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coatings Article

The Effect of Processing Parameters on the Formation and Properties of Al/Ni Core-Shell Pigments via a Galvanic Displacement Method Le Yuan 1,2, *, Min Zhou 1 , Can Wang 1 , Qinyong Zhang 1 , Xiaolong Weng 2 and Longjiang Deng 2 1

2

*

Key Laboratory of Fluid and Power Machinery of Ministry of Education, School of Materials Science and Engineering, Xihua University, Chengdu 610039, China; 18008224831@163.com (M.Z.); 17828187534@163.com (C.W.); zhangqy@mail.xhu.edu.cn (Q.Z.) National Engineering Research Center of Electromagnetic Radiation Control Materials, UESTC, Chengdu 610054, China; wengxl59@163.com (X.W.); denglj@uestc.edu.cn (L.D.) Correspondence: yuanle.cn@gmail.com; Tel.: +86-28-8772-9250





Received: 18 April 2018; Accepted: 22 May 2018; Published: 24 May 2018

Abstract: Al/Ni bimetallic core-shell pigments with flake Al particle as core and metallic Ni as shell were synthesized via a galvanic displacement method and studied with X-ray diffraction, scanning electron micrograph (SEM), specific surface area analysis (BET), thermogravimetry-differential thermalanalysis (TG/DSC), and visible-near infrared-infrared reflectance spectroscopy. The influence of reactant ratio (Al:Ni2+) and order of addition on phase structure, surface morphology, optical properties, and high temperature oxidation resistance properties were studied systematically. The results show that the local concentration of Ni2+ at solid-liquid interfaces can be effectively modulated by adjusting the reactant ratio and order of addition. A high local concentration of Ni2+ improves the rate of displacement reaction resulting in more metallic Ni on the surface of the flake Al powders. This increases the relative content of Ni in the shell. The change of displacement reaction rate also leads to a different surface morphology and roughness of the Ni shell. The thick and rough Ni shell has a strong absorption extinction due to the intense light scattering and absorption-this substantially reduces the spectral reflectance of the flake Al core. Infrared reflectance in particular is influenced by light scattering and absorption of the rough surface. In addition, the Ni shell can enhance the high temperature oxidation resistance of the Al core by preventing contact between the metallic Al substrate and oxygen. The oxidation resistance is also associated with the processing parameters of the galvanic displacement reaction. Keywords: flake aluminum; core-shell structure; galvanic displacement; nickel shell

1. Introduction Flake aluminum powders offer high spectral reflectance, good electric conductivity, excellent shielding capacity, and low cost. They are widely used in automobile metallized coatings, energy saving coatings, electromagnetic shielding coatings and infrared stealth coatings [1–6]. For the infrared stealth material, high concentrations of flakey aluminum powders can reduce infrared emissivity and restrain infrared radiation, but its high lightness and electrical conductivity challenge optics and radar stealth [7–9]. To overcome the multispectral stealth problem, we previously reported a bimetallic composite material with a core shell structure by wrapping a thin magnetic Ni shell on the surface of flake Al powders [9,10]. The prepared Al/Ni core-shell pigments may combine the properties of the core and the shell materials to offer particular new functions. Previous studies have shown that the optical, electrical, and magnetic properties of core-shell powders are associated with the relative content, thickness, and surface morphology of the shell layer. Coatings 2018, 8, 200; doi:10.3390/coatings8070200

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This can be influenced by the preparation process and method [10–13]. The galvanic displacement method is an effective method to prepare the bimetallic core-shell powder with controllability, high cladding rates, low costs, and simple processing [14–16]. This method allows low activity Ni2+ ions to be displaced by a highly active Al core at the solid-liquid interface. This ensures that the generated metal Ni is preferably deposited on the surface of the Al core rather than forming separate Ni particles. Thus, the quality and cladding rate of Al/Ni core-shell particles can be improved significantly. However, the covering effect and morphology of Ni shell is significantly affected by the rate of the galvanic displacement reaction [17,18] to ultimately influence the properties of the Al/Ni core-shell powders. Here, the rate of the galvanic displacement reaction was controlled by changing the reactant concentration and order of addition. The effect of processing parameters on phase structure, morphology, optical, and antioxidant properties of Al/Ni core-shell pigments were investigated systematically. 2. Materials and Methods 2.1. Materials Micrometer-scale flake aluminium pigments (particle size: ~20 µm) were procured from Toyo Aluminium K. K. (Zhaoqing, China). Nickel chloride (NiCl2), ammonium fluoride (NH4F), and ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemicals were A.R. grade. 2.2. Synthesis of Al/Ni Composite Material A detailed galvanic displacement method has been reported in our previous work [10]. In a typical synthesis, 2 g of flake Al powder was added to 200 mL of ethanol solution in a 500 mL conical flask and dispersed by ultrasonic vibration for 30 min. Then, 1.12 g of NH4 F was dissolved into 50 mL of a mixed solution of ethanol and distilled water with a volume ratio of 1:1 (this was Solution A). Next, NiCl2 (2.11–5.28 g) was dissolved into 60 mL of ethanol solution to obtain Solution B. To understand the mechanism of metallic Ni depositing on the surface of the flake Al core, two different preparation processes have been designed by changing the addition sequence of Solution A and Solution B. In this detailed processing step, Solution A (or Solution B) was quickly injected into the conical flask. After stirring for 30 min, Solution B (or Solution A) was added dropwise at 1 mL/min with a micro syringe pump under magnetic stirring (350 r/min) at 40 ◦ C. The color of the powders varied from brilliant silver to dark gray after a few minutes. After another hour, the products were separated by centrifugation and washed with ethanol and distilled water several times and then dried at 80 ◦ C for 12 h. This yielded black-gray Al/Ni core-shell pigments. According to the above order of addition, the prepared samples were labeled as “NH4 F→NiCl2 ” if Solution A was added first followed by dropwise addition of Solution B. Conversely, the samples were classified as “NiCl2 →NH4 F”. 2.3. Characterization The samples were characterized by X-ray diffraction (XRD, D2 PHASER with Cu Kα radiation, Bruker, Karlsruhe, Germany) and field emission scanning electron microscopy (FESEM, QUANTA 250, Thermo Fisher Scientific, Waltham, MA, USA). Thermogravimetry and differential scanning calorimetry (TG-DSC) analysis was performed using a STA 449F3 instrument (NETZSCH, Selb, Germany) under air atmosphere from 30 to 780 ◦ C at 10 ◦ C/min. The specific surface area was measured via specific surface area analyzer (BET, ASAP 2020HD88, Micromeritics Instrument Ltd., Atlanta, GA, USA). For optical characterization, paints containing Al/Ni composite pigments were prepared by mixing Al/Ni powder 16 wt. %, epoxy resin 12.5 wt. % and thinner 62.5 wt. %. Then the tested coating samples were painted by these mixtures onto microslides and cured at 60 ◦ C for 12 h. The VIS/NIR reflection spectrum (380–2500 nm) was measured with a UV/VIS/NIR spectrophotometer (Lambda 750, PerkineElmer,

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Waltham, MA, USA). The CIE LAB color data (L*) were calculated from the visible light reflection spectrum observation the (PerkineElmer, calculated spectrum range wassource, 400–700 nm). infrared reflectance spectrum by Color CIEangle; software CIE D65 photo and 10◦The observation angle; the calculated was measured with a Fourier transform infrared spectrometer (Tensor27, Bruker, Karlsruhe, spectrum range was 400–700 nm). The infrared reflectance spectrum was measured with a Fourier transform Germany) with an integrating infrared spectrometer (Tensor27,sphere Bruker,attachment. Karlsruhe, Germany) with an integrating sphere attachment. 3. Results and Discussion

Figure 1 presents XRD patterns of the Al/Ni Al/Ni products products obtained obtained at at different different preparation preparation processes 2+:Al and trace amounts of and different Ni2+ :Almolar molarratios. ratios.Only Onlythe theAlAlphase phase(JCPDS, (JCPDS,file fileNo. No.04-0787) 04-0787) and trace amounts 2+ 2+ the Ni phase (JCPDS, file No. 04-0850) were obtained in the XRD spectrum at a Ni :Al of 0.1:1. The of the Ni phase (JCPDS, file No. 04-0850) were obtained in the XRD spectrum at a Ni :Al of intensity the Ni peaks gradually the molar as ratio Ni2+: Al increases. 0.1:1. Theofintensity of the Ni peaksstrengthens gradually as strengthens theofmolar ratio of Ni2+This : Al illustrates increases. 2+ that illustrates Ni2+ ions that are successfully by replaced Al substances via the galvanic reaction This Ni ions are replaced successfully by Al substances via the displacement galvanic displacement 2+ 2+ without without any other reaction betweenbetween Al and Ni. The Ni. highThe molar of Ni to more reaction any other reaction Al and highratio molar ratio:Al of can Ni lead :Al can lead metallic Ni beingNi generated on the surface flake Alofpowders. This increases relativethe Ni relative content to more metallic being generated on theofsurface flake Al powders. Thisthe increases in the shell.in the shell. Ni content Figure 1 shows that the the Ni Ni peak peak intensity intensity of of “NiCl “NiCl22→ →NH of NH44F” F” samples samples are higher than that of 2+. We “NH44FF→NiCl reaction concentration concentration of of Ni Ni2+ the “NH →NiCl22”” samples samples even with the same reaction . concluded that the relative content of the the Ni Ni shell shell is is highly highly dependent dependent on on the the preparation preparation process process because because the the addition addition reactants, such such as as NH NH44F and NiCl22, would affect the rate of displacement. sequence of reactants,

Figure 1. 1. XRD XRD patterns patterns of of Al/Ni Al/Ni products preparation process process and and molar molar Figure products prepared prepared with with different different preparation 2+/Al: (a) the samples of “NH4F→NiCl2”; (b) the samples of “NiCl2→NH4F”. ratios of Ni 2+ ratios of Ni /Al: (a) the samples of “NH4 F→NiCl2 ”; (b) the samples of “NiCl2 →NH4 F”.

For the reaction process of “NH4F→NiCl2” samples, the surface oxide layer of flake Al core is For the reaction process of “NH F→NiCl ” samples, the surface oxide layer of flake Al core is first removed by a high concentration4 of NH4F2solution via the following reaction: first removed by a high concentration of NH4 F solution via the following reaction: Al 3+ +6F − → AlF63 − (1) Al3+ +6F− → AlF36− (1) Upon slow addition of NiCl2 solution, the displacement reaction can be performed on all of the bare Upon surfaces of addition the Al particles. overallthe displacement reaction equation slow of NiClThe solution, displacement reaction can be is: performed on all of the 2

bare surfaces of the Al particles. The overall equation is: 2Al + 3displacement Ni 2+ → 2Al 3+ +reaction 3Ni

(2)

+ core from the residual NiCl2 solution and 2+ However, the newly formed Ni2Al layer insulates the 3Al + 3Ni → 2Al + 3Ni (2) obstructs the subsequent displacement reaction. If the concentration of Ni2+ is relatively low in the reaction system, then the formed slow displacement reaction inhibits the growth rate of the shell layer, However, the newly Ni layer insulates therate Al core from the residual NiCl and 2 solution and this leads to a low content of Ni in the shell. If the concentration of Ni2+ is relatively low in the obstructs the subsequent displacement reaction. In contrast, for the of “NiCl 2→NH4rate F” samples, while the flake particles are reaction system, then the preparation slow displacement reaction inhibits the growth rate ofAlthe shell layer, immersed in ato high concentration of in NiCl the displacement reaction is hampered by the Al2O3 layer. and this leads a low content of Ni the2, shell. OnceInthe NH4F solution added dropwise, the AlNH 2O34layer is gradually the exposed contrast, for the is preparation of “NiCl F” samples, whiledissolved, the flake and Al particles are 2→ 2+ surface of in the Al substrate can react with Ni displacement ions to quickly formis ahampered Ni shell. by The immersed a high concentration of NiCl reaction thedisplacement Al2 O3 layer. 2 , the reaction by the high concentration Ni2+, and the reaction time isand extended due to Once therate NHis solution is added dropwise, the Al2 Oof is gradually dissolved, the exposed 4 Faccelerated 3 layer 2+ the low of removal of thecan Al2O 3 layer. Thus, metallic Niform can be deposited on displacement the surface of surface the Al rate substrate react with Ni more ions to quickly a Ni shell. The flake Al powder. Figure 2 shows SEM images of the as-prepared Al/Ni composite powders obtained under different preparation process and Ni2+:Al molar ratios. The surface of raw Al powders (Figure 2a) is

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reaction rate is accelerated by the high concentration of Ni2+ , and the reaction time is extended due to the low removal rate of the Al2 O3 layer. Thus, more metallic Ni can be deposited on the surface of flake Al powder. Figure 2 shows SEM images of the as-prepared Al/Ni composite powders obtained under different Coatings 2018,process 8, x FOR PEER 4 of 8 preparation and REVIEW Ni2+ :Al molar ratios. The surface of raw Al powders (Figure 2a) is smooth and flat. After the galvanic displacement process, a nickel shell is formed on the Al substrate and smooth and flat. After the galvanic displacement process, a nickel shell is formed on the Al substrate the surface becomes rougher. Figure 2b shows that a thin Ni layer is formed, and a small amount of and the surface becomes rougher. Figure 2b shows that a thin Ni layer is formed, and a small amount nanoparticle is distributed on the surface of the composite particles. Compared with the sample in of nanoparticle is distributed on the surface of the composite particles. Compared with the sample in Figure 2b, the Ni shell of the “NiCl2 →NH4 F” samples (Figure 2e) becomes a granular film, and the Figure 2b, the Ni shell of the “NiCl2→NH4F” samples (Figure 2e) becomes a granular film, and the 2+ :Al is 0.1:1. The surface roughness of the surface contains some NiNi nanoparticles when the ratio ofof NiNi 2+:Al is 0.1:1. The surface roughness of surface contains some nanoparticles when the ratio as-prepared compound particles is obviously enhanced. This results in in a significant the as-prepared compound particles is obviously enhanced. This results a significantincrease increaseininthe 2 /g. It verifies that the surface morphology of the Ni shells has a BET surface area from 24.1 to 42.9 m 2 the BET surface area from 24.1 to 42.9 m /g. It verifies that the surface morphology of the Ni shells corresponding changechange as a function of NHof and4FNiCl addition order. 4 FNH has a corresponding as a function and 2NiCl 2 addition order.

Figure 2. SEM photographs of Al and Al/Ni products prepared with different preparation process Figure 2. SEM photographs of Al and Al/Ni products prepared with different preparation process and Ni2+:Al = 0.1:1; (c) NH42+ F→NiCl2, and molar ratios of Ni2+:Al: (a) raw Al; (b) NH4F→NiCl 2+ :Al: 2+ :Al =2,0.1:1; molar ratios of Ni (a) raw Al; (b) NH F → NiCl , Ni (c) NH4 F→NiCl2 , Ni :Al = 0.2:1; 4 2 4F→NiCl2, Ni2+:Al = 0.3:1; (e) NiCl2→NH4F, Ni2+:Al = 0.1:1; (f) NiCl2→NH4F, Ni2+:Al = 0.2:1; (d) NH 2+ (d) NH NiCl2 →NH4 F, Ni2+ :Al = 0.1:1; (f) NiCl2 →NH4 F, Ni2+ :Al = 0.2:1; 4 F→NiCl2 , Ni :Al = 0.3:1; (e) 2→NH4F, Ni2+:Al = 0.3:1. Ni2+:Al = 0.2:1; (g) NiCl 2+ (g) NiCl2 →NH4 F, Ni :Al = 0.3:1.

As shown in Figure 2c, the quantity and size of the Ni nanoparticles on the surface of Ni shell 2+:Al As shown in Figure 2c, the quantity size of the Ni nanoparticles the surface of Ni shell gradually increases as the molar ratio of Niand increases—in particular, theiron shapes transform from 2+ gradually increases as long the molar ratio(Figure of Ni 2c). :Al These increases—in their shapes from nearly spherical into graininess changes particular, lead the BET surface areatransform of the Al/Ni 2/g (Figure 3). Continued increases in the molar ratio of Ni2+:Al to 0.3:1 (Figure 2d) sample to reach 45.1 nearly spherical intom long graininess (Figure 2c). These changes lead the BET surface area of the 2+ :Al to results in Ni particles develop flaky morphology. The increases transversein size reaches microns Al/Ni sample to reachthat 45.1 m2 /g a(Figure 3). Continued the molarthe ratio of Niscales although number Meanwhile, the BETasurface area increasesThe to 55.5 m2/g. size reaches 0.3:1 (Figurethe 2d) resultsdecreases. in Ni particles that develop flaky morphology. transverse However, for the “NiCl 2 →NH 4 F” samples, the size of Ni particle remains constant with to the microns scales although the number decreases. Meanwhile, the BET surface area increases 2+ increasing 55.5 m2 /g. Ni concentrations, but the particle number increases significantly (Figure 2f). When the rateHowever, of Ni2+:Alfor increased to 0.3:1 (Figure 2g), it causes serious aggregation of Ni nanoparticles, and a the “NiCl 2 →NH4 F” samples, the size of Ni particle remains constant with increasing small number of cubical particles are observed the sample surface. However, surface 2+ Ni concentrations, but the particle number on increases significantly (Figure the 2f). BET When the area rate of is barely affected by the morphological change of the Ni shell. 2+ Ni :Al increased to 0.3:1 (Figure 2g), it causes serious aggregation of Ni nanoparticles, and a small number of cubical particles are observed on the sample surface. However, the BET surface area is barely affected by the morphological change of the Ni shell.

Figure 3. The BET surface area of Al/Ni samples at different molar ratio of Ni2+:Al.

The morphology of the Ni shell is strongly affected by the preparation conditions (Ni2+:Al ratio and the addition sequence of reactant). This is because these changes the reactant concentration of

However, for the “NiCl2→NH4F” samples, the size of Ni particle remains constant with increasing Ni2+ concentrations, but the particle number increases significantly (Figure 2f). When the rate of Ni2+:Al increased to 0.3:1 (Figure 2g), it causes serious aggregation of Ni nanoparticles, and a small number of cubical particles are observed on the sample surface. However, the BET surface area Coatings 2018, 8, 200 5 of 9 is barely affected by the morphological change of the Ni shell.

2+:Al. Figure3.3.The TheBET BETsurface surfacearea areaofofAl/Ni Al/Ni samples samples at at different differentmolar molarratio ratioof ofNi Ni2+ Figure

The morphology of the Ni shell is strongly affected by the preparation conditions (Ni2+2+:Al ratio is strongly affected by the preparation conditions (Ni :Al 5ratio of 8 and the addition sequence of reactant). This is because these changes the reactant concentration of and the addition sequence of reactant). This is because these changes the reactant concentration of 2+.. It Ni2+ form aa relatively relativelysmooth smoothand andthin thinNi Nishell shellthrough through the heterogeneous nucleation Ni It is is easier to form the heterogeneous nucleation at 2+ 2+concentrations. at low concentrations.However, However,the thedisplacement displacementreaction reactionrate rateisisaccelerated accelerated with with an an increasing increasing low NiNi in the concentration of amount and inhibits growth of the Ni in of reactants. reactants.This Thisincreases increasesthe thenucleation nucleation amount and inhibits growth of the particles [17]. Ni particles [17]. In order order to to research research the the influence influence of of the the Ni shell on the spectral reflectivity of flake Al powders, In the VIS-NIR VIS-NIR diffuse diffuse reflectance reflectance spectra spectra of of Al/Ni Al/Ni core-shell samples and raw Al powders powders were were the measured. Uncoated Uncoated raw raw Al powders have extremely high reflectance reflectance value value in both VIS (400–800 nm) measured. and NIR NIR (0.8–2.5 (0.8–2.5 µm) μm) wavebands wavebands (Figure (Figure 4). 4). The spectral reflectance is remarkably decreased by and coating with with the the Ni Ni shell shell on on the the surface surface of of the the flake flake Al Al pigments. pigments. coating The morphology ofREVIEW the Ni shell Coatings 2018, 8, x FOR PEER

Figure 4. The diffusediffuse reflectance spectra of coating raw Al by andraw Al/Ni Figure 4. VIS-NIR The VIS-NIR reflectance spectra ofsamples coating prepared samples by prepared Alpigments. and Al/Ni pigments.

Regardless of the preparation process, the reflectance of Al/Ni samples declines gradually with 2+:Al; notably, the degree of decline is different. The “NH4F→NiCl2” samples at the increasing RegardlessNi of the preparation process, the reflectance of Al/Ni samples declines gradually with Ni2+increasing :Al = 0.1:1 have a VIS and NIR about 29% and 25% The lower, respectively. The VIS and the Ni2+ :Al; notably, thereflectance degree of decline is different. “NH 4 F→NiCl2 ” samples at NIR reflectance of “NiCl 2→NH4F” samples reduces by 35% and 29%, respectively. Furthermore, the 2+ :Al Ni = 0.1:1 have a VIS and NIR reflectance about 29% and 25% lower, respectively. The VIS and influence on the spectral is further exacerbated at higher ratios of Ni2+:Al. NIR reflectance of “NiClreflectance → 2 NH4 F” samples reduces by 35% and 29%, respectively. Furthermore, According previous research results exacerbated [10–12], the at incidence visible light is the influence on to theour spectral reflectance is further higher ratios of and Ni2+infrared :Al. difficult to penetrate Ni shellresearch becauseresults of the low spectral transmittance of the in the According to ourthe previous [10–12], the incidence visible andmetallic infraredNilight is VIS and NIR wavebands. This makes the reflectance of the Al/Ni core-shell samples more susceptible difficult to penetrate the Ni shell because of the low spectral transmittance of the metallic Ni in the VIS to the surface morphology of thethe Ni reflectance shell. The rough surfacecore-shell causes intense scatter the incident and NIR wavebands. This makes of the Al/Ni samples moreof susceptible to rays and excellent extinction coefficients at VIS and NIR wavelengths. A stronger extinction ability the surface morphology of the Ni shell. The rough surface causes intense scatter of the incident rays can be obtained with a higher Ni shell roughness. As shown in Figure 5, the lightness value L* of flake Al pigment can be significantly reduced via the cover treatment due to the close relationships between VIS reflectance and lightness. When the surface is only coated by the small number of metallic Ni (Ni2+:Al = 0.1:1), the color of the flake pigments changes from brilliant silver to pale gray, and the lightness value L* decreases by 16.2 and 21.4, respectively. The lightness declines gradually with increasing Ni2+ content. If the ratio of Ni2+:Al

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and excellent extinction coefficients at VIS and NIR wavelengths. A stronger extinction ability can be obtained with a higher Ni shell roughness. As shown in Figure 5, the lightness value L* of flake Al pigment can be significantly reduced via the cover treatment due to the close relationships between VIS reflectance and lightness. When the surface is only coated by the small number of metallic Ni (Ni2+ :Al = 0.1:1), the color of the flake pigments changes from brilliant silver to pale gray, and the lightness value L* decreases by 16.2 and 21.4, respectively. The lightness declines gradually with increasing Ni2+ content. If the ratio of Ni2+ :Al increases to 0.3:1, then the lightness decreases by 30.3 and 39.6, respectively, and the color becomes black gray. The infrared reflectance spectra of coating samples prepared by raw Al and Al/Ni pigments are shown in Figure 6. The reflectance between infrared reflectance spectra and VIS/NIR diffuse reflectance spectra are not approximately equal at λ = 2500 nm, which can be attributed to different test principle and instrumental error. Some characteristic infrared adsorption bands can be observed in the reflectance spectra. The absorption peaks at 2.85, 3.4, 6, 6.6 and 8 µm are attributed to the infrared spectra characteristics of the epoxy resin in the coating samples. It is similar to the VIS-NIR reflectance spectra that the infrared reflectance spectra can also be affected by the ratio of Ni2+ :Al and the preparation process. The reflectance gradually decreases with increasing Ni2+ concentration—especially for coating samples consisting of “NiCl Coatings 2018, 8, x FOR PEER REVIEW 6 the of 8 2 →NH4 F” pigments. Upon comparing Figures 3 and 6b, we see that variation in infrared reflectance is consistent with the BET surface area of the Al/Ni samples. The BET and thearea results that therelated infrared reflectance is mainly influenced by the light scattering and surface datashow are directly to the surface roughness, and the results show that the infrared absorption of the rough Ni shell. reflectance is mainly influenced by the light scattering and absorption of the rough Ni shell.

Figure 5. The and visible light color of coating raw Al and pigments. Figure 5. lightness The lightness and visible light color ofsamples coatingprepared samplesby prepared by Al/Ni raw Al and Al/Ni pigments.

Figure 7 shows the thermal analysis curves of raw Al and Al/Ni products. TG analysis illustrates the weight of raw Al powders remains constant from 40 to 500 °C, but the weight rises sharply by Figure 7 shows the thermal analysis curves of raw Al and Al/Ni products. TG analysis illustrates about 5.2% when the temperature increases from 570 to 620 °C. DSC cure shows that there is a sharp the weight of raw Al powders remains constant from 40 to 500 ◦ C, but the weight rises sharply by exothermic peak at 596 °C due to the oxidation of aluminum. There is also a weak decalescence peak about 5.2% when the temperature increases from 570 to 620 ◦ C. DSC cure shows that there is a sharp at 674 °C attributed to the crystalline phase transition of aluminum oxide [10]. exothermic peak at 596 ◦ C due to the oxidation of aluminum. There is also a weak decalescence peak In contrast, the weight of the Al/Ni core-shell samples consistently decrease with increasing at 674 ◦ C attributed to the crystalline phase transition of aluminum oxide [10]. temperature from 40 to 490 °C because of the loss of water molecules and thermal decomposition of In contrast, the weight of the Al/Ni core-shell samples consistently decrease with increasing organic residues. When the temperature exceeds 500 °C, the mass of the Al/Ni samples starts to rise temperature from 40 to 490 ◦ C because of the loss of water molecules and thermal decomposition gradually—this eventually leads to a drastic increase at 630 °C. However, as the concentration of Ni2+ of organic residues. When the temperature exceeds 500 ◦ C, the mass of the Al/Ni samples starts to increases, this phenomenon gradually declines because of the increased thickness of Ni layer. The thick rise gradually—this eventually leads to a drastic increase at 630 ◦ C. However, as the concentration Ni shell helps to prevent Al core contact with the oxygen and protects it from high temperature oxidation. of Ni2+ increases, this phenomenon gradually declines because of the increased thickness of Ni layer. The thick Ni shell helps to prevent Al core contact with the oxygen and protects it from high temperature oxidation.

In contrast, the weight of the Al/Ni core-shell samples consistently decrease with increasing temperature from 40 to 490 °C because of the loss of water molecules and thermal decomposition of organic residues. When the temperature exceeds 500 °C, the mass of the Al/Ni samples starts to rise gradually—this eventually leads to a drastic increase at 630 °C. However, as the concentration of Ni2+ increases, this phenomenon gradually declines because of the increased thickness of Ni layer. The thick Coatings 2018, 8, 200 7 of 9 Ni shell helps to prevent Al core contact with the oxygen and protects it from high temperature oxidation.

Figure 6. 6. The reflectance spectra spectra of of coating coating samples samples prepared preparedby byraw rawAl Al and and Al/Ni Al/Ni pigments: Figure The infrared infrared reflectance pigments: (a) Infrared reflectance spectra; (b) the average reflectance in the range of 8–14 μm. (a) Infrared reflectance spectra; (b) the average reflectance in the range of 8–14 µm.

In addition, the exothermic peak can be observed at about 630 °C from the DSC data. Compared In addition, the exothermic peak can be observed at about 630 ◦ C from the DSC data. with the raw flake Al powder, the oxidation temperature of the Al/Ni samples is delayed by 34 °C. Compared with the raw flake Al powder, the oxidation temperature of the Al/Ni samples is delayed This proves that the Ni shell has been successfully formed on the surface of flake Al powders via the by 34 ◦ C. This proves that the Ni shell has been successfully formed on the surface of flake Al powders galvanic displacement method. The compact Ni shell can prevent oxidation of flake aluminum core. via the galvanic displacement method. The compact Ni shell can prevent oxidation of flake aluminum Thus, the oxidation reaction is delayed. In contrast, the antioxidant property of Al core improves as core. Thus, the oxidation reaction is delayed. In contrast, the antioxidant property of Al core improves the thickness of the Ni shell increased. The “NH4F→NiCl2” sample at Ni2+:Al =2+ 0.3:1 has only a weak as the thickness of the Ni shell increased. The “NH4 F→NiCl2 ” sample at Ni :Al = 0.3:1 has only exothermic peak in the DSC curve. The antioxidant ability of the Al/Ni core-shell pigment is strongly a weak exothermic peak in the DSC curve. The antioxidant ability of the Al/Ni core-shell pigment affected by the content and surface morphology of the Ni shell. This is related to the preparation is strongly affected by the content and surface morphology of the Ni shell. This is related to the approach. preparation Coatings 2018, 8,approach. x FOR PEER REVIEW 7 of 8

Figure 7. TG-DSC curves of raw Al powder and Al/Ni samples. Figure 7. TG-DSC curves of raw Al powder and Al/Ni samples.

4. Conclusions 4. Conclusions Bimetallic Al/Ni core-shell pigments have been synthesized with a simple galvanic displacement Bimetallic Al/Ni core-shell pigments have been synthesized with a simple galvanic displacement reaction. The relative content and surface morphology of the Ni shell mainly depends on the rate of reaction. The relative content and surface morphology of the Ni shell mainly depends on the rate of displacement reaction, and this can be effectively modulated by the synthesis pathway and the ratio displacement reaction, and this can be effectively modulated by the synthesis pathway and the ratio of of Al:Ni2+. The synthesis pathway of “NiCl2 first, NH4F second” and high ratio of Al:Ni2+ helps form a granular shell and increases the relative content of Ni because the increased local Ni2+ concentration at solid-liquid interfaces accelerates the rate of the displacement reaction. At higher Ni2+ concentrations, a rougher Ni shell with greater specific surface area is obtained because there are more Ni particles and more agglomeration. Conversely, with an opposite synthesis pathway, a thin and dense Ni shell can

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Al:Ni2+ . The synthesis pathway of “NiCl2 first, NH4 F second” and high ratio of Al:Ni2+ helps form a granular shell and increases the relative content of Ni because the increased local Ni2+ concentration at solid-liquid interfaces accelerates the rate of the displacement reaction. At higher Ni2+ concentrations, a rougher Ni shell with greater specific surface area is obtained because there are more Ni particles and more agglomeration. Conversely, with an opposite synthesis pathway, a thin and dense Ni shell can be formed on the surface of the Al core. Although the grain size of the Ni shell significantly increases as the concentration of Ni2+ increases, the effect on specific surface area and surface roughness is relatively insignificant. The spectral reflectance of Al/Ni core-shell pigments is closely related to the content and roughness of the Ni shell. The thick and rough Ni shell intensely scatters the incident rays and results in an excellent extinction coefficient. The color of the core-shell pigments changes from brilliant silver to black gray. The infrared reflectance is mainly influenced by light scattering and absorption due to the rough Ni shell. In addition, the thick and rough Ni shell can also prevent Al core contact with the oxygen—this significantly enhances the high temperature oxidation resistance of the Al/Ni core-shell powders. Author Contributions: L.Y. conceived the experiments and wrote the paper; M.Z. And C.W. performed the experiments; Q.Z. And X.W. measured and analyzed the samples; L.D. carried out the manuscript modification. Funding: This research was funded by the Chunhui Program from Education Ministry of China (No. Z2017072 and Z2015082), the National Natural Science Foundation of China (No. 51402241 and 51501156), and the Open Research Subject of Key Laboratory of Special Materials and Manufacturing Technology (No. SZJJ2017-063). Conflicts of Interest: The author declares no conflict of interest.

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