Structural transformations in (CoFeNi)/Ti nanocomposite systems during prolonged heating

Romankov, S.,Park, Y.C.,Shchetinin, I.V. Elsevier 2018 Journal of alloys and compounds Vol.745 No.-

<P><B>Abstract</B></P> <P>A complex CoFeNi/Ti nanocomposite system with an average grain size of about 8 nm was fabricated on a Ti sheet under ball collisions. Heating experiments were performed at 400, 500, and 600 °C with hold times of up to 100 h at each temperature. The as-fabricated (CoFeNi)/Ti nanocomposite system demonstrated high thermal stability upon heating to 400 and 500 °C. Growth of the CoFeNi phases was retarded by closely spaced Ti particles. After heating to 600 °C, the system exhibited a bimodal nanograin structure due to coursing of the body-centered cubic (bcc) CoFeNi grains, which occurred more rapidly than with the face-centered cubic (fcc) CoFeNi grains. Upon heating, the diffusion that occurred between the phases tended to equilibrate the composition. Pairwise atomic interactions between the Co, Fe, and Ni components arose as chemical interactions in the corresponding binary alloy. The diffusive flux of elements in the system was outlined as Co from the fcc phase diffused into the bcc phase, Ni from the bcc phase diffused into the fcc phase, and Fe from the fcc phase diffused into the bcc phase. The activation energy for the reaction was calculated using time dependence curves of saturation magnetization. The activation energy coincided very closely with the value for the grain boundary diffusion activation energy for the Co, Fe, and Ni binary systems.</P> <P><B>Highlights</B></P> <P> <UL> <LI> (CoFeNi)/Ti nanocomposite system demonstrated high thermal stability during heating. </LI> <LI> Growth of the CoFeNi phases was retarded by closely spaced Ti particles. </LI> <LI> After heating to 600 °C, the system exhibited a bimodal nanograin structure. </LI> <LI> The diffusive flux of elements in the system was outlined. </LI> <LI> Activation energy was calculated using time dependence curves of magnetization. </LI> </UL> </P>

Relaxation and Phase-transition Characteristics of Relaxor Ferroelectric Potassium Lithium Niobate

전병옥,정중현,최병춘,황윤화 한국물리학회 2015 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.66 No.11

The electric modulus relaxations were considered to be coupled phenomena between the polarization fluctuations < P2 > due to local symmetry breaking and ionic hopping through nearest neighbor sites. The Nb-rich potassium lithium niobate (K5.595Li3.125Nb11.28O30) crystals exhibited a ferroelectric diffused phase transition around the dielectric maximum temperature Tmax = 350 C. The electric modulus relaxations were characterized by using the Cole-Davidson distribution of the electric modulus relaxation times at frequencies ranging from 100 Hz to 1 MHz. Although the lattice constants along the a-axes and the c-axes and the tetragonal unit cell volume showed linear expansions with increasing temperature T, the axial ratio c/a decreased with increasing T at temperatures below the Burn’s temperature TB = 550 C. The dielectric relaxation accompanied the high ionic conduction in the temperature range above Tmax = 350 C. The ac conductivity 0(!) was analyzed by using the formulae 0(!)/dc = 1 + (!/!O)n, where !O is the crossover frequency. It was considered that a double-Arrhenius behavior of the dc conductivity dc at temperatures around the Burn’s temperature TB = 550 C was suspected of thermally activated motions in the random distribution of oxygen vacancies and lithium ions. The ac universality could result in a disordered configuration of the mobile ions, i.e., the dc conductivity at temperatures above TB. The slow relaxations of the nano-sized polar regions may contribute to the dc conductivity dc at temperatures below TB. The characteristic relaxation time CD showed a change in activation energy from 1.08 eV to 0.80 eV at temperatures around TB. The scaling factor " ·T/"0 1 increased in proportion to (TB − T) at T < TB, but was almost constant at T > TB, where "0 1 is the high frequency dielectric constant and T is the temperature in Kelvin.

$Pb(Mg_{1/3}Nb_{2/3})O_3-PbTiO_3$ 계 완화형 강유전체의 특성에 미치는 $PbTiO_3$ 첨가량의 변화 -I.유전특성 및 초전특성-

박재환,흥국선,박순자 한국세라믹학회 1996 한국세라믹학회지 Vol.33 No.4

PMN계에서 유전적 성질에 관한 연구들은 광범위하게 이루어져 왔으나 전계인가 변위특성에 관한 폭넓은 연구는 매우 드물다. 본 연구에서는 완화형 강유전체의 가장 대표적인 PMN계에서 PT의 함량 및 측정 온도의 변화에 따른 유전적 성질 및 전계인가 변위특성을 광범위하게 조사하는 것을 목표로 하고 있다. 전형적인 columbite precur-sor법에 의해 분말을 준비하고 고상소결방법에 의하여 모든 시편을 제조하였다. PT의 함량이 증가하면서 직선적으로 (T$\varepsilon$max)가 증가하였으며 유전율은 13000~22000 사이의 값을 보였다. 유전율의 온도 의존성은 PMN의 경우 전형적인 완화형 강유전체의 유전 특성을 보이지만 PT의 함량이 증가될수록 보통 강유전체의 sharp한 거동을 보였다. 전계인가 변위특성에서는 단순히 유전율이 최대가 되는 온도에서 발생 strain이 최대가 되는 것은 아니며 tetragonal-cubic의 phase transition에서는 strain의 증가가 없고 rhombohedral에서 tetragonal 혹은 rhombohed-ral에서 cubic으로의 상전이에서만 strain이 크게 나타남을 알 수 있었다. In order to understand the electrostrictive behavior of Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT) solid solutions the dielectric constants and the electric-field-induced strains in (1-x)PMN-xPT (x=0.0-0.4) were investigated in the temperature range -5$0^{\circ}C$-20$0^{\circ}C$. Powder of (1-x)Pb(Mg2/3Nb2/3)O3-xPbTiO3 (x=0.0, 0.1, 0.2, 0.3, 0.35, and 0.4) were prepared from the oxide forms of Pb, Mg, Nb and Ti via a columbite precursor method As the amount of PbTiO3 increases the temperature of maximum dielectric constant(T$\varepsilon$max) increases and the phase transition become less diffusive. The strain maximum occurs only when the diffuse phase transition occurs from rhombohedral to cubic or rhombohedral to tetragonal as in x=0.1-0.35 The strains monotonically decrease with temperature in the materials in which phase transition occurs from tetragonal to cubic as in x=0.4.

Driven Contact Process with Temporal Feedback

하미순 한국물리학회 2020 새물리 Vol.70 No.7

We investigate a directed percolation (DP)-based cyclically coupled model with branching bias, namely, the driven contact process with temporal feedback (DCP-TF). In the DCP-TF, we control not only the temporal feedback by using its memory strength and the power-law tail exponent for the incubation period of the intermediate state but also the preferred direction of branching. The CP-TF in a one-dimensional lattice exhibits continuously varying scaling behaviors, which is attributed to the long-term memory caused by the power-law type feedback. This implies that the CP-TF does not belong to the DP universality class. However, its deviation from the DP exponent is not very big. To clarify this issue, we test the relevance of the external driving to the critical behaviors of CP-TF. If it belongs to the DP universality class, the branching bias is irrelevant to its critical exponents. We find that the branching bias does affect both the region of continuously varying scaling and the value of critical decay exponent. Based on numerical results and intuitive arguments in the DCP-TF, we discuss how the universality class of the CP-TF is related to the universality class of the pair CP with diffusion.

Phase Transition and Relaxor Behaviors in the Lead Magnesium Niobate-based Ferroelectrics

Y. J. KIM(김연중),J. H. LEE(이주호) 한국진공학회(ASCT) 2008 Applied Science and Convergence Technology Vol.17 No.2

PMN계 강유전체의 MPB 조성과 공존영역의 확산성을 이해하기 위해 유전 및 초전 특성을 분석하였다. PMN-PT계는 완만한 상전이를 보였으며, 이는 하나의 상유전과 2개의 강유전상이 공존함을 의미한다. 즉, 0.7PMN-0.3PT 조성 부근에서 능면정계와 정방정계의 공존 영역인 조성변태 상경계를 확인하였으며, 이들 조성은 연속적으로 능면정계→정방정계→입방정계로 상전이를 하였다. PT>0.325인 조성들은 측정된 진동수 범위에서 진동수 의존성을 보이지 않는 단일상의 강유전 특성을 보였으며, PMN>0.7인 조성들은 큰 진동수 의존성과 확산상전이의 전형적인 완화형 강유전 특성을 보였다. PMN-PT계의 조성변태 상경계 조성들은 우수한 유전 및 초전특성을 나타내었다. Dielectric and pyroelectric properties of relaxor ferroelectric in the PMN-PT solid solution series have been investigated. Features of the diffuse phase transition in PMN-PT system, typical relaxor ferroelectric materials, were studied as a function of temperature and frequency. The transition temperature of the ceramics with PT~0.325 did not depend on the measuring frequency. This can best realized in a relatively random environment that apparently is provided by PMN-rich complex perovskites, including those containing Pb. The composition with PT>0.35 show the characteristics of a normal single phase ferroelectric material. Thus the studies revealed that the morphotropic phase boundary in the PMN-PT system is in the vicinity of PT~0.3 and it has a small curvature and as a result the compositions near the morphotropic phase boundary show two phase transitions, rhombohedral→tetragonal→cubic, when the samples are heated up to higher temperature. The best optimum compositions are observed near the morphotropic phase boundary.

Magnetic Phase Transitions in Zn1−xMnxO

Kh. T. Igamberdiev,Sh. U. Yuldashev,권영해,강태원,S. A. Chizhik,A. V. Trukhanov,Sh. M. Rakhimova 한국물리학회 2014 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.64 No.10

Thermal diffusivity measurements have been used to study the magnetic phase transitions in ZnOdoped with Mn. The temperature dependencies of the thermal diffusivity in the wide temperaturerange from 70 to 300 K reveal an inverted-lambda-shaped peak at a temperature of 75 K forZn0.95Mn0.05O doped with nitrogen and additional peaks at 117 K and 122 K for the nitrogen-dopedand the undoped Zn0.95Mn0.05O samples, respectively. The lambda-shaped peak demonstrates theexistence of a second order phase transition in these samples. The thermal diffusivity peak at 75 Kwas attributed to a ferromagnetic-paramagnetic phase transition in the Zn0.95Mn0.05O doped withnitrogen, and the peaks at 117 K and 122 K were attributed to an antiferromagnetic-paramagneticphase transition in the MnO nanoclusters.

Diffused Phase Transition of Ba1−xEuxTiO3 Ceramics

전병억,최병춘,정중현,문병기 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.68 No.12

By applying the sol-gel method, we fabricated Ba1−xEuxTiO3 (BET) ceramics as a single peroveskite phase in the composition range of x = 0 0.20. The BET ceramics displayed a ferroelectric phase transition temperature that changed from 120 C to 80 C, and exhibited the coexistance of the tetragonal, and cubic structures as the Eu composition was increased. They also displayed anomalous dielectric behaviors related to structural relaxation in the temperature range from 200 C to 600 C. We considered the Arrhenius temperature dependence of the dielectric relaxation time by using the electric modulus formalism. The characteristic activation energy was thought to be related with the substitution of Eu (Eu2+, Eu3+) ions for Ba2+ or Ti4+ ions in the perovskite structure.

A Brief Review on Relaxor Ferroelectrics and Selected Issues in Lead-Free Relaxors

Chang Won Ahn,Chang-Hyo Hong,Byung-Yul Choi,Hwang-Pill Kim,Hyoung-Su Han,황영훈,Wook Jo,Ke Wang,Jing-Feng Li,Jae-Shin Lee,Ill Won Kim 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.68 No.12

Relaxor ferroelectricity is one of the most widely investigated but the least understood material classes in the condensed matter physics. This is largely due to the lack of experimental tools that decisively confirm the existing theoretical models. In spite of the diversity in the models, they share the core idea that the observed features in relaxors are closely related to localized chemical heterogeneity. Given this, this review attempts to overview the existing models of importance chronologically, from the diffuse phase transition model to the random-field model and to show how the core idea has been reflected in them to better shape our insight into the nature of relaxor-related phenomena. Then, the discussion will be directed to how the models of a common consensus, developed with the so-called canonical relaxors such as Pb(Mg1/3Nb2/3)O3 (PMN) and (Pb, La)(Zr, Ti)O3 (PLZT), are compatible with phenomenological explanations for the recently identified relaxors such as (Bi1/2Na1/2)TiO3 (BNT)-based lead-free ferroelectrics. This review will be finalized with a discussion on the theoretical aspects of recently introduced 0−3 and 2−2 ferroelectric/relaxor composites as a practical tool for strain engineering.

세라믹 엑튜에이터 응용을 위한 Sn 치환의 PLZT(7.5/70/30)의 변형특성

고태경,강현구,박재환 한국세라믹학회 1998 한국세라믹학회지 Vol.35 No.5

PLZT(7.5/70/30) was initially a cubic phase having diffuse phase transition at high temperatures. Sn-sub-stitution for PLZT(7.5/70/30) underwent an irreversible phase transition from cubic to rhom-bohedral structure. However PLZTS(7.5/70/30/y=5 & 10) could be reversibly switched from paraelectric to ferroelectric phase under electric field without showing a significant change in crystal structure. With in-creasing the amount of Sn-substitution the P-E behaviors of the PLZTS became more antiferroelectric which was similar to the effect of La-substitution of PLZT. Our study may suggest that Sn-substitution ef-fectively weakens a formation of long-range order between polar Ti-or Zr- containing octahedra which greatly affects strain properties.

Yttrium이 첨가된 BaTiO₃에서 형성된 core/shell 구조에서 shell의 TCC 거동: 독립적 관찰

전상채,Jeon, Sang-Chae 한국결정성장학회 2020 한국결정성장학회지 Vol.30 No.3

MLCC(Multi-Layer Ceramic Capacitor)의 유전체 층에 사용되는 BaTiO₃ 입자는 안정한 TCC(Temperature Characteristics of Capacitance) 거동을 갖기 위해 core/shell 구조를 갖는다. 지금까지 shell의 특성은 core/shell 구조의 전체 특성에서 유추해 왔다. 이는 core/shell 구조가 겨우 수 ㎛의 작은 크기로 shell 특성만 구별해서 측정하기가 어렵기 때문이다. 본 실험에서는 micro-contact법을 이용하여 확산쌍 시편의 계면에 형성된 확대된 core/shell 구조에 Pt 전극을 증착하여 35~135℃ 에서 shell 영역의 독립적인 TCC 거동을 측정하였다. 그 결과, 65℃에서 최대 유전율 값을 갖는 완만한 피크의 확산 상전이(Diffusion Phase Transition) 거동인 core의 특성과 구별되는 거동을 관찰하였으며, 이는 core/shell 구조의 온도-유전거동을 묘사하는 모델링에서 실험 자료로 활용될 것으로 본다. Grains in the BaTiO₃, which is used for a dielectric layer in MLCC(Multi-Layer Ceramic Capacitor) are necessary to form core/shell structure for a stable TCC(Temperature Coefficient of Capacitance) behavior. The shell property has been deduced from the whole TCC behavior of core/shell structure due to its tiny size, ~ few ㎛. This study demonstrates the individual TCC behavior of the shell phase measured by micro-contact measurement in a temperature range between 35 and 135℃. Pt electrode pairs deposited on an enlarged core/shell structure in a diffusion couple sample made the measurement possible. As a result, the DPT (Diffusion Phase Transition) behavior of the shell phase was revealed as a different TCC behavior from that of the core: a broad peak with T_m at 65℃. This would be also useful experimental data for a modelling that depicts dielectric-temperature behavior of core/shell structure.