Flexible Bacterial Cellulose Permalloy nanocomposite xerogel sheets size scalable magnetic actuator cum electrical conductor

Permalloy nanoparticles containing bacterial cellulose hydrogel obtained after reduction was compressed into a xerogel flexible sheet by hot pressing at 60 C at different pressures. The permalloy nanoparticles with an ordered structure have a bimodal size distribution centered around 25 nm and 190 nm. The smaller nanoparticles are superparamagnetic while the larger particles are ferromagnetic at room temperature. The sheets have a room temperature magnetisation of 20 emu/g and a coercivity of 32 Oe. The electrical conductivity of the flexible sheets increases with hot pressing pressure from 7 S/cm to 40 S/cm at room temperature.

The rapid strides made by the consumer electronics industry is mainly due to the development of materials processing technologies that result in the formation of nanoscale devices. These developments in turn have led to significant decrease in the average lifetime of consumer electronic products and has resulted in the accumulation of large amounts of electronic waste. 1 Handling or treatment of this electronic waste to recover precious materials does not require any changes to be made in the very processes that create this electronic waste. Although this recovery process is very essential, it cannot be a solution in the long run. An alternative however would be to develop materials and processes that are sustainable and recyclable without causing significant damage to the environment. Towards this goal, biodegradable polymeric substrates that can host electronic circuits or nanomaterials that have electronic functions have been intensively investigated. Typical biodegradable polymeric substrate materials arepolydimethylsiloxane, low density poly-ethylene, poly-vinyl alcohol, polyimide, polyethylene glycol and cellulose. [2][3][4][5][6][7] Except for cellulose, all other polymers need to be synthesized while cellulose is naturally occurring and is sustainable. It is produced by both plants and bacteria. The cellulose produced by bacteria has superior physical properties because it is pure and highly crystalline compared to plant cellulose. It is environmentally friendly biopolymer with a unique structure and physical properties 8 and is amenable to functionalization. Hence, bacterial cellulose (BC) has been chosen as the host /substrate material to develop a flexible, scalable, electrically conducting and magnetically actuating composite sheet in the present work. Bimetallic alloy nanoparticles such as Pt-Cu, Pt-Pd, Pd-Ag, Ag-Cu, Pt-Co, Fe-Co, Fe-Rh, Fe-Ni have been extensively studied for their catalytic, electronic, optical and magnetic properties. [9][10][11][12][13][14][15] The main advantage of these bimetallic nanoparticles is that their physical properties as well as their environmental stability can be tuned/ controlled to desired values. One typical example is Fe-Ni alloys whose magnetic behavior can be continuously varied between that of Fe and Ni while the environmental stability of Fe is significantly improved by the addition of Ni. More specifically, Fe-Ni alloys in the range 70-80 at. % Ni are of extreme technological importance and are known as 'Permalloys'.These alloys exhibit an extremely soft magnetic behavior with high saturation and high permeability. The magnetostriction coefficient and magnetocrystalline anisotropy in this composition range becomes extremely small and goes to zero around 78 at. % Ni which makes them highly suitable for applications that require large permeability. They also exhibit catalytic activity 12 while retaining high thermal stability and resistance to oxidation. Since these are alloys of metals they retain high electrical conductivity. Hence, the bacterial cellulose in the present work is functionalized with FeNi3 permalloy nanoparticles so that it can exhibit magnetic actuation-cum-electrical conductivity. This has been achieved by synthesizing the FeNi3 nanoparticles inside bacterial cellulose matrix using an inverse chemical reduction procedure. The resulting nanocomposite hydrogels have been converted into flexible sheets by hot pressing. These sheets can be used for a variety of applications ranging from simple magnetic diaphragms to electromagnetic radiation shields and magnetic sensors.
Materials: Food grade sugar, orange peel fibers, coconut water and high purity glacial acetic acid were used to grow bacteria and synthesize bacterial cellulose. High purity, 99 % NiCl2.6H2O, FeCl3.6H2O and NaBH4 procured from Merck were used for the synthesis of permalloy nanoparticles in the bacterial cellulose matrix.
Bacterial Cellulose Synthesis: Bacterial cellulose is synthesized as per protocol reported earlier. 16,17 The synthesis method consists of two essential steps -(i) preparation of cellulose synthesizing inoculum, and (ii) growth of cellulose using the inoculum in a static culture containing nutrients for the bacteria. The bacterial inoculum was harvested from wild culture made of orange peel fibres and sugar solution. This inoculum is allowed to grow and produce cellulose in a static nutrient medium consisting of coconut water, sugar and glacial acetic acid. The resulting cellulose is harvested and used for incorporation of FeNi3 nanoparticles after a thorough cleaning with aqueous solution of NaOH followed by distilled water. These sheets are designated as FNBC9 and FNBC26 respectively.
Chacterization: The structural characterization was performed by X-ray diffraction with Cu-Kα radiation, field emission scanning electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM). The magnetic studies were performed using a vibrating sample magnetometer equipped with a superconducting quantum interference device detector in the temperature range 5 K to 300 K and fields  20 kOe. The temperature dependent electrical conductivity was studied in the range 5 K to 300 K using the physical properties measuring system. Structure: The X-ray diffraction pattern of pristine BC obtained by hot pressing at 60 o C, This is further seen clearly in the cross-sectional micrographs which show nanoparticles in the spaces between the BC fibers. The nearly spherical nanoparticles have a lognormal distribution of sizes with average of  190 nm. Such a particle size distribution is commonly observed in growth kinetics controlled nanoparticles formation. 18  Magnetic Studies: The magnetic behavior of FNBC xerogel sheets was studied by two different methods -(i) field dependent isothermal magnetization variation and (ii) temperature dependent iso-field magnetization variation.
The field dependent magnetization M of FNBC9 and FNBC26 in the range  20 kOe at 300 K, 50 K and 5 K is shown in Figures 4(a) and 4(b) respectively. Both the xerogel sheets exhibit a clear hysteresis in magnetization at all the three temperatures as seen in the insets. At room temperature and 50 K the magnetization is nearly saturated whereas at 5 K it is not saturated at a field of 20 kOe. The magnetization maximum Mmax and coercivity HC at different temperatures for FNBC9 and FNBC26 are given in Table 1.
The magnetization maximum and the coercivity increase with decreasing temperature in both the cases, a typical ferromagnetic signature. The magnetization increases from ~ 19 emug -1 to 33 emug -1 while the coercivity increases from 32 Oe to 112 Oe in FNBC9 and 124 Oe in FNBC26 on decreasing the temperature from 300 K to 5 K. The magnetization is lower than the bulk saturation value of ~ 110 emug -1 while the coercivity is higher by about two orders of magnitude due to finite size effects.
The temperature dependent magnetization of the xerogel sheets in the range 5 K to 300 K in the presence of varying external magnetic fields was measured both in the zero-field cooled (ZFC) and field-cooled (FC) conditions. The magnetization, Figures 5 Electrical Studies: Since the objective of the work was to develop a flexible sheet which is not only magnetic but also electrically conducting, the resistivity of the sheets was studied in the temperature range 5 K to 300 K. The added advantage of studying the electrical conductivity is that it will also reveal the nature of interparticle magnetic interactionsdipolar or direct exchange that exist between the FeNi3 nanoparticles. The temperature dependence of conductivity of both FNBC9 and FNBC26 is shown in Figure   6. Both the xerogel sheets exhibit a typical metallic behavior with a negative temperature coefficient. The room temperature conductivity of FNBC9 is ~ 7 Scm -1 compared to 40 Scm -1 for the FNBC26 sheet. The higher conductivity observed in FNBC26 is due to greater inter-particle contact achieved by hot pressing at higher pressure. These values however are still far lower than the bulk value by about 4 to 5 orders of magnitude 20 indicating that the distribution of the metallic nanoparticles is just above the percolating threshold for conductivity. Also, the nanosize of the alloy particles leads to increased scattering and hence lower conductivity. This results in increasing the temperature coefficient far above the bulk value. The conductivity at 5 K and 300 K of both the sheets is given in Table 1 formation of a minimal network, percolation threshold, to exhibit electrical conductivity and at 26 MPa pressure better connectivity is achieved to realise a higher conductivity.

Conclusions:
A flexible xerogel nanocomposite sheet with permalloy nanoparticles distributed in bacterial cellulose matrix has been successfully synthesised. The synthesis procedure is extremely simple and can be easily scaled to make large flexible sheets. The formation of nanoparticles with a bimodal size distribution using this process ensures connectivity across the BC fibres which results in a percolation path to be established on hot pressing the hydrogel into a dry xerogel flexible sheet. The electrical conductivity is found to increase with increasing the hot pressing pressure while retaining the magnetic sensitivity. These properties make it highly suitable for applications such as magnetic sensors, electromagnetic radiation shielding and so on.       (b) in zero-field cooled (ZFC) and field-cooled (FC) condition shows irreversibility. The ZFC curve shows a low temperature peak, blocking temperature TB which shifts to lower temperatures with increasing external magnetic field (insets). shows a metallic behavior with a negative temperature coefficient. The photograph (inset) illustrates presence of electrical conductivity at room temperature.