African Journal of Science and Technology最新文献

{"title":"Donnees nouvelles sur les laves felsiques de ngaoundere (Adamaoua, ligne du Cameroun): chronologie K-Ar et petrologie","authors":"R. Temdjim, I. Njilah, P. Kamgang, C. Nkoumbou","doi":"10.4314/AJST.V5I2.15338","DOIUrl":"https://doi.org/10.4314/AJST.V5I2.15338","url":null,"abstract":"On the Adamawa plateau (Cameroon volcanic line), alkali basalt lava flows are dated at 6.5 ± 0.2 Ma by K-Ar method. Associated phonolitic and trachytic plugs were put in place 6.8 to 6.2 ± 0.2 Ma ago. The most recent volcanic episode aged 0.91 ± 0.06 Ma yielded maars and scoria cones with associated basaltic lava flows. The microlitic groundmass of miaskitic phonolites comprise of euhedral phenocrysts nepheline, anorthoclase, hastingsite, salite-ferrosalite, sphene and apatite. In the agpaitic phonolites, interstitial anhedral crystals of aegirine-augite, aenigmatite and lavenite mould euhedral phenocrysts of anorthoclase, nepheline, sodalite and noseane. Through the felsic lava series, pyroxene defines an evolutionary trend from salite to aegirine augite with gradual acmite enrichment. Amphibole trend shows a limited range (kaersutite in trachytic lavas and hastingsite in miaskitic phonolites). Genetically, phonolites are linked to trachytes by crystal fractionation mechanism but this mechanism alone cannot account for the overall Na 2 O and MnO enrichments in lavenite-bearing agpaitic phonolites. At this stage, a migration of dissolved volatilemetal complexes may have accompanied crystal fractionation to produce lavenite: (Na, Ca) 2 (Mn, Fe 2+ )(Zr, Nb, Ti)Si 2 O 7 (OH, F). Keywords : agpa itic phonolites, migration of volatile-metal complexes, lavenite Resume Dans la rgion de Ngaoundr, (plateau de l'Adamaoua, Ligne volcanique du Cameroun), des coules de basaltes alcalins dats 6,5 ± 0,2 Ma sont accompagnes de necks et pitons trachytiques et phonolitiques, phonolites dates 6,8 et 6,2 ± 0,2 Ma par la mthode K-Ar. L'pisode ultime des cnes stromboliens, maars et coules basaltiques remonte 0,91 ± 0,06 Ma. L'ensemble des laves appartient une srie alcaline sodique. La msostase des phonolites miaskitiques englobe des phnocristaux automorphes d'anorthose, nphline, sodalite, nosane, hastingsite, salite-ferrosalite, sphne et apatite. Dans les phonolites agpaitiques, les cristaux interstitiels xnomorphes de pyroxnes agyriniques, anigmatite et lavnite moulent les cristaux automorphes d'anorthose, sanidine, nphline, sodalite et nosane. Dans l'ensemble des laves felsiques, le pyroxne volue des salite et ferrosalite dans les trachytes et phonolites miaskitiques aux augite agyrinique et agyrine dans les phonolites agpaitiques. La variation des compositions des amphiboles est limite (kaersutite dans les trachytes hastingsite dans les phonolites). Sur le plan ptrogntique, les phonolites drivent des trachytes par un mcanisme de cristallisation fractionne. A la fin de la diffrenciation, ce mcanisme a t coupl la migration des complexes mtaux-volatiles dissous dans les fluides magmatiques pour former des phonolites hyperalcalines agpatiques lavnite, subnsosilicate complexe de Ti et Zr de formule (Na, Ca) 2 (Mn, Fe 2+ )(Zr, Nb, Ti)Si 2 O 7 (OH, F). Mots cls : phonolites agpaitiques, migration des complexes mtaux-volatiles, lavnite African Journal of Science and Techno","PeriodicalId":7641,"journal":{"name":"African Journal of Science and Technology","volume":"72 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2006-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"84527898","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Mathematical programming formulations for the examinations timetable problem: the case of the University of Dar es Salaam","authors":"Ar Mushi","doi":"10.4314/AJST.V5I2.15334","DOIUrl":"https://doi.org/10.4314/AJST.V5I2.15334","url":null,"abstract":"Examinations Timetabling Problem (ETP) is the problem of assigning courses to be examined and candidates to time periods and examination rooms while satisfying a set of constraints. Every University has a different set of constraints and structure of examinations. Thus there is no general ETP model for all Universities around the world [1]. ETP is NP-Hard [2] and therefore no optimal algorithm is known for this problem which can solve a general problem within reasonable time. However, exact methods can be used to provide a benchmark for the heuristic methods. There is no general model for University Timetabling Problems because the problem feature differs from one University to another. In this paper we focus in the formulation of the ETP for the University of Dar as salaam. We formulate, test and compare three Integer Programming models. It is concluded that, although exact methods cannot give a solution to a real-size problem, these models give a good benchmark for testing the performance of other approaches. This paper also gives a direction for better exact models for the University of Dar es salaam's ETP. African Journal of Science and Technology Vol. 5(2) 2004: 34-40","PeriodicalId":7641,"journal":{"name":"African Journal of Science and Technology","volume":"7 2 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2006-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83517879","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"FRACTURE STRENGTH OF POROUS CERAMICS: STRESS CONCENTRATION VS MINIMUM SOLID AREA MODELS","authors":"F. Nyongesa, B. O. Aduda","doi":"10.4314/AJST.V5I2.15336","DOIUrl":"https://doi.org/10.4314/AJST.V5I2.15336","url":null,"abstract":"In this study, we have reviewed recently published strength-porosity data of porous ceramics, and compared these data with those computed from both the minimum contact solid area (MCA) and the pore stress concentration effect (SCE) models. We observed that the theoretical data (MCA model) matched better the experimental results of ceramics in the low volume fraction porosity range (P 0.25), the SCE model better predicts the experimental results. African Journal of Science and Technology Vol. 5(2) 2004: 19-27","PeriodicalId":7641,"journal":{"name":"African Journal of Science and Technology","volume":"112 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2006-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73316378","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Quasistationary sequences in Hilbert spaces","authors":"J. Muriuki","doi":"10.4314/AJST.V5I2.15333","DOIUrl":"https://doi.org/10.4314/AJST.V5I2.15333","url":null,"abstract":"In this paper the concept of covariance differences of a sequence is introduced and its relationship with the covariance function is established. The criteria of linear representability of sequences in Hilbert space are proved. The necessary and sufficient conditions for a linearly representable sequence to be quasistationary are given and proved. The properties of dissipative random sequences are studied. African Journal of Science and Technology Vol. 5(2) 2004: 83-91","PeriodicalId":7641,"journal":{"name":"African Journal of Science and Technology","volume":"14 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2006-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"80953359","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Préparation et caractérisation de catalyseurs au palladium supporté: catalyseurs monométalliques et bimétalliques","authors":"T Mbang, JM Ketcha, C. Geron","doi":"10.4314/AJST.V5I2.15332","DOIUrl":"https://doi.org/10.4314/AJST.V5I2.15332","url":null,"abstract":"Preparation and characterisation of supported palladium catalysts: monometallic and bimetallic. A series of alumina and silica-supported palladium containing catalysts with weight content of palladium varying from 0.90 to 1.50% was prepared from Pd(C 5 H 7 O 2 ) 2 , i.e., Pd(II) bisacetylacetonate and Pd(NH 3 ) 4 (NO 3 ) 2 . These constitute the monometallic catalysts. Similarly a series of alumina and silica supported palladium-gold and palladium-copper containing catalysts with weight content, Pd-Au and Pd-Cu varying from 0.85 to 1.42%Pd- 0.14 to 0.29%Au, and from 0.85 to 1.42%Pd-0.21 to 0.28%Cu were prepared from Pd(NH 3 ) 4 (NO 3 ) 2 , HAuCl 4 , 2H 2 O and Cu(NO 3 ) 2 .3H 2 O, respectively, constituting the bimetallic catalysts. These supported metal catalysts were prepared either by the coimpregnation technique and/or by the surface oxidoreduction method. The catalysts were characterized by using infrared absorption spectrometric of adsorbed carbon monoxide and measurements of the metal dispersion on the supported catalysts techniques. All catalysts contained highly dispersed small particle-sized palladium oxides such as PdO and/or PdO 2 in the case of the monometallic catalysts, and Pd-Au or Pd-Cu alloys in the case of the bimetallic catalysts, which are easy to reduce in the presence of hydrogen. The results from CO absorption and metal dispersion studies are presented and discussed. Keywords : palladium, FTIR spectrometry, carbon monoxide, coimpregnation, dispersion oxidoreduction, bimetallic, copper and gold Resume Une serie de catalyseurs monometalliques au palladium supportes sur l'alumine, Al 2 O 3 et la silice, SiO 2 avec des teneurs en palladium (%Pd) variant entre 0,90 et 1,50%Pd a ete preparee a partir des sels precurseurs bis-acetylacetonate de palladium II, Pd(C 5 H 7 O 2 ) 2 et de nitrate de tetramine palladium II, Pd(NH 3 ) 4 (NO 3 ) 2 ; ils constituent les catalyseurs de base. Les catalyseurs bimetalliques supportes sur alumine et silice palladium-or et palladium-cuivre avec des teneurs Pd-Au et Pd-Cu variant entre 0,85 a 1,42%Pd- 0,14 a 0,29%Au et 0,85 a 1,42% Pd- 0,21 a 0,28%Cu respectivement a partir des sels precurseurs de Pd(NH 3 ) 4 (NO 3 ) 2 , tetrachloraurate d'hydrogene, HAuCl 4 , 2H 2 O et nitrate de cuivre II a 3 molecules d'eau, Cu(NO 3 ) 2 , 3H 2 O, ont ete prepares par la technique de coimpregnation et/ou par la methode de reaction d'oxydoreduction de surface. Les catalyseurs ainsi prepares ont ete caracterises par spectrometrie d'absorption infrarouge de monoxyde de carbone adsorbe et par mesure de la dispersion metallique sur la surface du support. Tous ces catalyseurs contiennent en surface une grande quantite de particules d'oxydes de palladium telles PdO et/ou PdO 2 dans le cas des catalyseurs monometalliques, et d'alliages Pd-Au ou Pd-Cu dans le cas des catalyseurs bimetalliques, particules tres faciles a reduire par le dihydrogene. Les resultats obtenus a partir des etudes d'absorption IR de CO adsorbe et","PeriodicalId":7641,"journal":{"name":"African Journal of Science and Technology","volume":"74 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2006-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"77178536","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Lightning signature as an index for the determination of the beginning of the planting season in Nigeria","authors":"E. Nymphas, M. Adeniyi, E. Ogolo, E. Oladiran","doi":"10.4314/AJST.V5I2.15335","DOIUrl":"https://doi.org/10.4314/AJST.V5I2.15335","url":null,"abstract":"West African rains are mainly due to the squall-line activities. Squall lines are groups of active thunderstorm clouds that are aligned with the ITCZ in front. The clouds are electrically active and 90% of lightning recorded occurs during squall lines associated thunderstorms. The results of measurements of lightning count from 1986 to date for Nigeria are presented in this work. Data obtained from other West African countries are also compared with them. The maximum lightning count occur mid-way between the north most position to the ITCZ and the ocean; shows a diurnal variation dependent on the movement of ITCZ that has very strong dependence on solar activities via evaporation and wind speed and direction. The minimum count is recorded during the ITCZ's recession and behind it. The applications to agricultural practices in West Africa are discussed. African Journal of Science and Technology Vol. 5(2) 2004: 28-33","PeriodicalId":7641,"journal":{"name":"African Journal of Science and Technology","volume":"111 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2006-05-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"89518512","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"EFFECT OF BLENDING VARIOUS COLLECTORS AT BULK FLOTATION OF COPPER AND COBALT SULPHIDE ORES FROM NKANA CONCENTRATOR","authors":"L. Witika, R. Chibulu","doi":"10.4314/AJST.V5I1.15319","DOIUrl":"https://doi.org/10.4314/AJST.V5I1.15319","url":null,"abstract":"Nkana Concentrator under the ownership of the then Zambia Consolidated Copper Mines Ltd (ZCCM) had been using Sodium Ethyl Xanthate (SEX) mainly as a collector, but with the coming of new Mopani Copper Mines Plc (M.C.M), it was felt that there was a need to test alternative collectors in an attempt to improve the production of the copper and cobalt concentrates. This work reports a series of laboratory testworks which were conducted on four other collectors which included, Potassium Amyl Xanthate (PAX), Sodium Isopropyl Xanthate (SIPX), Phosphorodithioic acid, O,Odiethyl ester, sodium salt or Ethyl - dtp-Na (DANAFLOAT 123) and Phosphorodithioic acid, O,O-bis (2-methylpropyl) ester, Sodium salt or i-butyl-dtp-Na (DANAFLOAT 245). The parameters considered in the evaluation were grade and recovery of both copper and cobalt minerals. Bulk float was characterised by an improvement in grades and recoveries by use of both individual collectors and their blends. Most collectors performed well except for SIPX and SEX: SIPX for the 20:80 ratio which did not give improved grades. Cobalt grades at bulk float were very poor, well below that obtained from the plant practice. It was established that the use of the other reagents such as PAX, SIPX, DANFLOAT123, DANAFLOAT 245 would be costly as compared to SEX. Cost of DANFLOAT 245 and 123 are $1.28 and $1.33 per kg respectively as compared to SEX at $1.23 per kg whereas SIPX and PAX are also slightly higher than SEX making them less favourable. Keywords : Collectors, recovery, grade, flotation. African Journal of Science and Technology Vol.5(1) 2004: 57-66","PeriodicalId":7641,"journal":{"name":"African Journal of Science and Technology","volume":"7 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2005-02-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"75460307","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"SIMULATION NUMERIQUE DES PERFORMANCES D'UN DISTILLATEUR SOLAIRE FONCTIONNANT EN MODE INDIRECT","authors":"R. Tchinda, E. Kaptouom","doi":"10.4314/AJST.V5I1.15322","DOIUrl":"https://doi.org/10.4314/AJST.V5I1.15322","url":null,"abstract":"RESUME :- Les auteurs proposent une evaluation des performances thermiques d'un distillateur solaire fonctionnant en mode indirect dans les conditions climatiques du mois de mai a Garoua (9°23' N; 13°23'E ; altitude 241 m). L'eau saumâtre a distiller est chauffee dans un collecteur solaire plan separe de la chambre de distillation, laquelle est partitionnee par un echangeur plan a tube continuellement refroidi. Les equations de transfert dans la chambre de distillation sont rappelees et sont numeriquement resolues. Les effets des parametres climatiques ainsi que ceux du systeme sont etudies. Le rendement journalier et la production horaire de la masse d'eau condensee sont particulierement analyses. Notre analyse theorique est en bon accord avec les resultats experimentaux publies pour des puissances thermiques echangees generalement rencontrees dans les distillateurs solaires. Mots cles : Capteur solaire plan, Chambre de distillation, vitesse du vent, eau de refroidissement, saumure, rendement journalier, masse d'eau condensee. ABSTRACT :- The thermal performance of solar still with an indirect evaporator-condenser has been studied for the Garoua (9°23' N; 13°23'E ; altitude 241 m) climatic conditions. The brine is heated in a flat plate heater coupled with an evaporator-condenser. Equations describing heat and mass exchanges in evaporator-condenser are recalled and numerically solved. The effects of climatic and other system parameters have also been studied. The studies have been made in terms of overall thermal efficiency as well as hourly yield. Our theoritical analyses reasonably agree with experimental results published in the literature for the practical heat fluxes encounted in solar stills. African Journal of Science and Technology Vol.5(1) 2004: 79-91","PeriodicalId":7641,"journal":{"name":"African Journal of Science and Technology","volume":"37 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2005-02-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"76417581","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"TEMPERATURE DEPENDENCE OF THE THERMAL CONDUCTIVITY OF A GROG MODIFIED KENYAN KAOLINITE REFRACTORY","authors":"J. Kimani, BO Aduda","doi":"10.4314/AJST.V5I1.15313","DOIUrl":"https://doi.org/10.4314/AJST.V5I1.15313","url":null,"abstract":"Thermal conductivity values, in the temperature range 300 – 1200 K, have been measured in air and at atmospheric pressure for a Kenyan kaolinite refractory with 0% - 50% grog proportions. The experimental thermal conductivity values were then compared with those calculated using the Zumbrunnen et al [1] and the Litovsky and Shapiro [2] theoretical models. The experimental values for samples prepared without or low percentages of grog increased with increase in temperature as predicted (reasonably good fit observed) by both the theoretical models, with the Litovsky and Shapiro's model giving theoretical values that were generally closer to experimental values than the Zumbrunnen et al's model. On the contrary, the conductivity values for the sample containing iÝ 40% decreased with increase in temperature in a manner consistent with the Eucken law. African Journal of Science and Technology Vol.5(1) 2004: 6-14","PeriodicalId":7641,"journal":{"name":"African Journal of Science and Technology","volume":"201 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2005-02-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"75988477","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A model for introducing and implementing e-learning for delivery of educational content within the African context.","authors":"Ea Omwenga, T. Waema, P. Wagacha","doi":"10.4314/AJST.V5I1.15317","DOIUrl":"https://doi.org/10.4314/AJST.V5I1.15317","url":null,"abstract":"Africa, where we have enormous and varied challenges in accessing higher education, there is need for relevant and customized content that is specific to our needs and challenges. Most of the models that exist to address these challenges have their limitations in terms of flexibility, time and space constraints and hence the need to address the mitigating factors. A blend of different types of information and communication technologies can be used in innovative ways in order to resolve some of these limitations. However, the same technologies brings about other challenges such as the costs that are associated and change of attitude and training on the part of the e-content developers. These challenges must be addressed for successful introduction of the same in an educational setup. In this paper, electronic learning is proposed as one of the main ways of overcoming some of the key challenges in accessing education. We argue that electronic learning models should be sensitive to the level of availability of infrastructure, technical support, and clear policy on implementation, evaluation and curriculum re-orientation. We propose an e-learning implementation model that can be used by educational institutions in introducing e-learning technologies to their staff and students. The model is a modification of Rogers'model of diffusion of innovation in organizations and has been developed from experiences and experiments conducted over a period of three years at the University of Nairobi. The steps involved in the proposed model are described and for each step factors that meliorate the situation are identified and put into perspective. System flowcharts have been developed and used to visualize the processes and the interrelatedness of the steps. We have contextualized the model to suit various parametric values that are dependent on cost, level of infrastructural support and staff motivation and commitment. Key Words: E-learning development process model, e-learning modes, e-content, infrastructure African Journal of Science and Technology Vol.5(1) 2004: 34-46","PeriodicalId":7641,"journal":{"name":"African Journal of Science and Technology","volume":"193 7 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2005-02-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"73143870","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}