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12 pages, 5006 KiB

Open AccessArticle

Phase Structure, Bond Features, and Microwave Dielectric Characteristics of Ruddlesden–Popper Type Sr₂TiO₄ Ceramics

by Jun Yang, Jinbiao Pang, Xiaofang Luo, Laiyuan Ao, Qiang Xie, Xing Wang, Hongyu Yang and Xianzhong Tang

Materials 2023, 16(14), 5195; https://doi.org/10.3390/ma16145195 - 24 Jul 2023

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Abstract

This work studied the phase constitution, bond characteristics, and microwave dielectric performances of Sr₂TiO₄ ceramics. Based on XRD and Rietveld refinement analysis, pure tetragonal Ruddlesden–Popper type Sr₂TiO₄ ceramic is synthesized at 1425~1525 °C. Meanwhile, the microstructure is [...] Read more.

This work studied the phase constitution, bond characteristics, and microwave dielectric performances of Sr₂TiO₄ ceramics. Based on XRD and Rietveld refinement analysis, pure tetragonal Ruddlesden–Popper type Sr₂TiO₄ ceramic is synthesized at 1425~1525 °C. Meanwhile, the microstructure is dense and without porosity, indicating its high sinterability and densification. Great microwave dielectric performances can be obtained, namely an ε_r value of 39.41, and a Q × f value of 93,120 GHz, when sintered at 1475 °C. Under ideal sintering conditions, the extrinsic factors are minimized and can be ignored. Thus, the intrinsic factors are considered crucial in determining microwave dielectric performances. Based on the P–V–L complex chemical bond theory calculation, the largest bond ionicity, and proportions to the bond susceptibility from Sr–O bonds suggest that Sr–O bonds mainly determine the dielectric polarizability. However, the Ti–O bonds show lattice energy about three times larger than Sr–O bonds, emphasizing that the structural stability of Sr₂TiO₄ ceramics is dominated by Ti–O bonds, and the Ti–O bonds are vital in determining the intrinsic dielectric loss. The thermal expansion coefficient value of the Sr₂TiO₄ structure is also mainly decided by Ti–O bonds. Full article

(This article belongs to the Special Issue Advances in Dielectric Ceramics and Their Applications)

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10 pages, 4029 KiB

Open AccessArticle

Role of Density and Grain Size on the Electrocaloric Effect in Ba_0.90Ca_0.10TiO₃ Ceramics

by Lavinia Curecheriu, Maria Teresa Buscaglia, Vlad Alexandru Lukacs, Leontin Padurariu and Cristina Elena Ciomaga

Materials 2022, 15(21), 7825; https://doi.org/10.3390/ma15217825 - 6 Nov 2022

Cited by 2 | Viewed by 1404

Abstract

Pure perovskite Ba_0.90Ca_0.10TiO₃ ceramics, with a relative density of between 79 and 98% and grain sizes larger than 1 µm, were prepared by solid-state reaction. The dielectric and electrocaloric properties were investigated and discussed considering the density and [...] Read more.

Pure perovskite Ba_0.90Ca_0.10TiO₃ ceramics, with a relative density of between 79 and 98% and grain sizes larger than 1 µm, were prepared by solid-state reaction. The dielectric and electrocaloric properties were investigated and discussed considering the density and grain size of the samples. Room temperature impedance measurements show good dielectric properties for all ceramics with relative permittivity between 800 and 1100 and losses of <5%. Polarization vs. E loops indicates regular variation with increasing sintering temperature (grain size and density), an increase in loop area, and remanent and saturation polarization (from P_sat = 7.2 µC/cm² to P_sat = 16 µC/cm²). The largest electrocaloric effect was 1.67 K for ceramic with GS = 3 µm at 363 K and electrocaloric responsivity (ζ) was 0.56 K mm/kV. These values are larger than in the case of other similar materials; thus, Ba_0.90Ca_0.10TiO₃ ceramics with a density larger than 90% and grain sizes of a few µms are suitable materials for electrocaloric devices. Full article

(This article belongs to the Special Issue Advances in Dielectric Ceramics and Their Applications)

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20 pages, 9405 KiB

Open AccessArticle

Impedance Spectroscopy of Pr-Doped BaBi₂Nb₂O₉ Aurivillius Ceramics

by Michał Rerak, Jolanta Makowska, Małgorzata Adamczyk-Habrajska and Lucjan Kozielski

Materials 2022, 15(18), 6308; https://doi.org/10.3390/ma15186308 - 11 Sep 2022

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Abstract

Herein this study, the polycrystalline nature of the Aurivillius type structure is studied; primarily, the main objective is to observe the influence of dopant Pr³⁺ at the Ba²⁺-site of BaBi₂Nb₂O₉ (BBN) ceramics. The ceramics under [...] Read more.

Herein this study, the polycrystalline nature of the Aurivillius type structure is studied; primarily, the main objective is to observe the influence of dopant Pr³⁺ at the Ba²⁺-site of BaBi₂Nb₂O₉ (BBN) ceramics. The ceramics under investigation were fabricated via the conventional solid-state reaction method. Scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) techniques were used to analyse their morphological structure. It was found that the chemical composition of the ceramic samples corresponds well to the initial stoichiometry of the ceramic powders. An increase in praseodymium content caused a slight decrease in the average size of the ceramic grains. The obtained ceramic materials are described by a tetragonal structure with the space group I4/mmm. The electrical properties of the material have been studied using complex impedance spectroscopy methods in wide temperature and frequency ranges. The analysis of obtained results showed grains and grain boundaries contribute to conductive processes in the material. A possible ’hopping’ mechanism for electrical transport processes in the system is evident from the analysis of results based on Joncher law. Full article

(This article belongs to the Special Issue Advances in Dielectric Ceramics and Their Applications)

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11 pages, 4252 KiB

Open AccessArticle

Hard Carbon Embedded with FeSiAl Flakes for Improved Microwave Absorption Properties

by Xiaogang Sun, Yi Liu, Daitao Kuang, Jun Lu, Junyi Yang, Xiaomin Peng and Anru Wu

Materials 2022, 15(17), 6068; https://doi.org/10.3390/ma15176068 - 1 Sep 2022

Cited by 6 | Viewed by 1471

Abstract

Carbon-based composites have been proven to be strong candidates for microwave absorbers in recent years. However, as an important member, magnetic hard carbon (HC)-based composites have rarely been studied in the field of microwave absorption. In this study, HC embedded with FeSiAl (FeSiAl@HC) [...] Read more.

Carbon-based composites have been proven to be strong candidates for microwave absorbers in recent years. However, as an important member, magnetic hard carbon (HC)-based composites have rarely been studied in the field of microwave absorption. In this study, HC embedded with FeSiAl (FeSiAl@HC) was synthesized by pyrolyzing a mixture of FeSiAl flakes and phenolic resin (PR). The as-synthesized HC-FeSiAl exhibited a layered structure, and the detailed microstructures were modified by changing the mass ratio of FeSiAl flakes and PR. Thus, the as-synthesized HC-FeSiAl exhibited tunable magnetic properties, wealthy functional groups, excellent thermal stability, and enhanced microwave absorption properties. The optimal minimum reflection loss is lower up to −36.1 dB, and the effective absorption bandwidth is wider up to 11.7 GHz. These results indicated that HC-FeSiAl should be a strong candidate for practical applications of microwave absorption, which may provide new insight into the synthesis of magnetic HC-based composites. Full article

(This article belongs to the Special Issue Advances in Dielectric Ceramics and Their Applications)

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15 pages, 5089 KiB

Open AccessArticle

The Effect of Pr Doping Contents on the Structural, Microstructure and Dielectric Properties of BaBi₂Nb₂O₉ Aurivillius Ceramics

by Michał Rerak, Jolanta Makowska, Katarzyna Osińska, Tomasz Goryczka, Anna Zawada and Małgorzata Adamczyk-Habrajska

Materials 2022, 15(16), 5790; https://doi.org/10.3390/ma15165790 - 22 Aug 2022

Cited by 3 | Viewed by 1298

Abstract

Aurivillius BaBi₂Nb₂O₉ and Ba_1-xPr_xBi₂Nb₂O₉ ceramics were successfully synthesized by a simple solid state reaction method. Ceramics were prepared from reactants: Nb₂O₅, Bi₂O₃ [...] Read more.

Aurivillius BaBi₂Nb₂O₉ and Ba_1-xPr_xBi₂Nb₂O₉ ceramics were successfully synthesized by a simple solid state reaction method. Ceramics were prepared from reactants: Nb₂O₅, Bi₂O₃, BaCO₃ and Pr₂O₃. The microstructure, structure, chemical composition, and dielectric properties of the obtained materials were examined. Dielectric properties were investigated in a wide range of temperatures (T = 20–500 °C) and frequencies (f = 0.1 kHz–1 MHz). The obtained ceramic materials belong to the group of layered perovskites, crystallizing in a tetragonal structure with the space group I4/mmm. Modification of the barium niobate compound with praseodymium ions influenced its dielectric properties and introducing a small concentration of the dopant ion causes a slight increase in the value of electric permittivity and shifts its maximum towards higher temperatures. Full article

(This article belongs to the Special Issue Advances in Dielectric Ceramics and Their Applications)

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19 pages, 10908 KiB

Open AccessArticle

Effect of Alumina on Crystallization Behavior of Calcium Ferrite in Fe₂O₃-CaO-SiO₂-Al₂O₃ System

by Rui-Feng Xin, Yu Du and Xing-Min Guo

Materials 2022, 15(15), 5257; https://doi.org/10.3390/ma15155257 - 29 Jul 2022

Cited by 5 | Viewed by 1401

Abstract

Al₂O₃ is a gangue component in iron ores, significantly influencing the formation and crystallization of calcium ferrite in the sintering process. But the mechanism of the Al₂O₃ effect on the crystallization of calcium ferrite is rarely reported. [...] Read more.

Al₂O₃ is a gangue component in iron ores, significantly influencing the formation and crystallization of calcium ferrite in the sintering process. But the mechanism of the Al₂O₃ effect on the crystallization of calcium ferrite is rarely reported. In this work, a crystallization device was designed to investigate the crystallization behavior of calcium ferrite in Fe₂O₃-CaO-SiO₂-Al₂O₃ melt under non-isothermal conditions. XRD, SEM-EDS, and optical microscopy were used to identify the crystalline phase and the microstructure of samples. The result shows that the crystal morphology of SFCA changed in the order of strip, column, and needle as the Al₂O₃ content increased. The crystallization sequence of samples containing Al₂O₃ was observed as Ca₄Fe₁₄O₂₅ (C₄F₁₄) → Fe₂O₃ → Ca_3.18Fe_15.48Al_1.34O₃₆ (SFCA-I) → CaFe₂O₄ (CF) → Ca₅Si₂(Fe, Al)₁₈O₃₆ (SFCA) → γ-Ca₂SiO₄ (C₂S). The generation pathway of SFCA-I was found to be C₄F₁₄ + Si⁴⁺ + Al³⁺ → SFCA-I. Increasing the cooling rate can promote the formation of C₄F₁₄, SFCA-I, Fe₂O₃ and the amorphous phase. However, it prevented the crystallization of CF and SFCA while inhibiting the transformation of β-C₂S to γ-C₂S. When the Al₂O₃ content reached or exceeded 2.5 mass pct, the viscosity of Fe₂O₃-CaO-SiO₂-Al₂O₃ melt increased sharply, resulting in the decrease in the crystal size of calcium ferrite. Full article

(This article belongs to the Special Issue Advances in Dielectric Ceramics and Their Applications)

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Review

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17 pages, 1378 KiB

Open AccessEditor’s ChoiceReview

Research Progress of Ba(Zn_1/3Nb_2/3)O₃ Microwave Dielectric Ceramics: A Review

by Sen Peng, Yu Zhang and Tulin Yi

Materials 2023, 16(1), 423; https://doi.org/10.3390/ma16010423 - 2 Jan 2023

Cited by 4 | Viewed by 2007

Abstract

Ba(Zn_1/3Nb_2/3)O₃ (BZN) microwave dielectric ceramics have attracted great attention due to their high-quality factor (Q), near-zero temperature coefficient of resonant frequency (τ_f), and suitable dielectric constant (ε_r), making them promising materials for [...] Read more.

Ba(Zn_1/3Nb_2/3)O₃ (BZN) microwave dielectric ceramics have attracted great attention due to their high-quality factor (Q), near-zero temperature coefficient of resonant frequency (τ_f), and suitable dielectric constant (ε_r), making them promising materials for application in microwave devices. Due to their superior dielectric properties, composite perovskite ceramics are widely used in the field of microwave communication, base stations, navigation, radar, etc. This article summarized the latest research progress of BZN ceramics and discusses the main preparation methods and performance modifications. Furthermore, the problems faced by BZN ceramics and solutions to improve their performance, as well as their potential applications, are analyzed. This article provides a reference for the design and preparation of BZN ceramics. Full article

(This article belongs to the Special Issue Advances in Dielectric Ceramics and Their Applications)

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21 pages, 5105 KiB

Open AccessEditor’s ChoiceReview

Metal Oxide Heterostructures for Improving Gas Sensing Properties: A Review

by Fan-Jian Meng, Rui-Feng Xin and Shan-Xin Li

Materials 2023, 16(1), 263; https://doi.org/10.3390/ma16010263 - 27 Dec 2022

Cited by 16 | Viewed by 2610

Abstract

Metal oxide semiconductor gas sensors are widely used to detect toxic and inflammable gases in industrial production and daily life. The main research hotspot in this field is the synthesis of gas sensing materials. Previous studies have shown that incorporating two or more [...] Read more.

Metal oxide semiconductor gas sensors are widely used to detect toxic and inflammable gases in industrial production and daily life. The main research hotspot in this field is the synthesis of gas sensing materials. Previous studies have shown that incorporating two or more metal oxides to form a heterojunction interface can exhibit superior gas sensing performance in response and selectivity compared with single phase. This review focuses on mainly the synthesis methods and gas sensing mechanisms of metal oxide heterostructures. A significant number of heterostructures with different morphologies and shapes have been fabricated, which exhibit specific sensing performance toward a specific target gas. Among these synthesis methods, the hydrothermal method is noteworthy due to the fabrication of diverse structures, such as nanorod-like, nanoflower-like, and hollow sphere structures with enhanced sensing properties. In addition, it should be noted that the combination of different synthesis methods is also an efficient way to obtain metal oxide heterostructures with novel morphologies. Despite advanced methods in the metal oxide semiconductors and nanotechnology field, there are still some new issues which deserve further investigation, such as long-term chemical stability of sensing materials, reproducibility of the fabrication process, and selectivity toward homogeneous gases. Moreover, the gas sensing mechanism of metal oxide heterostructures is controversial. It should be clarified so as to further integrate laboratory theory research with practical exploitation. Full article

(This article belongs to the Special Issue Advances in Dielectric Ceramics and Their Applications)

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