Pechini yöntemi ile lityum niyobat sentezi ve karakterizasyonun incelenmesi
Investigation of lithium niobate synthesis and characterization via Pechini method
Yazar: PINAR İŞAT
Danışman: PROF. DR. MELEK TÜTER
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı / Kimya Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2018
Konu: Kimya Mühendisliği = Chemical Engineering
Dizin: Lityum = Lithium ; Niyobyum = Niobium ; Pechini yöntemi = Pechini method ; Polimer jel = Polymer gel

Özet:
Lityum niyobat (LiNbO3) (LN) mükemmel ferroelektrik, piezoelektrik, piroelektrik ve elektro-optik özellikleri sebebiyle elektronik endüstrisinde son yıllarda kullanımı artan bir maddedir. Czochralski metodu ile kristali üretilmekte, Nb2O5 ve Li2CO3 kullanılarak katı hal  reaksiyonu ile yüksek sıcaklıklarda (>1000 ºC) toz olarak elde edilmektedir. Düşük sıcaklıklarda (<600 ºC) sol-jel,  hidrotermal reaksiyon, eriyik tuz yöntemleri ile de LN sentezi mümkündür. Çalışmada kullanılan Pechini yöntemi, sitrik asit gibi çok fonksiyonlu bir karboksilik asidin etilen glikol varlığında metal iyonlarının şelatlanması esasına dayanmaktadır. Elde edilen polimerde metaller moleküler seviyede, homojen olarak dağılırlar ve polimerik jelin yakılması ile LN, toz olarak elde edilir. Polimerik jelin oluşumu ve yakma sonucunda elde edilen toz LN'ın özellikleri;  polimerizasyon sıcaklığı ve süresi, sitrik asit/metal (CA/M+) ve sitrik asit/etilen glikol (CA/EG) mol oranlarına bağlı olarak değişiklik gösterir.  
Bu çalışmada, LN, Pechini metodu ile Li2CO3 (lityum karbonat), amonyum niyobat (v) okzalat hidrat  (C4NNbO9.xH2O, ANO), sitrik asit (CA) ve etilen glikol (EG) kullanılarak sentezlenmiştir. CA/M+ mol  oranı 3 olarak sabit tutulmuş  olup; CA/EG mol oranı  0.25, 0.54, 1, 1.86 ve 4 şeklinde değiştirilmiştir. Polimerizasyon 75ºC'de 4 saatte gerçekleştirilmiştir. Elde edilen polimerik jel etüvde 110ºC'de polimerizasyonun gelişmesi için bekletilmiş, daha sonra 400-800ºC'ler arasında kalsine edilerek LN tozu saf olarak elde edilmiştir. Köpüklenme derecesinin tespiti için jeller fırında 250°C sıcaklıkta 30 dakika süresince tutulmuştur. Çalışmada, CA/EG oranının, jelin oluşumu ve sonrasında yakılması ile elde edilen LN tozunun yapısal ve şekilsel özelliklerine etkileri incelenmiştir. Polimerik jelin yapısı, Fourier Dönüşümlü Kızılötesi Spektroskopisi (FT-IR) ile ısıl davranışı, Isıl Gravimetrik Analiz (TGA) yöntemiyle incelenmiştir. Toz ürünün yapısı,  X-ışını Kırınımı (XRD) yöntemiyle belirlenmiştır. Yüzey morfolojisi; Taramalı Elektron Mikroskobu (SEM) ile incelenerek, LN'ın yapısal karakterizasyon işlemi tamamlanmıştır. Yapılan incelemelere göre, artan CA/EG oranlarında  relatif kristalinite artmış, tanecik boyutunda farklılık oluşmuştur. Son olarak, LN'ın kullanım alanlarından biri olan fotokatalitik etkinlik denemeleri yapılmıştır. Tekstil atık sularında bulunan bir boyar madde olan metiloranjın, LN kullanılarak giderilmesi incelenmiştir. LN ilavesi ile metiloranj çözeltisinde UV lambası altında kaydadeğer bir fotokatalitik etkinlik izlenememiştir. LN tozlarına gümüş (Ag+) eklenerek elde edilen yeni ürün ile fotokatalitik etkinlik artırılmıştır. Relatif kristalinitesi yüksek olan tozlarda %48.16 gibi yüksek etkinlik değeri izlenirken, CA/EG oranı 4 olan çalışma için % 9.66 gibi düşük bir değer bulunmuştur.

Abstract:
Lithium niobate (LiNbO3) (LN) is an increasingly used material in the electronics industry in recent years due to its excellent ferroelectric, piezoelectric, pyroelectric and electro-optical properties. LN crystal; pyroelectric, piezoelectric, and high Curie temperatures. Due to its electro-optic compatibility, it is used in many technological applications. It can also be found in optical devices such as optical waveguides, optical modulators and high-density storage devices. In color televisions, surface acoustic wave (SAW) devices are used. SAW devices are a key component for many electrical circuits used in signal processing applications. SAW devices use the piezoelectric properties of the substrate material to convert an electrical signal into an acoustic signal propagating on the surface of the material and then convert it back into an electrical signal. The most common applications are bandpass filters in devices such as radios and mobile phones. The electro-optical properties of LN are used in a variety of devices to modulate light polarization or to modulate light amplitude or phase in marking applications
Crystals are produced by Czochralski method and are obtained as powders at high temperatures (> 1000ºC) by solid state reaction using Nb2O5 and Li2CO3. LN synthesis is also possible with chemical techniques (sol-gel technique, hydrothermal reaction, molten salt method) at low temperatures (<600 ºC).The Pechini method is also a chemical technique in which a multifunctional carboxylic acid such as citric acid is based on chelating metal ions in the presence of ethylene glycol. The metals in the obtained polymer are homogeneously dispersed at the molecular level, and LN is obtained as powder by polymeric gelatin burning. Properties of the powder LN obtained as a result of the formation of polymeric gel and combustion; the polymerization temperature and time depend on the molar ratios of citric acid/metal (CA/M+) and citric acid/ethylene glycol (CA/ EG). The binding of the citrate metal depends on the pH of the solution, the low pH value resulting in the protonation of the citrate. If it is a high pH value, the risk of metal hydroxides collapses. The pH control in the sol-gel method is very important because of the homogeneity of the gel and the resulting particle size control. Similarly, in the Pechini method, the pH of the starting metal-citrate complex solution can be optimized using ammonia (NH3), ammonium hydroxide (NH4OH) or other bases. One of the methods for adjusting citrate binding in the Pechini method is to use urea, instead of using a base such as ammonium hydroxide, due to the risk of hydroxide precipitation. This method provides better pH control. The Pechini method is notable for its low cost and easy temperature control. The most important advantage is; the aqueous solutions of the substances can be used. Because it is a watery technique, it is environmentally friendly and has lower toxicity. One of the major advantages of the Pechini method is the ability to form a polymeric gel (precursor) in which two or more metals can be homogeneously distributed throughout the network.The disadvantages are that significant amounts of organic reagents are used per unit of product mass and that some of the elements (bismuth, silicone, etc.) do not form stable citrate complexes.
Chelating is the process of attaching a multi-threaded ligand to a metal ion. Chelate is a complex formed in this process. Ligands are ions or neutral molecules that bind to the central metal atom or ion. The term ligand is meant to bind. The monodentate (unidentate) ligand term means that the ligand is centered via only one atom. When the chloride ions are the ligand, they are called chloro, when water is water (aqua), when hydroxide ions are hydroxo and when ammonia is ammonia. Bidentate ligands have two donor atoms that allow them to attach to a central metal atom or ion at two points. Common examples of double-threaded ligands are ethylene diamine and oxalate ion. Polydentate ligands are attached to a central metal atom or ion in excess of one.
In this study, LN was synthesized by the Pechini method using Li2CO3 (lithium carbonate), ammonium niobate (v) oxalate hydrate (C4NNbO9.xH2O, ANO), citric acid (CA) and ethylene glycol (EG). CA/M+ mole ratio was kept constant at 3; CA/ EG molar ratio was changed to 0.25, 0.54, 1, 1.86 and 4. Polymerization was carried out at 75ºC for 4 hours. The polymeric gel obtained was stored at 110ºC for polymerization, and then calcined at 400-800ºC to obtain pure LN dust. 
For determination of the degree of foaming, it was kept in a jelly oven at 250°C for 30 minutes. It is normal for the foam heights to be low extreme values that CA and EG, which the main components of the polymerization are decrease and increase greatly. The amount of foaming is important in the Pechini method. When the studies done in the literature are examined, it is seen that CA/M+ molar ratio is mostly taken as 3. The same rate was kept constant in this study. In the studied studies, the molar ratio of CA / EG was generally taken as 0.25, and these range of ratios were studied in very few studies. Using ANO and Pechini with LN synthesis, this rate change was never studied. In studies carried out at different molar CA/EG molar ratios (0.25, 0.54, 1, 1.86, 4), the amounts of Li2CO3, ANO and CA were kept constant and the amounts of EG and water were changed. By varying the amount of added water, the liquid / solid ratio for each run was kept constant at 17 in mol. In further studies to monitor the degree of foaming, about 3 g of polymeric gel was placed in a 25 mL beaker and kept at 250°C for 30 minutes, then the foam height was measured.
The highest foam value in the runs was observed when the mole ratio of CA / EG was 0.54 and 1. In addition, these two samples are lighter than the other samples. In this study, it is aimed to investigate the effect of different variables (temperature, time and CA/EG ratio) on the structural and morphological properties of LN powder obtained by gel formation and subsequent burning. In the study, the effects of the CA/EG ratio on the structural and morphological properties of LN powder obtained by gel formation and after burning were investigated.
The polymeric gel structure was investigated by Fourier Transform Infrared Spectroscopy (FT-IR) and Thermal Gravimetric Analysis (TGA). The structure of the powder product was determined by the X-ray Diffraction (XRD) method. Surface morphology; by scanning electron microscope (SEM), structural characterization of LN was completed. According to the studies done, working at different CA/EG ratios greatly affects the properties of the obtained LN powder.
The dyeing wastewater, which is highly abundant in the textile industry, exhibits toxicity and is often not biodegradable. Colored wastewater is treated by conventional physico-chemical methods such as chemical precipitation, separation of pollutants, electrocoagulation and adsorption with activated carbon, but the pollution is not completely eliminated.
Finally, photocatalytic activity experiments, one of the uses of LN, have been made. Removal of methylorange, a dye present in textile wastewater, using LN has been investigated. No significant photocatalytic activity was observed under the UV lamp in the methyloranges solution with the addition of LN. In the case of dust with high relative crystallinity, 48.16% of the highest activity value was observed, while for the study with a CA / EG ratio of 4, it was found to be as low as 9.66%.