence properties of nanomaterials depend on the activator concentration and crystallinity.
Dependence of the emission intensity of europium ions upon the doping concentration (x) in the crystalline Ca 2(1-
x)Eu 2x V 2 O 7 (900°C annealed) excited by 394 nm is shown in fig. 2(b). It is found that the PL emission intensity
increased with the increase in the concentration of Eu 3+ , reaching a maximum value with 4 mol % doping of
Eu 3+ . Usually, an over-doping concentration results in the enhancement of non-radiative relaxation between the
neighboring Eu 3+ ions which indicates the concentration quenching.
Photoluminescence excitation spectrum of Ca2V 2 O 7 : Eu 3+ sample (fig.2c), include a broad peak centered at 305
nm followed by a series of peaks beyond 390 nm. The dominant broad peak is ascribed to charge transfer band
(CTB) which corresponds to an electron transfer from an oxygen 2p orbital to an empty 4f orbital of europium
ions ( O 2- → Eu 3+ ). The sharp lines in the range above 390 nm are intra-configurational 4f-4f transitions of Eu 3+
in the host lattices, peak with maxima at ~ 395 nm ( 7 F 0 → 5 L 6 ) being the dominating.
The morphology and particle size of the Ca 2 V 2 O 7 : Eu 3+ nanoparticles with 4 mol % doping of both as prepared
and annealed at 900°C have been investigated by SEM images as shown in Fig.3. The as-synthesized products by
the combustion process show an unusual morphology i.e. forming cracks and porous network due to rapid release
of gases by-products during the combustion. This type of porous network is typical of combustion synthesized
powders. For the powders synthesized at 500°C, the particle size was very small and the particles tend to
agglomerate. With an increase of temperature, particle size increased and agglomeration decreased.
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