Püskürtmeli kurutma yöntemi ile PVA katkılı TiO2 tozlarının granülasyonu ve sinterlenmiş numunelerin karakterizasyonu
Optimization of processing parameters of TiO2 based ceramics by sprey drying
Yazar: CELAL AVCIOĞLU
Danışman: DOÇ. DR. BURAK ÖZKAL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Malzeme Bilimi ve Mühendisliği Ana Bilim Dalı / Seramik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Seramik Mühendisliği = Ceramic Engineering

Özet:
Püskürtmeli kurutma, seramik ürünlerin kuru preslenerek şekillendirme süreci öncesindeki bir adım olup ve püskürtmeli kurutucular günümüzde en yaygın kullanılan kurutucu tiplerinden biri olmuştur. Püskürtmeli kurutma temel olarak, solüsyon olarak hazırlanmış karışımın sıcak kurutma ortamına püskürtülerek sıvı halden granül haline dönüştürülmesine dayanan bir yöntemdir.
Granülasyonun üç avantajı vardır. Bunlar; tozun iyi akışkanlığa, yüksek paketleme yoğunluğuna ve güçlü preslenme yeteneğine sahip olmasıdır. Püskürtmeli kurutma sürecinde, kullanılan bağlayıcı elde edilen ürünün akışkanlığını, kütle yoğunluğunu ve granülün sıkıştırılma davranışını etkileyen temel parametredir.
Bu çalışmada TiO2 tozlarının paketlenme özelliklerinin iyileştirilmesi amacıyla, püskürtmeli kurutucu süreç parametrelerinin, bağlayıcı türünün ve miktarının son ürün üzerindeki etkileri araştırılmıştır.
Nano TiO2 başlangıç tozu kullanılarak yapılan çalışmalarda, presleme ve sinterleme evrelerinde yaşanan problemlerin ve bu problemlerin son ürün üzerine etkilerinin giderilmesi amacıyla başlangıç tozlarının püskürtmeli kurutma yöntemi ile granülasyonu gerçekleştirilmiştir. Optimum granül eldesi için farklı katı/sıvı ve TiO2/PVA oranına sahip sistemler ile çalışılmıştır. Üretilen granüllerin morfolojisi taramalı elektron mikroskobu (SEM), yoğunlukları ise piknometre analizleri ile incelenmiştir.
Granülasyon sonrası preslenen tozlar basınçsız sinterleme yöntemi kullanılarak farklı sıcaklıklarda sinterlenmiştir. Sinterleme sonrası numunelerin yoğunlukları arşimet yöntemi ile ve faz içerikleri X-ışınları analizi ile karakterize edilmiştir. Granüllerden üretilen numunelerin yaş ve sinterlenmiş yoğunluklarının, püskürtmeli kurutucu süreci uygulanmayan referanslarına kıyasla daha yüksek olduğu tespit edilmiştir. Sonuçlar yoğunluk ve faz kompozisyonu göz önünde bulundurularak karşılaştırmalı olarak değerlendirilmiştir.

Abstract:
Particle size is a very important parameter for nano systems in the field of nanotechnology which is commonly used today. Nanosized particles possess unique physical and chemical properties and hold considerable interest. However, some technical issue is occurred in the compaction process of nanosized powders because of the inherent tendency of fine particles to form cohesive powder assemblies that contain hard agglomerates with a poor flowability. lt was experimentally established that pressing nanopowders is much more difficult task than pressing powders with coarser particles. It is difficult to compact because of their small powder size which brings some problem such as lower density, poor mechanical property and fracture of compacted specimen
Spray-drying is a primary processing step in producing ceramic products by dry pressing. This process consists of the transformation of an aqueous slurry into dry spherical powders, often called granules, by spraying the slurry, which contains ceramic powders, a dispersant, binder, plasticizer, antifoaming agents, and if necessary a lubricant, into a hot drying medium. There are three main advantages of granulation: the resulting powders have flowability, a high packing density, and strong compactability.
The purpose of this study, exemine the effects of process parameters and the quantity of additive materials on the granulation, compaction and sintering behavior of TiO2 ceramics. Additionally, ensuring control of these parameters is to improve the engineering properties of TiO2 ceramics. 
Spray drying process, were performed in order to eliminate the problems occurred in studies using nano TiO2 starting powders while pressing and sintering process and for the elimination of the effects of these problems on final product. In order to obtain optimum granule properties, multiple spray drying processing parameters like different solid/liquid and TiO2/PVA ratios are selected and studied.
The binder is the most important ingredient for spray-drying, since the binder determines the flowability, bulk density, and compaction behavior of the granules. An adequate binder for spray-drying should confer high green strength to the compact article at the lowest possible addition The parameters controlling binder performance are the glass transition temperature (Tg), polymer backbone structure, molecular weight, viscosity, and hygroscopicity. Polyvinyl alcohol (PVA) is the water-soluble binder commonly used in spray-drying. The selection criteria for the binder are based on its ability to form granules that readily deform during compaction, to burn-out cleanly before sintering, and to give a high compact density and strength. In addition, to prevent a hard granule surface from forming, the binder should undergo minimal migration onto the granule surface while spray-drying the powder.
Nanostructured TiO2/PVA composite granules were prepared by spray drying technology. The effects of processing parameters on the microstructure and properties of composite powders were investigated. TiO2 granules were prepared by spray-drying a TiO2 suspension composed TiO2 nanoparticles with an average diameter of 292 nm dispersed in distilled water with the binder content in the slurries ranges from 1,5 to 4 wt%. In general, the powder content in the slurries used for spraydrying ranges from 20 to 40 wt%. It was followed by mixing in a planetary ball mill  initially at 300 rpm milling speed with 1 hour milling time using a 250 mL polymeric pot and zirconia balls. The suspension was spray-dried with BuchiTM Mini Spray Dryer at %15 flow rate and %100 extractor fan rate. The air temperatures at the inlet and outlet of the drying apparatus were 200°C and 100°C, respectively.
All composite powders have been successfully prepared in granulated form and pressureless sintering technique was employed to densify composites. Samples were characterized byscanning electron microscope (SEM), X-ray diffraction (XRD) measurements.
The results show that with increasing the slurry solid content, the particle size of powders increases, the particle size distribution of powders narrower, and the bulk density of powders increases,. With increasing the binder content, the particle size of powders increases, and the bulk density of powders increases. With increasing the slurry solid content, the particles have better sphericity. However, when the slurry solid content is too high (40 wt.%), the number of irregularly shaped particles increases and worse sphericity.
The reason for the relationship between bulk density and slurry solid content is as follows. When the slurry solid content is low, the hollow particles are formed with the water migrating to the surface of droplets in the spray drying process. If a flexible film with low permeability is formed on the surface of droplets, the temperature of droplets increases due to the low evaporation rate, and the water evaporates from the internal of droplets, and therefore the droplets bulge. For the above two cases, the sphericity of the particles will be destroyed, resulting in hollow or broken particles. Therefore, the lower slurry solid content will reduce both the bulk density and flowability of powders. With increasing the slurry solid content, the drying time of slurry is shorten and the binder content migrating to the surface of particles together with the water is reduced, which could avoid that the binder concentration in the surface of particles is greater than that of the core of particles, and hard shell would not be formed on the surface of particles. Powder particles with high density and better sphericity would be formed. The powders, therefore, have high bulk density and good flowability. However, when the slurry solid content is too high, on the one hand, the slurry has high viscosity and poor flowability, which result in poor droplet atomization and powders sticking the wall of spray dryer, and lead to hard atomization and even the spray drying process can stop; on the other hand, the powder particles have wider particle size distribution and worse sphericity, and irregularly shaped particles increase and the surface of particles become rough. Therefore, the bulk density and flowability of powders decrease. Based on the herein reported analysis, the appropriate slurry solid content under the present experimental conditions of spray drying is 35wt.%.
The influence of the amount of binder on granule shapes investigated. In spray granulation process, the binder bonds the small nanoparticles into a spherical particle with a certain size. Meanwhile, in the slurry atomization and drying process, the binder can improve the bonding strength of the powder particles. In the present experiments, the polyvinyl alcohol (PVA) was selected as the binder. binder content amounts were in the composite powder ranges from 1,5 to 4 wt% respectively. When the binder content is 2 wt.%, the granule size is small and the powder particles have wide particle size distribution, and some irregularly shaped formed also some TiO2 powders left without granulation. That indicates that low binder content implies non uniform particle size and low bonding strength. With increasing the PVA content to 3 wt.%, the powder particle size increases, and the granules have better sphericity. However, when the slurry solid content increases to 3.5 wt.%, the powder particles have worse sphericity and broken particles are formed. Moreover, when the binder content is too high, the pressure nozzle of the spray dryer may clog, and therefore the binder content should not be too high. The reason may be that when the binder content is low, it's not enough to bond the particles together or the bonding force between the particles is small, resulting in hollow or broken particles. Based on the above analysis, the appropriate binder content is 3 wt.%.
The most appropriate processing parameters under the present experimental conditions of spray drying are the slurry solid content of 30 wt.%, the binder content of 3 wt. The as-prepared composite powders have large particle size,good sphericity, and flowability.
At the second part of this study granules compacted with cold pressing in a tool-steel die at a pressure of 230 MPa into cylinder shaped green compacts with a diameter of 12.7 mm for 30 seconds by using a 10 tons hydraulic press. Zinc stearate was applied onto the walls of the die in order to take the samples out of the die easily. Compacted TiO2/PVA composites were sintered in air at 1000°C, 1200°C, and 1400°C respectively for 4 hours. Green and final density values has been found to be higher compared to the reference samples that spray drying process is not applied.