Yalıtım uygulamaları için toz esaslı malzemelerin geliştirilmesi ve karakterizasyonu
Development and characterization of powder based materials for insulation applications
Yazar: OZAN KOVANCI
Danışman: PROF. DR. ŞAFAK YILMAZ
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Malzeme ve İmalat Bilim Dalı
Derece: Yüksek Lisans, 2013
Konu: Makine Mühendisliği = Mechanical Engineering
Dizin: Dolgu maddeleri = Fillers ; Dolgu malzemeleri = Filling materilas ; Gözenekli silisyum = Porous silicon ; Isı geçişi = Heat transfer ; Vakum = Vacuum ; Yalıtım = Insulation ; Yalıtım malzemeleri = Insulating materials

Özet:
Çevresel sorunların artması nedeniyle uygulamaya konulan yönetmelikler sonucunda enerji tüketimini azaltmaya yönelik teknolojik çözümlerin arayışına girilmiştir. Bu amaç doğrultusunda izolasyon sistemlerinin geliştirilmesi ihtiyacı doğmuştur. Yalıtım uygulamalarında toz esaslı malzemeler en etkin ısıl yalıtım performansına sahip olmasının yanısıra diğer yalıtım malzeme gruplarına göre daha çevrecidir.Vakum izolasyon panelleri (VİP), kesit kalınlığı az olmakla birlikte düşük ısı iletimkat sayısına sahip iyi bir yalıtım malzemesidir ve enerji tüketiminde oldukça iyi tasarruf sağlamaktadır. Geleneksel yalıtım tekniklerine güçlü bir alternatif olan vakumlu izolasyon panelleri, mikroporoz silika tozu esaslı bir iç dolgu malzemesine mukavemet artırıcı ve opaklaklaştırıcı katkılar ilave ederek preslenmesi ve preslenip panel haline getirilmiş bu iç dolgu malzemesinin bir bariyer içerisine konularak vakumlanması ve sızdırmaz bir şekilde atmosfere kapatılması ile elde edilmektedir.Bu çalışma kapsamında, vakumlu izolasyon panelleri üretilecek bir altyapı kurularak, ısı iletim katsayısı oldukça düşük bir iç dolgu malzemesi geliştirilmiş ve karakterizasyonu yapılmıştır. Etkin ısı iletim katsayısının temel parametreleri olan panel iç basıncı, iç dolgu yoğunluğu ve yüzey alanı etkileri incelenmiştir. Elyaf takviyesinin iç dolgu malzemesinin mekanik özelliklerini nasıl etkilediği karşılaştırmalı olarak açıklanmıştır.Vakum İzolasyon Malzemelerinde kullanılan alternatif elyafların karakterizasyon çalışmaları yapılmıştır. DSC analizi kullanılmış, camsı geçiş sıcaklığı ve erime noktasından yararlanılarak elyafların çeşidi belirlenmiştir. Vakumlu izolasyon panellerinde kullanılan iç dolgu malzemelerinin FTIR analizleri yapılmış, analiz sonuçlarına göre iç dolgu malzemelerinin kimyasal yapıları ve bileşenleri belirlenmiştir. İç dolgu malzemelerine TGA analizi yapılarak nem yüzdesi ve sıcaklık uygulandıktan sonra geriye kalan inorganik malzeme miktarı tespit edilmiştir. Karakterizasyon yöntemlerinden SEM analiz yöntemi kullanılarak elyaf ile tozlar arasındaki etkileşim gözlenmiştir. Aynı zamanda SEM analizi ile elyafların boy ve çapları belirlenmiştir.Çalışma sonucunda, 0,5 mbar'da en düşük ısı iletim katsayısı 170 g/L yoğunlukta saptanmıştır. Bu değerden daha düşük yoğunluklarda ısı transferinde ışıma, daha yüksek yoğunluklu değerler için ise katısal iletim baskındır. Panel iç basıncının ısı iletimine olan etkisinin araştırıldığı S-eğrisi çıkarma çalışmalarında kritik basıncın 7 mbar olduğu görülmüştür. Kritik basınç altında ısı iletim katsayısı 4,3 mW/m.cK mertebelerinde iken atmosferik basınçta 22 mW/m.cK'dir. Vakumsuz ortamda dahi geleneksel yalıtım malzemelerinden daha iyi sonuçlar alınmıştır. Öte yandan vakumlama süresinin kritik basınç altında dramatik olarak arttığı gözlenmiştir.Geliştirilen iç dolgu malzemesinin basma mukavemeti testleri yapılarak, mekanik özellikleri araştırılmıştır. Farklı elyaf miktarları kullanılarak ilgili standarda göre yapılan testler sonucunda elyaf içeriğinin %10, %20, %40 ve %60 deformasyona uğradığında elde edilen mukavemet değerleri karşılaştırılmıştır. Sonuç olarak, iç dolgu malzemesi %20 deformasyona kadar birbirine yakın sonuçlar vermiştir. Ancak %20 deformasyondan sonra kesitte artma sonucu okunan kuvvet değerlerinde yüksek artışlar görülmüştür. Sonuç olarak, artan elyaf takviyesi artan tokluğa sebep olduğu gözlenmiştir.Son olarak, tez çalışması konusunda ileriye dönük çalışma önerileri sunulmuştur. Gözenekli malzemelerde, özellikle ışıma ile ısı iletim mekanizmasının açıklanabilmesi için malzeme sönümleme katsayısının tayini gerekmektedir. Öte yandan katısal iletimi mekanizması için liflerin çapı ve boylarının etkisi analiz edilmelidir. İç dolgu malzemesi bilgisayar ortamında sonlu elemanlar yoluyla modellenmesi, ileride yapılacak çalışmalar için son derece yararlı olacaktır.

Abstract:
As a consequence of increasing ecological concerns and regulations, in search of technological solutions that promise to decrase the energy consumption deals big importance in the industry. For this purpose, the development of new insulation systems and materials were needed. In insulation applications, the powder-based materials has the most effective thermal insulation performance as well as being more environmentallu friendly compared to other conventional insulation materials.Vacuum insulation panels (VIP) are great thermal insulation materials which have very low thermal conductivity coefficient with a lower cross-sectional thickness and provide very good savings in energy consumption. As a strong alternative to the conventional insulating techniques, vacuum insulation technology is based on wrapping microporous open celled core material that reinforced with fibers and opacifiers in a barrier material and then evacuating it before sealing to the atmosphere.VIPs are commonly used in construction sector especially, between the facades of the buildings. Vacuum insulation panels are also used in refrigerators in order to reduce the energy consumption. Lately, there are numerous of applications in the frigorific trucks and vassels.Generally, conventional thermal insulation materials? thermal conductvity are between 20 and 30 mW/m.K. The effective thermal conductivity of these materials are affected from conduction, convection and radiation. The cell structure of the conventional thermal insulation materials are closed type. Therefore, evacuation of the air could not be applied. Thermal conductivity of the air is approximately 24 mW/m.K, thus in heat transfer mechanism, convection is dominant. Nevertheless, the closed-cell sizes are larger than the core material of the VIPs. The conduction has also big effect to the thermal conductivity. As a result of this, the conventional materials such as PU, XPS, glass wool or etc. have their own advantages in terms of manufacturability, strength, cost but the thermal performance incomparable with VIPs.VIPs have porous media and open cell structures core in order to evacuate the gasses in the core material. When the vacuum level decreases, the convection heat transfer also reduces. There is a crucial pressure where the effective thermal conductivity of the VIPs are reduce dramatically and it can be nagliable. Below that vacuum level, the heat transfer and the thermal conductivity of the material is about 5 mW/m.K.VIPs have two main components: The core material that mixed of nanoporous powders and glass fibers, the metallized film that protects the internal vacuum of the core.In the core, silicon based nanoporous powders, which has a avarage particle size of 20 nm, are used to obtain very low thermal conductivity coefficient and SiC is used as opacifier in order to reduce the radiation. As a reinforcement glass fiber is used so as to increase the strength and provides great manufacturability of the specimen. Glass fiber also has a low thermal conductivity below the crucial pressure of the core. Thus, it also helps insulation properties of the VIPs.For manufacturing a vacuum insulation panel specimen, initially powders and glass fibers are weighing with precision scales. All of the measured materials are placed to mixer afterwards. Mixing process is approximately 3 minutes. The mixture is formed to a panel by pressing. Moisture have great influence to thermal conductivity, to get rid of the moisture effect, the insulation panels must be dried. Lastly, dried panels are evacuated in a metallized film by vacuum chamber.The core materials which was used in this study are fumed silica products with minor differences, specially in the nature of the fibres used as a reinforcement. The core material is a porous material. Therefore, the porosity of core material should be at least 90%, and the specific surface area should be at least 200 m²/g. Moreover, it was observed that high sorption capability results from the huge specific surface area; at 75% relative humidity, 0.05 kg of water per kg material is generally adsorbed. Because of this, fumed silica can act as a desiccant.In this study, a manufacturing infrastructure was established in order to obtain vacuum insulation panel specimens. A core material was developed and charachterized with a very low thermal conductivity coefficient. The basic parameters of the effective heat transfer coefficient in terms of panel interior pressure, the density of inner core and the surface area of the particles were investigated. Besides, the effects of the fiber supplementation to mechanical properties were explained.Resulting this study, lowest thermal conductivity coefficient measured at a density of 170 g/L with a vacuum pressure of 0.5 mbar. Below this density, heat transfer of the core material has radiation tendency, and above, conduction heat transfer is dominant. Another research topic was the infulence of the vacuum level over thermal conductivity. This reasearch shows us the crucial pressure of the core material is about 7 mbar. Below the crucial pressure, the thermal conductivity of the vacuum insulation panel is 4,3 mW/m.oK. At the atmospheric pressure (1 bar) the thermal conductivity of the core is about 22 mW/m.K. This shows the developed core has better performance even at the atmospheric pressure in comparison to conventional insulating materials.As mentioned above, Vacuum Insulation Panels are available to the refrigerator industry. They are composite material with a core and an enclosure. However, barier material has to prevent gas permeation from the core material. In addition, it should have a minimized thermal bridge effect around the edge of a panel.Compression tests are also performed for developed internal filling material in order to investigate the mechanical properties. Test specimens were prepared and the compression test was set according to standart EN 826. Compression strength values are compared in 10%, 20%, 40% and 60% deformations. As a result, the inner core materials have similar mechanical properties to 20% deformation. After 20% deformation, different increase in the area with the fiber content has observed. Consequently, the core material has better toughness with the increase in the fiber content.Scanning electrone microscopy is used for determination of the particle size of the alternative materials, the diameter and lenght of the glass fibers and the elemental anaylsis of the powders. The characterization is completed with the FTIR, TGA and DSC analysis.Differential scanning calorimetry (DSC) has been used to characterize fiber thermal stability, overall conformation, and which fiber was used in other VIP's. Understanding the method precision can help determine what differences between samples are significant and also establish the acceptance criteria for comparability and other characterization studies. In this study, we identify the parameters for the qualification of DSC for determining the type of fiber. The experiments and data analyses were performed by different analysts using different instruments at different sites. The results show that the (apparent) melting point (Tm) values and glass transition (Tg) obtained for the fibers. In addition, Tg and Tm values were found to look at literature. Therefore, types of the fibers which are used in VIP's were Polypropylene (PP), polyethyleneteraphatale (PET) and polyamid (PA) fibers.Thermal properties were determined according to Thermogravimetric Analysis (TGA) and Differantial Scanning Calorimetry (DSC). TGA was used to obtain percentage of water and percentage of inorganic material in the VIP. It was obtained that first weight loss occured near 100 ?C. This is associated to loss of structural water. It was found that the remaining percentage of inorganic material. DSC was used to determine type of fiber. Therefore, It was performed to obtain glass transition temperature (Tg) and melting temperature (Tm). Type of fiber was determined according to this Tg and Tm values.Fourier Transmission Infrared (FTIR) analysis was performed to obtain structure and chemical composition of VIP. The resulting spectrum represented the the molecular absorption, creating a molecular fingerprint of the sample which corresponds to the frequencies of vibrations of molecular bonds and chemical environments in which these bonds present. The most intense near about 1000 cm-1 band and in this spectrum is attributed to Si-O-Si vibrations. The presence of peaks in the range of 3200-3700 cm-1 also indicates the existing of stretching vibration of Si-OH groups. The Si-OH band seen in the FTIR spectrum indicates that the surface hydroxyls still exist, even though the material shows a hydrophobic behavior.Scanning electrone microscopy (SEM) was used to provide the particle morphology of the VIP, its diameter and also the particle size distribution. Images obtained by SEM provided information on the physical properties of VIP core materials and particulates including particle size, shape, surface morphology, and adherence to surfaces. SEM was performed to obtain the diameter and length of fibers and their attraction with other core materials such as fumed silica, SiC. Because mixture of powder agglomerates round of fiber. Therefore, fiber and powder interaction was obtained by using SEM technique.And finally, prospective studies and future suggestions are proposed about the thesis study field. Determination of the specific extinction coefficient has a vital importance for radiative heat transfer mechanism in porous materials. On the other hand, fibers length and diameter effect over the effective thermal conductivity so as to clarify the solid conduction. By means of finite element computer modeling of the internal filling material will be extremely useful for future studies.