Materials Science and Engineering / Malzeme Bilimi ve Mühendisliği
Permanent URI for this collectionhttps://hdl.handle.net/11147/4719
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Research Project Ferroelektrik (1-x)BaTiO3-xBi(Li1/3Ti2/3)O3(0≤x≤0.2) sisteminin elektrokalorik özelliklerinin belirlenmesi(2018) Adem, Umut; Çağın, TahirBu projede BaTiO3 bazlı iki farklı malzeme sisteminin sentezi, yapısal, dielektrik, ferroelektrik, elektrokalorik karakterizasyonları yapılarak, farklı mekanizmaların elektrokalorik etkiye etkisi ayrıntılı biçimde incelenmiştir. İlk olarak (1-x)BaTiO3-xBi(Li1/3Ti2/3O3) (0.02?x?0.08) sistemi çalışılmıştır. Bu sistemde literatürde gözlenen kabuk-çekirdek yapısının elektrokalorik etkinin mekanizmalarının anlaşılmasını engelleyeceği fikrinden hareketle, sinterleme öncesi parçacık boyutu yüksek enerjili bilyalı öğütücü yardımıyla düşürülerek difüzyonun daha hızlı gerçekleşmesi sağlanmış ve kabuk-çekirdek yapısı gözlenmemiştir. Sıcaklığa bağlı dielektrik ve ferroelektrik polarizasyon ölçümleri, bu sistemde kompozisyona göre değişen sıcaklıklarda ferroelektrik-antiferroelektrik ve antiferroelektrik-paraelektrik olmak üzere iki farklı faz geçişi olduğuna işaret etmektedir. Bu ilginç ve değişik faz geçişlerini doğrulamak için akım yoğunluğu-elektrik alan grafikleri çizdirilmiş ve gerinme elektrik alan ölçümleri yapılmıştır. Ferroelektrik antiferroelektrik faz geçiş sıcaklıklarında göreceli olarak yüksek elektrokalorik sıcaklık değişimleri gözlenmiştir (22 kV/cm elektrik alan altında yaklaşık 0.66 Kelvin). İkinci olarak ise faz diyagramı literatürdeki bir çalışmadan alınan Ba0.80Sr0.20Ti1-xZrxO3 (0?x?0.10) sistemi çalışılmıştır. Bu sistemde de faz diyagramındaki farklı bölgelerde çalışan farklı mekanizmaların elektrokalorik etkiyi nasıl etkilediği ortaya çıkarılmıştır. Elektrokalorik etki, faz geçişinin keskinliği (1. dereceden faz geçişi olup olmadığı), farklı ferroelektrik fazların bir arada olduğu noktalara yakınlık ve tane boyutu ile kontrol edilebilir. Bu projede en yüksek elektrokalorik etki perovskit yapının B pozisyonunda Zr içermeyen Ba0.80Sr0.20TiO3 örneğinde görülmüştür. Bunun nedeni bu kompozisyonunda faz geçişinin keskinliğidir. Zr katkısıyla faz geçişi yayvanlaşmış, fakat aynı zamanda farklı ferroelektrik fazların bir arada ya da yakın olduğu kompozisyonlar için elektrokalorik etki göreceli olarak yüksek çıkmıştır.Article Ferroelectricity of Ca9fe(po4)7 and Ca9mn(po4)7 Ceramics With Polar Whitlockite-Type Crystal Structure(Sivas Cumhuriyet Üniversitesi, 2020) Adem, UmutCa9Fe(PO4)7 is a member of the double phosphate family having polar whitlockite-type crystal structure. The phase transition from the room temperature polar R3c to the high temperature non-polar R c phase has been called a ferroelectric phase transition using complementary experiments such as temperature dependent second harmonic generation and dielectric constant measurements however no ferroelectric hysteresis measurement has been reported. In order to be able to call these polar materials ferroelectric, measurement of a saturated ferroelectric hysteresis loop is necessary to demonstrate that the electrical polarization of these materials is switchable. In order to realize this goal, we have synthesized Ca9Fe(PO4)7 as well as structurally identical Ca9Mn(PO4)7 using solid state synthesis. Crystal structure of the ceramics were confirmed using Rietveld refinement of the x-ray diffraction (XRD) patterns. Differential scanning calorimetry (DSC) measurements revealed phase transition temperatures of 848 and 860 K for Ca9Fe(PO4)7 and Ca9Mn(PO4)7, respectively. Our ferroelectric hysteresis measurements and current electric field loops (I-E) derived from the hysteresis loops showed that the loops cannot be saturated and the direction of the electrical polarization of both materials cannot be switched up to the largest applied electric field of 100 kV/cm. Possible origins of this behaviour are discussed.Article Citation - WoS: 20Citation - Scopus: 21Data-Driven Computational Prediction and Experimental Realization of Exotic Perovskite-Related Polar Magnets(Nature Publishing Group, 2020) Han, Yifeng; Wu, Meixia; Gui, Churen; Zhu, Chuanhui; Sun, Zhongxiong; Zhao, Mei-Huan; Adem, Umut; Li, Man-RongRational design of technologically important exotic perovskites is hampered by the insufficient geometrical descriptors and costly and extremely high-pressure synthesis, while the big-data driven compositional identification and precise prediction entangles full understanding of the possible polymorphs and complicated multidimensional calculations of the chemical and thermodynamic parameter space. Here we present a rapid systematic data-mining-driven approach to design exotic perovskites in a high-throughput and discovery speed of the A(2)BB'O-6 family as exemplified in A(3)TeO(6). The magnetoelectric polar magnet Co3TeO6, which is theoretically recognized and experimentally realized at 5 GPa from the six possible polymorphs, undergoes two magnetic transitions at 24 and 58 K and exhibits helical spin structure accompanied by magnetoelastic and magnetoelectric coupling. We expect the applied approach will accelerate the systematic and rapid discovery of new exotic perovskites in a high-throughput manner and can be extended to arbitrary applications in other families.Article Citation - WoS: 14Citation - Scopus: 17Electrical Characteristics of Low Temperature Densified Barium Titanate(Elsevier, 2020) Karacasulu, Levent; Tokkan, Melike; Bortolotti, Mauro; Ischia, Gloria; Adem, Umut; Ahmetoğlu, Çekdar VakıfA low temperature densification technique, i.e. reactive hydrothermal liquid-phase densification (rHLPD) was followed to obtain highly dense BaTiO3 components at temperatures <= 240 degrees C. The formed ceramics were characterized concerning not only the structural features but also the electrical properties. The increase of both reaction time and temperature resulted in enhanced densification of BaTiO3 components reaching about 90% of theoretical density. The presence of the tetragonal BaTiO3 was demonstrated by both XRD and TEM analysis. Despite the low reaction temperatures, the samples showed promising dielectric, ferroelectric and piezoelectric functionality without additional annealing. A broad dielectric peak was observed around 135 degrees C at the Curie temperature; saturated hysteresis loops and corresponding butterfly-shaped strain-electric field loops were obtained. BaTiO3 sample subjected to hydrothermal reaction at 240 degrees C for 72 h yielded a piezoelectric coefficient of 84 pC/N.Article Citation - WoS: 13Citation - Scopus: 16Electrocaloric Properties of Ba0. 8sr0. 2ti1-Xzrxo3 (0? X? 0.1) System: the Balance Between the Nature of the Phase Transition and Phase Coexistence(Elsevier Ltd., 2020) Şanlı, Keriman; Adem, UmutWe investigate the electrocaloric effect of Ba0.8Sr0.2Ti1-xZrxO3 (0 ≤ x ≤ 0.1) system by comparing the electrocaloric temperature change (ΔT) of different compositions belonging to the different regions of the phase diagram. We show that as the amount of Zr increases, electrocaloric temperature change initially decreases as the phase transition gets diffuse then increases again as the composition of the samples are located closer to the critical point where different ferroelectric phases coexist. Since the critical point is reached at relatively low Zr substitution levels (i.e. around x = 0.07), the phase transition doesn't get too diffuse and thefore the compositions between x = 0 and x = 0.10 (which contains higher Zr than the critical point composition) have comparable ΔT values. Electrocaloric efficiency of these compositions (x = 0.03, 0.05 and 0.07) is around 0.20 K mm/kV at 20 kV/cm. We discuss the results in terms of the balance between the nature of the phase transition and proximity to the critical point, based on the phase diagram.Article Citation - WoS: 33Citation - Scopus: 33Influence of Aging on Electrocaloric Effect in Li+ Doped Batio3 Ceramics(Elsevier Ltd., 2019) Güvenç, Çetin Meriç; Adem, UmutAging creates significant changes in the properties of the ferroelectric materials such as dielectric and piezoelectric properties. However, the influence of aging on the electrocaloric effect (ECE) has not yet been investigated. In this work, we investigate the effect of the aging on the ECE in acceptor (Liþ) doped BaTiO3 ceramics. We observe that aging induced defect polarization (PD) reduces the saturation polarization of the doped samples until Tc ¼ 115 C. Above that temperature PD loses its effectiveness and material behaves like a fresh sample. Suppression of polarization below TC due to aging effect results in a sharper slope change in the temperature dependence of electrical polarization in aged samples which causes an increase in the electrocaloric temperature change of Liþ doped BaTiO3 ceramics up to 23% at TC. Above a critical Li doping amount, both negative and positive electrocaloric effect are observed in the same sample.Article Multiferroic Materials: Physics and Properties(Elsevier Ltd., 2016) Buurma, A. J. C.; Blake, G. R.; Palstra, T. T. M.; Adem, UmutMultiferroics are materials in which magnetism and ferroelectricity coexist. They are of fundamental interest to understand electronic behavior coupling magnetic interactions and electric dipolar order. Moreover, they are of applied interest because they allow various types of novel magnetic and electric device structures. We distinguish two important classes of multiferroic materials and discuss mechanisms and materials belonging to each class separately. In the first group of multiferroics, magnetization and polarization arise independently from each other. In the second category of multiferroics, ferroelectricity is induced by magnetic order, resulting in strong magnetoelectric coupling. We also briefly discuss multiferroic thin film heterostructures showing interfacial magnetoelectric coupling interactions with potential applications in memory devices.