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Infrared Physics and Technology 93 (2018) 162–170
Contents lists available at ScienceDirect
Infrared Physics & Technology
journal homepage: www.elsevier.com/locate/infrared
Regular article
Influence of sodium dodecyl sulphate on the surface morphology and
infrared emissivity of porous Ni film
T
⁎
Jiacheng Guoa, Xingwu Guoa,b, , Jiyong Zenga, Lewen Niea, Jie Donga,b, Liming Penga,b,
Wenjiang Dinga,b
a
National Engineering Research Center of Light Alloy Net Forming and Key State Laboratory of Metal Matrix Composites, School of Materials Science and Engineering,
Shanghai Jiao Tong University, Shanghai 200240, China
b
Shanghai Innovation Institute for Materials, Shanghai 200444, China
A R T I C LE I N FO
A B S T R A C T
Keywords:
Sodium dodecyl sulphate
Dynamic hydrogen bubble template
Infrared emissivity
Hydrogen bubbles
Roughness
The purpose of this study is to investigate the effect of sodium dodecyl sulphate (SDS) concentration on the
surface morphology as well as infrared emissivity of porous Ni film prepared through the method of dynamic
hydrogen bubble template (DHBT). The results indicate that the addition of SDS can enhance the hydrogen
evolution reaction and increase the volume of evolving hydrogen bubbles during electrodeposition process,
resulting in the increase of pore density, depth and diameter of Ni film. These factors affect the roughness greatly
and the relationship between roughness and infrared emissivity has been inferred. A wide range of infrared
emissivity (0.36–0.80) of porous Ni film was obtained through control of the concentration of SDS. It is concluded that the addition of SDS has a significant effect on the improvement of roughness of the porous Ni film
and is beneficial for the film to obtaining high thermal emission.
1. Introduction
The emissivity is the ratio of energy radiated by the material to the
energy radiated by a blackbody at the same temperature, which represents the key parameter to measuring the thermal radiation properties of materials [1–5]. High infrared emissivity coatings have attracted a lot interests in the field of spacecraft applications, radiative
cooling applications and electrical insulation [6–10]. Most high infrared emissivity materials are insulators while the conductive materials like metals exhibit low infrared emissivity and high reflectivity.
For example, most of the non-transparent insulators exhibit infrared
emissivity up to 0.8. On the other hand, a polished copper block shows
infrared emissivity of the order of 0.02–0.03 [11,12]. Numerous studies
about the relationship between the macroscopic surface structure and
the thermal radiation properties have been reported. The thermal
emission properties of a surface coating are altered profoundly when
the geometric scale (s) of the radiators approximately equals the wavelength (λ) of the measured radiation (namely s ≈ λ) due to the
electromagnetic interactions at the surface [13–19]. Therefore, the pure
metallic and alloy films with characteristic surface microstructure will
possess special thermal emission property and become the potential
application materials of high infrared emissivity [20]. It is known that
the electronic device housing coated by high infrared emissivity insulating coatings will not process the electromagnetic shielding performance, and the electronic device will be interfered by electromagnetic wave in space. As a result, a coating with both high infrared
emissivity and electrical conductivity is urgently required in order to
lower their temperature and protect them from electromagnetic interference in space.
Dynamic hydrogen bubble template (DHBT) method in aqueous
solution is the most convenient method to prepare the metallic film
with macropores. In aqueous solution H+ is reduced to H2 at sufficient
cathodic overpotential, which actually can be used as a dynamic template of metallic film during the electrodeposition process [21–29].
When a metal surface possesses the porous structure, its infrared
emissivity increases dramatically. The infrared emissivity is closely
related to the distribution characteristic of pores on the surface, which
is dependent upon the bubble behavior during electrodeposition process [21,22,28]. A number of directing agents that affect bubble behavior has been reported to successfully adjust the nanostructured and
macroporous morphology of electrodeposits when employing the DHBT
method. Halides (including NaCl, HCl and NH4Cl) [28,30–32], surfactants (including SDS, CTAB and alkyphenol polyxyethylene) [33,34]
and other additives (including PEG, polyethylene glycol and MPSA, 3-
⁎
Corresponding author at: National Engineering Research Center of Light Alloy Net Forming and Key State Laboratory of Metal Matrix Composites, School of
Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China.
E-mail address: xingwuguo@sjtu.edu.cn (X. Guo).
https://doi.org/10.1016/j.infrared.2018.07.029
Received 15 May 2018; Received in revised form 21 July 2018; Accepted 22 July 2018
Available online 23 July 2018
1350-4495/ © 2018 Elsevier B.V. All rights reserved.
Infrared Physics and Technology 93 (2018) 162–170
J. Guo et al.
mercapto-1-propane sulfonic acid sodium salt) [31,35,36] are commonly used for copper electroplating. They can play a crucial role in
forming 3D interconnected Cu foams by changing the pore size, shape
and density of Cu deposits. Obviously, surfactants which can greatly
affect the quantity and the size of hydrogen bubbles are key factors in
the preparation of porous metal films. However, the mechanism of
surfactant and its influence on the H2 bubble behavior still lack of detailed research. In present work, the anionic surfactant sodium dodecyl
sulphate (SDS) has been employed to investigate how it affects the
bubble behavior, the surface morphology of the porous Ni film and its
infrared emissivity during electrodeposition process.
(a)
700
2
i (mA/cm )
400
300
0
-1.75
600
2
i (mA/cm )
500
0
-1.75
240 s
25
0 mM SDS
0.25 mM SDS
0.5 mM SDS
1 mM SDS
2 mM SDS
0
-1.50
-1.25
-1.00
-0.75
Potential (VSCE)
-0.50
-0.25
H+(aq) + H(ads) + e− → H2
(2)
2H(ads) → H2
(3)
In addition to the aforementioned source, the NH4Cl in the electrolyte can also be regarded as another source for generating H2 bubbles
through the following Eq. (4) [21,37].
2NH+4 + 2e− → H2 + 2NH3
(4)
These H2 bubbles generated through these hydrogen evolution reactions can actually be used as a dynamic template during the electrodeposition process. In order to investigate the effect of SDS on these
hydrogen evolution reactions, the polarization curve test was carried
out in solution containing 4 M NH4Cl and different concentration of SDS
in absence of any metal ions. The polarization curves were shown in
Fig. 1a. For the cathodic scan (−0.25 VSCE ∼ −1.5 VSCE), the current
increased dramatically at -0.75 VSCE, which indicates that the hydrogen
evolution reactions intensely occur under this electric potential (Eqs.
(1)–(4)). It can be seen that the current density for hydrogen evolution
reactions increases significantly with the increase of SDS concentration
under the same potential. It is indicated that the addition of SDS can
enhance the hydrogen evolution reactions. The polarization curves for
hydrogen evolution reaction in Ni2+ solutions with different concentration of SDS have also been shown in Fig. 1b for comparison. Similar results have been obtained, that is, the addition of SDS can enhance the hydrogen evolution reactions as well.
It is known that the generation of H2 bubbles on the surface of
cathodic electrode involves three steps: nucleation, growth and
(1)
3
-0.25
Fig. 1. (a) Polarization curves for hydrogen evolution reaction in Ni-free solution with 4 M NH4Cl and different concentration of SDS. (b) Polarization
curves for hydrogen evolution reaction in 0.2 M NiCl2·6H2O and4 M NH4Cl
solution with different concentration of SDS. Scan rate is 50 mV/s.
Table 1
The bath composition and deposition parameters of porous Ni films.
0.2 M
4M
0.25–2 mM
300
-0.50
100
In aqueous solution, H+ is reduced to H2 at sufficient cathodic
overpotential, and it is usually considered that H2 is generated through
the following Eqs. (1), (2) and (3) [21].
NiCl2·6H2O
NH4Cl
SDS
400
-1.25 -1.00 -0.75
Potential (VSCE)
200
3.1. The analysis of hydrogen evolution reaction with the addition of SDS
Temperature (°C)
-1.50
(b) 700 2 mM SDS
3. Results and discussions
Deposition
time (s)
0
100
In this work, the pure Cu with an area of 2 cm was chosen as the
substrate material, and a Pt foil was served as the anode. Analytical
pure nickel chloride hexahydrate (NiCl2·6H2O), ammonium chloride
(NH4Cl) and the anionic surfactant (sodium dodecyl sulfate, SDS) were
purchased from Sinopharm Chemical Reagents Co. Ltd. without further
purification. The Cu substrate for electrodeposition were wet ground
with SiC papers down to 2000 grit, and then degreased ultrasonically in
acetone for 10 min. Before being electrodeposited, the specimens were
etched in 40 wt% HNO3 solution for 1 min, then rinsed with deionized
water, and subsequently dried in hot air. Bath composition and deposition parameters were shown in Table 1. The current density of
electrodeposition was fixed at 3 A/cm2 to form a porous film, and the
deposition time was 240 s.
Polarization curve test was carried out by PARSTAT 2273 electrochemical system in a three-electrode cell system. A pure Pt wire with
0.5 mm diameter was used as a working electrode, saturated calomel
electrode (SCE) as the reference electrode and a pure Pt network with
the size of 30 mm × 30 mm as counter electrode. All experimental
electrodes were cleaned enough and dried before all measurements. In
addition, all polarization curves were performed at the temperature of
25 °C.
The characterization of surface morphology of the porous Ni film
was analyzed by scanning electron microscope (Phenom XL). The surface roughness were studied and measured by Zeiss Profiler. The phase
compositions were determined by X-ray diffraction (XRD, D/MAX
2000 V, Rigaku, Japan) with a Cu-Kα target. 2θ/θ scanning mode was
executed in the range of 2θ = 20 ∼ 80° at a scanning speed of 4°/min.
The diameter and density of pores in the film were calculated by
Ipwin32 Software. The total hemisphere emissivity of the infrared
spectrum with the wavelength in the range of 3–30 μm was evaluated
by TEMP 2000A (AZ Technology, Inc. 7047 Old Madison Pike, Suite
300 Huntsville, AL 35806) at room temperature.
Deposition
current density
(A/cm2)
Hydrogen evolution reaction
200
2
Composition
0 mM SDS
0.25 mM SDS
0.5 mM SDS
1 mM SDS
2 mM SDS
500
2. Experimental
H+(aq) + e− → H(ads)
2 mM SDS
600
163
Infrared Physics and Technology 93 (2018) 162–170
J. Guo et al.
(a)
(b)
Electrolyte
Electrolyte
Detachment
+Ead
γLSair
Attachment
γsSair
CATHODE
(c)
θΗ2
γS
γSL
θ
γsLSair
CATHODE
γL
θ
Fig. 2. The schematic diagram of the effect of SDS (surface tension) on the growth of bubbles on the electrode. (a) Electrolyte without SDS; (b) electrolyte with SDS;
(c) The balance among surface tensions to obtain Young’s relation. γS, γL and γSL are surface tension of the H2-cathode, H2-liquid and cathode-liquid interfaces,
respectively. Ead, adhesion energy of the bubble; Sair, the area of interface and θH2 contact angle of bubble.
Without SDS
0.25mM SDS
1mM SDS
2mM SDS
0.5mM SDS
Fig. 3. The SEM images of the porous Ni films electrochemically deposited at 25 °C and a 3 A/cm2 cathodic current density in a 0.2 M NiCl2·6H2O and 4 M NH4Cl
solution with different concentration of SDS in 240 s. The inset images are high magnification (20,000×).
detaching the bubble from the electrode, Sair is the area of interface, γS,
γL and γSL are surface tension of the H2-cathode, H2-liquid and cathodeliquid interfaces, respectively. The schematic diagram was shown in
Fig. 2. The adhesion energy per unit area of interface (Wad = Ead/Sair)
of the bubble on the electrode is
detachment. The H2 bubbles initially generated tend to adhere to the
cathode instead of detaching from the cathode immediately, so their
presence on the cathode will reduce the reactive area and play the role
of template for the porous film. Furthermore, once the old H2 bubbles
leave the cathode, the new H2 bubbles was generated in quick succession at the same site and production of continuous hydrogen evolution.
The following equation can be used to express the energy balance
within the area of interface between attachment and detachment of the
bubble [38]:
γS Sair + Ead = (γL + γSL )Sair
Wad = Ead /Sair = γL + γSL−γS
(6)
The balance between the surface tension along the horizontal axis is
γS = γSL−γL cosθH2
(5)
(7)
where θH2 is the contact angle of the H2 bubble on the cathode surface.
Therefore, following equation can be used to express the
where Ead is adhesion energy which is defined as the work required
164
Infrared Physics and Technology 93 (2018) 162–170
J. Guo et al.
Percentage
(a)
0.20
0.16
0.12
0.08
0.04
0.00
0.15
0.12
0.09
0.06
0.03
0.00
0.12
0.10
0.08
0.06
0.04
0.02
0.00
0.10
0.08
0.06
0.04
0.02
0.00
0.10
0.08
0.06
0.04
0.02
0.00
0 SDS
0.25 mM SDS
0.5 mM SDS
1 mM SDS
2 mM SDS
10
20
30
40
50
60
Diameter of pores (μm)
0.40
60
50
0.35
40
0.30
30
0.25
20
0.20
10
0
Density of pores
Average diameter of pores (μm)
(b)
0.0
0.5
1.0
1.5
Concentration of SDS (mM)
2.0
0.15
Fig. 4. (a) The pore size distribution of porous Ni film electrodeposited from the solution containing 0.2 M NiCl2·6H2O and 4 M NH4Cl with different concentration of
SDS at 3 A/cm2 and 25 °C in 240 s. (b) The dependence of average pore diameter and pore density on the concentration of SDS.
area Wad of the interface will be decreased based on Eq. (8) because of
the surfactant SDS can reduce the surface tension γL of aqueous solution
greatly. As a result, the H2 bubbles can easily escape from the cathode
when SDS was used. The similar phenomenon was observed directly by
high-speed photography, which was reported by Mobius and coworkers [39]. Therefore, the addition of SDS can improve the hydrogen
evolution and provide a higher volume of H2 bubbles on the cathode,
which subsequently affect the porous structure of Ni film.
44
η(%)
42
40
3.2. Effect of SDS on the characteristics of porous Ni film
38
Fig. 3 shows the SEM images of the typical 3D foam structure of Ni
film electrodeposited from the bathes with different concentration of
SDS. It can be seen that all these films appear honeycomb-like structure,
which consists of a continuous matrix and pores distributed on the
whole surface. The inset images show that the addition of SDS can
significantly affects the morphology of the deposit. Numerous relatively
small nodular Ni particles tightly close together and form smooth and
compact wall of pores in the absence of SDS. With the addition of SDS,
the wall of pores becomes cauliflower-like shape more clearly. This can
be explained by that SDS functions as an effective foaming agent for
nickel electrodeposition. A lot of small hydrogen bubbles generated on
the cathodic surface and then detached, which leads to such specific
morphology of deposit.
The variation of pore size shown in Fig. 3 by the SEM images directly gives the evidence of the growth or coalescence of H2 bubbles in
36
0.0
0.5
1.0
1.5
Concentration of SDS
2.0
Fig. 5. The current efficiency for electrodeposition porous Ni film as a function
of concentration of SDS.
relationship between Wad, γL and θH2:
Wad = Ead /Sair = γL (1 + cosθH2)
(8)
If the contact angle θH2 is a constant, the adhesion energy per unit
165
Infrared Physics and Technology 93 (2018) 162–170
J. Guo et al.
(a)
Porous Ni film
(b)
104μm
Porous Ni film
Cu substrate
Cu substrate
(d)
(c)
Porous Ni film
103μm
104μm
Porous Ni film
Cu substrate
134μm
Cu substrate
(e)
Porous Ni film
Cu substrate
132μm
100μm
Fig. 6. The cross-section morphology of porous Ni films electrodeposited from solution containing 0.2 M NiCl2·6H2O and 4 M NH4Cl with (a) 0, (b) 0.25, (c) 0.5, (d) 1
and (e) 2 mM SDS at 3A/cm2 and 25 °C in 240 s.
104 μm. The average thickness increases to about 130 μm with SDS
increasing to 2 mM. Many pores embedded into the Ni film can be
observed in Fig. 6a. Under the high current density deposition condition
(3A)/cm2), the growth of the Ni film is very fast. The Ni film obtained
from the bath without any SDS is relatively smooth and the pore diameter is relatively small. However, the “V” shaped pores with larger
diameter and depth can be observed obviously from the cross-section
morphology and corresponding surface morphology when SDS was
added into the solution.
the absence and presence of SDS. The pore diameter in the film electrodeposited will be approximately the same size as the diameter of the
bubbles released. Fig. 4a shows the different pore size distribution of
porous Ni film electrodeposited from the solution with different concentration of SDS. The feature size of the pores in Ni films electrodeposited from the solution without SDS is in the range of 2–15 μm.
When SDS was used, the range of pore diameter widens obviously. In
addition, the average pore diameter increases from 10 to 30 μm when
the concentration of SDS increases from 0 to 2 mM as shown in Fig. 4b.
The average pore diameter almost increases linearly with the concentration of SDS. The enlargement of the pore size indicates that more
hydrogen bubbles coalesce during electrodeposition process. In addition, more bubbles with different sizes which served as the dynamic
template, results in the variation of pore size and shape in the film. If
the pore density (θ) was approximatively defined as the ratio of the
projected area of pores (Ap) to the whole surface area (As) of the film,
Ap
that is θ = A . It can be seen that the value of θ increases from 0.20 to
s
0.38 with the increase of SDS from 0 to 2 mM in the solution as shown
in Fig. 4b.
There are two main cathodic reactions during electrodeposition
process, that is, the reduction of H ± and Ni2+ on the cathode.
Therefore, the current efficiency of electrodeposition porous Ni film can
be calculated according to the method reported in detail by Marozzi
and co-workers [22,23]. Fig. 5 shows the current efficiency for electrodeposition porous Ni film from different concentration of SDS. It can
be seen that the current efficiency extremely decreases when SDS was
used, and a slight change in current efficiency is observed with the
increase of SDS. The decrease of current efficiency suggests that a
higher volume of H2 bubbles were generated and used as the dynamic
template during electrodeposition process.
Figs. 6 and 7. show the cross-section morphology and corresponding
surface morphology of the Ni film electrodeposited from the bathes
with different concentration of SDS respectively. As expected, the addition of SDS has a great effect on the surface morphology including
pore depth, density, diameter and surface roughness. It can be seen that
the average thickness of the Ni film in the absence of SDS is about
3.3. Effect of SDS on the growth of porous Ni film
At high current density, H ± and Ni2+ were reduced concurrently at
the cathode and the H2 bubbles can be used as the dynamic template
during the electrodeposition process. Ni deposit settled down on the
substrate and grew around the evolved bubbles. With the continuous
progress of nickel electrodeposition, there are two possible ways of
growth for Ni deposit. One way is that Ni deposit grows around the
small H2 bubbles and become sufficiently dense before the H2 bubbles
leave the cathode. As a result, the small H2 bubbles left inside the Ni
deposit and can be seen from the cross-section image [22]. The other
way is that some bubbles will leave the cathode before the dense Ni
deposit completely covers the bubble, so the bowl-shaped void is
formed. Once the bubble left the cathode, new hydrogen bubbles were
evolved off at the same site continuously and the streams of H2 bubbles
were formed on the cathode. The steady stream of H2 bubbles can act as
the dynamic template during electrodeposition process, and the Ni
deposit would grow around the steady stream of H2 bubbles. In addition, these bubbles formed at the beginning can coalesce as they rose.
The diameter of pores formed at the initial phase in the film gradually
becomes larger at the later phase. Finally, the transverse “V” shaped
void came into being. The schematic diagram of the formation of
porous structure with and without SDS was shown in Fig. 8. Both of the
growth ways coexist in the electrolyte without any SDS. When SDS was
added into the bath, the reduction of surface tension greatly changed
the bubble behavior and enhanced the second growth ways during the
electrodeposition process. More hydrogen bubbles would continuously
166
Infrared Physics and Technology 93 (2018) 162–170
J. Guo et al.
(a)
(b)
20
Ra:2.7607 μm
-10
-20
-30
0
200
400
600
Distance (μm)
800
-20
-40
-60
0
1000
(c)
200
400
600
Distance (μm)
800
1000
800
1000
(d)
20
40
Ra:9.4217 μ m
0
Depth (μm)
Depth (μm)
Ra:7.2595 μm
0
0
Depth (μm)
Depth (μm)
10
-20
-40
Ra:12.786 μm
20
0
-20
-40
-60
-60
0
200
400
600
Distance (μ m )
800
1000
800
1000
0
200
400
600
Distance (μm)
(e)
Depth (μm)
40
Ra:15.288 μm
20
0
-20
-40
-60
0
200
400
600
Distance (μm)
Fig. 7. Surface topography of porous Ni films electrodeposited from solution containing 0.2 M NiCl2·6H2O and 4 M NH4Cl with (a) 0, (b) 0.25, (c) 0.5, (d) 1 and (e)
2 mM SDS at 3 A/cm2 and 25 °C in 240 s, which were measured by Zeiss Profiler. The images below are the depth and roughness of the corresponding porous Ni film
according the diagonal line.
167
Infrared Physics and Technology 93 (2018) 162–170
J. Guo et al.
Electrolyte without SDS
Uncoalesced
bubbles
Table 2
Texture coefficient TC(hkl) and relative texture coefficient (RTC) for porous Ni
films obtained from 0.2 M NiCl2·6H2O + 4 M NH4Cl with different concentration of SDS additions.
Electrolyte with SDS
Coalesced
Coalesced
bubbles
bubbles
Plane (hkl)
TC(hkl)
0 mM SDS
0.25 mM SDS
0.5 mM SDS
1 mM SDS
2 mM SDS
(1 1 1)
(2 0 0)
(2 2 0)
1.173
0.681
0.758
1.179
0.657
0.777
1.163
0.670
0.845
1.168
0.676
0.805
1.191
0.630
0.777
Fig. 8. Schematic diagram of the growth of porous Ni film electrodeposited
from different electrolytes.
Plane (hkl)
RTC(hkl)100%
evolve from the pores or even substrate, leading to the formation of
column stream of H2 bubbles approximate perpendicular to the cathode
surface. which were used as the dynamic template. As a result, the
growth direction of Ni deposit was perpendicular to the substrate rather
than thickening of the side wall of the pores. That is why more deep “V”
shaped voids can be observed and the film thickness increased with the
addition of SDS. Although the surfactant in the bubble surface can
imped the bubble coalescence to some extent, the coalescence cannot
be inhibited completely especially when the surfactant concentration is
below the critical micelle concentration (CMC). The CMC for SDS in
water is 2.3 g/L (∼8 mM).
In addition, according to the Laplace equation, ΔP = 4γ/R, (ΔP is
the pressure difference inside and outside the bubble; γ is the surface
tension of the liquid membrane; R is the radius of the bubble.), the
smaller the R is, the bigger the ΔP is. This means that the pressure inside
the smaller H2 bubbles are higher than that inside the bigger H2 bubbles. There is a tendency for gas to diffuse from small H2 bubbles
through the liquid membrane to large H2 bubbles. Once diffusion occurs, the smaller the small H2 bubbles become, the bigger the big H2
bubbles become. High volume H2 bubbles increase the probability of
bubble collision and coalescence during the electrodeposition process.
That is the addition of SDS has a significant effect on the growth of
porous Ni film.
(1 1 1)
(2 0 0)
(2 2 0)
CATHODE
CATHODE
Ni deposit
Ni deposit
0 mM SDS
0.25 mM SDS
0.5 mM SDS
1 mM SDS
2 mM SDS
44.91
26.07
29.02
45.12
25.14
29.74
43.43
25.02
31.55
44.09
25.52
30.39
45.84
24.25
29.91
concentration of SDS at 3 A/cm2 and 25 °C in 240 s was shown in Fig. 9.
For all the porous Ni films, the pure Ni was detected. The three peaks
appearing at around 44°, 52° and 76° can be assigned to the crystal
plane (1 1 1), (2 0 0) and (2 2 0) of Ni film. It can be seen that SDS has
little effects on the Ni peak position. In order to compare the preferred
orientation of the crystal growth of porous Ni films with different
concentration of SDS, the (1 1 1), (2 0 0) and (2 2 0) texture coefficients
(TC) of Ni films are calculated by the following Eq. (9) [41,42]:
TC(hkl) =
∑ I0 (hkl)
I(hkl)
×
× 100%
I0 (hkl)
∑ I(hkl)
(9)
where I(hkl) is XRD intensity in experimental data, I0(hkl) is the intensity in JCPDS cards. The relative texture coefficients (RTC) of (1 1 1),
(2 0 0) and (2 2 0) in Ni films are calculated by equation (10) [41]:
RTC(hkl) =
Tc(hkl)
∑ Tc(hkl)
(10)
The values of the texture coefficient TC(hkl) and relative texture
coefficient RTC(hkl) for the five porous Ni films investigated are present
in Table 2. The value of TC (1 1 1) greater than 1 indicates that a preferred orientation of the (1 1 1) reflection. The RTC(hkl) value presented in Tab.2 show that the preferred orientation for all porous Ni
films is (1 1 1) reflection. The addition of SDS has little effect on the
crystal preferred growth orientation, and the porous Ni films preferred
orientation is (1 1 1) reflection.
3.4. XRD analysis on Ni films
The phase compositions of these pure Ni films were investigated by
XRD method. The XRD patterns of Ni films electrodeposited from solution containing 0.2 M NiCl2·6H2O and 4 M NH4Cl with different
Ni (111)
Ni (200)
Intensity
Ni (220)
2 mM SDS
1 mM SDS
0.5 mM SDS
0.25 mM SDS
0 SDS
20
30
40
50
2θ(degrees)
60
70
80
Fig. 9. XRD patterns of porous Ni films electrodeposited from solution containing 0.2 M NiCl2·6H2O and 4 M NH4Cl with different concentration of SDS at 3 A/cm2
and 25 °C in 240 s.
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Infrared Physics and Technology 93 (2018) 162–170
J. Guo et al.
0.8
(a)
0.7
Infrared emissivity
Infrared emissivity
0.8
0.6
0.5
0.7
0.6
0.5
0.4
0.4
0.3
(b)
0.3
0.0
0.5
1.0
1.5
Concentration of SDS (mM)
2.0
2
4
6
8
10
Roughness (μm)
12
14
16
Fig. 10. The dependence of infrared emissivity of porous Ni film on (a) the concentration of SDS and (b) the roughness.
deviation of the profile from the mean. For most rough surfaces, δ can
be expressed in terms of the mean arithmetic deviation, Ra, of the
profile according to the relation δ≈1.25Ra [40]. Therefore, the effective emissivity of the rough surface can be expressed as
3.5. Infrared emissivity
Infrared emissivity is highly sensitive to the detailed surface geometry especially for the porous structure of Ni film. The dispersed pores
which have characteristic diameters on the order of the infrared wavelength and microscale roughness possess significant effect on the
infrared emissivity. The mechanism responsible for the enhancement of
the emissivity is due to the multiple reflections in these cavities which
decrease overall reflectance and increase more absorption. This is
called cavity effect. According to Kirchhoff’s law, an arbitrary opaque
body at a definite temperature, the ratio of emissivity to absorptivity at
every wavelength is equal to the ratio of emissivity to absorptivity of a
black body at the same fixed temperature. This means the emissivity
will equal to the absorptivity when the thermal radiation of emitting
and absorbing of an arbitrary opaque body is at the thermodynamic
equilibrium [1–5].
Emissivity is the ratio of the thermal radiation from a surface to the
radiation from an ideal black body surface at the same temperature as
given by the Stefan–Boltzmann law. The calculated thermal radiation at
a given surface is thus determined when we take the difference between
the number of bundles absorbed and the number of bundles emitted by
that surface. If radiant energy is envisioned as being transported by ray
bundles, the ray bundles are released successively from various locations on the actual area (Ar). If there is N ray bundles, then each ray
bundle is assigned an energy content E* given by
E∗ =
εσT 4Ar
N
εr ≈ (1 + 1.252π 2n2R 2a)
Eout
N E∗
N A
= out4
= ε out r
σ T 4As
σ T As
NAs
4. Conclusions
The porous pure Ni film with a wide range of infrared emissivity
from 0.36 to 0.80 was electrodeposited at 3 A/cm2 from the bath
containing 0.2 M NiCl2·6H2O and 4 M NH4Cl with different concentration of SDS. The reduction of surface tension caused by the addition of
SDS is able to enhance the hydrogen evolution reaction and thereby
increase the number of H2 bubbles acting as the dynamic template
during the electrodeposition process. The bubble behavior directly decides the surface morphology and roughness of porous Ni film including
pore density, depth and diameter, which increase with the concentration of SDS. The addition of SDS has a marked effect on improvement of
the roughness of porous Ni film. The formula between infrared emissivity and roughness indicates that the increase in roughness can increase the infrared emissivity accordingly. The infrared emissivity of
porous Ni film can be designed and tailored through the control of SDS
concentration.
(11)
(12)
where Nout is the number of bundles that exit through the surface, and
the denominator, σT4As, represents the radiant energy emitted by a
blackbody having a temperature equal to the equivalent smooth surface
are As.
The effective emissivity of a rough surface given in Eq. (12) is related to the roughness factor R = Ar/As. In addition, the roughness
factor can be calculated from a profilogram of the surface and the
roughness factor can be expressed as
R = 1 + π 2n2δ 2
(14)
According to Eq. (14), the increase in surface roughness can increase
the effective emissivity accordingly. The surface roughness dramatically increased when SDS was used due to the increase of depth, diameter and density of pores. Fig. 10a shows the dependence of infrared
emissivity on the concentration of SDS. It shows that the infrared
emissivity increases from 0.36 to 0.80 when the concentration of SDS
increases from 0 to 2 mM. The relation between the infrared emissivity
and the roughness was shown in Fig. 10b. Obviously, the infrared
emissivity increases with the increase of roughness, which is consistent
with Eq. (14). The presence of SDS has a great effect on the roughness of
film, subsequently on the infrared emissivity, and a wide range
(0.36–0.80) of infrared emissivity of pure Ni film was obtained through
the control of concentration.
where ε is the intrinsic emissivity of the material, T is the temperature
of the emitting surface, Ar is the total surface, σ is the Stefan-Boltzmann
constant and N is the total number of energy bundles emitted by all
surfaces.
Thus the effective emissivity (εr) of the rough surface can be expressed as
εr =
εNout
N
Conflict of interest
None.
Acknowledgments
(13)
The authors gratefully acknowledge the National Natural Science
Foundation of China (grant no. 51371116), the Joint Research Center of
SJTU-SAST for Advanced Aerospace Technology (grant no.
where n is the number of intersections of the surface profile with the
mean per unit length of the mean line, and δ is the mean-square
169
Infrared Physics and Technology 93 (2018) 162–170
J. Guo et al.
USCAST2013-23) and the 111 Project (Grant No. B16032) for financial
support. The helpful assistance to film analysis from the Instrumental
Analysis Center of Shanghai Jiao Tong University (SJTU) is sincerely
acknowledged.
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