Spark plazma sinterleme yöntemi ile ışık geçirgenliğine sahip magnezyum alüminat spinel seramiklerinin üretimi ve karakterizasyonu
Production and characterization of magnesium aluminate spinel ceramics with ligth transmission by spark plasma sintering
Yazar: SEYRAN SARIDAŞ
Danışman: PROF. DR. FİLİZ ŞAHİN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: Seramik Mühendisliği = Ceramic Engineering

Özet:
Bu çalışmada optik, medikal ve balistik amaçlı kullanımlara uygun saydam MgAl2O4 seramiklerinin MgO ve Al2O3 karışımlarından hareketle Spark Plazma Sinterleme yöntemiyle eldesinde sinterleme sıcaklığı, kalıp içi eleman olarak kullanılan molibden kaplı grafit folyonun ve SiC disklerin saydamlığa etkisi, ve B2O3 katkısının malzeme üzerindeki etkisi incelenmiştir.
Spark Plazma Sinterleme yöntemi ile yapılan deneylerde başlangıç spinel tozu eldesi için MgOve Al2O3 tozu kullanılmıştır. Birinci grup deneyler katkısız MgO + Al2O3 tozlarıyla 1200 – 1400 °C arasında iki basamaklı basınç profili (oda sıc – 1000 °C arasında 40MPa, 1000 °C – sinterleme sıc. arasında 80 MPa) uygulanarak 20 dk bekleme süresiyle gerçekleştirilmiştir. Mo kaplı grafit folyo ve SiC disk ile yapılan deneyler ise karbon kirlenmesinin malzeme üzerindeki etkisini belirleyebilmek için aynı spark plazma sinterleme parametreleri ile gerçekleştirilmiştir.
Katkılı deneylerde B2O3, MgO + Al2O3 karışımına ağırlıkça % 0,15 oranında H3BO3 asidik formunda katılmıştır. Bu grup deneyler 1250 – 1400 °C arasında gerçekleştirilmiştir.
Tüm deneyler karşılaştırıldığında en yüksek rölatif yoğunluk değeri 1350 °C'de sinterlenmiş katkılı numuneye ait olup % 99,7 olarak ölçülmüştür. Bu numune aynı zamanda 550nm dalga boyunda % 65 ışık geçirgenliği ile en yüksek ışık geçirgenliğine sahiptir. Yapılan deneylerde ışık geçirgenliğinin sıcaklığa bağlı olarak arttığı ancak 1300 °C'nin üzerinde tane boyutunun irileşmesi nedeniyle azalmaya başladığı belirlenmiştir.
En yüksek sertlik değeri 1200 °C'de sinterlenen katkısız magnezyum alüminat spinel numunesinden 15,74 GPa olarak ölçülmüştür. Sıcaklık arttıkça tane büyümesiyle sertlik değerlerinde de azalma gözlenmiştir.

Abstract:
In this study, magnesium aluminate spinel ceramics (MgAl2O3) that are suitable for optical, medical and ballistic uses were manufactured by spark plasma sintering. Besides the effect of sintering temperature, novel precautions against to carbon contamination were examined and sintering dopant was analyzed.
Transparent spinel was demonstrated already in the late 1960's and by the end of the 1980's high transmission parts were obtained in small numbers. In present, research and development efforts continue. One of the main goals of this researchs is the developing of satisfactory spinel fabrication technologies. A satisfactory technology means one able to generate commercially aviable transparent spinel parts, ones which both exhibit good performance, the desired size and shape, and are available in sufficient amounts at a reasonable price [2-4]. 
Other research topic is the development of transparent spinel exhibiting an optimal balance between the transmittance and the mechanical properties. In fact, obtainment of fine grained that is a need for good mechanical properties transparent spinel was tried early. Specimens exhibiting an average grains size of just 1 μm have been obtained, using a special incremental hot pressing schedule, but the level of transparency attained was modest. In the following decades sintering at high temperatures was used by most research teams, in order to improve optical transmission. The large grains size (50–300 μm) thus generated had a harmful effect on mechanical properties; hardness values reported were limited to around 12 GPa. More recent research, in which new, better processing approaches were developed, succeeded in dramatically reducing the grain size while keeping transmittance close to the theoretical level; as a result hardness was raised toward the 15 GPa level. Further improvements of the balance between transparency and mechanical properties are one of the main goals of current efforts [3,4].
In this study, spark plasma sintering method was choosed against hot press and hot isostatic press. Because spark plasma sintering has important advantages to other pressure sintering method. For instance, sintering period of SPS significantly short and leads to reach high tempreratures in short time. Besides, SPS provides homogeneous heating, because of direct current which is applied directly on the graphite mold. 
Magnesium aluminate spinel attracts a great deal of attention,because of its  properties such as high melting point (2135 °C), high hardness (16 GPa), high mechanical strength both at room (135–216 MPa) as well as at  elevated  temperatures (120–205 MPa at 1300 °C), high resistance against chemical attack, wide band gap energy, high 
electrical resistivity, relatively low thermal expansion coefficient (9x10-6 °C-1 between 30 and 1400 °C), high thermal shock resistance, low dielectric constant (ε=7.5), low loss tangent (tan δ=4x10-4) and low density (3.58g cm-3) [34].
Spinel is transparent to electromagnetic radiation from the ultraviolet through the mid-infrared (0.2 - 5.5 μm). Spinel has a distinct transmission advantage over sapphire and AlON from 4.5 to 5.5 μm, a region of particular importance for seeker and electro-optic imaging systems. The combination of unique optical and mechanical properties makes spinel one of the most outstanding optically transparent ceramics. Given its high strength and excellent transmittance properties up to the mid-IR wavelength, spinel is a very attractive material to manufacture high performance optical components, such as lenses, IR windows and domes. Very good UV-vis and medium wavelength IR transparency coupled with high hardness and high resistance to erosion makes spinel an excellent material for protective windows for aircraft, ship and submarine IR sensors, as well as missile domes. Spinel is also very   promising for applications  as  windows  or  windshields  for military vehicles such as large and bullet-proof windows, due to its mechanical and ballistic properties [2].
MgAl2O3 spinel powder as starting raw material was synthesized from MgO and Al2O3 powders. Experiments were carried out between 1200 – 1400 °C and under  two step pressure profile which are 40 MPa pre-load and 80 MPa final load with 20 min. holding time. Wheras first group of experiments were made without dope, second group experiments were carried out with molibdenium graphite foil, third group experiments were obtained with SiC discs and fourth group experiments made with B2O3 (wt. 0,15%) dope. 
Relative densities were measured and calculated by Archimedes method and results were between 95,2 – 99,7 %. It was observed that relative density increased with increasing temperature from 1200 °C to 1400 °C with same holding time and sintering pressure. The highest relative density was obtained from sintered magnesium aluminate spinel with B2O3 dope. This sample was sintered at 1350 °C under 40 MPa pre-load and 80 MPa final load with 20 min. holding time. 
Mechanical properties that are hardness and fracture toughness were investigated. Hardness was measured via Vickers hardness method and fracture was  measured with Anstis calculation. Hardness values were between 12,44 – 15,74 MPa and fracture toughness were changed between 1,23 – 1,98 MPa·m1/2 . Magnesium aluminate spinel without dope that was sintered at 1200 °C under 40 MPa pre-load and 80 MPa final load with 20 min. holding time had the highest hardness value (15,74 MPa) and it is obsereved that hardness values was dicreased with increasing temperature. Additionally, the highest fracture toughness (1,98 MPa·m1/2) value achieved in the magnesium aluminate spinel sample without dope which was sintered at 1400 °C with the same sintering parameters. 
Microstructure of the samples were analyzed with scanning electron microscope  from polished and thermallyy etched surface. The linear intercept method was used for average grain size measurement from scanning electron micrographs. It was showned that average grain size changed from 0,75 μm to 69 μm. This huge difference of grain sizes is explained by neck transformation during sintering. 
Grain growth causes improvment of optical properties but effect mechanical properties negatively. The most challenging subject is keeping grains small when improving tranparency to produce transparent ceramics.
High performance means for transparent seramics is a combination of high in-line optical transmission (ILT) with good mechanicalproperties.Various features of the structure, at the atomic or nano/micro scales, determine the performance controlling properties. In-line transprency of the samples was measured by UV-spectrophotometre from mirror like polished surface. Best result was obtaine in the sample with 0,15% B2O3 dope. It was measured 70% at 680 nm wavelength and 65% at 550nm wavelength. Grain growth causes improvment of optical properties but effect mechanical properties negatively. The most challenging subject is keeping grains small when improving tranparency to produce transparent ceramics.