oC In Vacuum Research Articles

Impact of cryogenic aging and test-environment on mechanical properties and tribological performance of PI-based materials

This study investigates the effect of test-environment, temperature, and cryogenic aging in liquid nitrogen for 5 months on mechanical and tribological performance of polyimide (PI) and PI composites. Tribological tests were conducted at 25 °C in air, and at both 25 °C and −100 °C in a high vacuum (10-5 Pa) environment. Impact of cryogenic aging, testing condition on PI-based materials varied depending on the polymer structure and composition. Cryogenic aging led to embrittlement, increasing the coefficient of friction and wear rate up to 77 % and 165 %, respectively. Some polyimides exhibited the lowest coefficient of friction (0.04) in vacuum at 25 °C, while the temperature reduction to −100 oC in vacuum generally decreased the tribological performance, increased contact stresses and abrasive wear of the materials.

Wetting and spreading of Cu(Cr) melt over the Cr2AlC MAX phase

• The wetting and spreading of Cu-based melt with chromium on the surface of Cr2AlC MAX phase was studied for the first time; • Good wetting of Cr2AlC by Cu (0.8 at.%Cr) melt at 1150 oC in vacuum was discovered; • Rapid Al redistribution beteween MAX phase and Cu+ 0.8 at.% Cr melt was found upon contact. The wetting and spreading of Cu(0.8 at.% Cr) melt over Cr 2 AlC MAX phase were measured in 10 -3 Pa vacuum at 1150 °C. The measurements were carried out in-situ , by the dispensed drop method from a graphite dispenser. The contact angle established during 1 s was 35° and the alloy is found to be suitable for brazing of Cr 2 AlC MAX-phase. Studies of the chemical and phase compositions near the contact surface showed an intense chemical interaction during the spreading, associated with the dissolution of aluminum in the penetrated copper melt.

Control of O-H bonds at a-IGZO/SiO2 interface by long time thermal annealing for highly stable oxide TFT

We report two-step annealing, high temperature and sequent low temperature, for amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) to improve its stability and device performance. The annealing is carried out at 300 oC in N2 ambient for 1 h (1st step annealing) and then at 250 oC in vacuum for 10 h (2nd step annealing). It is found that the threshold voltage (VTH) changes from 0.4 V to -2.0 V by the 1st step annealing and to +0.6 V by 2nd step annealing. The mobility changes from 18 cm2V-1s-1 to 25 cm2V-1s-1 by 1st step and decreases to 20 cm2V-1s-1 by 2nd step annealing. The VTH shift by positive bias temperature stress (PBTS) is 3.7 V for the as-prepared TFT, and 1.7 V for the 1st step annealed TFT, and 1.3 V for the 2nd step annealed TFT. The XPS (X-ray photoelectron spectroscopy) depth analysis indicates that the reduction in O-H bonds at the top interface (SiO2/a-IGZO) by 2nd step annealing appears, which is related to the positive VTH shift and smaller VTH shift by PBTS.

Electrochemical Performance of C-Centered Lepidocrocite-Type Titanate for Na-Ion Battery

After the discovery of electrochemical Na+ insertion into non-graphitizable carbon[1], the hard carbon has been recognized as the most promising candidate of a negative electrode material for sodium ion batteries. On another research front, various titanium oxides and titanates including TiO2[2], spinel-type Li[Li1/3Ti5/3]O4[3], NASICON-type NaTi2(PO4)3[4] and Na2Ti3O7[5] have also attracted much attention owing to its average redox potential where metal dendrite formation would not occur. Among them, Na2Ti3O7 delivers 177 mAh g-1 at first deinsertion with approximately 50% in coulombic efficiency, and its insertion potential of 0.3 V possibly realizes a safer practical Na-ion battery system compared to hard carbon. However, the specific capacity of titanium-based electrode is generally much lower than that of hard carbon, which causes consistent drawback to bring them into first choice. Recently, as a new titanium-based negative electrode candidate, Doeff and her colleagues have reported lepidocrocite-type titanate with a primitive lattice (P-type) and proposed Na0.8[Ti1.73Li0.27]O2･nH2O with a bilayer arrangement of water[6]. The material has a large theoretical capacity of 192 mAh g-1 and delivers more than 140 mAh g-1 of reversible capacity. In this study, since there is a possibility that the presence of interlayer water disturbs insertion of Na+ ions, and P-type Na0.8[Ti1.73Li0.27]O2･nH2O transforms into Na0.8[Ti1.73Li0.27]O2 with a C-centered lattice (C-type) as removal of interlayer water by drying, we synthesize C-type Na0.8[Ti1.73Li0.27]O2 to investigate the electrochemical performance and reaction mechanism for the first time. First, P-type Na0.9[Ti1.7Li0.3]O2･nH2O was prepared by ionic-exchange from K0.9[Ti1.7Li0.3]O2 with 5 M NaCl solution, and then the obtained white powder was dried at 80 to 500 ºC. The as-prepared materials were characterized by XRD, SEM, TG-DTA and their electrochemical property was measured in Na system. The working electrodes, consisting of Na0.9[Ti1.7Li0.3]O2, acetylene black and binder with the mass ratio of 80:10:5, were prepared with N-methylprrolidone and the obtained slurry was casted on a Al foil current collector. After drying the electrodes at 80 ºC in vacuum for overnight, 2023-type coin cells were assembled in a Ar-filled glove-box with 1 mol m-3 NaPF6 in PC against sodium metal. The charge and discharge measurements were carried out at room temperature at a current density of 10 mA g-1 in the voltage range between 2.0 and 0.1 V vs. Na/Na+. From XRD pattern of the synthesized sample, C-type Na0.9[Ti1.7Li0.3]O2 with lattice parameters of a = 3.7867(2), b = 14.4096(8) and c = 2.96935(19) Å was obtained. As shown in Fig.1(a) and (b), C-type Na0.9[Ti1.7Li0.3]O2 shows first reversible capacity of 145 mAh g-1 with nearly 80% in coulombic efficiency, and demonstrates excellent capacity retention at the 40th cycle. In addition, the reversible capacity even at 10C rate reached to 81% of that with C/17.5 (= 10 mA g-1) (Fig. 2). Detailed results of electrochemical performances and its charge and discharge mechanism will be discussed in the conference.

Diffusion of the High Melting Temperature Element from the Molten Oxides for Copper Alloys

To alloy high melting point elements such as boron, ruthenium, and iridium with copper, heat treatment was performed using metal oxides of B2O3, RuO2, and IrO2 at the temperature of 1200 oC in vacuum for 30 minutes. The microstructure analysis of the alloyed sample was confirmed using an optical microscope and FE-SEM. Hardness and trace element analyses were performed using Vickers hardness and WD-XRF, respectively. Diffusion profile analysis was performed using D-SIMS. From the microstructure analysis results, crystal grains were found to have formed with sizes of 2.97 mm. For the copper alloys formed using metal oxides of B2O3, RuO2, and IrO2 the sizes of the crystal grains were 1.24, 1.77, and 2.23 mm, respectively, while these sizes were smaller than pure copper. From the Vickers hardness results, the hardness of the Ir-copper alloy was found to have increased by a maximum of 2.2 times compared to pure copper. From the trace element analysis, the copper alloy was fabricated with the expected composition. From the diffusion profile analysis results, it can be seen that 0.059 wt%, 0.030 wt%, and 0.114 wt% of B, Ru, and Ir, respectively, were alloyed in the copper, and it led to change the hardness. Therefore, we verified that alloying of high melting point elements is possible at the low temperature of 1200 oC.

Enhanced Cycleability of LiNi0.5Mn1.5O4 Cathode with Zn Substitution

Introduction In the last decade, lithium-ion batteries are expected to be applied in electric vehicles. High voltage or capacity cathode materials are required to achieve high energy batteries. Spinel LiNi0.5Mn1.5O4 draws much attention owning to a voltage plateau as high as 4.7 V vs. Li/Li+, accepted capacity, excellent rate capability and good safety.[1] However, high voltage plateau brings side reactions between the cathode and the electrolyte at high voltage which cause capacity fade of spinel LiNi0.5Mn1.5O4.[2] Partial substitution of Mn or Ni by metal elements such as Al, Cr, Fe, Co etc.[3] has been proven to be successful to enhance its cycleability. Series substitutions of Mg instead of Ni are achieved to modify the spinel. The cycleability has been much improved by Mg substitution even charged to 5.1 V. Experimental LiZn x Ni0.5-x Mn1.5O4 was prepared via a sol–gel method assisted by citric acid as chelating agents. Citric acid was firstly dissolved in distilled water. Stoichiometric LiNO3, Zn(NO3)2•6H2O， Ni(NO3)2•6H2O and Mn(NO3)2•4H2O were added to the solution above with continuous stirring. The final solution was evaporated at 80 ºC until a gel was obtained and then dried at 140 ºC for 10 h. The powders was preheated at 400 ºC for 10 h and then calcined at 850 ºC for 24 h in air. The structure and morphology of the samples were studied by X-ray diffraction (XRD). Electrochemical measurements were carried out with CR2032 coin cells. The obtained materials were mixed with C-black and PVDF (80:10:10 wt %) in N-methyl pyrrolidinone solvent. After coating the above slurries on Al foils, the electrodes were dried at 80 ºC in vacuum. In the coin-cell tests, metallic lithium foil was used as the counter and reference electrodes; the electrolyte was 1 M LiPF6in a 1:1 solvent mixture of ethylene carbon (EC)/dimethyl carbonate (DMC). Results and Discussion Figure 1 displays XRD patterns for LiZn x Ni0.5-x Mn1.5O4 (x=0, 0.02, 0.04, 0.08, 0.25, 0.5) powders. The powders all show cubic structure. Rocksalt impurity phase Ni1-x O can be found in the samples of x=0 and 0.02. Impurity Ni1-x O, Li1-x Ni x O, or Ni6MnO8 as a well-known secondary phase in samples of LiZn x Ni0.5-x Mn1.5O4 can be observed at around 37.6˚ and 45.7˚[1–3] when x=0 and 0.02. The impurity intensity decreases with increasing Zn content and the x=0.04 and 0.08 samples show pure spinel phase. While x=0.25 and 0.5, a new impurity phase ZnMn2O4 emerges. In spinel structure, Zn ions prefer occupying tetrahedral sites (8a), such as ZnFe2O4 and ZnAl2O4. As a result, Zn ions are supposed to exist in tetrahedral sites (8a) instead of Li ions. Cycling performance for the electrodes made of LiZn x Ni0.5-x Mn1.5O4 (x=0, 0.02, 0.04, 0.08) at 0.5 C between 3.5–4.9 V is shown in Figure 2. The capacity mainly origin from Ni2+/Ni3+ and Ni3+/Ni4+ redox couples for the spinel LiNi0.5Mn1.5O4. As a result, the capacity decreased with increasing Zn substitution content instead of Ni. However, the cycleability has been much improved by Zn substitution especially when x = 0.08. Only 65 % capacity retention was obtained for the bare electrode. Compared to the pristine electrode, the LiZn0.08Ni0.42Mn1.5O4 electrode delivers 98 mAh/g for the first cycle with remarkable capacity retention of 95 % after 100 cycles. XPS data reveals that a Li-permeable SEI layer formed on the surface of the electrode which protects the electrolyte from oxidation at high voltage[4]. In conclusion, LiZn x Ni0.5-x Mn1.5O4 spinels (x=0, 0.02, 0.04, 0.08, 0.25, 0.5) have been successfully prepared by sol-gel method. Zn substitution significantly improved the cycleability of the spinels. A plausible explanation is that a Li-permeable SEI layer formed on the surface of the Zn substitution electrode which protects the electrolyte from oxidation at high voltage. Figure 1

Gold-Free InAlN/GaN Schottky Gate HEMT On Si (111) Substrate with ZrO2 Passivation

The electrical properties of Ti/Al/Ni/W (25/200/50/100 nm) ohmic contact and Ni/W (350/50 nm) Schottky contact to InAlN/GaN HEMT grown on high resistive Si (111) substrate are reported in this study. The specific contact resistivity of Ti/Al/Ni/W contacts has been investigated as a function of annealing temperature and Ti/Al thickness ratio, and has achieved the lowest value of 1.06×10-6 Ω∙cm2 after annealing at 900 oC in vacuum. The maximum Schottky barrier height of the Ni/W (350/50 nm) contact achieved is 0.72 eV at room temperature. With ZrO2 passviation, the InAlN/GaN-on-Si HEMT exhibits a higher extrinsic transconductance of 160 mS/mm (by ~33.3%) and a lower on-resistance of 10 Ω/mm (by ~16.7%) than that without passivation.

Thickness Dependent Electrical Characteristics of InAlN/GaN-On-Si MOSHEMTs with Y2O3 Gate Dielectric and Au-Free Ohmic Contact

Thickness dependent electrical properties of Y2O3 gate dielectric based metal-oxide-semiconductor high electron mobility transistors (MOSHEMTs) on In0.18Al0.82N/GaN-on-Si are reported. A Au-free Ti/Al/Ni/W (25/200/40/50 nm) ohmic contact compatible for Si fab industry has been developed, which demonstrates a contact resistance of 0.56 Ω.mm and a specific contact resistivity of 1.45×10-6 Ω.cm2, obtained after annealing at 900 oC in vacuum for 60 sec. A positive shift in threshold voltage with decreasing Y2O3 film thickness is demonstrated while an improved MOSHEMTs transconductance and maximum saturation drain current have been obtained for thicker one. Besides, Y2O3 film thickness and annealing dependent interfacial properties have been investigated.

PVD of N-CuIn3Se5 Photoabsorber Films

Thin films of Cu-In-Se (CISe) photoabsorber with overall composition of CuIn3Se5 were deposited onto glass/ITO substrates by using physical vapour deposition (PVD) technique. Thermal conditions for the substrates during deposition process and following thermal annealing were selected with the purpose to prepare polycrystalline n-CuIn3Se5 photoabsorber layers for the hybrid photovoltaic structures based on inorganic photoabsorber and conductive polymer functional layers. It was found, that the CISe layers deposited at the temperature of substrate of 200 °C and annealed at the temperature range of 450-500 oC in vacuum and double annealed in argon and vacuum at 500 oC demonstrate high photosensitivity and photoconductivity under white light illumination of 100 mW/cm2 intensity. Obtained results show the chalcopyrite structure of prepared photoabsorber films with good adhesion to the glass/ITO substrate.

Microstructure and Properties of Porous CMCs Prepared by HIPing the Pyrolyzed ZrO<sub>2</sub>/ Si / Phenol Resin Composite

A novel two-step process was used to investigate the microstructure and properties of porous CMCs prepared by HIPing the pyrolyzed composites of commercially available ZrO2 (TZ- 3YS), silicon powders and phenolic resin. In the first step, preforms with 70, 80 and 90 mass % of ZrO2 powders were prepared by the pyrolysis of ZrO2 / Si / Phenol Resin composites at 850 oC in vacuum. And, then the pyrolyzed preforms were HIPed at 1400 oC for 30 minutes in Argon atmosphere under a pressure of 50 MPa to fabricate the porous CMCs, in the second step. XRD analyses suggested the formation of β-SiC in the composites below the melting point of silicon. SEM photographs showed that spherical pores of several μm in diameter were uniformly distributed in the matrices of composites and crystals of β-SiC with facets were observed in the pores. EDS analyses showed that the crystals were composed of Si and C with 1 : 1 atomic ratio suggesting that crystal are SiC. The maximum hardness (13.78 GPa) was achieved from the composite with 90 mass % ZrO2, which is more than that of bulk hardness of ZrO2 (10-12 GPa).

The formation of low temperature Cu3Si in Ag(Cu)/Si structure upon annealing and its effects on adhesion and resistivity

AbstractThe annealing of Ag(40 at.% Cu) alloy films deposited on a Si substrate at 200 – 800 oC in vacuum has been conducted to investigate the formation of Cu3Si at the Ag-Si interface and its effects on adhesion and resistivity of Ag(Cu)/Si structure. Auger electron spectroscopy(AES) analysis showed that annealing at 200°C allowed a diffusion of Cu to the Si surface, leading to the significant reduction in Cu concentration in Ag(Cu) film and thus causing a rapid drop in resistivity. In addition, the segregated Cu to the Si surface reacts with Si, forming a continuous copper silicide at the Ag(Cu)/Si interface, which can contribute to an enhanced adhesion of Ag(Cu)/Si annealed at 200 oC. However, as the temperature increases above 300°C, the adhesion tends to decrease, which may be attributed to the agglomeration of copper silicide beginning at around 300°C.

IS THE HISTORY OF AN ANCIENT POTTERY WARE CORRELATED WITH ITS MÖSSBAUER SPECTRUM ?

IS THE HISTORY OF AN ANCIENT POTTERY WARE CORRELATED WITH ITS MÖSSBAUER SPECTRUM ?