The characterization by X ray diffraction was performed in a Rigaku Miniflex 600 diffractometer, with a Cu K_α (λ = 1.54 Å) lamp at 40 kV and 15 mA. All the measurements were performed in symmetric geometry (θ-2θ), with scans of 3 to 100 degrees at a velocity of 3 °C/min. The characterization by scanning electronic microscopy was performed in a microscope JEOL, model JSM-7800F with a resolution of 1.2 nm. at 1 kV of acceleration and 0.8 nm at 15 kV.

After the phase transformation by microwave, an Al₂O₃/C/ß-Bi₂O₃ composite for each sample was elaborated, alumina was employed as support due to it doesn't have any value of capacitance in it pristine state. 0.39 g of alumina was blended with 0.01 g of the sample of graphite and bismuth oxide obtained after microwave irradiation, the homogeneous mixture was placed in a press and it was applied a force of 2 ton. This composite was placed between two aluminum sample holders for SEM of 12.5mm diameter, 3.2 mm χ 8 mm type pin and wrapped in parafilm to avoid air currents and fastened with metal clips. Resistance and capacity of each composite was measure with a Digital LCR meter model LCR700 in resistivity and capacitance mode, respectively, all the samples were measured at 1 kHz of frequency. All this system is shown in Figure 1. The measurements of the electric parameters were determined in cylindrical capacitors of 2mm thickness and 13 mm of diameter for all the samples.

Figure 1

The results of the next two samples corresponds to the study of the reactant's blending time previous to the irradiation stage, all of them have a weight ratio 1:1 and they were irradiated with microwaves during 30 s.

XRD analysis of the two samples at different time of blending of the reactants before irradiation (Figure 3a and 3b, black lines), demonstrated that the initial samples were formed by hexagonal graphite and monoclinic bismuth oxide agree with PDF charts 89-7213 and 27-0053 respectively, after irradiation of 30 s, the sample blended during 1 min previously, change the phase of the reactants from α-Bi₂O₃ to a product of ß-Bi₂O₃ (Figure 3a, blue line), the results corresponds to 65-1209 PDF chart of tetragonal Bi₂O₃ with lattice of a= 7.7425 Å, c=5.6313 Å with space group P-421-c(114), Figure 3a (red dotted line) also shows the reflections of hexagonal graphite in the planes (002) and (101); SEM micrograph of Figure 3a let observe the distribution of the particles in the mixture, the dark particles belong to the graphite and the clearest ones corresponds to bismuth oxide with tetragonal system and irregular morphology. In the sample blended during 2.5 min before irradiation the change was to ß-Bi₂O₃ and metallic bismuth as is shown in Figure 3b, where the XRD pattern demonstrated the presence of a mixture of phases in the green line, where a phase change for bismuth oxide from monoclinic to tetragonal Bi₂O₃ with lattice of a=7.7425 Å, c=5.6313 Å with space group P-421-c(114), metallic bismuth with rhombohedral crystalline system with red lattice of a=4.53 Å, c=11.81 Å and alpha bismuth oxide didn't react is observed, also the red dotted line correspond to the main reflections of 2H graphite, pink ones correspond to tetragonal bismuth oxide and the dotted blue lines for metallic bismuth, agree with charts 89-7213, 65-1209 and 85-1329 respectively The product obtained after irradiation is a mixture of irregular particles of graphite, monoclinic and tetragonal Bi₂O₃ and metallic bismuth. SEM micrographs show a graphite structure major than 10 μπι with structures of 1 micrometer of bismuth oxide with morphology type needle. In both cases were demonstrated that the time of 1 min of blend before irradiation is enough to achieve a phase change in the bismuth oxide, while the graphite doesn't have any structural or morphological change, the rest of the peaks correspond to α-Bi₂O₃ not transformed.

Figure 3

The resistance and capacitance were measured in both samples after irradiation, the results with numeric values and their behavior are shown in Table 2. An increase in the capacitance is observed in both cases, however, the sample blended prior to irradiation for only 1 min shows a 97.37% increasing, it is due the phase transition of monoclinic to tetragonal system in the bismuth oxide compound, in the second sample the value of capacitance only increases 46.73% and this is attributed to the characteristic phase change of the bismuth oxide system and the presence of metallic bismuth that prevents the increase of the capacitance. From the results obtained in this experimental section was verified that the ideal blending time before irradiating with microwaves is 1 min to avoid the formation of metallic bismuth.

Table 2

Blending time (min)	Resistance (ΜΩ)		Capacitance (pF)		Percent capacitance increase (%)
	Before MW	After MW	Before MW	After MW
1	6.32	1.90	29.7	58.62	97.37
2.5	5.51	2.63	30.92	45.37	46.73

Once the optimum blending time prior to irradiation is determined (previous section), the next study corresponds to the power level effect of three samples, they were carried out with a weight composition of equal parts of reagents (0.1 g each one), mixture of 1 min before irradiation and time of 30 s inside the microwave, the difference between each one of these samples is the power level of the microwave oven used for each synthesis which are 600, 960 and 1200 W respectively. The X-ray diffraction analysis of the sample irradiated at 600 W of microwave's power level demonstrated no change in the crystalline structure of the Bi₂O₃ (Figure 4a), SEM micrographs show the product of reaction of the samples at this condition, where is observed bismuth oxide agglomerates of 30 - 50 μπι on graphite particles of 30 μm; the sample irradiated at 960 W, as in the previous sample no change in the crystalline structure of Bi₂O₃ was observed, (Figure 4b). Finally, at 1200 W, the irradiated sample shows a change of morphology as is observed in the micrograph of the Figure 4c, where type needle micro structures of 1 micrometer average was obtained. XRD pattern demonstrated also a structural transformation, where a mixture of bismuth oxide phases was obtained, majority a change of phase from alpha to beta bismuth oxide was determined by this technique, pink dotted lines in Figure 4 show this transformation, however, characteristic reflections of metallic bismuth were found too, corresponding to blue dotted line in Figure 5c, agree with chart 85-1329.

Figure 4

Figure 5

Capacitance of each sample, before and after irradiation during 30 s at 600, 900 and 1200 W of microwave's power level was measured, the results are showing in Table 3, where values of resistance decreases after irradiation in each sample and capacitance increase in the three samples. At 600 and 960 W, the change is not significant due to no change in phase was obtained, or it was in very little portion to be measured by XRD. The sample irradiated at 1200 W demonstrated the power level enough to get a phase transition from a- Bi₂O₃ to ß- Bi₂O₃ and metallic bismuth, for this reason, all the experiments was made at 1200 W and other variables were modified in order to obtain only bismuth oxide phase change.

Table 3

Power level (W)	Resistance (MΩ)		Capacitance (pF)		Percent capacitance increase (%)
	Before MW	After MW	Before MW	Before MW
600	7.52	2.40	24.32	25.04	2.97
960	6.646	2.00	27.22	28.34	4.11
1200	6.32	1.90	29.70	58.62	97.37

The first sample for this experimental section was formed by 0.01 g of α-Bi₂O₃ and 0.1 g of graphite and blended during 1 min before irradiation, after, this sample was irradiated during 1 min at 1200 W of microwave's power level, X Ray Diffraction shows a blend of the reactants in Figure 5 a (black line), the analysis demonstrated that the initial sample is formed by the monoclinic phase of bismuth oxide and hexagonal graphite, while the green line shows the phase transformation from a or monoclinic to ß or tetragonal bismuth oxide, with lattice parameters of a=7.7425 Å, c=5.6313 Å and space group P-421-c(114) agree with PDF chart 65-1209. Graphite doesn't have any modification in his crystalline structure. The micrographs of the samples synthesized with different time of irradiation demonstrated not homogeneous distribution and morphology; the samples irradiated during 1 min show particles of graphite of approximately 3 to 5 μm surrounded by Bi₂O₃ microspheres of 500 nm to 1 μm of size (Figure 5a). The sample irradiated for 2.5 min, shows the same results from a to β bismuth oxide by XRD (Figure 5b) and the micrograph shows graphite particles surrounded by bismuth oxide with appearance of melted material. And finally, at 5 min of irradiation with microwaves, XRD results, shows the characteristic transformation from a to ß bismuth oxide, and the peaks of the bismuth oxide in the diffractogram are overshadowed by the graphite signal; this sample shows a structure with pyramidal morphology as is shown in Figure 5c.

The capacitance and resistance of the samples of this group of experiments are shown in the Table 4, where the effect of the time of irradiation was studied. The section of the XRD results shows the change of monoclinic to tetragonal phase of bismuth oxide in all the samples, then, the change of the capacitance in each sample could be due to its morphology and size, in the sample irradiated for 1 min the structures of bismuth oxide are smaller than the other ones and they are closer between them with a capacitance value of 209.8%, if the sample is irradiated during 2.5 min, the result is a group of structures with no defined morphology bigger than the previous one and with an increasing of capacitance value of 88.28% respect to the no irradiated sample, finally, the third sample of this group was irradiated during 5 min and increase 57.12% the capacitance in this system, the structure is more compacted and is the biggest of this three samples, with bismuth oxide structures of size from 2 to 5 μπι average, which proves that at bigger contact area there is a greater capacitance as it was demonstrated for other researchers ^[14].

Table 4

Irradiation time (min)	Resistance (MΩ)		Capacitance (pF)		Percent capacitance increase (%)
	Before MW	After MW	Before MW	Before MW
1	7.48	1.44	22.80	70.65	209.80
2.5	7.52	1.56	24.83	65.58	164.11
5	7.66	0.83	24.91	54.79	119.95

The next serial of experiments was synthesized at 1200 W and irradiated for 1 min, for the sample with weight composition of 0.1 g of graphite and 0.01 g of Bi₂O₃ the results obtained by XRD demonstrated the transition of a to ß Bi₂O₃ in the first sample and the presence of phase transition of bismuth oxide and metallic bismuth in the rest of the samples. For the first sample, the micrograph shows semi-spherical structures with 500 nm. of size surrounding graphite spheres of 3 - 5 μι^ as can observe in Figure 6a. The next sample (0.1 g of graphite and 0.02 g of Bi₂O₃) presents similar results in the morphology to the previous one, however, Figure 6b shows the diffractogram of the blend of reactants (black line) and the characteristic change of phase of the monoclinic Bi₂O₃ to tetragonal Bi₂O₃ and the metallic bismuth growth (green line), the lattice parameter and space group of the metallic bismuth and ß-Bi₂O₃ coincide with the crystallographic charts 85-1330 and 65 1209 respectively, for both samples, the micrographs obtained show graphite particles from 5 to 20 μm surrounded by smaller spherical particles from 500 nm to 2 μm composed of Bi₂O₃ or metallic bismuth agree with XRD results. Finally, for the sample with equal quantity in weight of each reactant (0.1 g), the XRD diffractograms allowed to identify the currency of metallic Bi and the change of monoclinic Bi₂O₃ obtaining as result a tetragonal crystalline system; the characteristics signals of metallic Bi match with the information of the PDF chart 85-1330, where the rhombohedral crystalline structure is reported, with a space group R-3m (166) and lattice parameters: a = 4,535 Å and c = 11,814 Å. Micrographs obtained by SEM (Figure 6c) show particles of metallic bismuth or Bi₂O₃ with irregular shape and approximately size of 1 micrometer.

Figure 6

The capacitance and resistance of the materials in this experimental section show an increasing of the capacitance in all the samples and the values are observed in table 5, the behavior demonstrated an increasing in the capacitance values when the ratio between carbon and bismuth oxide in the sample is observed, this is due to the property of energy absorbance which results in an increase of temperature, the capacitance increase in a larger proportion of 209.8% when a ratio of the reactants is 1:10, where a complete transformation of α-Bi₂O₃ to ß-Bi₂O₃ is observed and no other products are obtained, 43.63% for the sample with reactants ratio 1:5 and finally, 27.65% for the sample with equal weight ratio of the reactants; in the last two samples a transformation of α-Bi₂O₃ to ß-Bi₂O₃ is observed, however, a reduction to metallic bismuth is achieved, which reduces the capacitance of the composite as is known.

Table 5

Reactants ratio (Bi2O3:C)	Resistance (MΩ)		Capacitance (pF)		Percent capacitance increase (%)
	Before MW	After MW	Before MW	Before MW
1:10	7.48	1.44	22.8	70.65	209.8
1:5	6.18	3.34	29.38	42.2	43.63
1:1	4.74	2.44	44.66	57.01	27.65

In the next group of experiments the quantity of reactants and the ratio between them were analyzed as in the previous section, however, in this case the time of microwave's irradiation was changed at 2.5 min. First, the composition of the sample was 0.1 g of graphite and 0.01 g of α-Bi₂O₃; in the second sample was increased the quantity of bismuth oxide to 0.02 g and constant weight of 0.1 g of graphite; finally, in the third experiment the quantity of reactants is the same, weight of 0.1 g each one. XRD shows the initial sample with the mixture of reactants, where is demonstrated the presence of hexagonal graphite and monoclinic bismuth oxide (PDF charts 89-7213 and 27-0053, respectively) as is observed in Figure 7, black lines. After microwave irradiation, the XRD results of the first two samples in Figure 7-a and -b, show the change of phase of bismuth oxide from a to β and Figure 7c shows the bismuth oxide change and the reduction of the Bi⁵+ to Bi⁰. The morphology of the structures obtained with 0.1 g of graphite and 0.01 g of Bi₂O₃ show melted structures, as in the micrograph of the Figure 7a; forthe sample with 0.1 g of graphite and 0.02 g of Bi₂O₃, the Figure 7b shows big structures of ß-Bi₂O₃ with size from 10 to 40 μm and irregular shape; finally, the micrograph of the Figure 7c show the sample with the same weight composition of reactants, where hemispherical ß-Bi₂O₃ is observed (as the XRD result demonstrated), the size of bismuth oxide particles are approximately 1 micrometer average.

Figure 7

Bismuth oxide - aluminum oxide - carbon composites were tested as capacitors, giving the results of Table 6, were the better results are obtained at 1:10 ratio of reactants with a percent of increasing of 88.28%, the results are similar to the sample with ratio 1:5 with 80.87% of capacitance increasing, and finally, the capacitance decrease to 23.82% in the sample with reactants ratio of 1:1 due to during the phase transformation of bismuth oxide, the quantity of photons of microwave is enough to heat the carbon and create metallic bismuth, which is not a good material to be used as capacitor.

Table 6

Reactants ratio (Bi2O3:C)	Resistance (MΩ)		Capacitance (pF)		Percent capacitance increase (%)
	Before MW	After MW	Before MW	Before MW
1:10	7.52	1.56	24.83	65.58	164.11
1:5	4.32	2.88	31.79	57.5	80.87
1:1	3.62	1.43	49.83	61.7	23.82

In the samples where the change of phase was from alpha bismuth oxide to beta bismuth oxide, the process of phase transformation is due to a rearregement from a monoclinic crystalline system to one with tetragonal crystalline system, the symmetry in all the samples was increased, as well as its capacitance.