Doğal hammadde ve atıklardan alümina-silika esaslı aerojel üretimi
Alumina-silica based aerogel production from natural raw materials and wastes
Yazar: ÖZGE KILINÇ
Danışman: PROF. DR. NİL TOPLAN
Sakarya Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2024
Konu: Metalurji Mühendisliği = Metallurgical Engineering ; Mühendislik Bilimleri = Engineering Sciences

Özet:
Artan nüfus ve kentleşme yanında birçok endüstriyel operasyonda, ekonomik değer kazanma potansiyeli olan atık malzemeler açığa çıkmaktadır. Oluşan tonlarca atık malzemelerin bertaraf edilmesi yönünde çalışmalar artarak sürdürülmekte olup; alümina-silika esaslı aerojel üretimi için literatürdeki çalışmaların yetersiz olduğu görülmüştür. Alümina-silika aerojellerin üretiminde, sentetik başlangıç malzemeleri kullanıldığında yüksek maliyetten dolayı çalışmalar doğal ve atık malzeme kullanımına doğru eğilim göstermektedir. Atık sorunlarının üstesinden gelmek için eloksal atık gibi çevreye zararlı ve alümina içeriği yüksek olan bu kaynak alümina başlangıç malzemesi olarak tercih edilmiştir. Ayrıca üretim rezervi ülkemizde bol olan perlit, diatomit, şamot tuğlası, vollastonit gibi ekonomik silika hammadde kaynaklarınının, katma değeri daha yüksek olan hafif, dayanıklı ve gözenekli malzeme sınıfı olan aerojellere dönüştürülebilirliği incelenmiştir. Ticari kimyasallar yerine başlangıç hammaddeleri olarak çok daha ekonomik olan bu doğal hammadde kaynaklarının ve atığın alümina-silika esaslı kompozit aerojel üretiminde kullanılarak değer kazandırılması bu tez çalışmasının temel hedeflerindendir. Aerojeller düşük termal iletkenlik ve yoğunluğun yanında yüksek yüzey alanına ve nano boyutlu gözenekliliğe sahip olduklarından günümüzde çeşitli alanlarda önem kazanmış ve birçok kullanım potansiyeline sahip olmaya başlamıştır. Alümina aerojeller, yüksek sıcaklıklara (1700°C'ye kadar) dayanabilen, yüksek bir mekanik mukavemete sahip, birçok kimyasala karşı dirençli olmakla birlikte; darbelere karşı kırılgan ve hassas olabilmektedir. Silika aerojeller, alümina aerojellere göre daha düşük mekanik mukavemete sahip olmakla birlikte, yüksek sıcaklıktaki kırılganlığı, sinterleme davranışı, yüksek maliyeti gibi tüm bu olumsuzluklara çözüm olmak ve termal kararlılığı arttırmak için alümina-silika esaslı kompozit aerojel üretimi özellikle doğal hammadde kaynaklarımız ve atık malzeme kullanılarak yapılmıştır. Böylece silika aerojele daha yüksek termal ve boyutsal kararlılığa sahip bir refrakter malzemenin (alümina gibi) eklenmesi ile oluşturulan kompozit aerojelin termal kararlılığının arttırılması sağlanacaktır. Mevcut çalışmada pahalı sentetik başlangıç kimyasalları yerine ülkemizde rezervi bol olan hammaddelerden şamot tuğlası, eloksal atık, perlit, zeolit, diatomit ve vollastonit kullanılarak farklı Si ve Al elementel oranlarına sahip hafif, düşük yoğunluklu, iyi mekanik özelliğe, termal dayanıklılığa ve düşük termal iletkenliğe sahip yalıtım alanında kullanılanlara alternatif olarak kullanılabilecek alümina-silika esaslı aerojel tozları üretilmiştir. Sol-jel tekniği ile çözeltiye alınan doğal ve atık malzemeler, jelleşme ve yaşlandırma prosesleri tamamlandıktan sonra atmosferik şartlarda kurutulmuş ve üretilen aerojel tozlarının karakterizasyonu, FT-IR, SEM ve FESEM-EDS, XRD, BET, termal iletkenlik ve yoğunluk analizleri ile gerçekleştirilmiştir.
Yapılan analizler sonucunda sentezlenen alümina-silika esaslı aerojel tozlarının nano boyutlarda, mikro ve mezo gözenekli yapıya (~1,9-2,1 nm) sahip oldukları görülmüştür. FTIR analizlerinde görülen -OH pikleri, yapıda fiziksel suyun adsorblandığını, Si-O-Al ve Si-O-Si pikleri, üretilen aerojel tozlarının kuvvetli Al ve Si bağlarını içerdiğini göstermektedir. BET analiz sonuçlarına göre, alümina-silika esaslı aerojel tozlarının yüzey alanları, partikül boyutları (gözenek çapı ve genişliği), gözenek hacimleri belirlenmiş olup; yüzey alanları 13,36-248,35 m2/g aralığında, gözenek çapları 18,783-20,01 Å aralığında, gözenek genişliği 20,574-20,968 Å aralığında, ortalama partikül boyutları 241,598-4492,363 Å aralığında ve gözenek hacimlerinin 0,006416-0,12199 cm3/g aralığında olduğu görülmüştür. Termal iletkenlik testinde alümina-silika esaslı aerojel tozlarında beklenen düşük termal iletkenlik katsayıları elde edilmiş olup; 0,0608-0,0950 W/mK aralığında olduğu tespit edilmiştir. Hacmi belli olan bir kap yardımıyla alümina-silika esaslı aerojel tozlarının yoğunlukları ölçülmüş ve oldukça düşük değerler (0,176-0,825 g/cm3 aralığında) tespit edilmiştir. Termal iletkenlik ve yoğunluk değerlerinin daha düşürülebilmesi için yeterli tozun bulunduğu numuneler için ekstra bir yıkama adımı daha yapılmış olup yeni yoğunluk değerleri 0,14-0,34 g/cm3 aralığında, termal iletkenlik katsayıları da 0,058-0,0778 W/mK aralığında tespit edilmiştir.

Abstract:
Increasing population and urbanization have led to many waste material generation problems, and efforts to dispose of tons of waste materials are continuing to increase. As an alternative to overcome waste problems, raw material resources such as perlite, diatomite, chamotte brick, vollastonite, which have high alumina content such as anodized waste and whose production reserves are abundant in our country, can be economically cheap and can be converted into aerogels, which are light, durable and porous material class with much higher added value. The conversion of these natural raw materials, which are very economical as raw materials, is one of the main objectives of the current thesis. Aerogels have gained importance today and have the potential to be used in various fields since they have high surface area and nano-sized porosity as well as low thermal conductivity and density. Alumina aerogels sinter above 1000°C due to their brittleness as well as poor mechanical strength, which limits their high temperature applications. The production of alumina-silica based composite aerogels is planned to solve all these problems such as high temperature brittleness, sintering behavior and high cost of silica aerogels and to increase thermal stability. By adding a refractory material (such as alumina) with higher thermal and dimensional stability to the silica aerogel, the thermal stability of the composite will be increased. The surface areas of pure alumina aerogels are generally lower than pure silica aerogels and their bulk densities are not as low as silica. However, alumina aerogels exhibit better thermal stability at high temperature compared to pure silica aerogels. Alumina-silica aerogel powders find applications in many fields due to their superior properties such as large surface area, well-developed pore structure, low density, low dielectric constant, low thermal conductivity coefficient and high porosity. 
For use in different applications, alumina-silica based aerogels can be developed with different ratios of precursor materials. The properties of alumina-silica aerogels enable them to be used in many areas such as thermal insulation, catalyst carrier, adsorption and biomedical applications. Especially in thermal insulation applications, they have very low thermal conductivity coefficients due to their low density and nanoporous structure. Alumina-silica aerogels have shown equivalent catalyst performance when compared to reference catalysts. In the field of thermal insulation, these materials have great potential, especially in high temperature environments due to their thermal dimensional stability and inherently low thermal conductivity. The addition of alumina phase as adsorbent to silica aerogels results in higher stability and adsorption capacity and these materials are suitable for repeated adsorption/desorption cycles. These properties mean that they can be used to increase energy savings and efficiency in industrial processes. In addition, atmospheric drying technique was used in the study, which has the advantage of reducing the capillary forces acting on the nanostructures and increasing the ability of the nanostructures to withstand these forces compared to other drying methods such as supercritical drying and freeze drying, which are more complex, expensive and require energy consumption. This reduces the production costs and therefore, it can be stated that the composite alumina-silica aerogels produced within the scope of this thesis are much more economical thanks to both the raw material and the selected drying method. 
In the present study, alumina-silica based aerogel powders were produced and characterized by using natural raw materials and wastes such as chamotte brick mortar, anodizing waste, perlite, zeolite, diatomite, diatomite and wollastonite, which are abundantly reserves in our country, instead of expensive synthetic starting chemicals to prepare alumina and silica rich sol solution by sol-gel technique, using oven drying technique under atmospheric conditions. In the study, two different starting precursors used as alumina (fireclay brick mortar and anodizing waste) and silica sources (perlite, diatomite, zeolite and vollastonite) were dissolved separately in NaOH base solution at 120°C for 3 hours using a heater stirrer and the sol solutions were combined by filtering when the solutions cooled. Sieving the powder that will be included in the solvent in a sol solution to a smaller powder grain size range will facilitate the reaction of the powders and make the starting material more soluble in the sol solution. Therefore, the powders were sieved through a 45 µm sieve before being taken into the sol solution. During the aging process, the solution was kept sealed at room temperature for 2 weeks. Since the aging time is one of the important production parameters, the effect of waiting time on the structure and properties of the gel can be investigated by increasing or decreasing this time. During the gel aging step, ethanol solution was used to strengthen the skeletal structure and bonds. The gels were dried in an oven at 120°C for 2 days under atmospheric conditions and after drying, light, low density, good mechanical properties with different Si and Al elemental ratios, Alumina-silica based aerogel powders, which can be used as an alternative to those used in insulation with thermal resistance and low thermal conductivity, were characterized by FT-IR, SEM (scanning electron microscopy) and FESEM (field emission scanning electron microscopy), EDS, XRD, BET, thermal conductivity and density analysis.
The alumina-silica based aerogel powders synthesized as a result of the analyses were obtained at different Al/Si ratios and in different amounts after drying under atmospheric conditions. The reason for this difference is the gelation efficiency of the solution depending on the starting materials used and the production parameters. For all aerogel powders produced, raw material cost was eliminated as raw material sources and wastes were used. 
Although the aerogel powders produced were found to have nano-sized grain size; the grain size appears large due to agglomeration in microstructural examinations. It has been determined that it has a homogeneous distribution, very low density caused by the 3-dimensional network structure, powder grain size at nano level, porous and spongy microstructure. While the aerogel powder coded N1 consists of small grain size powders, it was observed that in the sample coded N2, in addition to small grain size powders, large spherical grains were also present in the structure. This was confirmed by FTIR analysis and the presence of -OH peak in the structure caused physical adsorption of water and thus agglomeration of the powders. The powder coded N3 has a large grain size and mixed grain structure, while N4 is composed of smaller sized powder grains. Therefore, when the concentration was increased, small grain size, uniform structure and agglomerated grains were observed and it can be stated that the effect of concentration on grain size is consistent with the results obtained in various studies in the literature. Samples coded N5, N9 and N10 have small powder grain size, while the other samples have a wide grain size range and coarse and small mixed powder grain structure. In the EDS analysis, it is seen that Si, Al and O elements are concentrated in the structure of alumina-silica based aerogel powders synthesized by drying under atmospheric conditions. In aerogel powders coded N3 and N4, where diatomite was used as silica precursor, the silica content was higher than the alumina content. This is thought to be due to the higher gelling ability of the diatomite structure; similar dominant silica effect was observed in powders produced with diatomite (such as N9). It was observed in the EDS analysis that the additions of NaOH and HCl used in aerogel production with washing processes with distilled water remained in the structure and the washing should be increased. The -OH peaks seen in FTIR analyses indicate that physical water is adsorbed in the structure, while Si-O-Al and Si-O-Al and Si-O-Si peaks indicate that the synthesized aerogel powders contain strong Al and Si bonds. The peaks caused by the vibration of Si-O-Al bonds observed in the spectra of the aerogel powders produced are due to the presence of Si-O-Si strong bonds as well as Al-O in the structure. 
XRD analysis revealed the presence of kaolinite, blolite, quartz and corundum phases in chamotte brick, which is one of the initial precursors used to produce alumina-silica based aerogels, and SiO2 peaks as well as aluminum oxide hydroxide peaks in anodized waste. While SiO2 peaks were observed in the structure of perlite, diatomite and wollastonite used as silica precursors; the presence of clinoptilolite phase was observed in the structure of zeolite. In the alumina-silica based aerogel powders produced with these raw materials, results close to the general XRD results of alumina-silica aerogels produced in the literature were obtained and the presence of large SiO2 peaks, which are indicative of the predominantly amorphous structure, and in addition, in the powder coded N2 produced from chamotte brick and perlite precursors, blolite; in the powder coded N7 produced from chamotte brick and anodizing waste precursors, boehmit; bayerite and berythite in powder coded N8 produced from anodized waste perlite precursors; aluminum hydroxide and berythite in powder coded N9 produced from anodized waste and diatomite precursors; aluminum hydroxide and berythite in powder coded N10 produced from anodized waste zeolite precursors; bayerite, gibbsite, berythite and aluminum hydroxide phases were also detected in powder coded N11 produced from anodized waste and vollastonite. These phases have been observed in various studies in this field in the literature and the present thesis is similar in this direction. In addition, it can be stated that the volume of base and solution directly affects the salt formation in the structure, which is clearly seen in N3 and N4. Aerogel powder coded N3 produced with 1 M 250 ml contains more NaCl peaks in XRD analysis and higher Na content in EDS analysis compared to aerogel powder produced with 3 M 100 ml base/acid solution. Similar situation was observed in aerogel powders coded N1 and N2 produced from the same precursors (chamotte brick and perlite) in 1 M 250 ml and 3 M 100 ml base/acid solution volumes, respectively; the Na content of aerogel powder coded N1 is higher. If the number of washing processes is increased, the salt content in the structure can be reduced. 
Surface areas, particle sizes and pore volumes of alumina-silica based aerogel powders were determined by BET analysis. According to these data, adsorption average pore diameters ranged between 18,783-20,01 Å, pore widths ranged between 20,492-20,968 Å, average nanoparticle size results ranged between 241,598-4492,363 Å; The lowest adsorption average pore diameter was determined in the sample produced with N10 coded anodized waste-zeolite precursors, the highest pore width was determined in the sample produced from N8 coded anodized waste-perlite, the highest pore width was determined in N8 coded anodized waste-perlite and the lowest was determined in N11 coded anodized waste-zeolite. The lowest average nanoparticle size was found in the sample obtained from N2 coded chamotte brick-perlite and the highest particle size was found in the alumina-silica aerogel powder obtained from N4 coded chamotte brick-diatomite. Surface areas were 13.36-248.35 m2/g and pore volumes were in the range of 0.006416-0.12199 cm3/g. The N2 coded sample produced from fireclay brick-perlite has the largest pore volume value with 0.12199 cm3/g, while the N4 coded sample produced from fireclay brick-diatomite has the smallest pore volume value with 0.006416 cm3/g. When the data of BET surface area analysis were analyzed, it was observed that the sample with the highest surface area was obtained from perlite-chamotte brick precursors coded N2 with a surface area of 248.35 m2/g, while the sample with the lowest surface area was the sample coded N4 produced from chamotte brick-diatomite precursors with a surface area of 13.36 m2/g. While the Langmuir surface area data ranged between 20.77-383.665 m²/g, the sample coded N2 had the highest surface area result with a value of 383.665 m²/g and the sample coded N4 had the lowest surface area result with a value of 20.77 m²/g. In the BJH adsorption surface area results, the highest value belongs to the alumina-silica aerogel powder obtained from perlite-chamotte coded N2 with 133.3291 m2/g, while the lowest value belongs to the sample obtained from diatomite-anodized waste coded N9 with 5.4267 m2/g. It was observed that the particle size data coincided with the data obtained as a result of FESEM analysis at high magnifications and the powders were spongy and nano-sized as targeted. When the grain sizes of the aerogel powders produced were compared with the literature, it was determined that the powder grain sizes were within acceptable limits. 
Low thermal conductivity coefficients were obtained in the thermal conductivity test of alumina-silica based aerogel powders synthesized using anodizing waste and different natural raw materials as waste sources. As a result of the thermal analysis performed at 25°C for the powders dried under atmospheric conditions, the thermal conductivity coefficient of the sample coded N4 produced from chamotte brick and diatomite was measured as 0.0608 W/mK and it can be said that it has the most successful thermal conductivity coefficient and the lowest density (0.176 g/cm3) among the samples produced, so it can be used as a potential insulation material in thermal insulation. The thermal conductivity coefficient of chamotte brick before treatment was measured as 0.1874 W/mK and diatomite as 0.0687 W/mK; 2.7 times regression was observed in composite aerogel powder compared to chamotte brick and a lower value was obtained from both values. The value of 0.0723 W/mK was measured in aerogel powder coded N10 produced from anodizing waste and zeolite; the second lowest density value (0.2465 g/cm3) among all samples was seen in aerogel powder coded N7. It was determined that the thermal conductivity coefficient of aluminum anodizing waste before treatment was 0.8 W/mK and zeolite was 0.1188 W/mK, and after the treatments, the conductivity coefficient of aluminum anodizing waste decreased approximately 11 times and zeolite decreased approximately 1.6 times. The thermal conductivity of aerogel powder coded N2 produced from chamotte brick and perlite decreased approximately 2 times compared to anodized waste and was found to be 0.0929 W / mK. The thermal conductivity of N3 coded aerogel powder produced from fireclay brick diatomite at 1 M 250 ml base/acid volume was 0.0920 and its density was higher due to the higher Na content compared to N4 coded aerogel produced with the same precursors at 3 M 100 ml base/acid volume. The thermal conductivity (0.0950 W/mK) of aerogel powder coded N9 produced from anodized waste and diatomite was higher than the others, which can be attributed to the density of the aerogel powder (0.3646 g/cm3) and the highest Na content (12.88% w/w) in the samples produced. The aerogel powder coded N9 was found to be the most efficient (excess gel amount and minimum shrinkage rate) production in the scope of the study; this density ratio can be significantly reduced if the number of washes with pure water is increased due to the excess gel amount.  Thermal conductivity values could not be obtained from N1, N5-N8 and N11 due to insufficient amount of powder. When compared with the literature, it is understood that this value can be considered successful. Since the increase in the amount of air-filled pores in the structure makes heat conduction difficult, the thermal conductivity coefficient value may decrease. After the 2nd washing process, the thermal conductivity values for N2, N3, N4, N9 and N10 were 0.0697 W/mK, 0.0694 W/mK, 0.058 W/mK, 0.0778 W/mK and 0.0602 W/mK, respectively, and very low values were obtained compared to the first washing.
Density values of alumina-silica based aerogel powders produced with the help of a container with a certain volume were measured and found to be quite low. The density of N6 could not be measured due to lack of powder; the densities between N1-N11 were 0.176-0.825; 0.2647, 0.2565, 0.407, 0.176, 0.2536, 0.3565, 0.4869, 0.3646, 0.2485, 0.825 g/cm3 respectively. The lowest density was N4 coded aerogel powder produced from fireclay brick diatomite precursor at 3 M 100 ml base/acid condition. The density of N3 coded aerogel powder produced with the same precursor and different base/acid volume is about 2.3 times, which is attributed to the Na content from the high base volume and insufficient washing. The high density of the aerogel powder coded N11, which has the highest density, is attributed to the high Al content and high Na content. A similar situation was observed in N1 and N2 coded samples produced with the same precursors; N1 had a higher density than N2 due to the higher Na content due to the base/acid solution in 250 ml volume. The densities were measured again after the second washing process on the aerogel powders (for N2, N3, N4, N9 and N10), which contained sufficient amount of powder for thermal conductivity analysis, and a decrease in density values was observed due to the decrease in sodium content.
Research on the production, characterization and applications of aerogel materials can make significant contributions to the national industry by enabling the discovery and development of more effective and diverse applications. In order to support the wider use of these materials in the future (in biomedical applications such as wound dressings, drug delivery systems and tissue engineering scaffolds due to their biocompatibility and porous structure) and their wider adoption in industrial processes, waste and natural raw materials were investigated and used in the production of aerogel powder in this thesis. In this way, it will find widespread use at affordable costs and will provide significant benefits in human life.