Chang Jun Jeon

Effects of Crystal Structure on Microwave Dielectric Properties of Ceramics

Microwave dielectric properties of MgTiO₃, MgWO₄, MgNb₂O?, and MgTa₂O? were investigated based on the structural characteristics. The dielectric constant (K) was dependent on the dielectric polarizabilities of the specimens, and the deviation of the observed dielectric polarizabilities (α obs. ) from the theoretical dielectric polarizabilities (α theo. ) were decreased with increasing of Mg-site bond valence. Quality factors (Qf) were affected by the sharing type of MgO? and BO? octahedra. Temperature coefficient of resonant frequency (TCF) was decreased with increasing of average octahedral distortion.

Effects of Ceramics on Dielectric Properties of Polystyrene / Ceramics Composites at Microwave Frequencies

Dependencies of dielectric properties on MgTa₂O?, MgNb₂O?, and MgWO₄ (Mg-based ceramics) fillers of the polystyrene (PS) matrix composites were investigated as a function of frequency. With increasing frequency from 1 ㎓ to 7.3 ㎓, the dielectric constant (K) of the composites was not changed significantly, while the dielectric loss (tanδ) of the composites was slightly decreased. The K, tanδ, and temperature coefficient of resonant frequency (TCF) of the composites were dependent on the type and amount of ceramics at 11 ㎓. Also, several theoretical models have been employed to predict the effective dielectric constant of the composites and the results were compared with experimental data. Typically, a K value of 6.67, tanδ of 0.56×10?³, and TCF of ?4.99 ppm/℃ were obtained for the PS composites with 0.4 volume fraction of MgNb₂O? at 11 ㎓.

Dependence of dielectric properties on the crystallinity of ceramic/polymer composites

Effects of ZnNb₂O₆ content and crystallinity of polymers on the dielectric properties of ZnNb₂O₆/polytetrafluoroethylene (PTFE), polypropylene (PP), and polystyrene (PS) composites were investigated at microwave frequencies. With increasing ZnNb₂O₆ content, the dielectric constant (K) of the composites increased, whereas the dielectric loss (tanδ) and temperature coefficient of resonant frequency (TCF) decreased. The tanδ of the composites with amorphous PS was lower than those of the composites with semi-crystalline PP and PTFE. For the composites with semi-crystalline PTFE and PP, the tanδ was strongly dependent on the degree of crystallinity of composites. Several types of theoretical models were applied to predict the effective dielectric properties of the composites. Typically, K of 5.73, tanδ of 1.45 x 10^-3, and TCF of 2.66 ppm/°C were obtained for the PP composites with 0.5 volume fraction V_f of ZnNb₂O₆.

DEPENDENCE OF MICROWAVE DIELECTRIC PROPERTIES ON STRUCTURAL CHARACTERISTICS OF ILMENITE, TRI-RUTILE AND WOLFRAMITE CERAMICS

Effects of crystal structures on dielectric properties of ATiO3 (trigonal ilmenite), ATa2O6 (tetragonal tri-rutile) and AWO4 (monoclinic wolframite) (A = Ni, Mg, Co) ceramics with A- and B-site oxygen octahedra were investigated at microwave frequencies. The dielectric constant (K) of the specimens was affected by the dielectric polarizabilities of composition and cation bond valence between octahedral cation and oxygen ion per molar volume (Vm). The quality factor (Qf) of ATiO3 was appreciably larger than those of ATa2O6 and AWO4 due to the different sharing types of oxygen octahedra. The temperature coefficient of resonant frequency (TCF) of the specimens was dependent on the octahedral distortion per Vm.

Dependence of Thermal Properties on Crystallization Behavior of CaMgSi 2 O 6 Glass-Ceramics

The effects of thermal properties on the crystallization behavior of CaMgSi2O6 glass-ceramics were investigated as a function of sintering temperature from 800oC to 900oC. The crystallization behavior of the specimens depended on the sintering temperature, which could be evaluated from the differential thermal analysis, X-ray diffraction and Fourier transform infrared spectroscopy. With increasing sintering temperature, the thermal conductivity of the sintered specimens increased, while the coefficient of thermal expansion (CTE) of the sintered specimens decreased. These results could be attributed to the increase of crystallization, confirmed from the estimation by density measurements. Also, the thermal diffusivity and specific heat capacity of the sintered specimens were discussed with relation to the sintering temperature. Typically, a thermal conductivity of 3.084 W/moC, CTE of 8.049 ppm/oC, thermal diffusivity of 1.389 mm2/s and specific heat capacity of 0.752 J/gC were obtained for CaMgSi2O6 specimens sintered at 900 C for 5 h.

Effect of Connectivity on Microwave Dielectric Properties of Low Loss Ceramics Filled PTFE Composites

Effects of connectivity patterns on the dielectric properties of polytetrafluoroethylene (PTFE) composites filled with low-loss Zn-based ceramics (ZnWO4, ZnMoO4) were investigated at microwave frequencies. The dielectric constant (K), dielectric loss (tan δ) and temperature coefficient of resonant frequency (TCF) of the 0–3 connectivity composites were dependent on the type and amount of ceramics. For the composites with 0.2 volume fraction (Vf) of ZnMoO4, the K and tan δ increased with number of connectivity and internal strain, respectively. Typical results of K = 2.99, tan δ = 3.155×10−3 and TCF = 0 ppm/°C were obtained for the 0–2 connectivity composites with 0.2 Vf of ZnMoO4.

Structural Properties of Nickel Manganite Thin Films Fabricated by Metal Organic Decomposition

Thin thermistor films of solutions with nickel and manganese oxides were prepared by metal-organic decomposition (MOD). The structural properties of the thin films were investigated as a function of annealing temperature. Field emission scanning electron microscope (FE-SEM) results indicated that the thin films had a thin thickness, smooth and dense surface. The crystallization temperature of was confirmed from thermogavimetric-differential thermal analysis (TG-DTA) curve. A single phase of cubic spinel structure was obtained for the thin film annealed from to , which was confirmed from the X-ray diffraction (XRD). From the selected area electron diffraction (SAED) in high resolution transmission electron microscope (HRTEM), the nano grains (2~3 nm) of spinel phase with (311) and (222) planes were detected for the thin film annealed at , which could be applicable to read-out integrated circuit (ROIC) substrate of the uncooled microbolometer with low processing temperature.

Spin Spray-Deposited Spinel Thin Films for Microbolometer Applications

Spinel thin films were prepared by the spin spray technique to develop new thermal imaging materials annealed at low temperature for uncooled microbolometer applications. The spinel thin films were deposited from [(Ni0.30Co0.33Mn0.37)1-xCux]3O4 (0.1≤x≤0.2) solutions and then annealed at 400°C for 1 h in argon. Effects of Cu content (x) and deposition time on the electrical properties of the annealed films were investigated. With increasing deposition time, the resistivity of the annealed films increased. For the annealed films deposited for 1 min, the resistivity of x=0.15 films was lower than that of x=0.1 films due to the different grain sizes. The high temperature coefficient of resistance (TCR) of the annealed films could be obtained at temperature below 50°C. Typically, the resistivity of 127 Ω·cm and TCR of -5.69%/K at 30°C were obtained for x=0.1 films with deposition time of 1 min annealed at 400°C for 1 h in argon.

Micro-structure and NTCR Characteristics of Copper Manganite Thin Films Fabricated by MOD Process

Copper manganite thin films were fabricated on substrate by metal organic decomposition (MOD) process. They were burned-out at and annealed at various temperatures () for 1h in ambient atmosphere. Their micro-structure and negative temperature coefficient of resistance (NTCR) characteristics were analyzed for micro-bolometer application. The copper manganite film with a cubic spinel structure was well developed at which confirmed by XRD and HRTEM analysis. It showed a low resistivity () at room temperature and high NTCR characteristics of and at room temperature and , implying a good thin film for micro-bolometer application. Furthermore, its crystallinity was enhanced with increasing temperature to . However, the appearance of secondary phase at temperatures higher than lead to deteriorate the NTCR characteristics.

Structural and Electrical Properties of [(Co 1-x Cu x ) 0.2 (Ni 0.3 Mn 0.7 ) 0.8 ] 3 O 4 Spinel Thin Films for Infrared Sensor Application

Co1-xCux)0.2(Ni0.3Mn0.7)0.8)3O4 (0≤x≤1) thin films prepared by metal organic decomposition process were fabricated on SiN/Si substrate for infrared sensor application. Their structural and electrical properties were investigated with variation of Cu dopant. The ((Co1-xCux)0.2(Ni0.3Mn0.7)0.8)3O4 (CCNMO) film annealed at 500℃ exhibited a dense microstructure and a homogeneous crystal structure with a cubic spinel phase. Their crystallinity was further enhanced with increasing doped Cu amount. The 120 nm-thick CCNMO (x=0.6) thin film had a low resistivity of 53 Ω·cm at room temperature while the Co-free film (x=1) showed a significantly decreased resistivity of 5.9 Ω·cm. Furthermore, the negative temperature coefficient of resistance (NTCR) characteristics were lower than -2%/℃ for all the specimens with x≥0.6. These results imply that the CCNMO (x≥0.6) thin films are a good candidate material for infrared sensor application.