Isı yalıtımı uygulamaları için cam nanolif üretimi ve karakterizasyonu
Fabrication and characterization of glass nanofibers for thermal insulation applications
Yazar: AHSEN ÜNAL
Danışman: PROF. DR. SADRİYE OSKAY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2017
Konu: Kimya Mühendisliği = Chemical Engineering
Dizin: Cam seramikler = Glass ceramics ; Deney tasarımı = Experimental design ; Elektroeğirme yöntemi = Electrospinning method ; Isı yalıtımı = Heat insulation ; Nanolif = Nanofiber ; Nanoteknoloji = Nanotechnology

Özet:
Nanoteknoloji alanı; filtrasyon, ilaç salınım sistemleri ve doku mühendisliği uygulamalarında, elektronik, optik, kozmetik ve enerji gibi çeşitli endüstriyel alanlarda kullanılmak üzere nano ölçekte üstün özellik ve performansa sahip malzemelerin geliştirilmesini kapsamaktadır. Lif çapının nanometre düzeyine indirilmesi, malzemelerin yüzey/hacim oranını artırmakta ve liflerin mekanik özelliklerini geliştirmektedir.Elektrospinning yöntemi, çok çeşitli sayıda malzemeden nanolif üretimi için çok yönlü, basit ve ekonomik olarak uygulanabilir bir prosestir. Sol-jel ve elektrospinning yöntemlerinin birleştirilmesiyle düşük sıcaklıklarda, yüksek saflıkta ve homojenlikte nanolifler üretilebilmektedir. Cam yünü gibi geleneksel ısı yalıtım malzemelerine eşsiz bir alternatif olarak, cam nanolifler yüksek düzeyde ısı yalıtımı sağlamaktadır. Nanoliflerde lif tabakaları arasında havanın sıkıştırılması sayesinde, daha iyi bir ısı yalıtımı sağlanmaktadır. 
Silisyum, titanyum ve zirkonyum gibi inorganik malzemelerden üretilen cam seramik membranların yüksek mekanik dayanıklılık, yüksek sıcaklıklarda yüksek termal ve kimyasal kararlılık gösterdikleri bilinmektedir. Buradan yola çıkarak bu çalışmada, sol-jel ve elektrospinning yöntemleri birlikte uygulanarak ısı yalıtım malzemesi olarak kullanılabilecek SiO2-TiO2-ZrO2-CeO2 sistemindeki cam nanoliflerin üretilmesi amaçlanmıştır. Deneysel çalışmanın ilk kısmında; nanolif morfolojisini ve çapını önemli derecede etkileyen çözelti değişkenleri olarak, polimer derişimi, sol/pol karışım oranı ve % asetik asit miktarı ile lif çapı arasındaki ilişki, üç değişkenli ve üç seviyeli Box-Behnken deney tasarımı uygulanarak belirlenmiştir. Sol-jel çözeltisi ( TEOS, TTIP, ZrOCl2.8H2O, Ce(NO3)3.6H2O, etil alkol ve asetik asit) ve polivinilpirolidon (PVP)/etil alkol çözeltileri, Box-Behnken deney tasarımıyla belirlenen optimum değişken seviyelerine göre hazırlanmış ve birbiriyle karıştırılmıştır. Hazırlanan çözeltiye elektrospinning işlemi uygulanarak cam nanolifler üretilmiştir. Yapılan ön çalışmalar ile, %65TiO2-%28SiO2-%6ZrO2-%1CeO2 sistemi iyi derecede mekanik dayanıklılık gösteren cam nanoliflerin üretimi için en uygun sol-jel bileşimi olarak belirlenmiştir. Nanoliflere 900°C'de uygulanan ısıl işlem sonrasında; SEM, FTIR ve XRD analizleri, yoğunluk ölçümü, ısı iletim katsayısı ölçümü ve yanmazlık testi uygulamalarıyla cam nanoliflerin karakterizasyonları yapılmıştır. Sonuç olarak, bu çalışmada üretilen yüksek yoğunluk, düşük ısı iletim katsayısı ve yanmazlık gibi üstün özellikler gösteren cam nanoliflerin, ısı yalıtım malzemesi olarak endüstriyel uygulamalarda kullanıma uygun olduğu görülmüştür.

Abstract:
The field of nanotechnology involves developing a wide range of applications such as filtration, drug delivery, tissue engineering, protective clothing, electrical and optical, biomedical, cosmetic and energy applications. Reducing the fiber diameter into nanometer scale increases the surface-to-volume ratio of fibers and improves the mechanical properties of the fibers. This feature provides better mechanical qualities, like wetting behavior and fiber strength. Nanofibers have attracted considerable attention due to their high surface to volume ratios, high porosity and high mechanical strength in various applications such as filtration, drug delivery, tissue engineering, electronics, insulation and many other fields. Insulation is a highly energy efficient technique that conserves energy and provide thermal comfort. Fiberglass, mineral wool, foam and some other materials are typically used materials as an insulator. "Thermal insulation" is defined as a design process which limits the heat loss and gain in buildings and installations. Technically, the thermal insulation is achieved to reduce the heat transfer between two media at different temperatures. For example; different insulation materials are used in the ovens, windows, cold air tanks and walls. On the other hand, there may be more than one insulation material available for the same application. Various materials are used for thermal insulation in the buildings. Foams are used as coating materials in coating systems for exterior masonry. Double glazed systems are used in windows, plastic pipes and glass wool are used for heating and hot water applications. The properties of the best heat insulating materials are cost-efficiency, environment friendly applicability, easy application, protection against acids, acid rain, insects and microorganisms, lightness and elasticity. Additionally, there should be no change in the thermal conductivity value and no degradation over time. Advantages of heat insulation are reducing energy consumption, contributing to the protection of the environment, providing thermal comfort, providing a healthy lifestyle, and reducing initial investment and operating costs. There are many kinds of insulation materials. Glass wool, stone wool, polystyrene foam, polyurethane, wood shavings, glass foam, phenol foam etc. can be given as examples for these materials. Glass ceramic membranes that are manufactured from inorganic materials such as silica, titania, zirconia have high thermal and chemical resistance and stability at high temperatures and high mechanical strength by comparison with polymeric membranes. With high temperature resistance of zirconia ceramic, zirconia nanofibers have great potential for thermal insulation applications. The composition of glass strongly affects the surface morphology of glass ceramic membranes. Basically, glass wool is used for insulation material. However, nanofiber glasses can be used for insulation purposes to be able to obtain insulation materials better then glass wool. As an unique alternative to traditional thermal insulating materials such as glass wool, glass nanofibers provides higher levels of thermal insulation. Nanoporous structure of glass nanofibers makes them highly thermal resistant materials. Compressing the air between the layers of nanofibers enhances the insulating facilities. From this point of view, in this study, glass nanofibers in the system of SiO2-TiO2-ZrO2-CeO2 were fabricated by combined sol-gel and electrospinning techniques.
The sol-gel technique is a very facile method for obtaining both inorganic and organic-inorganic hybrid materials. The sol-gel technique provides opportunities for producing polymer/glass composite materials, which allow the formation of glass network under mild conditions. The sol-gel process is quite different from the solid-state processes and provides the control of the final product at the molecular level during the reaction. Sol-gel chemistry is a rather complex process due to the large number of reaction variables (hydrolysis and condensation rate of metal alkoxide precursors, pH, temperature, mixing method, oxidation rate, etc.) that should be continuously checked to ensure good reproducibility of the synthesis protocol. The sol-gel procedure is a lab-scale method and used for industrial scale production. The sol-gel method has the main steps including the hydrolysis of the initiator, condensation of sol-gel active species such as alcohol or water, gelation, aging, drying and high temperature treatment. Both single and multi-component glasses with uniform morphology can be prepared by sol-gel process at much lower temperatures than those required for traditional melt-quenched derived glasses. The main steps of producing glass ceramic nanofibers are: (1) preparation of the sol with suitable metal alkoxide precursor and polymer matrice, (2) electrospinning of the as-prepared solution to obtain polymer/glass nanofibers, (3) calcination of the as-spun nanofibers to yield glass ceramic nanofibers. Recently, new TiO2-doped glass-ceramic nanofibers have been fabricated by sol-gel electrospinning method. Because these-ceramics comprise TiO2 in its structure, it keeps peeling of the TiO2 from the substrate. Fibers produced by combined sol-gel/ electrospinning techniques have unique properties such as exceptional length, uniform diameter, various composition and high surface area, and can be applied to many fields. Electrospinning is remarkably simple, versatile and cost-effective method for producing nanofibrous structures. Parameters and processing variables that affect the electrospinning process are; molecular weight and structure of the polymer, and polymer solution properties (concentration, viscosity, conductivity, surface tension), process parameters (applied voltage, flow rate, distance between the tip of syringe and collector) and ambient conditions (temperature, humidity). Electrospinning method has recently become a topic of interest because the produced fibers at this method have diameters under micrometers in length and are suitable for a wide range of applications, such as filtering, reinforcing composites and biomedical devices. Advantages of this method can be explained as low cost, reproducibility and easy control of the fiber diameter. Applied voltages between 7 kV and 30 kV can overcome the surface tension of a polymer solution. The polymer solution, charged in the electric field generated by the high voltage, moves toward the grounded metal surface. During this movement, as the solvent evaporates from the medium, the polymer turns into a nanofiber structure by thinning as a result of various physicochemical processes. Polymer type, polymer concentration, solvent type, applied voltage during electrospinning, flow rate of the polymer solution, temperature and humidity of the air are very effective parameters on the morphological structure of the nanofibers. For this reason, determining the optimum conditions is very important for obtaining the desired properties of the nanofibers. As the first part of the study, response surface methodology based on a three-level, three-variable Box-Behnken design technique was used to model the resultant diameter of the as-spun nanofibers to be able to determine the optimum conditions for the production of the glass nanofibers. PVP concentration, content of acetic acid in the overall solvent and sol-gel (sol)/polymer (pol) mixing ratio which are the most effective electrospinning parameters on nanofiber size and morphology were determined. A second-order model was obtained to determine the relationship between the fiber diameter and electrospinning parameters. The individual and the interactive effects of these parameters on the fiber diameter were determined. It is aimed to obtain the minimum fiber diameter and the fiber structures without beads also, which has regular fiber morphology. As a result, 4% polymer concentration, 1:1 sol/ pol mixing ratio and 60.5% acetic acid content were determined as the optimum levels for which homogeneous fibers were obtained in a small fiber diameter and less beaded structure. In the second part of the study, sol-gel mixture (TEOS, TTIP, ZrOCl2.8H2O, Ce(NO3)3.6H2O, ethanol and acetic acid) and polyvinylpyrolidone (PVP)/ethanol solutions were prepared and then they mixed together. The obtained mixture was used to fabricate nanofibers by electrospinning method. 65% TiO2 - 28% SiO2 - 6% ZrO2 - 1% CeO2 galss system was determined as the most suitable sol-gel composition for the production of glass nanofibers with good mechanical resistance based on the pre-experimental studies. In order to remove the polymer from glass nanofibers, the heat treatment process was applied at 900°C. After calcination process, produced nanofibers were investigated by using SEM, FTIR and XRD analyses to determine whether the produced material provided the required properties. Insulation properties were investigated by applying nonflammability test, thermal conductivity measurement and density measurement. The nonflammability test of the sample was carried out with calorimeter bomb and according to ISO 1716 Standards. It was proved that the produced sample could be classified as A1 Class insulation material. According to CE Standards, materials that have thermal conductivity coefficients less than 0.065 W/mK are classified as insulation materials. Thermal conductivity meaurements were performed with Armfield HT10CX Heat Transfer Service Appliance and the thermal conductivity coefficient of the sample was measured as 0.02 W/mK. The density of the sample was determined with Archimedes densimeter, based on the determination of the density of the solid materials by flotation. Density of the sample was measured as 5682 kg/m3, which is 50 times greater than that of glass wool. It is observed from SEM images that the diameter of the 65% TiO2 - 28% SiO2 - 6% ZrO2 - 1% CeO2 nanofibers in the regular morphology was measured as 104 ± 39 nm after calcination. The fiber diameter decreases after calcination. It was also determined that the diameter of the fiber decreased with the addition of ZrO2 and CeO2 components in the glass system. The results of the FTIR analysis showed that all organic bonds belonging to PVP disappeared after calcination, and the polymer is removed from the nanofiber structure. According to the XRD analysis, when the calcination temperature increased, the amorphous structure disappeared and crystalline structure occured. XRD analysis results showed that TiO2 and ZrO2 anatase and rutile phases coexisted after calcination at 900 ° C. After the calcination process, it was observed that the glass-ceramic structure occured in the samples. It was concluded that produced glass nanofiber structure, which has high density, low thermal conductivity coefficient and nonflammability may be possible candidates for the industrial applications as an insulating material.