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Fosfat bağlı kalıp malzemelerinin geliştirilmesi

The Phosphate bonded investment materials

  1. Tez No: 39506
  2. Yazar: BURAK FENERCİ
  3. Danışmanlar: PROF.DR. M. NİYAZİ ERUSLU
  4. Tez Türü: Yüksek Lisans
  5. Konular: Metalurji Mühendisliği, Metallurgical Engineering
  6. Anahtar Kelimeler: Belirtilmemiş.
  7. Yıl: 1994
  8. Dil: Türkçe
  9. Üniversite: İstanbul Teknik Üniversitesi
  10. Enstitü: Fen Bilimleri Enstitüsü
  11. Ana Bilim Dalı: Belirtilmemiş.
  12. Bilim Dalı: Belirtilmemiş.
  13. Sayfa Sayısı: 46

Özet

ÖZET Fosfat bağlı kalıp malzemeleri, hassas dökümde kullanılan ve su ilavesiyle sertleşen refrakter malzemelerdir. Genellikle balmumundan yapılan modele, kalıp malzemesinin çamur şeklinde uygulanması ve sertleşen kalıptan balmumunun ergitilerek uzaklaştırılması ile kalıp boşluğu meydana getirilir. Fosfat bağlı kalıp malzemesini oluşturan refrakter bileşenlerden silis kumunun, sinterleşme özellikleri belirlenmiş, elek analizi uygulanarak AFS Tane İncelik Numarası 67.15 olarak tespit edilmiştir. Kristobalit ve toz kuvarsın tane boyut dağılımları incelenmiş, yoğunlukları sırasıyla 2.26 g/cm5 ve 2.6 g/cm3 olarak saptanmıştır. Fosfat bağlı kalıp malzemesini araştırmak amacıyla 20 adet karışım hazırlanmış ve karışımların fiziksel ve kimyasal özellikleri incelenmiştir. Diferansiyel termal analiz incelemesinde, bileşenlerin faz dönüşümleri tespit edilmiştir. Fosfat içeren karışıma su ilavesiyle ortaya çıkan ekzotermik reaksiyonların sertleşmeyi takiben kalıp malzemesinde gösterdiği ısınma durumları belirlenmiştir. Karışımların sertleşme süreleri tespit edilmiş ve tablet deneyleri uygulanarak plastisite suyu (P.S.), havada kurumada su kaybı (H.K.S.K.) ve pişmede ağırlık kaybı (P.A.K.) incelenmiş, pişmiş tabletlerin kırılma mukavemetleri saptanmıştır. Magnezyum oksit oranının artmasıyla sertleşme süresinin düştüğü, ancak kırılma mukavemetinin arttığı tespit edilmiştir. En yüksek kırılma mukavemetinin %10 MgO içeren karışımlarda görüldüğü belirlenmiştir. Kalsiyum oksitin de kırılma mukavemetine olumlu etki gösterdiği saptanmıştır.

Özet (Çeviri)

THE DEVELOPMENT OF THE PHOSPHATE BONDED INVESTMENT MATERIALS SUMMARY Investment materials are refractory compositions suitable for forming a mould into which molten metal can be cast. The cavity is formed in the mould by burning out an expendable wax pattern imbedded in the mould. Investment casting was found wide application in dental and jewellery industry. The several different investment materials consist generally of a powdered refractory, a binder, and modifiers. The refractory base is generally crystalline form of silica (Si02)j mainly quartz and cristobalite can be used. Investment materials containing cristobalite which approximately 30%, expand at lower temperatures and exhibit greater expansions than those containing only quartz. Quartz is found in great quantity in nature as silica sand which contains mostly quartz and other impurities. Cristobalite is prepared commercially by calcining selected quartz at 1500°C or by heating silicic acid at 1000°C for several hours. Zircon, zirconia, alumina, magnesia, sillimanite, molochite are refractory materials used in investment. The binders in investments serve to hold the sand grains together and impart strength, resistance to erosion. They may be classified in two groups; organic and inorganic. Inorganic silicates, calcined gypsum, and phosphates are commonly used in moulding investments. Inorganic binders are not combustible and may retain strength at high temperatures. Phosphate bonded investment materials which consist usually of monoammonium phosphate, diammonium phosphate, and monosodium phosphate, are generally used in dentistry and jewellery. The first major stage in foundry is the production of the mould. In these compounds, the bond is developed in aqueous solution. Phosphate bonding can be represented as follows [5]: MgO + NHAH2P04 > MgNH«P04 + H20 viMould drying was carried out slowly below 45 °C for 24h. This is followed by gentle heating below 120 °C before raising to firing temperature. Firing of moulds is normally completed at temperatures around 1010°C. Decomposition of magnesium-ammonium phosphate can be represented as follows [7]: 2MgNH4P04 > Mg2P207 + H20 + 2NH3 The phosphorus was characterized of its combustion product, phosphorus pentoxide. Throughout the first half of the nineteenth century, the importance of phosphates was recognized. Nowadays, fertilizers are still found the largest application for phosphorus derivatives on a tonnage basis. In addition, however, phosphoric acid and phosphates are used in detergents, animal feed supplements, dentifrices, metal treating, water softening, fire retardants, and as binder in refractory materials. Phosphates can be described broadly as compounds containing four phosphorus -oxygen (P-O) linkages. The term phosphate is employed here, referring to compounds in which phosphorus atoms are surrounded by a tetrahedron of four oxygen atoms. Since each tetrahedron can share up to three of its corners in polyphosphate formation, multidimensional networks can result. Compounds containing monomeric P04 ions are known as orthophosphate. Linear P-O-P chains as polyphosphate, cyclic rings as metaphosphate, and branched polymeric materials and cage anions as ultraphosphate. In the structural investigation, crystalline condensed phosphates have been found that each P04 group remains approximately tetrahedral with O-P-0 bond angles of 95-125°; P-O-P angles was observed to be between 120-180°. Phosphates have been represented stoichiometrically as combinations of oxides. It is generally shown that the phosphates described, in terms of P205 content and oxide ratios. The amount of cationic oxides (such as Na20, K20, and CaO) to anionic oxide (P205) determines phosphate type. If the oxide mole ratio is three, the substance is an orthophosphate, generally considered as phosphate, if it is two, a pyrophosphate. A ratio between two and three represents a mixture of orthophosphate and pyrophosphate. Linear polyphosphate fall into the range between one and two. A ratio of exact unity gives a metaphosphate; the substance is an ultraphosphate when the ratio lies between zero and one. vxiThe linear polyphosphate formula is as follows; where M represents a univalent metal [2]. It encompasses most commercially important phosphate salts. For n=0, the formula represents the metal oxide; for n=l, orthophosphate; values of n=2,3,4,... represents the classic chemical divisions of polyphosphate. It has been common practice to designate polyphosphate with very long chains as metaphosphate. The term metaphosphate is generally reserved for cyclic structures with the exact composition (MP03)n. In phosphate salts, any or three acidic protons of phosphoric acid are replaced by cationic species. The phosphate salts of most elements are known and, if all minerals, salts, hydrates, and polymorphic varieties are considered. The commercial phosphates include alkali metal, alkaline earth, heavy metal, mixed metal, and ammonium salts of phosphoric acid; sodium phosphates are the most important, followed by calcium, ammonium, and potassium salts. There are three simple sodium phosphate which result from neutralization of the acidic protons of phosphoric acid; monosodium dihydrogen phosphate, di sodium monohydrogen phosphate, and trisodium phosphate. Monosodium phosphate is prepared commercially by neutralization of phosphoric acid with sodium carbonate. The main use of monosodium phosphate (MSP) is a water-soluble, solid acid and pH buffer. Mixtures of mono and disodium phosphates are used in textile processing, food manufacture, and other industries to control pH at 4-9. The ammonium phosphates are presented as solid phases between 0 and 75°C. Owing to thermally unstable nature of ammonium phosphates, applications are related to flame retarding and fire extinguishing. Monoammonium phosphate (MAP) is a common fire-extinguishing ingredient. Ammonium phosphates are used as a flame retardant for viiicellulosic materials, including plywood, papers, and fabrics, to prevent afterglow in matches, to control forest fires, and also refractories. In order to prepare phosphate bonded investment materials, silica sand, magnesium oxide, cristobalite, powder quartz, monoammonium phosphate, diammonium phosphate, monosodium phosphate, and calcium oxide were used. First of all, components were observed to establish their physical properties. Silica sand which was used in the present work was placed into sintering furnace to establish its sintering temperature. The sintering furnace was heated to 550 °C and waited this temperature in five minutes. The temperature of furnace was progressively increased to 1450°C. The sample was observed in stereo microscope in order to determine its sintering properties after it was taken off from the furnace. It was observed that the sintering starts about 1400 °C. The absolute sintering temperature of the silica sand was found to be 1450 °C and it had been gained refractory characteristic phenomenon of the investment materials. The size and distribution of sand grains are determined with the AFS sieve-analysis test. The sand grain characteristics are listed in Table 5.1. The sieving duration was selected to be 15 minutes. After sieving operation, amount of silica sand that could be place on each screen, was taken and weighed. In the scrutiny of sieve-analysis of the silica sand, it was understood that most of the grains had been at the top of the 100 mesh screen, and the size was over 0.149 mm. AFS Grain Size Number was established as 67.15. Having used the apparatus (SA-CP2) as centrifugal grain size analyzer the observed grain size distribution of cristobalite and powder quartz, cristobalite had been shown more regularity than the powder quartz. It was also observed that most of the grains of cristobalite the sizes were between 10-20(im. It was understood the grain size of powder quartz were found to vary between 10-30um. In the present work the specific gravity of cristobalite was found to be 2.26 g/cm3 by using pycnometer for the specific gravity analyze. This is ixclosely proportional to the value of 2.27 g/cm3 given by LANGE [9]. The specific gravity of powder quartz was determined to be 2.6 g/cm3. This is found to be proportional to the value of 2.653 g/cm3 [9]. In the present investigation, it was observed that magnesium oxide concentration was the main factor for controlling duration to get harden phosphate bonded materials. The maximum period of hardening process was observed with the formula of MgO 6%, MAP 10%, DAP 2%, MSP 6%. The hardening duration was shorten rapidly with the increase of the ratio of excess magnesium oxide. Having kept the amount of MgO, MAP, and DAP constant, it was found that the hardening period was increased in accordance with the increase of MSP. When the ratios of MgO and MAP were kept constant, the increase of the DAP was observed to shorten hardening duration. Similar observation was made by adding of calcium oxide. Phosphate bonded investment materials demonstrate heat due to exothermic reactions during and after the hardening process. It has been determined that the mixture number 1 shown slow heating. The mixture has been reached to maximum temperature in the 37 minutes. In spite of this phenomenon, the second mixture was found to show rapid heating. It was arrived at maximum temperature in the 20 minutes. Differential Thermal Analysis (DTA) curves reveal all energy changes occurring in the phosphate bonded investment sample on heating. The curves are, therefore, a function of crystal structure, and chemical composition of sample. The water added mixture was dried in the furnace at 110°C after hardening. Then the sample was ground before differential thermal analyze process.There were three important endothermic reaction peaks on the differential thermal analyze (DTA) curve. The peak observed on 65 °C was related to removal of humidity of the investment material. The peak at 290 °C which was related with the mass loose was due to probably the transformation of sodium acid pyrophosphate to the insoluble sodium metaphosphate (IMP). The peak seemed at 565°C, probably was due the transformation of J3-quartz to a-quartz. The value of plasticity water which is required for phosphate bonded investment materials was determined. The maximum plasticity water value of 17.67% was observed forthe mixture of MgO 10%, MAP 8%, DAP 1%, MSP 4%. It was also observed that with the increase of MgO, the plasticity water value generally was decreased. The plasticity water was decreased with the increase of MSP. Having kept constant the amount of the MgO and MAP, it was observed that the increase of DAP and MSP decreased the plasticity water value. The water loss of air dried samples was determined. The maximum value was observed to be 13.9 7% for MgO 8%, MAP 10%, DAP 2%, MSP 6% mixture. It was found that the curves for water loss and plasticity water curves demonstrate similar tendency. The gravity loss in the firing of the samples was calculated for 1010°C. When the total phosphate ratios were increased, the gravity loss in the firing were observed to increase. This can be described by the transformations of phosphates as a result of the loss of the vapour and the ammonia. It was confirmed that the maximum gravity loss in the firing was found to be 11.82% for MgO 9%, MAP 10%, DAP 2%, MSP 6% mixture. The breaking strength was observed to increase with the increase of magnesium oxide. In contrast to this phenomenon, opposite observations were made with the increase of MSP. DAP decreased the breaking strength. The maximum breaking strength value as 17.25 kg/cm2, was observed for the mixture of MgO 10%, MAP 9%, DAP 2%, MSP 4%. xx

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