Hasil Pencarian  ::  Simpan CSV :: Kembali

"Lantanum manganat LaMnO3 (LMO) adalah material yang sedang menjadi perhatian banyak peneliti sampai saat ini karena memiliki potensi untuk diterapkan pada berbagai aplikasi terutama pada bidang magnetik-elektrik. Modifikasi struktur kristal senyawa LMO melalui subsitusi parsial ion La dan Mn dapat menginduksi sifat elektrik-magnetik seperti giant magneto resistance (GMR) atau colossal magneto resistance (CMR). Berdasarkan telusuran literatur, diketahui bahwa substitusi parsial ion La oleh ion Sr dan ion Mn oleh ion Fe dapat menimbulkan sifat baru, selain GMR atau CMR, juga memiliki kemampuan menyerap gelombang elektromagnetik, khususnya dalam rentang frekwensi ultra-tinggi (GHz). Dengan demikian senyawa LMO termodifikasi adalah merupakan salah satu radar absorbing materials (RAM) yaitu suatu material berkemampuan menyerap gelombang radar. Pada penelitian ini, dipelajari rekayasa struktur senyawa LMO dengan komposisi (La1-xSrx) (Mn0,25Fe0,5Ti0,25)O3 dimana x =0,25; 0,5; 0,75 dan 1,0. Pada tahapan sintesis material diperkenalkan teknik penggabungan antara pemaduan mekanik (mechanical alloying) dan destruksi sonikasi daya tinggi untuk menghasilkan ukuran rata-rata partikel skala nanometer. Karakterisasi material mencakup observasi struktur mikro, identifikasi fasa, sifat magnetik dan sifat absorbsi. Hasil penelitian menunjukkan bahwa material hasil pemaduan mekanik memiliki distribusi ukuran rata-rata partikel bimodal dengan waktu penghalusan relatif panjang (puluhan sampai ratusan jam) untuk memperoleh ukuran partikel rata-rata terendah. Bila sintesis melibatkan destruksi ultrasonik, distribusi ukuran partikel bersifat monomodal dengan ukuran partikel rata-rata mencapai <100 nm dalam waktu kurang dari 10 jam. Pola difraksi sinar X material memperlihatkan bahwa keseluruhan komposisi memiliki fasa tunggal dikarenakan jari-jari ion La dan Sr setara, demikian juga ion Fe dan Mn. Hasil evaluasi karakteristik serapan gelombang mikro material berdasarkan pengujian Vector Network Analyzer (VNA) memastikan bahwa keseluruhan material bersifat penyerap gelombang mikro dalam jangkau frekwensi 8 ? 15 GHz. Serapan tertinggi terjadi pada frekwensi 14,8 GHz dengan nilai Reflection Loss ~ 1 dB atau 10 % gelombang yang datang diserap oleh material. Efek ukuran partikel dengan nilai rata-rata 90 nm meningkatkan kemampuan penyerapan hingga mencapai lebih dari 60 %. Penggabungan material ini dengan senyawa magnetik hexaferrite pada jaringan komposit memperlihatkan dua serapan setara pada dua frekwensi yang berbeda (10 dan 14,8 Ghz). Pengaruh komposisi pada sistem komposit memberikan efek pelebaran terhadap kedua puncak serapan hingga terbentuk sebuah serapan dengan jangkau frekwensi yang lebar (8-15 GHz). Kesimpulan pada penelitian ini adalah sintesis material penyerap gelombang mikro senyawa (La1-xSrx) (Mn0,25Fe0,5Ti0,25)O3 dengan ukuran rata-rata kristal berskala nanometer diperoleh secara efektif melalui penggabungan teknik pemaduan mekanik dan destruksi ultrasonik. Efek ukuran partikel adalah meningkatkan daya serap material. Penggabungan material ini dengan material magnetik hexaferrite dalam sistem komposit menghasilkan suatu material penyerap gelombang mikro dalam rentang frekwensi serapan yang lebar.

---

Lanthanum manganites, LMO especially those doped LaMnO3, have attracted attentions of many researchers, due to their significant potential for applications in the field of magnetic electronic functional materials. Structural modification either through doping of La with Ca, Sr, and Ba or Mn with Fe, Cu, and Ti has been reported to induce electromagnetic properties such as giant magneto resistance (GMR) or colossal magneto resistance (CMR). A partial substitution of La with Sr or Mn with Fe gives rise to new properties, in addition to the GMR or CMR, in which the substituted LMO has the ability to absorb electromagnetic waves, especially in the ultra-high frequency range (GHz). Thus, doped LaMnO3 can be considered as one of radar-absorbing materials (RAM).

In this study, structural modification of LMO with designated compositions (La1-xSrx) (Mn0.25Fe0.50Ti0.25)O3 where as x = 0.25; 0.50; 0.75 and 1.0 is reported. The materials were prepared by mechanical alloying assisted with high-power sonication to produce particles with mean size in a nanometer scale. Material characterization includes the observation of microstructures, identification oh phase materials, magnetic properties and microwave absorption characteristics. It was found that mechanically alloyed of doped LMO have a bimodal particle size distribution and required a relatively long milling time (tens to hundreds of hours) to obtain the lowest average particle size. It was also found that when sintered mechanically alloyed powders were further treated under the application of a high power sonicator, a monomodal particle size distribution with mean particle size of less than 100 nm was obtained within less than 10 hrs. X-ray diffraction traces indicated that synthesized materials are single phase due to ionic radii of La and Sr ions are almost similar. This is also applicable to Fe and Mn ions.

Results of microwave absorption characteristics as evaluated by Vector Network Analyzer ensure that the entire materials have capability to absorb the microwaves in the frequency range 8-15 GHz. The highest absorption was occurred at 14.8 GHz with a Reflection Loss ~ 1 dB. It means that only 10% of the incident wave energy was absorbed by the material. However, materials with the average particle size ~ 90 nm increased the absorption up to 60%. Incorporation the doped LMO with hexaferrite particles in a composite structure has resulted two similar absorption peaks at two different frequencies (10 and 14.8 GHz). Furthermore, variation in composition of composite system was widening the absorption peak into a single peak with a wide range frequency (8-15 GHz).

It is concluded that mechanical alloying coupled with ultra sonication can be an alternative route for the preparation of fine and homogeneous powder materials leading to nanoparticle-based materials. Effect of fine particles in the materials is to increase the microwave absorbing properties. Where as the composite structure is to affect the frequency absorption width."

"Upaya kodoping ZnO dengan dua jenis unsur logam transisi yang berbeda diyakini mampu meningkatkan kualitas sifat room temperature ferromagnetic (RTFM) dari diluted magnetic semiconductor (DMS) ZnO yang didoping menggunakan satu jenis logam transisi saja. Oleh sebab itu pada penelitian ini, dilakukan studi mengenai efek penambahan unsur Mangan (Mn), Kobalt (Co), Nikel (Ni) dan Tembaga (Cu) pada nanopartikel Fe - doped ZnO terhadap perubahan struktur, sifat optik dan sifat magnetiknya.Pembuatan sampel dilakukan dengan metode ko-presipitasi pada temperatur ruangan. Analisis struktur sampel dilakukan menggunakan pengukuran Energy Dispersive X-ray (EDX), X-ray diffraction (XRD), dan Fourier Transform Infrared (FT-IR), sedangkan studi mengenai sifat optiknya dilakukan berdasarkan hasil spektroskopi Uv-vis. Adapun sifat magnetik dari sampel dipelajari melalui pengukuran Electron Spin Resonance (ESR) dan Vibrating Sample Magnetometer (VSM).Pola difraksi XRD menunjukkan bahwa keempat sampel masih memiliki struktur hexagonal wurtzite ZnO dalam batas sensitivitas alat ukur. Hasil pengukuran Uv-vis menunjukkan adanya penurunan nilai celah energi akibat pembentukan mid-gap. Sementara itu, hasil pengukuran ESR menunjukkan adanya pengaruh ion-ion dopan sekunder (Mn, Co, Ni, dan Cu) dalam menentukan sifat magnetik sampel. Dan hasil pengukuran VSM menunjukkan adanya penguatan sifat RTFM yang signifikan.

---

The attempt of codoping ZnO with two different kinds of transition metal elements is believed to be the key to enhance the room temperature ferromagnetism (RTFM) of single transition metal - doped ZnO diluted magnetic semiconductor (DMS). Therefore, within the scope of this research, the effects of adding Manganese (Mn), Cobalt (Co), Nickel (Ni), and Cooper (Cu) regarding to the structural, optical, and magnetic properties change of Fe - doped ZnO nanoparticles have been studied.

The synthesis of the samples was done by co-precipitation method at room temperature. The structural analysis had been performed by Energy Dispersive X- ray (EDX), X-ray Diffraction (XRD), and Fourier Transform Infrared (FT-IR) measurements, meanwhile the optical properties were studied based on the result of Uv-vis spectroscopy. The magnetic properties were studied through Electron Spin Resonance (ESR) and Vibrating Sample Magnetometer (VSM) measurements.

The diffraction pattern of XRD shows that all of the samples still possess hexagonal wurtzite ZnO structure within the sensitivity limit of the spectrometer. The Uv-vis measurement results indicate the decrease in band gap due to the forming of mid-gaps. Meanwhile, ESR measurement results reveal the influence of secondary dopant ions (Mn, Co, Ni, and Cu) that affects the magnetic behavior. Moreover, the VSM measurement result shows a significant enhancement of RTFM."

"This book covers both basic physics of ferromagnetism, such as magnetic moment, exchange coupling, magnetic anisotropy, and recent progress in advanced ferromagnetic materials. Special focus is placed on NdFeB permanent magnets and the materials studied in the field of spintronics (explaining the development of tunnel magnetoresistance effect through the so-called giant magnetoresistance effect)."

"This book describes and provides design guidelines for antennas that achieve compactness by using the slot radiator as the fundamental building block within a periodic array, rather than a phased array. It provides the basic electromagnetic tools required to design and analyse these novel antennas, with sample calculations where relevant. The book presents a focused introduction and valuable insights into the relevant antenna technology, together with an overview of the main directions in the evolving technology of compact planar arrays. While the book discusses the historical evolution of compact array antennas, its main focus is on summarising the extensive body of literature on compact antennas. With regard to the now ubiquitous slot radiator, it seeks to demonstrate how, despite significant antenna size reductions that at times even seem to defy the laws of physics, desirable radiation pattern properties can be preserved. This is supported by an examination of recent advances in frequency selective surfaces and in metamaterials, which can, if handled correctly, be used to facilitate physics-defying designs. "