THE EFFECT OF FREQUENCY VARIATION ON DIELECTRIC CONSTANT OF RAPHIA HOOKERI (PENGARUH VARIASI FREKUENSI KONSTANTA DIELEKTRIK PADA

This work sought to investigate the effect of varying the frequency of an applied electric field on the dielectric constant of raphia hookeri. The aim of the work was to ascertain the suitability of using the sample for engineering applications. It was revealed, from the recorded results, that the dielectric constant of the sample decays exponentially with frequency. Also, greater degree of polarisation was observed with decrease in frequency. Again, as a good thermal insulator, it was revealed that the values of the sample’s dielectric constant compared favourably with those of commonly used conventional dielectric materials. Thus, it is obvious that if put into production cycle, the sample can serve as a low-cost and environmental friendly material suitable for engineering applications within audio frequency range. This will, in turn, help to provide raw material for electronic industry and also improve the economy.


INTRODUCTION
Raphia Palm is a monocotyledonous evergreen plant that is cultivated for many purposes. In the history of mankind, this plant has played a significant role and according to (Aluasa et al., 2011), it belongs to the family called palmaceae. When cultivated in swampy or semiswampy areas, a typical full-grown raphia palm can be taller than 15m especially in Southern part of Nigeria. By nature, raphia palm produces flowers and then dies after a period of vegetative growth (Obuhiagham, 2009).
Raphia palm happens to be a very economic plant as all its parts are useful. For instance, the midrib of its leaflet can be used for broom making while its leaves, bamboo, stem and trunk can be used for the construction of local thatch houses, local wooden bridges and kidder (Oduah, and Ohiman, 2005). As posited by (Udofia, and Oworen, 2017), the petiole of raphia palm leaf yields fibrous pissava which are used for weaving as well as production of traditional textile anad various kinds of rope suitable for climbing and tying. In addition, raphia palm has been reported to have certain nutritional and health benefits. For example, the sap from it called palm wine contains vitamins, protein and some minerals and is drunk as beverages. Though (Ohimain, 2012) averred in their research work that this sap can be fermented and then distilled into alcohol, (Van Andel et al., 2012) noted that it can be used for treatment of measles, malaria and jaundice fontanel in babies. Equally reported is the use of extract from raphia palm leaves to inhibit carbohydrate utilizing enzymes and this is significant in reducing blood glucose level (Ogunsuyi et al., 2017).
It has been observed that among the useful parts of raphia palm, the leaves are the least utilized by man. As such, when the plant is to be used for other purposes, the leaves are usually discarded and then burnt as waste. Obviously, this practice leads to environmental pollution with the potential of posing risk to human health. This work, therefore, aims at investigating the component of raphia palm leaves called hookeri in order to ascertain its suitability for engineering applications. Specifically, the effect of frequency variation on the dielectric constant of the hookeri will be determined and then compared with the dielectric constant of conventional dielectric material used in fabricating capacitors. It is hoped that the findings from this work will help researchers and manufacturers/ users of dielectric materials in their decision-making process.

MATERIALS AND METHOD
Fresh leaves of raphia palm grown in Uyo Local Government Area of Akwa Ibom State were plucked and washed with clean water after which the raphia hookeri was extracted from them. The hookeri was then allowed to dry at room temperature without twisting before it was covered with a cellophane material prior to its use. This was necessary in order to prevent it from having contact with dust. Two equal plates of copper material were cut and used to form a parallel-plate capacitor with the hookeri as dielectric. Care was taken to ensure that the plates overlapped properly and the entire assembly was uniformly weighed down with a load of 20N, such that air gap and the possibility of by-passing the hookeri by linking the plates were completely prevented. Measurement of the thickness of the dielectric (raphia hookeri) was carried out using digital micrometer screw gauge. With the aid of Schering Bridge circuit arrangement as described by (Menkiti et al., 2010) . This portrays the fact that the influence on the capacitance is greater in case of frequency than when the sample's thickness is varied.  Figure 1 illustrates a linear relationship between the mean capacitance values and inverse values of the sample's thickness at each frequency. This is possible since the said capacitance values are in inverse trend with the thickness values of the sample as earlier stated based on the results presented. It can be inferred from the plot that when a fixed potential difference is applied at a particular frequency across the sample through plates of common area, the magnitude of charges on either of the plates decreases with increase in the sample's thickness. This results in an increasing tendency of the sample to inhibit the flow of current.   Table 2 Dielectric constant values of the sample (raphia hookeri) at various frequencies Figure 2 Plot of dielectric constant as a function of frequency. Table 2 shows the variation of dielectric constant of the sample with frequency. It can be observed from it that the values of the dielectric constant depend inversely on frequency. As can be deciphered from figure 2, the relationship between the dielectric constant values and the frequency values is an exponential decay. A seeming stability in the value tends to occur from frequency value greater than 1000 kHz. This means that if placed in an electric field, as the ability of the sample to store electric charge decays with increase in applied frequency, no significant decrease in the dielectric constant is possible at extremely high frequency.
Above all, the hygroscopicity of the sample is seen to play a vital role for the variation of its dielectric properties. Also, greater degree of polarisation is possible as the frequency values decrease.

CONCLUSION
The dielectric constant of the test sample (raphia hookeri) was observed to decay exponentially with the frequency of an external electric field applied. Within the audio frequency range (1.0 kHz to 20 kHz), the values of dielectric constant recorded in this work cut across the literature values for conventional dielectric materials such as polystyrene (2.6), waxed paper (3-5), mica (3-6), glass (7-8), and bakelite (4.5-5.5) (Shedha, 2008). Also, with the reported thermal resistivity of 17.730 W -1 mK [1], it is obvious that apart from being a good thermal insulator, the test sample can be used as a low-cost and environmentally friendly dielectric material for engineering applications within audio frequency range.