Effect of pore-forming agents on microstructure in La-BaTiO3 ceramics

Yüksek porozite ve elektriksel direnç, seramik esaslı sensör malzemelerin ana karakteristiklerinden birisi olup sinterleme parametrelerinin değişimi ve La+3 dop etme ile BaTiO3 yapısına söz konusu özellikler kazandırılabilmektedir. BaTiO3 esaslı seramiklerin elektriksel özellikleri, söz konusu seramiklerin çeşitli uygulama alanları ve elektriksel davranışının açıklanmasındaki güçlükler nedeniyle, günümüzde ilgi çekici bir araştırma konusu olmaya devam etmektedir. BaTiO3 esaslı seramiklerde elektriksel direnç, perovskit kafeste baryum veya titanyum konumlarına nadir toprak elementleri dop edilerek etkin şekilde kontrol edilebilmektedir. Porozite yapıcılar ile katkılandırma yönteminin de, sensör uygulamalarında kullanılacak poroz seramiklerin üretiminde etkili bir yöntem olduğu bilinmektedir. Bu çalışmada, grafit ve PMMA (Polimetil metakrilat) katkılarının La+3 katkılı BaTiO3 esaslı seramiklerin porozite özellikleri üzerine etkileri incelenmiştir. Farklı bileşimlerde grafit ve PMMA katkıları içeren örnekler için porozite çalışmaları ve mikroyapısal karakterizasyon yapılmıştır. BaTiO3 esaslı seramiklerde porozite yapıcı katkı türünün porozite ve mikroyapısal özellikler üzerine etkisi incelenmiştir. Buna göre, porozite yapıcı katkılar veya donör katkısının yapıda ikincil bir faz oluşturmadığı belirlenmiştir. PMMA katkısının, grafit katkısına göre daha yüksek porozite yüzdesi ve büyük por boyutları oluşturduğu gözlenmiştir. Grafit katkısı, ekzotermik reaksiyonlar nedeniyle tane büyümesine neden olmuştur ancak tane morfolojisinde belirgin herhangi bir değişim gözlenmemiştir. PMMA ile katkılandırılmış örneklerde ise tane boyutu incelmiştir. Üretilen BaTiO3 esaslı poroz seramiklerin, gaz veya nem sensörü uygulamalarında kullanılmaya elverişli olduğu belirlenmiştir.   Anahtar Kelimeler: BaTiO3, porozite, sensör, grafit, PMMA.High porosity and electrical resistance are among the main characteristics of ceramic based sensor materials, which are obtained by the variation of sintering parameters and La+3 doping for BaTiO3. Addition of pore-forming agents (PFA) is also an effective method to fabricate porous ceramics for sensor applications. Several approaches have been done recently for manufacturing porous ceramics. It has been reported that porous BaTiO3 can be prepared by the incorporation of graphite, polyvinylalcohol (PVA), polyvinylbutyral (PVB), borides, silicides, carbides, corn-starch, potato-starch and partially oxidized titanium powder addition to BaTiO3 or by the thermal decomposition of barium titanyl oxalate BaTiO(C2O4)2-4H2O. Oxygen can be adsorbed at the grain boundaries due to the presence of pores in the porous ceramics, which are more favorable to form surface acceptor states compared with ordinary dense ceramics. The addition of organic particles, which vaporize at relatively low temperatures forming small and homogeneously distributed pores, is one of these methods. Porous structures having gas / liquid permeability, which can serve as gas or humidity sensors, could be fabricated via this method. The electrical properties of BaTiO3 based ceramics are still a matter of considerable interest due to their various applications and the difficulty in explaining the electrical behaviour. The electrical resistance of BaTiO3 based ceramics can be controlled effectively by rare earth doping into the barium or titanium sites in the perovskite lattice. In the present study, the effect of low amount of donor doping on the microstructural and electrical properties of BaTiO3 based ceramics was investigated. Instead of applying lower sintering temperatures and/or compaction pressures, using low amounts of additives, BaTiO3-based ceramics were fabricated with sufficient mechanical stability to fit long-term room temperature humidity environment applications. On the other hand, low amount of additives provided the porosity percentages, required for humidity sensing process, which is mainly a surface reaction including the adsorption of water vapour to the ceramic surface. In the present study, the effects of graphite and PMMA (Polymethyl metacrylate) additions on the porosity characteristics of La+3 doped BaTiO3 based ceramics were observed. Porosity and microstructural studies were carried out for PFA containing samples. The effects of the type and PFA content on the porosity and microstructural features of BaTiO3 based ceramics were examined. It was observed that there were not any secondary phases due to the existance of PFAs and donor dopant. PMMA additions caused a more pronounced effect in porosity percent and pore size values than graphite. Donor doping and variable graphite addition concerning BT, BTL and BTLC compositions, did not change the overall microstructural features and grain morphology was similar to BT compositions. However, graphite addition enhanced grain growth due to exothermic reactions. On the other hand, PMMA caused grain refinement. Besides, organic based additive affected the grain morphology and microstructural features. Grain morphology became more homogeneous, regular and globular due to PMMA addition. This result is of great importance from the view that porous ceramics with low density could be manufactured easily by organic addition. SEM images showed that spaces between the pores and intergranular pore morphology was present. Since intergranular pores were connected by larger pores, pore network can be regarded as interconnected spaces forming a type of capillary tube. One of the main advantage of employing additives with low percentages is that it provides economical benefit for the porous ceramics. Stable microstructures related to high temperature processes were obtained by organic addition and porosity requirements were satisfied with homogeneous distribution of high porosity percentages throughout the microstructures of the porous ceramics. The effects of the type of PFA on the porosity and microstructure of BaTiO3 ceramics were examined. It was observed that there is no evolution of a secondary phase due to the existance of PFAs and La+3 dopant. PMMA additions caused a more pronounced effect in porosity and pore size than graphite addition. It was concluded that based on electrical conductivity results, porous BaTiO3 ceramics are effective candidate materials for humidity sensing applications.  Keywords: BaTiO3, sensor, porosity, graphite, PMMA

Binder removal and characterization of zirconia ceramics prepared by hot molding

Seramik tozlarının çeşitli şekillendirilme yöntemleri vardır. Bu yöntemler soğuk veya sıcak, yaş veya kuru olmak üzeri çeşitli şekillerde sınıflandırılabilir. Bu çalışmada, mühendislik seramiklerinin şekillendirilmesinde diğer şekillendirme yöntemlerine nazaran daha düşük maliyetli ve daha kolay bir üretim prosesine sahip yeni bir şekillendirme yöntemi denenmiştir. Bu yöntem iki farklı şekillendirme yönteminin birleştirilmesi esasına dayanır. Yaş döküm yöntemi ile enjeksiyon kalıplama yönteminin birleşimi şeklindedir. Polimer katkılı sıcak akışkan seramik toz karışımı bir kalıp içine basınç uygulamadan dökülerek şekillendirilir. Zirkonya (ZrO2), magnezya (MgO) ve kalsiya (CaO) tozlarından farklı bileşimlerde mekanik alaşımlama tekniği ile karışımlar hazırlanmıştır. Bağlayıcı sistemi,  parafin (%95 ağ.) ve oleik asit (%5 ağ.) olan, hacimce %60 bağlayıcı ve %40 seramik tozu içeren karışımlardan basınçsız sıcak şekillendirme yöntemiyle numuneler hazırlanmıştır. Hazırlanan numunelerden bağlayıcı giderme çalışmalarında iki kademeli bir işlem kullanılmıştır. Bağlayıcı giderme çalışmalarının ilk aşamasına ait şartlar, 120°C’de 5 saat olarak belirlenmiştir. Ağırlıkça yaklaşık %33 bağlayıcı giderilen söz konusu şartlar, çalışılan tüm numunelere uygulanmıştır. İkinci aşamada ise, geri kalan bağlayıcı, atmosfer şartlarında bağlayıcı giderme fırınında 600°C’nin al-tında tamamen giderilmiştir. Bu işlem tamamlandıktan sonra numuneler, 1600°C’de 3 saat sinterlenmiş ve oda sıcaklığına soğutulmuştur. Sinterlenmiş bazı numuneler 1450°C’de 4 saat yaşlandırmaya bırakılmıştır. Sinterlenmiş numunelerin XRD ve SEM kullanılarak faz analizi ve mikroyapısal karakterizasyonu yapılmıştır.  Anahtar Kelimeler: Stabilize zirconia, basınçsız sıcak şekillendirme, bağlayıcı giderme, sinterleme, karakterizasyon.Oxide ceramics include alumina, zirconia, silica, aluminum silicate, magnesia and other metal oxide based materials. Oxide ceramics have high melting points, low wear resistance, and a wide range of electrical properties. Oxide ceramics are used in a variety of applications. Examples include chemical and materials processing, electrical and high voltage power applications, Radio Frequency (RF) and microwave applications, and foundry and metal processing. "Zirconia-Ceramic steel" The title of the first scientific paper to highlight the possibilities offered by the "transformation toughening" mechanism which occurs in certain zirconia ceramics. Since the publication of this seminal work in 1975, considerable research, development, and marketing efforts have been expanded on this single material which offer the traditional ceramic benefits of hardness, wear resistance and corrosion resistance, without the characteristic ceramic property of absolute brittleness. Zirconia, ZrO2 exists as a monoclinic crystal at room temperature inverting to tetragonal phase above approximately 1200°C. The addition of large amounts of a stabilizer such as magnesium oxide will induce a cubic crystal structure during firing that does not revert to the monoclinic phase upon cooling. The addition of generally less than 10% by weight of stabilizers yields high-density ceramic bodies known as transformation toughened zirconia. Two fundamentally different microstructures exist depending upon the added stabilizer. The addition of MgO yields a relatively coarse grained (50-100 micron) microstructure known as Mg-PSZ. The grains are predominantly cubic morphology with a fine precipitate of tetragonal phase dispersed within the grain. Yttria, Y2O3, additions yield an extremely fine grained (less than 1 micron) microstructure known as Y-PSZ or TZP or tetragonal zirconia polycrystal. The individual grains are completely tetragonal. In present study, Calcia and Magnesia stabilized fine Zirconia powders in various ratios were studied. Powder mixtures (Ca(OH)2, Mg(OH)2 and ZrO2) were prepared in acetone and milled as wet with YSZ (yitria stabilized zirconia) balls of 10 mm Ø for 3 h. Particle size of Calcia, Magnesia and Zirconia powders were submicrometer. The pressureless hot molding technique has been selected for forming of the partial stabilized zirconia with MgO and CaO. Binder system for pressureless hot molding was a mixture of paraffin wax as primary binder and oleic acid as surfactant. Paraffin wax/oleic acid ratio was maintained at a value close to 95/5 as weight. Typical feedstock contained 40 vol. % ceramic powders, and 60 vol. % binder phases. A two-step process was used for debinding of green samples. In wicking step by capillary action, samples embedded in same ZrO2 powder in a copper box were put in an furnace in the 120, 150 and 180°C temperatures for 1, 3, 5 and 10 hours. The variation of sample weight with temperature and time was measured to calculate the debinding ratio. In the second step of debinding process, partially debinded samples were heated to 600°C at a slow heating rate. After completing this process, green ceramics were fired at 1600 °C for 3 hours in an electric-heated furnace and then cooled quickly. Some of the fired ceramics were aged in the same furnace for 4 hours at 1450 °C. After annealing, the some specimens were sectioned, ground, polished to 1 mm surface-finish and finally thermally etched in air for 30 min. at a temperature 1420 °C. The density and porosity of sintered samples were measured by Archimedes principle. The microstructural characterization of the sintered samples was carried out using scanning electron microscopy. The morphological parameters of the various phases were characterized by using a semiautomatic image analyzer, EDS and the formed phases were analyzed by X-ray powder diffractometer using Co K? radiation. Ca-ZrO2 and Mg-ZrO2 ceramics have been prepared by solid state reaction synthesis. The beneficial effects on the binder removal, microstructural characterization and phase structure of zirconia ceramics prepared by pressureless hot molding have been analyzed. t- ZrO2 and c- ZrO2 phase value increased with the addition of Calcia (CaO) and fired temperature and time. The addition of Calcia (CaO) to Zirconia (ZrO2) matrix the highest the value of t- ZrO2 and c- ZrO2. Keywords: Stabilized zirconia, pressureless hot molding, debinding, sintering, characterization

Synthesis of B4C thin films by plasma-enhanced magnetron sputtering

Bor karbür, yüksek sertliği, mekanik, tribolojik, elektronik, optik özelliklerinin yanı sıra yüksek nötron absorblama özelliği ile de dikkat çeken bir malzemedir. Düşük yoğunluğu, yüksek Young modülü, çok yüksek termal ve kimyasal kararlılığı vb. özellikleri olan B4C, elmas ve c-BN’den sonra bilinen en sert malzeme olmasına karşın bu durum oda sıcaklığında geçerlidir. Elmas ve c-BN’ün sertliği artan sıcaklıkla kademeli olarak düşerken B4C termal kararlılığı sayesinde yüksek sıcaklıklarda sertliğini muhafaza etmektedir ve özellikle 1100 °C’nin üzerindeki sıcaklıklarda bilinen en sert malzemedir. Bu özellikleriyle B4C ince filmler, kesici takımların, fren balatalarının, sabit disklerin ve çeşitli makine parçalarının kaplanması gibi mekanik, tribolojik uygulamaların yanı sıra, yüksek sıcaklık ortamları gibi zorlayıcı şartlarda çalışacak transistörler vb. elektronik ve optik uygulamalarda da kullanılmaktadır. Bu çalışmada, bor karbür tozlarının sıcak preslenmesiyle elde edilmiş olan bor karbür hedef malzeme kullanılarak, plazma-destekli doğru akım manyetik alan sıçratma tekniğiyle 350–400 nm kalınlığında homojen ve taban malzemeye iyi yapışan bor karbür ince filmler üretilmiştir. Biriktirme sıcaklığı tüm kaplamalar için 250 °C olarak sabitlenmiş ve 0–250 V arasında uygulanan taban malzeme voltajının kaplama yapısına olan etkileri incelenmiştir. Elektron sondası mikro analizleri (EPMA), kaplamaların elementel bileşimlerinin üretim şartlarından bağımsız olduğunu ortaya koymuştur. Kesitten gerçekleştirilen taramalı elektron mikroskobu (SEM) incelemeleri sonucunda, bu çalışma şartlarında bor karbür filmlerin kolonsuz yapıda biriktiği tespit edilmiştir. Nanosertlik testleri neticesinde, taban malzeme voltajının arttırılmasıyla sertlik ve Young modülü değerlerinde önemli bir artışın meydana geldiği tespit edilmiştir. Anahtar Kelimeler: Bor karbür, ince film, sıçratma, iyon bombardımanı, nanomekanik.Boron carbide (B4C) is the third hardest material at room temperature, which has many other attractive properties such as good wear resistance, high modulus, high chemical and thermal stability. These properties make boron carbide a promising candidate as hard and protective coating for cutting tools, automobile parts, hard disk drives and other wear-resistant applications. Within the range of 8-20% of carbon, boron carbide is ideally described by a rhombohedral unit cell with icosahedral arrangement of 12 atoms, in addition to a three-atom chain along the crystallographic c-axis that interconnect the icosahedra. For amorphous boron carbide films deposited by sputtering, it is believed that the structure is still based on a random icosahedral network at a carbon content less than 50%. Several techniques have been used to synthesize boron carbide thin films, including chemical vapor deposition, plasma-enhanced chemical vapor deposition, cathodic arc, atmospheric plasma spraying, electromagnetically accelerated plasma spraying, RF magnetron sputtering, and DC magnetron sputtering. Among these, magnetron-sputtering techniques have been successfully commercialized in a large scale because of their high film-deposition rate and low-temperature features. In this study, boron carbide powders were obtained from the carbothermal reduction of boric acid in a graphite resistance furnace at 2000 ºC. The powder thus obtained was hot pressed in pure nitrogen atmosphere with 100 MPa applied force at 2100 °C for 15 minutes to obtain boron carbide target used in this study. The sputtering target was 15 cm in diameter and 7 mm in thickness. Boron carbide thin films were deposited by plasma-enhanced DC magnetron sputtering of hot-pressed boron carbide target. AISI M2 steel and Si (100) wafers were used as substrates in each deposition. High-purity (99.999%) Ar was used as precursor and was introduced into the vacuum chamber through a mass flow controller to establish the desired working pressure, which was 0.3 Pa. The cathode power was fixed at 500 W for all the experiments. Microstructural examinations revealed the presence of continuous and homogeneous B4C films with 350-400 nm thickness. Elemental composition of the films was measured by EPMA. Grazing-angle XRD of the coatings over showed no characteristic peaks for boron carbide, indicating that the coatings were amorphous. Infrared spectra presented two broad bands, one centered at ~1100 cm-1 and the other at ~1570 cm-1. The band at 1100 cm-1 is attributed to B?C bonds in the icosahedra and is characteristic of B4C thin film structure. The band at 1570 cm-1 has been attributed either to the presence of graphite or free carbon in boron carbide structure or to the stretching in the linear chains that interconnect the icosahedra. Nanoindentation results demonstrated that boron carbide films deposited are remarkably hard and the increase in the negative bias voltage led to an increase in the measured hardness from 32 GPa for the film deposited at floating potential to a maximum value of ~40 GPa for the coating deposited at 100 V bias voltage. Further increase in the bias voltage to 250 V resulted with a decrease in the hardness to 32 GPa. The same tendency of increase was observed for the Young's modulus, from 270 GPa for the films deposited at floating potential, it reached its maximum value for 300 GPa at 100 V bias voltage and then decreased thereafter to 265 GPa for 250 V bias voltage. Modified sputtering configuration led non-columnar, featureless microstructures with smooth surface morphologies. There was no significant effect of the deposition parameters on the thickness and elemental composition of the films deposited. Films were amorphous and exhibited remarkably high hardness and Young's modulus values with high elasticity. With the increase in the bias voltage, increases in the hardness and Young's modulus of boron carbide films were observed.  Keywords: Boron carbide, thin film, sputtering, ion bombardment, nanomechanics

Turkey Investigation of Grain Boundary Compositions and Magnetic Properties of Hot-Worked Nd 18 Tb 1 Fe 66

Magnetic properties which are related to the compositions on the grain boundaries of the NdFeB permanent magnets were studied at dierent hot working temperatures applied to the cast Nd18Tb1Fe66.5Co5Al1.5B8 ingots. Since the addition of Al caused the formation of hard magnetic µ phase between the magnetic NdFeB crystals after the pressing 800 • C Hc value increased to the highest value of 8.21 kOe. According to the SEM micrographs and EDS analysis, the increase in pressing temperature, the atomic ratios in the NdFeAl compositions approached to hard magnetic µ phase with the increase in pressing temperature and its exact composition was obtained in the sample hot deformed at 800 • C. Although the secondary µ phase with a high melting temperature is detremental to the alignment of c axis of the magnetic NdFeB grains along the pressing direction, we found that the magnetic properties of the magnets improve due to the presence of this hard magnetic phase on the grain boundaries

An imaging SIMS study on the tribological properties of boron carbide thin films

International audienceIn this study, boron carbide thin films were deposited on AISI M2 grade high-speed steel substrates by plasma-enhanced DC magnetron sputtering of an 'in-house' produced boron carbide target material. Tribological properties of the coatings have been evaluated by a 'pin-on-disc' tribometer. Wear tracks on boron carbide thin films were investigated by scanning electron microscopy-energy dispersive spectroscopy, electron probe microanalysis and secondary ion mass spectrometry (SIMS) elemental ion imaging. The results of analyses provided information about the reliability of the SIMS ion imaging compared to the other investigation techniques on nano-layers and nano-sized thin films

Investigation of Grain Boundary Compositions and Magnetic Properties of Hot-Worked Nd18Tb1Fe66.5Co5Al1.5B8Nd_{18}Tb_1Fe_{66.5}Co_5Al_{1.5}B_8 Magnets

Magnetic properties which are related to the compositions on the grain boundaries of the NdFeB permanent magnets were studied at different hot working temperatures applied to the cast $Nd_{18}Tb_1Fe_{66.5}Co_5Al_{1.5}B_8$ ingots. Since the addition of Al caused the formation of hard magnetic μ phase between the magnetic NdFeB crystals after the pressing 800C $H_{c}$ value increased to the highest value of 8.21 kOe. According to the SEM micrographs and EDS analysis, the increase in pressing temperature, the atomic ratios in the NdFeAl compositions approached to hard magnetic μ phase with the increase in pressing temperature and its exact composition was obtained in the sample hot deformed at 800°C. Although the secondary μ phase with a high melting temperature is detremental to the alignment of c axis of the magnetic NdFeB grains along the pressing direction, we found that the magnetic properties of the magnets improve due to the presence of this hard magnetic phase on the grain boundaries

Gamma Attenuation Behavior of h-BN and h-BN-TiB2TiB_2 Composites

Hexagonal boron nitride (h-BN) and hexagonal boron nitride-titanium diboride (h-BN-$TiB_2$) composites are advanced materials for high-tech applications. They were investigated against gamma radioisotope sources Cs-137 and Co-60 which have gamma peaks 0.662 MeV for Cs-137, and 1.17 and 1.33 MeV for Co-60. Materials have been produced at 1800C temperature in argon atmosphere without pressure during 2 h time. Linear and mass attenuation coefficients were calculated for the materials. Gamma transmission technique was used in the experiments. The experimental investigated mass attenuation coefficients of the materials for Cs-137 and Co-60 gamma radioisotope sources were compared with XCOM computer code. For h-BN-$TiB_2$ composites the ratio of $TiB_2$ in the composites is 55% by weight. So gamma attenuation effects of adding $TiB_2$ to h-BN were discussed. It could be said that adding $TiB_2$ to h-BN increases the linear gamma attenuation of the samples

Gamma Attenuation Behavior of h-BN and h-BN-TiB 2

Hexagonal boron nitride (h-BN) and hexagonal boron nitride-titanium diboride (h-BN-$TiB_2$) composites are advanced materials for high-tech applications. They were investigated against gamma radioisotope sources Cs-137 and Co-60 which have gamma peaks 0.662 MeV for Cs-137, and 1.17 and 1.33 MeV for Co-60. Materials have been produced at 1800C temperature in argon atmosphere without pressure during 2 h time. Linear and mass attenuation coefficients were calculated for the materials. Gamma transmission technique was used in the experiments. The experimental investigated mass attenuation coefficients of the materials for Cs-137 and Co-60 gamma radioisotope sources were compared with XCOM computer code. For h-BN-$TiB_2$ composites the ratio of $TiB_2$ in the composites is 55% by weight. So gamma attenuation effects of adding $TiB_2$ to h-BN were discussed. It could be said that adding $TiB_2$ to h-BN increases the linear gamma attenuation of the samples