Bor karbür oluşum koşullarının belirlenmesi ve toz karakterizasyonu

Abstract

Bu çalışmada, grafit dirençli fırında karbatermik yöntemle har karbür üretimi ve taz karakterizasyanu ger çekleştirilmiştir. Deneysel çalışmalar iki aşamada top lanabilir. Birinci aşama, reaksiyon oluşum koşullarının belirlenmesi ve toz üretimi, ikinci bölüm ise tozların yüzey alanlarının büyütülmesine yönelik çalışmaları içermektedir. Deneysel çalışmalarda hammadde olarak değişik oranlar da borik asit ve petrol koku içeren şarj karışımları kul lanılmıştır. Bu karışımlar sabit elektrod çapı ve benzer ısıtma rejimlerinde farklı miktarda katalizör ilavesi ile reaksiyona sokulmuştur. Şarj bileşimi ve katalizör ilave sinin reaksiyon verimi ile sistemin enerji dengesine etki si saptanmıştır. Elde edilen sonuçlar bor karbür oluşumunun, reaktör de sınırlı bir hacimde gerçekleştiğini göstermiştir. Bor oksitin redüklenmesi sıvı ve gaz fazdan olmak üzere iki mekanizma ile gerçekleşmektedir. Üretilen tozların yüzey kalitesinin geliştirilmesini amaçlayan ikinci aşamada tane boyutunun küçültülmesine çalışılmıştır. Bir miktar toz farklı besleme hızlarında beş kez öğütülerek her çevrimden alınan numunelerin yüzey alanları BET cihazı ile saptanmıştır. Sonuçta mev cut imkanlarla tozların serbest birim yüzeyi 2m2/gr. değe rine kadar düşürülebilmiştir. Yapılan tarama elektron' mikroskop incelemeleride tozların sivri köşeli "bir morfo lojiye sahip olacak şekilde inceldiğini göstermiştir.

In this study, optimization of boron carbide reaction formation conditions and powder characterization have been investigated using boric asid and petroleum coke. In most nan metallic hard materials, boron carbide has a specific place. This compound was first discovered in 1858. Then joly on 1883 and Moisson in 1894 prepared- and identified the compounds EUC and Bfi0 respectively. The stoichiometric formula "B, u" was only assigned in 193A- and than many diverse formula were; proposed by Russian authors, which have not been confirmed. A lot of controversial phase diagrams have been proposed in the period of 1955-1960 about B-C. system. The most common phase diagram devoloped by Elliot and Kieffer assumed a wide phase homogenity range for boron carbide (9-20 at°6C ) and eutectic between B.C and C lies at 26 at % C and 24uO°C). " * The crystal structure of boron canbide have been known for a long time. The rombohedrol unit contains 15 atom corresponding to B^pC.,. Nucleer Manyetic Resonans studies carried out on a crystal of stoichiometry close to B.pC.,, have shown that the central position in the C-C-C cram (b site) was partially occupied by boron. Using IR absorbsion spectroscopy Becher and The'venot confirmed the existence of the C-B-C chain in compounds such as B, C and Bc "C. 4- 5.52 Consequently, because of the perfect physical and chemical properties boron carbide can be utilized in many different industries. VI Uses of baran carbide can be explained as fallows 1- Uses based dh the hardness The major industrial use of boron carbide is as abrasive grit or powder. Particle size are available from 1 Urn to 1D mm, used as polishing, lapping and grinding media for hard materials such as comented carbides, technical ceramics etc. A second category is wear rezistance components made of hot pressed sintered pieces. Baron carbide sand blasting nozzles are characterized by minimum wear, even with silicon carbide or corundum grit. Post-HIP sintered components have the best wear resistance. Baran carbide ceramic nozzles are used for water jet cutting. Other wear applications include sintered B, C wheel dressing sticks to produce new cutting edges, mortars and pestles. Lightweight armour plates have been used for the protection of helicopters or as breastplates far the protection of personnel against piercing bullets. Chemical uses 3oron carbide powders, activated by fluorides (and other halides) are used to diffuse baron at the surface of steels, the resulting Fe"B thin layer (1 G-2D0 Urn) is very hard and wear resistant. 3- Electrical application Boron carbide-graphite thermacuple consists of a graphite tube, a B.C bar and a BN sleeve between them. It can be used until 22GD C, under inert gas or vacum atmosphere, there is. a linear relationship between its voltage and the temperature at 600-2200 C. Vll k- NQclear applications The main part of nuclear pouer is now produced in reactors controlled by tuo kinds of absorbing materials boron carbide (B,C) or a ternary alloy (Ag-In-Cd). Boron carbide is a neutron absorber widely used because of its high B content, its good chemical inertness and high recraf jtoriness. The neutronic absorption of B,D is due to B, which participates according to the fallowing capture reaction. 10 B + k He + 7Li + Z.k MeV The cross section of this reaction varies from 3B5D barns for thermal neutrons to a few ba.rns for fast neutrons, and allows use of boron carbide with the.* naturel isotopic content US. 8% of B-the rest being B) or enriched up to 9D at % B for reactor guiding. The absorbent nucleer material depends on the specific reactor. Reduction of boronanyhidride with carbon proceeds in two stages according to the fallowing reactions'.B203 + 3C0 = 2B + 3C02 4 B + C = B, C The first stage of reduction of B"0, with carbon monoxide becomes thermodynamically feasible only above 140D C. However the reduction temperature is required to be maintained beyond -2000 C to achieve an enhanced rate of reduction accompained by the formation of B, C at the second stage. There are several methods for the production of B.C. The method used for our investigations was a gr'atphite resistance type furnace. vm In the initial stages, furnace temperature was kept lam. Ta allow boric acid to dehydrated to boron anyhidrides. As the reaction progressed, the- furnace was charged to full capacity at regular intervals when the bluish flame subsided' the reaction was taken to be complete. In the process of heating and reduction, the graphite core was slowly consumed, there by increasing power density and temperature at the core when the flame over the furnace charge subsided. Heating was discontinued and the furnace was allowed to cool to room temperatur-e. The hard upper surface was broken with a pneumatic chipper and boron carbide powder and fused chunklets from inside the furnace were recovered. Crude carbide from the furnace was crushed in three stages and grounded. Then this powder was subjected to magnetic seperation for the removal of iron and finally subjected to the aqueous processing to get rid of the residual impurities. In the leaching process, powders weresub jected to % 1 G H_SD, solution and dried at 90 C for 1 hour. A purified sample of carbide was analysed for total boron and carbon content and BET analysis. The results obtained from experiments can be explained as follows: 1 - Boron carbide reaction takes place in a reaction chamber in the graphite resistance furnace. 2- Reduction reaction occurs in two ways. Firstly, reduction from.the liquid phase of B"0_ and secondly reduction from the vapour phase of B?0,. 3- Boron carbide formation reaction contains three visible stages. The first is evoparation of water in boric acid. Reduction. stage with bluish flame is the second and the third stage is. when. the bluish flame has subsided. k- Water of boric acid has a corrosive effect on to graphite resistance. IX 5- The resulting products will contain high level free graphite if the furnace runs in an arc mode. 6- High level free graphite of the product can be eliminated by using an effective acid solution to oxidize carbon and other impurities. 7- The H,BQ,/C ratio is one of the most important criteria of the product quality.. Uhen this ratio was fixed at 3.3 and the- amount of catalysis was % 1.5. The maximum yield was obtained. 8- As the H"B0_/C ratio increases, yield decreases but boron concentration of product increases. 9- Grinding experiments showed that a 2 m2/gr free surface area could be obtained. 1 G- Boron carbide powders which have been produced from this investigation can be suggested for using in composite materials production investigations.