Characteristics of silica coating on copper substrate using coconut oil as a dispersion medium

The object of this study was to develop an easy and simple technique to fabricate the coating solutions based on Viet Nam rice husk ash-derived water glass (VRHA-WG) and Viet Nam coconut oil (VCO). The coating solutions with different mixing ratios of VRHA-WG and VCO such as 1:5 and 1:10 (v/v) coated on copper substrates.

CHARACTERISTICS OF SILICA COATING ON COPPER

SUBSTRATE USING COCONUT OIL AS A DISPERSION MEDIUM

Pham Trung Kien1, *, Le Ngoc Long1, Ngo Minh Van1, Kieu Do Trung Kien1,

1

Faculty of Materials Technology, Ho Chi Minh City University of Technology (HCMUT),

268 Ly Thuong Kiet Street, Ward 14, Dist 10, Ho Chi Minh City

2

Laboratory of Biofuel and Biomass Research, Ho Chi Minh City University of Technology

(HCMUT), 268 Ly Thuong Kiet Street, Ward 14, Dist 10, Ho Chi Minh City

*

Email: phamtrungkien@hcmut.edu.vn

Received: 15 July 2019; Accepted for publication: 27 September 2019

Abstract The object of this study was to develop an easy and simple technique to fabricate the

coating solutions based on Viet Nam rice husk ash-derived water glass (VRHA-WG) and Viet

Nam coconut oil (VCO) The coating solutions with different mixing ratios of VRHA-WG and

VCO such as 1:5 and 1:10 (v/v) coated on copper substrates After coating, the substrates were

heated at 500 oC for 2 hours to enhance the adherence of silica layer and substrates The coating

layer were studied by X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) while the surface

morphology of the substrate before and after coating was studied be Scanning Electron

Microscope and Energy-dispersive X-ray spectroscopy (SEM/EDX) Volume (bulk) resistivity

(VR) of the coated substrate increased up to 33.33 % compared to un-coating samples

Keywords: rice hush ash, silica, coconut oil, coating, environment materials

Classification numbers: 2.5.3, 2.9.2, 2.10.2

1 INTRODUCTION

Viet Nam is an argiculture country which annually produces a huge amount of rice husk

(RH) and rice husk ash (RHA) Viet Nam rice husk ash (VRHA) is considered as an argicuture

waste since there is no extra use of Viet Nam rice ash The amount of VRHA is around 8 million

tons/year [1] It needs to emphasize that the main component of RHA is silica whose the content

varies from 80 to 95 % depending on the burning mode of Viet Nam RH [2-6] Researchers

around Viet Nam and ASEAN countries mainly focus on the synthesis of silica powder from RH

as well as RHA, and pay few attention on the application of silica powder [7-9] Other

researches also pay attention on using silica gel as coating agent to protect the metal substrate

such as stainless steel [10-11] With this regard, our research group focuses on the use of silica

coating solution on metal substrate, for example copper substrate, in order to extend the

application of VRHA-dervied silica The silica was kept as water glass then mixed with Viet

Nam coconut oil as medium dispersed, the mixture then was used to coat on copper substrate

The coating layer is expected to enhance the volume resistivity (VR) of copper substrate

materials, thus make copper from electrical conducting materials to electrical isolating materials

2 MATERIALS AND METHODS 2.1 Preparation of Viet Nam rice hush ash-derived water glass (VRHA-WG)

The VRHA-WG was prepared as described in the previuous publication [3] In brief, the

VRH supplied by Loc Troi Group Corpration (LTGC) was burnt continously in a steam boiler at

500 oC for 4 hours to obtain VRHA The VRHA was reacted with 160 mL of 2.5 M NaOH

solution according to the molar ratio of SiO2:NaOH of 1:3.5, and the mixture was stirred at

90 oC for 3 hours using magnetic stirring with the speed of 120 rpm Then the stirring mixture

was filtered by vacuum pump to obtain the water glass according to the equation (1):

SiO2 + 2 NaOH  Na2SiO3 + H2O (1)

2.2 Source of coconut oil (VCO)

The Viet Nam coconut oil was supplied by Vietcoco Ltd Co., and used without further

purification

2.3 Preparation of the VRHA-WG/VCO mixture

The VRHA-WG and VCO were mixed together by stiring at 70 oC with the speed of 120

rpm using different mixing ratios of VRHA-WG to VCO, i.e of 1:5 and 1:10 (v/v) The liquid

mixture after cooling was coded as VRHA-WG/VCO and characterized The liquid mixture was

also heated at 500 oC for 2 hours, and then characterized by chemical element analysis using

X-ray Fluoresence (XRF) and powder phase composition using X-X-ray Diffraction (Powder XRD)

2.4 Coating of mixture of VRHA-WG/VCO on copper substrate

The copper plates with the size of 75 mm (L) x 25 mm (W) x 1 mm (D) were used in this

experiment The mixture of VRHA-WG/VCO was coated on copper substrate by dip coating

with the dipping speed of 2 mm/s and soaking for 30 s The coated sample was dried at 110 oC

for 3 hours, then followed by heat treatment at 500 oC for 2 hours, and the product analyzed

using a thin-film phase composition analyzer (Thin film XRD)

2.5 Characterization of the sample

Viscosity of liquid mixture of VRHA-WG and VCO was evaluated as follows 100 mL

mixture of VRHA-WG and VCO with mixing ratio of VRHA-WG to VCO of 1:5 and 1:10 (v/v),

were passed through the Ford viscosity standard cup with the orifice diameter of 3 mm, and the

passing time was measured in unit of second at 25 oC

Chemical composition of VRHA and the silica coating layer were analyzed using XRF

(MESA-50, Horiba) The samples were compressed in a 10 mm mold and exposed to X-ray

irradiation, to analyze the oxide composition

Phase composition of VRHA and the silica coating layer were analyzed using XRD (Bruker D8 Advance) at  = 1.54 A and 2-theta variation from 5o to 50o with the step scan of 0.02 s The samples were compressed in a 10-mm mold and exposed to X-ray irradiation under accelerator

of 15 kV

Surface morphology of the coating layer was observed using scanning electron microscope (SEM) (JSM 5400LV, JEOL Co Ltd) under accelerating voltage of 2 kV after being coated with Pt The thickness of coating layer was observed by EDX mapping technique with the Cu, Si, and Na element In addition, the optical images of samples before and after coating were observed by light microscope (Olympia S800) using magnification of 100X

Measurements of Volume Resistivity of sample before and after coating were performed as follows The samples before and after coating were measured of the electrical resistance by Digital Ohm meter (Megger, Warmup Ltd Co) The samples were kept in between the positive and negative electrode (measure between two surfaces) as guided in the Megger guideline to measure the volume resistance

3 RESULTS AND DISCUSSION

The chemical composition of VRHA obtained after a heat treatment of VRH at 500 oC for 4 hours given by XRF analysis is shown in Table 1

Table 1.Chemical composition of VRHA (weight percent).

Oxide SiO2 K2O CaO P2O5 Other Total

% weight 92.7 3.16 1.33 0.59 2.22 100 %

These data indicate that dominant oxide in VRHA is silica Therefore VRHA can be used

as a source of qualified silica for the next experiment to produce VRHA-WG

The phase analysis of VRHA is also given in Fig 1, indicating that VRHA is composed of low degree of crystallinity SiO2 according to PDF card #39-1425

-0.05 0.15 0.35 0.55 0.75 0.95 1.15

2theta / degree

Figure 1.XRD pattern of VRHA obtained by burning VRH at 500 oC for 4 hours

Data of viscosity and density of mixture of VRHA-WG/VCO by Ford viscosity standard cup are shown in Table 2 The kinematic viscosity of 1: mixture ratio is faster than 1: 10 mixture ratio whereas the density of 1:5 mixture ratio is of 2 % higher than that of 1:10 mixture ratio Table 3 shows the chemical composition of different mixing liquid ratios of VRHA-WG to VCO (v/v) after a heat treatment at 500 oC for 2 hours

Table 2 Data of viscosity and density of mixture of VRHA-WG/VCO

Parameter Single liquid Mixing liquid ratio of VRHA-WG to

VCO (v/v)

Kinematic viscosity (s) for

100 mL of single liquid

and/or mixture liquid

Table 3.Chemical composition of different mixing liquid ratios of VRHA-WG to VCO heated at

500 oC for 2 hours

Mixing ratio

of

VRHA-WG to VCO

(v/v)

Oxide Na2O SiO2 K2O P2O5 Other(s) Loss on

ignition (LOI)

Total

1:5 % weight 62.08 27.79 1.10 0.42 5.69 2.92 100

1:10 % weight 57.82 31.68 1.00 0.51 5.19 3.80 100

2theta / degree

2theta / degree

a) SiO2 reference b) 1:5

c) 1:10

e) 1:5 f) 1:10

d) Non coatin g

Figure 2.XRD patterns of the copper substrate coated with VRHA-WG/VCO mixture after heat treatment

at 500 oC for 2 hours: a) SiO2 reference; b) with mixing ratio of VRHA-WG to VCO of 1:5; c) with mixing ratio of VRHA-WG to VCO of 1:10; d) Un-coated substrate; e) with mixing ratio of VRHA-WG to VCO of 1:5; f) with mixing ratio of VRHA-WG to VCO of 1:10

Note that a, b, c is powder XRD patterns while d, e, f is thin film XRD patterns

Figure 2 shows the XRD pattern of sample with different mixing ratios of VRHA-WG to VCO after heat treatment at 500 oC for 2 hours The XRD patterns of the copper substrate coated with VRHA-WG/VCO mixture after heat treatment at 500 oC for 2 hours (on the left Figure 2), analyzed by XRD powder technique (Figure 2b and 2c) show a presence of the silica peak at 30o

On the other hand, the XRD pattern of thin film non coating XRD displayed in Figure 2d does not show the characteristic peaks of silica, however the peak can be displayed at 2-theta = 30o upon coating with the mixing ratio of 1:5 and 1: 10 as seen in Figure 2e and 2f The XRD data are particularly evident to prove the forming of silica coating on the copper substrate In addition, the XRD data are also in agreement with SEM data as displayed in Figure 3a, b, c to confirm the forming of silica coating on copper substrate

Figure 3 shows the morphology of samples before and after coating with VRHA-WG/VCO mixture after heat treatment at 500 oC for 2 hours The sample before coating has the polished surface, while coating with different coating ratios display the precipitated needle-like crystal on the surface In this research, although the thickness of coating layer can not be identified by SEM on the copper substrate surface, we can recognize the homogeneous coating on the surface

at Figure 3 The needle-like crystal on the surface is quite well distributed, indicating the homogeneous coating Figure 4 shows the optical microscope (OM) of the copper substrate (Figure 4a) with mirror polish, while the VRHA mixing with WG of ratio of 1:5 and 1:10 coating on copper substrate is shown in Figure 4b and 4c, respectively The coating layer is shown by black arrow, with well distribution of silica layer, while the non-coating area is indicated by red arrow The OM data also are in quite well agreement with SEM data shown in Figure 3

b) Coating with WG:VCO is 1:5

a) Non coating

c) Coating with WG:VCO is 1:10

Figure 3 Morphology of copper substrate: a) before coating and b) after coating with mixture of

VRHA-WG to VCO of ratio of 1:5 and c) after coating with mixture of VRHA-WG to VCO of ratio of

1:10 The copper substrate acre coated follow by heat treatment at 500 oC for 2 hours

The thickness of coating layer is depicted in Figure 5 by using mapping EDX technique at the cross-section layer of the copper substrate When the cross surface exposed to X-ray beam used in EDX technique, the mapping area of elements such as Cu, Si, O, Na and C (Figure 5a, b,

c, d and e) are displayed From the EDX mapping, we can see the well distribution of Si on the coating surface (Figure 5b) Elements O and Na are coming from silica (SiO2) and water glass

(Na2O.SiO2) The peak of C comes from the carbon tape originating from the preparation of

SEM sample The thickness of silica coating was identified based on the cross-section

observation of Cu, O and Na element with a value approximately of 15~20 µm The elements

contributed to the forming of water glass as the surface coating on copper substrate

O-K

%Wt Cu = 32.30%

%Wt O = 23.72%

%Wt Si = 1.44%

Na-K %Wt Na = 5.78% C-K %Wt C = 36.77%

15-20µm

Copper layer

15-20µm 15-20µm

Figure 5 EDX mapping of copper substrate at cross surface after coating with the mixture of VRHA:WG

at mixing ratio 1:5 and heat treatment at 500oC for 2 hours A) Mapping analysis of Cu-L; b) Si-K; c) O-K; d) Na-K and e) C-K The thickness of coating layer is estimated at 15-20 µm based on the

thickness of Cu, O and Na element

Figure 6 Definition of volume resistivity (left) and surface resistivity (right) of sample [12]

Table 4 shows the volume resistivity (VR) of copper substrate before and after coating with

different mixing ratios of VRHC-WG and VCO By its definition, the volume resistivity, is the

resistivity measured when the current applied perpendicularly to the sample plane as shown in

Figure 6 As shown, the coating layer of VRHA-WG and VCO can enhance the volume

resistance of copper substrate up to 33.33 %, thus indicating the silica coating layer on copper plate can contribute to electrical resistance as well as anticorrosion property

Table 4.Volume resistivity of copper substrate before and after coating with different mixing ratios of

VRHC-WG and VCO (number of sample n = 3)

Parameter Before coating Mixing ratio of VRHA-WG to

VCO

The meaning of this research should be put in the context of research context of synthesis silica from VRH and VRHA As mentioned in the introduction, most of the works just focused

on the synthesis of silica powder from VRH and VRHA The process including the convert of VRHA to water glass by alkaline activator, then treatment with acidic condition to get the silica gel The silica gel is treated at high temperature to obtain the silica powder [10-11; 13-17] It is inconvenient for a company user wanting to coat silica powder on substrate, since silica powder cannot be attached on the surface of substrate In order to extend the application of silica product, our research group suggests to use water glass as coating agent, then mixed with VCO

at different ratios of 1:5 and 1:10 The mixtures were coated on copper substrate and increased the volume resistivity up to 33.33 % Further study on the volume resistivity as well as anti-corrosion property of copper plate coated with VRHA-WG and VCO is under investigation to commercialize the research results

4 CONCLUSIONS

Our research group succeeded in fabricating the coating solutions from VRHA-WG and coconut oil The results indicated that the coating mixture with different mixing ratios of

VRHA-WG to VCO can give rise a well distribution of silica on the copper substrate The coating layer supported to increase the volume resistivity of copper substrate up to 33.33 %, promising to be used in weld industry

Acknowledgements. This research is funded by Nippon Sheet Glass Foundation The authors also thank the equpment support from Ho Chi Minh City University of Technology (HCMUT), Vietnam National University Ho Chi Minh City (VNU-HCM).

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