Effects of Colour-filtering Solar Beam on Reaction Product from d-group Transition Metals in N2 Gas Environment under Heating with Concentrated Solar Beam

Recently, Noda and collaborators at Kyoto University reported possible enhancement of photo-voltaic cell (PVC) energy conversion efficiency by allowing penetration of preferential range of wavelength components alone to PVC by filtering the solar beam through a special semiconductor thin film developed at their laboratory. During the course of our recent experimental attempts ofnitridingd-group transition metals in N2 gas environment, we detected intriguing effects of colour-filtering on reaction product. In the present report, these effects of colour-filtering are reviewed under new light of recently reported evidence by Noda and co-workers. It appears that, by colour-filtering the selected wavelength range in solar beam, undesirable secondary reactions might be suppressed to result in promotion of a target reaction under heating using concentrated solar beam as the source of reaction heat.


Introduction
In a recent issue of Nature Photonics [1], an interesting report by collaborators of Noda at Kyoto University suggesting possible enhancement of photo-voltaic cell (PVC) energy conversion efficiency by allowing penetration of preferential range of wavelength components alone to PVC by filtering solar beam using special semiconductor thin film developed at their laboratory. Ve ry essence of this significant informat ion was published immed iately in some Japanese newspaper [2].
During the course of recent attempts of nitride synthesis using a solar furnace at PROM ES-CNRS (Laboratoire Procédés Materiaux et Energie Solaire) in Odeillo (France) undertaken in July 2010, the authors [3][4][5] detected also some intriguing effects of colour filtering of solar beam on reaction products.
In a standard experimental setup at PROM ES-CNRS, graphite crucible is used as sample holder. In an earlier report fro m Flamant and co-workers at PROM ES-CNRS [6], solar-induced fluorescence (SIF) of C 2 rad ical plu me was reported to yield fro m graphite under concentrated solar beam irradiation. This does not do any harm for synthesis of carbides of d-group transition metals. However, in our experimental campaign at PROM ES-CNRS during July 2010, objective was nitride formation by using concentrated solar beam as the reaction heat source for which presence of C 2 radical in gas phase was definitely not favourable.
According to Badie et al. [6], C 2 Swan band emission was detected at around 517 n m. Thus, Sky blue filter (Ref.# 068) supplied fro m Lee Filters (Andover, UK) [7] showing low transmittance of wavelength range 600 ± 100 n m was tried to be inserted for suppression of C 2 radical emission from graphite crucible (see Fig. 1). The consequence was significant suggesting satisfactory extent of suppression of carburizing fo r M C 1-x N x type carbo-nitrides for IVa-group element (Ti) and Va-group elements (V, Nb and Ta) [3]. For VIa-g roup elements (Mo and W) [4], starting material M remained in metallic state after 30 min heating at 2000ºC in N 2 gas environment without showing no evidence of carburization. Mo and W are known to be very difficult to be converted to nitride with N 2 [8,9] but relatively easy to be carburized. Thus, the acquired experimental results were interpreted as the evidence of effective C 2 radical yield suppression by insertion of the Sky b lue filter.
As the supplemental experiment, we undertook filtering experiment using Medium yellow (Ref.# 010) filter of Lee [10] wh ich cut wavelength component shorter than 450 nm almost completely. The effect of this filter on C 2 plu me emission did not seem very significant unlike the Sky blue filter. However, we detected unmistakable evidences of surface morphological modification for carbo-nitrides of Ti, V, Nb and Ta by insertion of the Mediu m yello w filter. In this report, the results so-far-acquired during the course of colour-filtering experiments [3][4][5] are interpreted under new light of recently reported evidence by Noda and co-workers [1,2].

Implications of the Published Work by de Zoysa et al.[1]
Detailed contents of the published work by de Zoysa et al. [1] were quite physical and not at all easy to understand straightforwardly the pragmatic significance by readers like myself who are not familiar with proper physics but digested newspaper introduction of this article [2] concisely demonstrated the essence of the reported work.
According to de Zoysa et al. [1], Al 0.3 Ga 0.7 As/GaAs quantum well (QW) inter-sub-band transistors (ISB-T) of ISB absorption wavelength λ = 9.7 µm was synthesized with GaAs QW layer of thickness 6.8 n m and Al 0.3 Ga 0.7 As barrier layer of thickness 13 n m. By filtering the solar beam through the developed QW ISB-T to allo w penetration of only limited range of wavelength component with certain extent of intensity enhancementby converting the other wavelength components into the preferred wavelength onto PVC surface, PVC energy conversion efficiency was enhanced up to remarkable 40% fro m conventional 20% gained using non-filtered white spectrum solar beam [2].
This appears to be a quite significant achievement with pragmatic convenience imply ing that PVC energy conversion efficiency was enhanced through suppressed side reactions taking place alongside the principal reaction contributing to the conversion of thermal energy to electricity. As reported in Ref. [3], N content x in the synthesized carbo-nitride MC 1-x N x under concentrated solar beam colour-filtered through the Sky blue filter was appreciably higher than the counterpart synthesized under heating with concentrated solar beam with no insertion of the Sky blue filter for M = Ti, V, Nb and Ta. For the VIa-group elements, Mo and W which are known to be extremely difficu lt to be converted to nitride in N 2 gas environment [8,9], reaction product remained in metallic M without being converted to neither carbide nor n itride by 30 min heating to 2000ºC using the Sky blue co lour-filtered concentrated solar beam [4] imply ing that C 2 plu me yield was well suppressed to inhibit carburizing reaction while chemical activ ity a(N) of N 2 gas mo lecules were too low to pro mote nitrid ing reaction.

Evi dences
By the heating in N 2 gas environment with the concentrated solar beam filtered through the Sky b lue filter, certain extent of grain size refinement was detected for M = V (Fig. 3 in Ref. [5]) and for M = Ta (Fig. 5 in Ref. [5]).

Select ive Pro mot ion of Certain Reaction Route by Inserting the Medium Yellow Filter
As reported in Ref. [5], intrigu ing consequence of inserting the Medium yello w filter was detected as the surface morphology modifications of the synthesized carbo-nitride by solar heating to 2000ºC for several transition metals of IVa-and Va-groups although the effect of the Medium yellow filter on C 2 plu me suppression was insignificant judging fro m the lattice parameters for the synthesized MC 1-x N x evaluated by X-ray d iffraction (XRD).
On the contrary to the g rain size refinement effect detected for M = V and Ta by the Sky b lue filtered concentrated solar beam rad iation in N 2 gas environment as pointed out above in 2.2.1., certain extent of crystal grain coarsening appeared to take place by irradiat ion of concentrated solar beam filtered through the Medium yello w filter fo r M = Nb (Fig. 4 in Ref. [5].
Although the detailed mechanisms lead ing to distinguishable surface morphologies among the reaction products MC 1-x N x with and without the Medium yello w filter remained unknown, it seemed certain that, by inserting the Medium yellow filter, some routes of secondary reactions competing the principal react ion were retardedto result in promotion of the rate of the principal reaction wh ich was, in some sense, analogous to the enhanced PVC energy conversion efficiency by allo wing arrival of the intensified monochromat ic solar beam alone to the PVC surface using the special quantum effect filter [1,2].

Temperature Drop Caused by Inserting the Colour Filter
By insertion of either the Medium yello w or Sky blue colour filter, temperature drop was measured to be insignificant imp lying that the heating effect by concentrated solar beam was largely contributed by IR (infra-red) range of wave components not by UV (ultra-v iolet) and shorter wavelength components.
By the special QW ISB-T thin film filter developed by Noda and collaborators [1,2], light component of λ ≈ 10 µm alone is selectively t ransmitted to PVC surface. The light component of this wavelength range would not contribute to unfavourable heating of PVC element and this fact might also contribute favourably for enhancement of PVC energy conversion.

Discussion
According to Noda and co-workers [1,2], energy conversion efficiency of PVC might be nearly doubled by allo wing the transmission of intensified wave co mponents favourable for the energy conversion process selectively by filtering the solar beam through the special QW ISB-T thin film filter. The cut wavelength co mponent by the filter must have included the IR component that would contribute the heating of PVC element.The exclusion of the IR co mponent might have contributed to certain extent the enhanced energy conversion efficiency of PCV through minimizat ion of undesirable heat-up effect fo r the PVC element besides the retarded unfavourable secondary reactions taking place competitively alongside the principal photo-voltaic process of the PVC element under exposure to the solar beam.
The QW ISB-T thin film developed by de Zoysaet al. [1,2] is not a simple filter but has capacity to convert thermal energy of solar beam with white spectrumto the narrow band spectrum at around the preferable frequency for theenergy conversion function of the PVC element and th is must have also contributed to the enhanced PVC energy conversion efficiency.

Concluding Remarks
The evidences so far acquired through colour-filtering experiments in our recent solar nitride synthesis [3][4][5] were still very scarce and thence further systematic investigation is definitely desirable. Anyway, as well as for PVC energy conversion efficiency enhancement, selective filtering of solar beam must be of pragmatic merit on usage of concentrated solar beam as the heat source for synthesis of refractory materials including nitride and carbide.
In conventional presentation of equilibriu m phase diagram for mu lti-co mponent system, p ressure P and temperature T are employed as the state variables to define the composition C. However, the recent experimental evidences acquired through heating using concentrated solar beam with deliberately cut wavelength components by colour-filtering seemed to suggest necessity for introduction of new state parameter, wavelength, for defin ition of phase relationship for some co mpounds synthesized at elevated temperatures.