Figure 1 – uploaded by Kalarical Janardhanan Sreeram

Figure 1 5 °C/min and 7reoo1=5 ‘C/min) have been employed in all the studies reported in this work as baseline. The XRD pattern of the colorant oxide with and without Ce and Pr/Nd doping is given in Fig. 1. In the absence of doping, (Fig. 1(a)) at the calcination temperature o 1000 °C, the only crystalline phase that appeared was CoAl,0,4 spinel, whose diffraction pattern matched with that of JCPDS file No. 82-2252 (spinel cubic Fd3m, lattice size of 8.104 A). The formation of CoA1,0, like structures has been reported earlier from the direct interaction of Co with alumina to form the precursor of CoAlI,O, [18]. Copper added to the reaction mixture did not appear as a_ separate crystalline CuO phase, instead interacted with the

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Table 1 The Color coordinates and band gap energy for cool blue pigment The onset of optical absorption in cobalt aluminate occur around 2eV, with shifts observed with preparatory conditions. Characteristic absorptions at 543 nm, 578nm and 627 nm, typical of Co** ions present in a Ty ligand field with 3d’ configuration were observed in the case of cobalt aluminate (Fig. 3). According to literature, the triple band is attributed to The crystal size estimated from the most intense peak (311) in the case of Fig. 1(a) and (111) for Figs. 1(b)-1(d) varied as (3942), (4641), (4142) and (36+ 1) nm for Al:Cu:Co, Al: Cu:Co:Ce, Al:Cu:Co: Ce:Pr and Al:Cu:Co:Ce:Nd respectively. The hydrodynamic diameter varied as 1224, 1622, 890 and 822 nm _ respectively. It can be seen that the introduction of Ce and Nd into the lattice in the place of Ce and Pr, provided for a reduction in crystal and particle size. Scanning electron microscopy provided information to substantiate the claim on particle size (Fig. 2(a)). The SEM image of the sample with Nd doping (Al: Cu: Co: Ce: Nd=2.0: 0.2: 0.2 : 0.15 : 0.05) showed morphological and size homogeneity. The grain size was around 34-53 nm. Energy dispersive X-ray diffraction analysis (Fig. 2(b)) confirmed the presence of the rare earth metal ions. The corresponding mole ratio was found to be Al:Cu: Co:Ce:Nd:O=2:0.06: 0.06: 0.012: 0.0001 :6.18. A variation from the initial mole ratio could be attributed to the losses during homogenization or during calcination reactions.

Fig. 2 (a) Scanning electron microscopic image of oxide of Al, Cu, Co, Ce, Nd; (b) Corresponding EDX spectra.

*A(F)>"T) (P) transitions arising from Jahn-Teller distortion of Tj structure [22]. No change in the spectral features was observed by the addition of Ce and Pr/Nd. Cerium and praseodymium will be in the 4+ oxidation state and hence absorbance in visible region will be absent. Cu’* and Nd" are expected to have absorbance in the visible region. However, due to small concentration employed and thus present in the matrix, a significant absorbance is not detected. No major changes were also observed in the energy which remain at around (2.4+0.2) eV cases, indicating that a maj pigments on doping is not the earlier reports that the d jor shift in the co band gap in all the or of the likely. This is in tune with ensity of localized states in the band gap is lesser at higher temperatures of annealing due to structura defects [23]. Al samples were blue (Table 1) with a luminosity (L’) value ranging from 35 to 49, w F * * hile a and b va between —6.5 to —9.5 and —14.5 to —21.5 res A slight positive shift in b’ values when Ce and Pr are doped into cobalt aluminate is an indication of yellowness. CIELAB based color difference (AZ) between undoped and doped sample was the lowest with Nd doping. ues were pectively. A wtwiamoant urban avnncad ta liaht ic avnacrtan ta

Fig. 4 Solar irradiance spectra of oxides (a) Al, Cu, Co; (b) Al, Cu, Co, Ce; (c) Al, Cu, Co, Ce, Pr and (d) Al, Cu, Co, Ce, Nd.

Table 2 Test methods for rare earth doped blue pigment (Ce-Nd) As a prerequisite to evaluation of performance of the developed Ce-Nd pigment for various applications, bleed resistance on treatment with acid, alkali and water, oil absorption, light fastness and heat stability were evaluated. For acid and alkali resistance, the pigment was treated with 2% H2SO,4 and 2% NaOH solution overnight. Subsequently, the pigment was washed with water, filtered, dried and color parameters determined. The Ce-Nd pigment remained color fast in the presence of strong acids and bases. Bleed resistance of the pigment was carried out with water mineral turpentine, and isopropyl alcohol, methanol and ethanol etc. The pigments are non-migratory, and do not dissolve or bleed in contact with various solvents. The blue pigment shows excellent bleed résistance. The water soluble matter determined for the blue pigment was found to be 0.04% as per the procedure adopted by (ASTM D1208). Light fastness of the pigment carried out by irradiating the pigment for 6 h and the reading monitored for every one hour indicated a high level of stability. The salient observations from tests carried out as per ASTM D262 are reported in Table 2. A paint formulation developed as per procedures ey ey determined. The Ce-Nd pigment remained color fast in

$Fig. 5 Color profile of plastic containing blue colored pigment based on neodymium doped cobalt aluminate. In essence, cobalt blue, an ideal inorganic pigment with blue hue and spinel structure has Co”* and Al** ions occupying tetrahedral and octahedral positions. On doping with rare earth and transition metal ions like copper, the stable spinel structure remains intact differing only slightly in the size of the lattice. For instance, doping with a transition metal ion like Cu" at appropriate ratios, resulted in a mixed spinel with relative XRD reflection intensity of 0.6, similar to that of cobalt aluminate. Doping with rare earth elements provides for shielding effect of 5s and 5p energy levels resulting in host independent optical properties. The band gap energy remained at (2.4+0.2) eV, indicating that the color of the resultant oxide would remain blue. The same was confirmed from the absorbance bands at 540 nm, 578 nm and 627 nm, which is characteristic of *A(F)>"T\(P) Co(II) transitions. CIELAB color coordinates are in concurrence to the above observation. One of the observed effects of rare earth doping was a reduction in size of the crystals and particles. Though literature suggests that NIR reflectance of a pigment can increase with a reduction in size, from our observations it can be concluded that the changes to the crystal size and/or crystal orientation on doping rare earth metal ions, more so ee ee ee Se a A a ee ee a er eS ae © In a similar manner, incorporating 7% of the pigment into PMMA produced uniform color distribution (Fig. 5). The color coordinates measured on three exposed surfaces of the disc produced were compared with each other and the AE value was below 2. It is clear that the pigment was uniformly dispersed into the matrix and produced more or less the same color that it possessed prior to dispersing into the plastic.$

Fig. 5 Color profile of plastic containing blue colored pigment based on neodymium doped cobalt aluminate. In essence, cobalt blue, an ideal inorganic pigment with blue hue and spinel structure has Co”* and Al** ions occupying tetrahedral and octahedral positions. On doping with rare earth and transition metal ions like copper, the stable spinel structure remains intact differing only slightly in the size of the lattice. For instance, doping with a transition metal ion like Cu" at appropriate ratios, resulted in a mixed spinel with relative XRD reflection intensity of 0.6, similar to that of cobalt aluminate. Doping with rare earth elements provides for shielding effect of 5s and 5p energy levels resulting in host independent optical properties. The band gap energy remained at (2.4+0.2) eV, indicating that the color of the resultant oxide would remain blue. The same was confirmed from the absorbance bands at 540 nm, 578 nm and 627 nm, which is characteristic of *A(F)>"T\(P) Co(II) transitions. CIELAB color coordinates are in concurrence to the above observation. One of the observed effects of rare earth doping was a reduction in size of the crystals and particles. Though literature suggests that NIR reflectance of a pigment can increase with a reduction in size, from our observations it can be concluded that the changes to the crystal size and/or crystal orientation on doping rare earth metal ions, more so ee ee ee Se a A a ee ee a er eS ae © In a similar manner, incorporating 7% of the pigment into PMMA produced uniform color distribution (Fig. 5). The color coordinates measured on three exposed surfaces of the disc produced were compared with each other and the AE value was below 2. It is clear that the pigment was uniformly dispersed into the matrix and produced more or less the same color that it possessed prior to dispersing into the plastic.

Figure 1 – uploaded by Kalarical Janardhanan Sreeram

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