共查询到20条相似文献,搜索用时 15 毫秒
1.
Wübbenhorst M Murray CA Dutcher JR 《The European physical journal. E, Soft matter》2003,12(Z1):S109-S112
The local and cooperative dynamics of supported ultrathin films ( L = 6.4 - 120 nm) of isotactic poly(methyl methacrylate) (i-PMMA, Mn = 118 x 10(3) g/mol) was studied using dielectric relaxation spectroscopy for a wide range of frequencies (0.1 Hz to 10(6) Hz) and temperatures (250 - 423 K). To assess the influence of the PMMA film surfaces on the glass transition dynamics, two different sample geometries were employed: a single layer PMMA film with the film surfaces in direct contact with aluminum films which act as attractive, hard boundaries; and a stacked polystyrene-PMMA-polystyrene trilayer film which contains diffuse PMMA-PS interfaces. For single layer films of i-PMMA, a decrease of the glass transition temperature T(g) by up to 10 K was observed for a film thickness L < 25 nm (comparable to R(EE)), indicated by a decrease of the peak temperature T(alpha) in the loss epsilon(")(T) at low and high frequencies and by a decrease in the temperature corresponding to the maximum in the apparent activation energy E(a)(T) of the alpha-process. In contrast, measurements of i-PMMA sandwiched between PS-layers revealed a slight (up to 5 K) increase in T(g) for PMMA film thickness values less than 30 nm. The slowing down of the glass transition dynamics for the thinnest PMMA films is consistent with an increased contribution from the less mobile PMMA-PS interdiffusion regions. 相似文献
2.
H. Huth A. A. Minakov A. Serghei F. Kremer C. Schick 《The European physical journal. Special topics》2007,141(1):153-160
A differential AC-chip calorimeter capable to measure the
glass transition in nanometer thin films is described. Due to the
differential setup pJ/K sensitivity is achieved. Heat capacity can
be measured for sample masses below one nanogram even above room
temperature as needed for the study of the glass transition in
nanometer thin polymeric films. The calorimeter allows for the
frequency dependent measurement of complex heat capacity in the
frequency range from 1 Hz to 1 kHz. The glass transition in thin
films of polystyrene (PS) (100–4 nm) and polymethylmethacrylate
(PMMA) (400–10 nm) was determined at well defined experimental
time scales. No thickness dependency of the glass transition
temperature was observed within the error limits (±3 K) -
neither at constant frequency nor for the traces in the activation
diagrams (1 Hz–1 kHz). 相似文献
3.
We have used ellipsometry to measure the glass transition temperature T(g) of high molecular weight (M(w)=790 x 10(3)), freely-standing films of atactic poly(methyl methacrylate) (a-PMMA), as well as films of the same polymer supported on two different substrates: the native oxide layer of silicon (Si) and gold-covered Si. We observe linear reductions in T(g) with decreasing film thickness h for the freely-standing PMMA films with 30 nm < h<100 nm, which is qualitatively similar to previous results obtained for freely-standing polystyrene (PS) films. However the magnitude of the T(g) reductions for PMMA is much less than for freely-standing films of PS of comparable molecular weight and thickness. We also find that for films supported on either substrate, with thicknesses as small as 30 nm, the T(g) values do not deviate substantially from the value measured for thick films. 相似文献
4.
The film thickness dependence of both the glass transition temperature (T(g)) and the 1 kHz alpha relaxation were studied for thin films of isotactic Poly (methylmethacrylate) (i-PMMA) supported on aluminium substrates. Films in the thickness range 7-200 nm were studied. The ellipsometrically determined T(g) was found to show reductions for films thinner than 60 nm, with the largest observed reduction being 12 K for a 7 nm thick film. Measurements of the T(g) were also performed on i-PMMA films supported on silicon substrates. Dielectric studies of the temperature dependent 1 kHz alpha relaxation peak, showed that the position (T(alpha)) and shape of the peak have no film thickness dependence. This was shown to hold for films with one free surface and films with a 30 nm thermally evaporated capping layer. Capping the films was shown to have no effect on the thickness dependence of either T(g) or T(alpha). The implications of these results are discussed further and the different film thickness dependencies of T(g) and T(alpha) are discussed. This is done within the framework of the Vogel-Fulcher-Tamann (VFT) theory of glass forming materials and also in the context of the existence of a dynamic correlation length xi. 相似文献
5.
用Mg/B多层膜退火的方法制备了一系列MgB2超导薄膜,研究了退火温度、退火时间和薄膜厚度对于MgB2薄膜性质的影响.厚度为250 nm的Mg/B多层膜经400 ℃低温退火后已经生成超导相,此厚度薄膜750 ℃下退火20—30 min实现最佳超导转变温度(Tc).前驱膜分层厚度相同时,随着薄膜厚度减小MgB2薄膜Tc明显降低,而且较薄的膜Tc
关键词:
2超导薄膜')" href="#">MgB2超导薄膜
电子束蒸发
超导成相 相似文献
6.
The glass transition temperature of poly (maleic anhydride-alt-1-octadecen) and poly (styreneco-maleic anhydride) cumene-terminated
thin films has been measured by mechanical relaxation of Langmuir films of these polymers. The dynamical properties show glass-like
features (non-Arrhenius relaxation times and non-Debye mechanical response) interpreted by the coupling model. The glass transition
temperature values determined by a mechanical relaxation experiment (step-compression) agree very well with those obtained
by surface potential measurements. It is found that the glass transition temperature values in thin films decrease by about
100K as compared with those corresponding to the bulk polymers. The coadsorption of the water-insoluble surfactant DODAB decreases
the glass transition temperature. 相似文献
7.
Hartmann L Gorbatschow W Hauwede J Kremer F 《The European physical journal. E, Soft matter》2002,8(2):145-154
The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights
embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz
to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the
dynamic glass transition (α-relaxation) and a (local) secondary β-relaxation. While the latter does not depend on the dimensions
of the sample, the dynamic glass transition becomes faster (≤2 decades) with decreasing film thickness. This results in a
shift of the glass transition temperature T
g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and
a decrease of the dielectric strength is observed for the α-relaxation. This enables to deduce a model based on immobilized
boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T
g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements
yield a gradual increase of T
g with decreasing film thickness. The findings concerning the different thickness dependences of T
g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T
g shifts incorporates recently developed models to describe the glass transition in thin polymer films.
Received 12 August 2001 and Received in final form 16 November 2001 相似文献
8.
Ellison CJ Kim SD Hall DB Torkelson JM 《The European physical journal. E, Soft matter》2002,8(2):155-166
Fluorescence intensity measurements of chromophore-doped or -labeled polymers have been used for the first time to determine
the effects of decreasing film thickness on glass transition temperature, T
g, the relative strength of the glass transition, and the relative rate of physical aging below T
g in supported, ultrathin polymer films. The temperature dependence of fluorescence intensity measured in the glassy state
of thin and ultrathin films of pyrene-doped polystyrene (PS), poly(isobutyl methacrylate) (PiBMA), and poly(2-vinylpyridine)
(P2VP) differs from that in the rubbery state with a transition at T
g. Positive deviations from bulk T
g are observed in ultrathin PiBMA and P2VP films on silica substrates while substantial negative deviations from bulk T
g are observed in ultrathin PS films on silica substrates. The relative difference in the temperature dependences of fluorescence
intensity in the rubbery and glassy states is usually reduced with decreasing film thickness, indicating that the strength
of the glass transition is reduced in thinner films. The temperature dependence of fluorescence intensity also provides useful
information on effects of processing history as well as on the degree of polymer-substrate interaction. In addition, when
used as a polymer label, a mobility-sensitive rotor chromophore is demonstrated to be useful in measuring relative rates of
physical aging in films as thin as 10 nm.
Received 21 August 2001 相似文献
9.
Bisio F Terreni S Gonella G Floreano L Morgante A Canepa M Mattera L 《Physical review letters》2004,93(10):106103
We observe that ultrathin Fe/Cu(3)Au(001) films in the 6-13 A thickness range, beyond the thickness of pseudomorphism breakdown at room temperature, exhibit a temperature dependent structural phase transition in the range T(c) approximately 345-380 K. In the high temperature state the Fe film becomes pseudomorphic, while breakdown of pseudomorphism reversibly occurs as the system is cooled below the transition temperature. The difference between substrate and overlayer thermal expansion coefficient is highlighted as the driving force for the observed transition. 相似文献
10.
Nickel films of different thickness ranging from 15 nm to 350 nm were deposited on glass substrates, at different substrate temperatures (313-600 K) under UHV condition. The nano-structure of the films was obtained, using X-ray diffraction (XRD) and atomic force microscopy (AFM). The nano-strain in these films was obtained using the Warren-Averbach method. Their optical properties were measured by spectrophotometry in the spectral range of 190-2500 nm. Kramers-Kronig method was used for the analysis of the reflectivity curves. The absorption peaks of Ni thin films at ∼1.4 eV (transition between the bands near W and K symmetry points) and ∼5.0 eV (transition from L2′ to L1 upper) are observed, with an additional bump at about 2 eV. The over-layer thickness was calculated to be less than 3.0 nm, using the Transfer Matrix method. The changes in optical data are related to different phenomena, such as different crystallographic orientations of the grains in these polycrystalline films (film texture), nano-strain, and film surface roughness. 相似文献
11.
The effect of free surfaces on the glass transition temperature (T(g)) of thin polystyrene films was studied. Measurements were performed on films (8 nm相似文献
12.
M. K. Mundra C. J. Ellison P. Rittigstein J. M. Torkelson 《The European physical journal. Special topics》2007,141(1):143-151
Confinement effects in polystyrene and poly(methyl
methacrylate) films and nanocomposites are studied by fluorescence.
The ability to employ an intensive measurable, the excited-state
fluorescence lifetime, in defining the glass transition temperature,
Tg, of polymers is demonstrated and compared to the use of an
extensive measurable, fluorescence intensity. In addition, intrinsic
fluorescence from the phenyl groups in polystyrene is used to
determine the Tg-confinement effect in films as thin as
~15 nm. The decrease in Tg with decreasing film thickness
(below ∼60 nm) agrees well with results obtained by extrinsic
pyrene fluorescence. Dye label fluorescence is used to quantify the
enhancement in Tg observed with decreasing thickness (below
~90 nm) in poly(methyl methacrylate) films; addition of
2–4 wt% dioctyl phthalate plasticizer reduces or eliminates the
Tg-confinement effect in films down to 20 nm
thickness. Intrinsic polystyrene fluorescence, which is sensitive to
local conformation, is used to quantify the time scales (some tens
of minutes) associated with stress relaxation in thin and ultrathin
spin-coated films at Tg + 10 K. Finally, the shape of the
fluorescence spectrum of pyrene doped at trace levels in polystyrene
films and polystyrene-silica nanocomposites is used to determine
effects of confinement on microenvironment polarity. 相似文献
13.
利用混合物理化学气相沉积法(HPCVD)在MgO(111)衬底上制备了干净的MgB2超导超薄膜. 在背景气体压强, 载气氢气流量以及沉积时间一定的情况下, 改变B2H6的流量, 制备得到不同厚度系列的MgB2超导薄膜样品, 并测量了其超导转变温度 Tc, 临界电流密度Jc等临界参量. 该系列超导薄膜沿c轴外延生长, 表面具有良好的连接性, 且有很高的超导转变温度Tc(0) ≈ 35-38 K和很小的剩余电阻率ρ(42 K) ≈ 1.8-20.3 μΩ·cm-1. 随着膜厚的减小而减小, 临界温度变低, 而剩余电阻率变大. 其中20 nm的样品在零磁场, 5K时的临界电流密度Jc ≈ 2.3×107 A/cm2. 表明了利用HPCVD在MgO(111)衬底上制备的MgB2超薄膜有很好的性能, 预示了其在超导电子器件中广阔的应用前景.
关键词:
MgO(111)衬底
2超薄膜')" href="#">MgB2超薄膜
混合物理化学气相沉积 相似文献
14.
掺镱氟铅硅酸盐玻璃光谱性质研究 总被引:2,自引:1,他引:1
采用高温熔融法制备了组成为(80-x)SiO2·xPbO·(17~19)( R2O+RF) ·(1~3)Yb2O3(R=Li、Na、K)(mol%)的掺镱氟铅硅酸盐玻璃,测量了样品的吸收光谱、荧光光谱和荧光寿命,利用McCumber理论计算了该玻璃系统中Yb3+离子2F5/2→2F7/2能级跃迁受激发射截面.结果表明,该玻璃在1 013 nm处的σemi为0.21×10-20 cm2,荧光有效线宽为94.8 nm,荧光寿命为2.36 ms.激光性能评价结果表明,该玻璃是实现大能量超短脉冲激光的理想材料. 相似文献
15.
We study the glass transition in confined polymer films and present the first experimental evidence indicating that two separate mechanisms can act simultaneously on the film to propagate enhanced mobility from the free surface into the material. Using transmission ellipsometry, we have measured the thermal expansion of ultrathin, high molecular-weight (MW), freestanding polystyrene films over an extended temperature range. For two different MWs, we observed two distinct reduced glass transition temperatures (T(g)'s), separated by up to 60 K, within single films with thicknesses h less than 70 nm. The lower transition follows the expected MW dependent, linear T(g)(h) behavior previously seen in high MW freestanding films. We also observe a much stronger upper transition with no MW dependence that exhibits the same T(g)(h) dependence as supported and low MW freestanding polymer films. 相似文献
16.
Effects of Thickness and Temperature on Thermoelectric Properties of Bi_2Te_3-Based Thin Films
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Bi_2Te_3 thin films and GeTe/B_2Te_3 superlattices of different thicknesses are prepared on the silicon dioxide substrates by magnetron sputtering technique and thermally annealed at 573 K for 30 min. Thermoelectric(TE)measurements indicate that optimal thickness and thickness ratio improve the TE performance of Bi_2Te_3 thin films and GeTe/B_2Te_3 superlattices, respectively. High TE performances with figure-of-merit(ZT) values as high as 1.32 and 1.56 are achieved at 443 K for 30 nm and 50 nm Bi_2Te_3 thin films, respectively. These ZT values are higher than those of p-type Bi_2Te_3 alloys as reported. Relatively high ZT of the GeTe/B_2Te_3 superlattices at 300-380 K were 0.62-0.76. The achieved high ZT value may be attributed to the unique nano-and microstructures of the films,which increase phonon scattering and reduce thermal conductivity. The results indicate that Bi_2Te_3-based thin films can serve as high-performance materials for applications in TE devices. 相似文献
17.
The behavior as a function of temperature of very thin films (10 to 200 nm) of pentylcyanobiphenyl on silicon substrates is reported. In the vicinity of the nematic-isotropic transition we observe a coexistence of two regions of different thicknesses: thick regions are in the nematic state while thin ones are in the isotropic state. Moreover, the transition temperature is shifted downward following a 1/h(2) law ( h is the film thickness). Microscope observations and small-angle x-ray scattering allowed us to draw a phase diagram which is explained in terms of a binary first-order phase transition where thickness plays the role of an order parameter. 相似文献
18.
We have studied the effect of physical ageing in thin supported glassy polystyrene films by using ellipsometry to detect overshooting
in the expansivity-temperature curve upon heating of aged samples. Films with thickness 10-200 nm have been aged at 70°
C and 80°
C (below the bulk glass transition temperature). We observe clear relaxation peaks in the expansivity-temperature curve for
films thicker than 18 nm but not for the 10 nm film. The intensity of the relaxation peak is inversely proportional to the
film thickness, while the temperatures characteristic to the relaxation peak are almost independent of the film thickness.
These observations are successfully interpreted by the idea that the surface layer of the order of 10 nm has liquid-like thermal
properties.
Received 28 October 2002 / Published online: 1 April 2003
RID="a"
ID="a"Present address: Yokohama Research Center, Mitsubishi Chemical Corporation, 1000 Kamoshida-chou, Aoba-ku, Yokohama 227-8502, Japan; e-mail:
kawana@rc.m-kagaku.co.jp 相似文献
19.
K. Fukao Y. Oda K. Nakamura D. Tahara 《The European physical journal. Special topics》2010,189(1):165-171
The glass transition temperature and the dynamics of the α-process have been investigated using dielectric relaxation spectroscopy
for single and stacked thin films of poly(2-chlorostyrene) (P2CS). The stacked film consists of 10 layers of single thin films
with thickness of 12 nm or 18 nm. The glass transition temperature T
g
of the single thin films of P2CS is found to decrease with decreasing film thickness in a similar way as observed for polystyrene
thin films. The magnitude of the depression of T
g
for the stacked thin films is larger than that of the single thin films with corresponding thickness. The depression of the
temperature at which the dielectric loss shows a peak due to the α-process at a given frequency, T
α, is larger than that of the single thin films, although the magnitude is smaller than that of T
g
. Annealing at a high temperature could cause the T
g
and T
α of the stacked thin films to approach the values of the bulk system. 相似文献
20.
The glass transition temperature is known to increase with decreasing film thickness h for sufficiently thin poly(methyl methacrylate) films supported by silicon oxide substrates. We show that this system undergoes a CO2 pressure-induced devitrification transition, P(g), which is film thickness dependent, P(g)(h)=DeltaP(g)+P(bulk)(g). P(bulk)(g) is the bulk glass transition and DeltaP(g) can be positive or negative depending on T and P. The phenomenon of retrograde vitrification, wherein the polymer exhibits a rubbery-to-glassy-to-rubbery transition upon changing temperature isobarically, is also shown to occur in this system and it is film thickness dependent. 相似文献