Electrically conductive ceramic composites prepared with printer toner as the conductive phase

Electrically conductive ceramic composites prepared with printer toner as the conductive phase

Yongjia He^a,b, Bing Ping^a,c, Linnu Lu^a,c, Fazhou Wang^a,band Shuguang Hu^a,b

aState Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, China;^bSchool of Materials Science and Engineering, Wuhan University of Technology, Wuhan, China;^cSchool of Science, Wuhan University of Technology, Wuhan, China

ABSTRACT

Conductive ceramic composite was prepared by sintering the mixture of clay and printer toner at 1050°C and in the N₂atmosphere. The microstructure and mineral phases of the ceramic composite were characterised by SEM, EDX, TG and XRD, and its electrical conductivity and mechanical properties were also investigated. The results show that, in the sintering process, a series of physical and chemical reactions take place, and mineral phases with excellent electrical conductivity, such as metal iron, carbon and Fe–Al solid solution material, are formed. The electrical conductivity mechanism can be explained by the percolation theory.

The threshold value for electrical conductive percolation is between 3.5 and 7.0 wt-%. At the content of printer toner 10 wt-%, the volume electrical resistance of the ceramic composite is as low as 8.5Ωcm, and the composite exhibits excellent flexural strength higher than 14 MPa.

ARTICLE HISTORY Received 10 August 2016 Accepted 28 December 2016 KEYWORDS

Conductive ceramic composite; sintering;

electrical conductivity;

percolation threshold;

mechanical properties

Introduction

The use of conductive ceramic materials has been increasing in industrial and daily life applications because of their excellent performances, e.g. high anti-corrosion, heat resistance and long service life [1–6]. Many functional elements, for example, cell elec- trodes, chemical sensors, conductive thin films, electro- thermal heaters etc. are made of conductive ceramic materials [7–12].

Composite conductive ceramics rapidly developed in recent years. They are prepared by adding electro conductive fillers into the ceramic matrix. Compared to intrinsically conductive ceramics such as SiC, zirco- nia and LaCrO3 [13–17], composite conductive cer- amics show advantages of simple processing, cheap and ease of mass production. Now it is widely known in many studies that carbon series composite conduc- tive ceramic, using graphite, carbon powder or carbon fibre as conductive phases [18–23], has good conduc- tivity and chemical stability, however, commonly exhi- bits low mechanical strength. The low strength is mainly due to the presence of weak interfaces between conductive phases and the composite matrix. These interfaces allow for energy dissipation before fracture through mechanisms of crack deflection and crack bridging [13]. In Kanbara et al.’s work, carbon black was added into clay to prepare conductive ceramic, and when the mass content of carbon black was 20%, the resistivity of ceramic composite was 0.5Ωcm but the compressive strength was merely about 1000 Pa [24]. In Bondar and Iordache’s work, graphite particles

were homogenised with alumina and kaolin powder then sintered into ceramic composite, when the mass content of graphite was 20%, the resistivity of ceramic composite was about 3000Ωcm [25].

To improve the mechanical performances of con- ductive ceramic composite, the weak interfaces between conductive phases and the matrix should be strengthened. In this paper, a new method of preparing conductive ceramic composite by using printer toner as the conductive phase is first proposed. The main com- ponents of printer toner are Fe₃O₄, organic polymer and carbon black. Fe3O4 reacts with clay and form Fe–Al solid solution at high temperature, which can promote the sintering of the composite and improve the interface between conductive phases and the matrix. A part of Fe3O4is reduced to Fe in the process by carbon black, and organic polymer is carbonised to carbon, which significantly improves the electro con- ductivity of the composite.

Experimental Materials

A commercially available printer toner was used as the conductive phase, the main components of which were organic polymer (52 wt-%), Fe3O4(25 wt-%) and car- bon black (10 wt-%). Clay acquired from a clay pit was used as the matrix material. The main chemical compositions of clay were SiO2 (66 wt-%), Al2O3

(17 wt-%), Fe₂O₃ (6 wt-%) and K₂O/MgO/Na₂O (3 wt-%).

© 2017 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute.

CONTACT Linnu Lu lln@whut.edu.cn State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, China

VOL. 116, NO. 3, 158–164

http://dx.doi.org/10.1080/17436753.2016.1278339

Processing procedure

Clay was dried to water content less than 4%, and ground to powder sieved through 150μm sieve. Printer toner was mixed into clay powder in a rolling blender for 30 min, the mass contents of which were 0, 5, 10, 15 and 30% for different specimens. The mixtures were uniaxially pressed at 60 MPa for 30 s in a steel mould and formed prism specimens with the size of 35 mm × 5 mm × 5 mm. And then the specimens were sintered in a tube furnace filled with N2gas to pre- vent Fe3O4and carbon black in printer toner from oxi- dation. The temperature was elevated from room temperature to 200°C with the rate of 5°C min⁻¹, kept at 200°C for 10 min, then elevated to 1050°C with the same rate and kept for 30 min. After the heat- ing process, the specimens were cooled to room temp- erature in the furnace.

Analytical methods

A FEI/Quanta 450 scanning electron microscope (SEM) was used to observe the micro morphology of the fracture surface of the ceramic specimen. X-ray dif- fraction method (XRD) was used to determine the crystalline phases of the ceramic specimens. The XRD was performed using a Bruker D8 Advance dif- fractometer using Cu Kα radiation at room tempera- ture. The patterns were collected at 40 kV, 40 mA, with a step size of 0.02° (2θ) and a scanning rate of 3° min⁻¹. Energy dispersive spectrometer (EDS) tech- nology was applied to detect the element distribution

in the polished fracture surface of the ceramic speci- men. Thermogravimetric analysis (TGA) of the com- posite was carried out in air using a NETZSCHSTA449c/3/G thermal analyzer at a heating rate of 10°C min⁻¹from room temperature to 1000°C.

Electrical resistivity of the ceramic specimens was measured with the two-probe method using an ESCORT-3146A type precision multimeter. The two probes were attached on two ends of the prism speci- men by conductive silver paste. The mechanical per- formances of the ceramic specimens were characterised by hardness and flexural strength. The hardness was measured with a Vickers hardness tester under the loading of 300 N. The flexural strength was tested with the three-point loading method. The appar- ent porosity of the composite was determined by the measurement of the amount of water taken in the specimen, and calculated according to the following equation: porosity = (weight of H₂O/volume of compo- site) × 100%. All measurements of electrical resistivity, hardness, flexural strength and porosity, six specimens were used for each mixture and the average value was calculated.

Results and discussion XRD

Figure 1shows the XRD patterns of different ceramic composites mixed with 0–30 wt-% printer toner. It is observed that the main crystalline mineral phases are quartz, morganite and hercynite in the specimens

Figure 1.XRD pattern of ceramic composites with 0–30% printer toner: (a) 30% printer toner, (b) 15% printer toner, (c) 10% printer toner, (d) 5% printer toner, (e) 0% printer toner.

without printer toner. With the addition of printer toner, the diffraction peaks of (110) lattice plane of elemental iron appear, and the peaks are enhanced with the increase of printer toner content. In addition, it appears a small peak (marked by‘●’in the pattern), which is neither the characteristic peak of crystal phases of ceramic nor carbon. Its intensity increases at a higher conductive phase content. It may be attributed to the presence of a small amount of carbon-containing phase formed by the reaction among organic phase and other materials in the sintering process. The car- bon-containing phase is beneficial for the bonding of conductive phase and ceramic matrix, also for the for- mation of conductive network. However, the character- istic peaks of carbon are not found in the patterns. This is consistent with the results of Juana et al.’s research work [22], in which none of characteristic peaks of car- bon is found in the XRD pattern of conductive ceramic.

In their research they prepared the composite by adding coal tar into ceramic matrix materials and sintering the blend. The absence of carbon peaks may be due to the low content of carbon in ceramic composite and its amorphous forms. Characteristic peak of iron is detected in samples with printer tonner and its intensity increases as the content of printer toner increases. There is no elemental iron in printer toner, so it is inferred that

in the sintering process Fe₃O₄ was partly reduced to elemental iron by carbon black and the decomposition products of organic polymer; another part of Fe₃O₄ reacted with clay and formed mineral phases such as hercynite.

SEM

Figure 2shows the SEM images of the fracture surfaces of the reference sample without printer toner and the ceramic composite with 10% printer toner. It is obvious that the reference sample has high density and low por- osity (Figure 2(a,b)), whereas the ceramic composite with 10% printer toner is characterised by a coarse pore structure with an average diameter of 5μm (Figure 2(c,d)). It is regarded that during the sintering process, gas produced by the decomposition of organic polymer and Fe₃O₄components of printer toner leads to the formation of the porous structure.

Electrical resistivity

Figure 3shows the volume resistivity of ceramic com- posite at various printer toner contents. It is observed that the resistivity decreases with the increase of printer toner. As the content increases from 3.5 to 4.0%, the

Figure 2.SEM images of the fracture surface of ceramic composites: (a), (b) without printer toner, at lower and higher magnifi- cation, respectively, showing dense microstructure; (c), (d) with 10% printer toner, at lower and higher magnification, respectively, showing coarse pore structure.

resistivity decreases sharply from about 2.0 × 10⁴Ωcm to 158Ωcm. It continues to decrease with a moderate rate as the content increases from 4.0 to 7.0%. How- ever, as the content is above 7%, the resistivity of cer- amic composite decreases more slowly. It is concluded that the percolation threshold of printer toner content for the electrical conductivity of the cer- amic composite is between 3.5 and 7.0%, which is much lower than the percolation threshold of graph- ite–ceramic composite (10–20 wt-%) [25] and carbon nano-fibre–ceramic composite (7.5–13 vol.-%) [26].

The volume resistivity of the ceramic composite is about 8.5Ωcm at 10%, showing good electrical conductivity.

Ceramic sintered from clay is intrinsically an insu- lating material, and the addition of electrically Figure 3.Volume resistivity (log scale) of ceramic composites

versus printer toner content.

Figure 4.(a) BSE image of electrically conductive ceramic composite. (b), (c), (d) and (e) show corresponding EDX elemental map- ping images of C, Al, Si and Fe, respectively.

conductive components makes it conductive. The per- colation theory is usually used to explain the conduc- tivity of conductor-insulator materials. When the content of conductive fillers reaches a critical value, a sudden and remarkable decrease in the ceramic com- posite’s electrical resistivity is observed. The reason for that is the contact of the conductive filler particles forms conductive pathways directly, at the same time, among adjacent filler particles electron hopping and tunnel effects will happen, which leads to conductivity enhancement of ceramic composite.Figure 4shows the BSE image and elements distribution images of the polished fracture surface of the ceramic composite. It is observed that Fe and C elements are homogenously distributed in the matrix and the particles containing Fe or C element are in contact or in near distances.

Thus, it is speculated that a three-dimensional percola- tion network is formed by these electrically conductive particles when their contents reach a threshold value.

The porous structure of the matrix is not necessarily negative for the electrical conductivity of the ceramic composite. Actually, in the sintering process of electri- cally conductive composite ceramic, the dilation of the gas produced by the decomposition of polymer, organic materials and ferrite phases forms the pores, at the same time, makes the electrically conductive par- ticles distribute more homogenously and more closer in the partly melted matrix.

Mechanical strength

Figure 5 shows the tested hardness values vary with the content of printer toner. It is observed that the hardness decreases whereas its standard deviation increases with the increase of printer toner. At 10%

printer toner, the hardness of ceramic composite is about 1.5 GPa, exhibiting relatively high mechanical strength. The results are similar to that in Tang et al.’s work, in which the hardness of NiO–ZrO2

ceramic decreases with the increase of PMMA con- tents, and the standard deviation increases [27].

These phenomena are related to the increase of the porosity and average pore diameter at higher conduc- tive phase contents. As shown inFigure 6, the appar- ent porosity of the ceramic composite is increasing with the content of printer toner.

Figure 7 shows the flexural strength of ceramics composites with different conductive components.

These composites were prepared under the same con- ditions. It is observed that the flexural strengths of the sample added with printer toner are much higher than those of the samples added with nano-carbon black or carbon fibre powder. Even when the printer toner content is as high as 15%, the flexural strength is still higher than 11 MPa. The main reason is that the organic polymer in printer toner acts as sintering aid in the high temperature processing of composite ceramic, which improves the interfaces among phases Figure 5.Hardness of ceramic composites versus printer toner

content.

Figure 7.Flexural strength of ceramic composites versus con- ductive component content ((▪) printer tonner added; (●) nano-carbon black added; (▴) carbon fibre powder added).

Figure 6.Apparent porosity of ceramic composites versus prin- ter toner content.

in the matrix. However, carbon black particles and car- bon fibre powder are not so tightly bonded to ceramic matrix (Figure 8(a,b)), which leads to the lower flexural strength.

Thermogravimetric analysis

TGA was carried out to determine the oxidation behav- iour at high temperature. The thermogravimetric curve in Figure 9 is divided into two parts. The first part is from room temperature to about 350°C, which cor- responds to the mass loss due to the moisture evapor- ation and decomposition of organic carbon-containing residues. However at the second part from about 350 to 1000°C there is an obvious mass increase, which is mainly due to the oxidation of Fe in the air. But the mass change during the heating process is small, which indicates the stability of the ceramic composite in application.

Conclusions

A new type of electrically conductive ceramic composite was prepared and investigated in this paper. The mix- ture of printer toner and clay was sintered at 1050°C

under the protection of N2 to acquire the ceramic composite. The investigation of the microstructure and performances of the ceramic composite reveals that

(i) Complex chemical reactions occur between prin- ter toner and clay during the sintering process, which leads to the formation of elemental Fe and C acting as electrical conductive phases together with the unreacted Fe3O4;

(ii) The percolation theory is used to explain the con- ductivity of the ceramic composite. Over the threshold content of printer toner, three-dimen- sional conductive network is formed by the direct electron transportation among contacting Fe, C and Fe3O4particles, at the same time by the elec- tron hoping and tunnel effects among adjacent conductive particles, which leads to the good elec- trical conductivity of the composite ceramic. In this work, the threshold content of printer toner is between 3.5 and 7.0%, and the volume res- istivity can be as low as 8.5Ωcm at 10% content;

(iii) The ceramic composite shows relatively high mechanical strength. The Vickers hardness and flexural strength are 1.5 GPa and 14 MPa, respectively, at the printer toner content of 10%;

(iv) Owing to the good electrical conductivity, rela- tively high mechanical strength and low pro- duction cost, the ceramic composite prepared with clay and printer toner is expected to have promising application prospects.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was financially supported by National Natural Science Foundation of China (No. 51372183, 50902106, 51461135005).

Figure 8.SEM images of the fracture surface of ceramic composites with nano-carbon black and carbon fibre powder: (a) with 10%

nano-carbon black, (b) with 10% carbon fibre powder.

Figure 9.TGA curve of ceramic composites with 10% printer toner.

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