Influence of Nanocomposites of LDPE Doped with Nano-MgO by Different Preparing Methods on Its Dielectric Properties

1Key Laboratory of Engineering Dielectric and Its Application, Ministry of Education, Harbin University of Science and Technology, Harbin, Heilongjiang 150080, China 2College of Material Science & Engineering, Harbin University of Science and Technology, Harbin, Heilongjiang 150040, China 3College of Electrical and Electronic Engineering, Harbin University of Science and Technology, Heilongjiang 150080, China 4Zhongtian Technology Group Ltd., Nantong, Jiangsu 226000, China


Introduction
The contradiction between urban expansion and electricity demand is becoming more prominent.Long distance and effective and safe power transmission are the main developing direction to solve the conflict.Comparing direct current (DC) transmission with alternative current (AC) transmission, it has advantages of the lower line cost, lower power loss, transmission system stability, and so forth.But DC transmission of cross-linked polyethylene (XLPE) cable was hampered by its own shortcomings in which there is a lot of space charge accumulation leading to distorting electric field and even local electric breakdown of insulator of XLPE cable [1,2].It was surprising that base polyethylene (base PE) doped with nano-MgO particles can effectively suppress the space charge accumulation.At the same time the composite still has excellent electrical properties.So researches of the dielectric polymer nanocomposite have become one of hot spots in electric power field [3].
Nanoparticles are generally agglomerated as a result of their high surface energy and therefore the disaggregation of nanoparticles in nanocomposites proves to be a key issue in obtaining good filler dispersion and to bring the nanoparticles into full play.However, there is different dispersion effect for the nanoparticles with different preparation methods.Nano-MgO particles prepared with direct precipitation method and homogeneous precipitation method as well as microwave-assisted method were researched in this paper.Nano-MgO particles prepared with direct precipitation method and homogeneous precipitation method have poor dispersion and serious aggregation.However, the microwave-assisted method has been used to prepare nanooxides, which has good dispersion and disaggregation, and so forth [4][5][6].This paper systematically researched the fact that nano-MgO particle was prepared by microwaveassisted method and the influence of nano-MgO particles by different preparation methods on size, dispersion of nano-MgO, and dielectric properties of its nanocomposite.Then appropriate amount of aqueous ammonia was put into the three-necked bottle drop by drop at starting heating by a sheathed heater.White precipitate powder began to occur after dropwise addition of aqueous ammonia in the solution of the flask.After maintaining a certain period of time for heating the solution, the white precipitate powder was washed with water and alcohol and then Mg(OH) 2 powder was obtained.See [7,8].

Homogeneous Precipitation
was dissolved in distilled water (200 mL) to prepare a solution of certain concentration (0.2 mol/L) in a three-necked bottle.Then, magnesium nitrate aqueous solution was put into a three-necked flask.Once heated, a small amount of PEG dispersant (0.1 mL) was added into the flask and then stirred for 5 min; finally, urea (1.0 g) was added according to the predetermined ratio.After 6 h reaction at 125 ∘ C, a white precipitate was formed which was washed by water and alcohol, respectively, with filtration devices.Then the precipitate was put into a vacuum dry oven (at 80 ∘ C, 24 h) to dry to obtain Mg(OH) 2 powder.See [9].

Microwave-Assisted Method.
A certain amount of ammonium bicarbonate accompanied with a small amount of PEG200 (1 mL) was dissolved in distilled water (20 mL).Then the three-necked flask containing the solution was heated by microwave oven under 80 ∘ C, 625 W for 30 min.Then, magnesium nitrate solution was added into the flask step by step with a separatory funnel at a fixed speed of dropping in accordance with the addition of molar ratio (1 : 4) of magnesium nitrate and ammonium bicarbonate.A white precipitate was produced at 50 min and 80 ∘ C.And washed it with water and alcohol, respectively, with suction device.Finally, the precipitate was put into a vacuum dry oven (at 80 ∘ C, 24 h) to dry to obtain Mg(OH) 2 powder.

Preparation of Surface Coupling Treatment of Nano-MgO.
Nano-MgO was prepared by stepwise calcination.First, Mg(OH) 2 was put into the muffle furnace and calcinated at the temperature of 300-400 ∘ C for 1 hour.Then, it was taken out for ball milling in planetary ball mill at rotating speed of 200 rpm for 20 min (forward 10 min, reverse 10 min, resp.).Secondly, it was put into muffle furnace and calcinated again at the temperature of 500 ∘ C for 2 hours.Then it was fetched out and the nano-MgO powder was obtained.Nano-MgO particles' surface was treated with silane coupling agent [KH-550, NH 2 CH 2 CH 2 CH 2 Si(OC 2 H 5 ) 3 , 2 wt% of the particle loading] at about 60 ∘ C for 16 min (forward 8 min, reverse 8 min, resp.) using ball milling at a ball speed of 140 rpm.
2.4.Preparation of LDPE Nanocomposite.LDPE (40 g) and nano-MgO particle treated by using silane coupling agent (2 wt%, nano-MgO prepared by microwave-assisted method or direct precipitation method or homogeneous precipitation method, resp.) were blended to prepare three kinds of nanocomposites at 150 ∘ C and 80 rpm for 15 min in the mixer of torque rheometer.During the blending process, ethylenevinyl-acetate copolymer (EVA, 9 wt% of LDPE loading) was added into the mixer in order to further improve the dispersibility between nano-MgO and LDPE in the nanocomposite.

Analysis by SEM.
As for sample of the composite, it must be broken into small pieces, whereas the powder of MgO can be used directly.The brittle fracture processing of the composite sample was carried out in liquid nitrogen.After that, the fragments of nanocomposite sample or the nano-MgO powders were fixed on the tin block with conductive adhesive and then coated with gold.Finally, the dispersion and size of nano-MgO particles were observed by using a scanning electron microscope (SEM, FEI Sirion 200, Dutch Philips Co., Ltd.).

XRD Test Method.
The size of nano-MgO powder was analyzed by X-ray diffractometer (XRD, model D/max 2500, Rigaku Co., Ltd., Japan), whose scanning speed was 4 ∘ /min and angle range was 10 ∘ -90 ∘ .The size of nano-MgO powder was calculated according to the Scherrer formula: In the formula,  = 0.89 and  = 0.15406 nm. stands for width at half-intensity diffraction line degree;  for a diffraction angle ( ∘ );  for grain size (nm).

Volume Resistivity Test.
Volume resistance test of the composite specimen slice which was about 1 mm in thickness was carried by digital resistance meter (ZC-36) under testing voltage being 1000 V applied to specimen slice at threeelectrode system (Figure 1), and the volume resistivity was calculated according to formula (2), in which  V (Ω⋅m) was the resistivity,  V (Ω) the composite volume resistance,  1 (m) the protested electrode diameter,  (m) the electrode gap between the protested electrode and the shied electrode, and ℎ (m) the specimen thickness:

The Results and Discussion
3.1.SEM Analysis of Nano-MgO.Nano-MgO was made from Mg(OH) 2 by two-step calcination method in muffle furnace, whereas the Mg(OH) 2 was made by microwave-assisted method, direct precipitation method, and homogenous precipitation method, respectively.The dispersion and size of the nano-MgO particles made by three methods were observed by using scanning electron microscope (SEM), as shown in Figures 2(a), 2(b), and 2(c).The results indicated that the diameter size of MgO particle prepared by microwaveassisted method was about 49 nm, which was the minimum of the nano-MgO particles and disperses uniformly among three methods.In addition, the particles agglomeration phenomenon was not obvious (see Figure 2(a)), while the diameter size of MgO particle prepared by direct precipitation method was beyond the nanometer size, and it had serious agglomeration phenomenon (see Figure 2(b)).
The diameter size of MgO powder prepared by homogenous precipitation method ranges from 70 to 90 nm and it had obvious agglomeration (see Figure 2(c)).The reasons may be that the uniformity of particle size is influenced by multiple nucleation of precipitate phase.Different from the direct precipitation method and homogeneous precipitation method, the microwave-assisted method can provide uniform heat in a very short period of time to greatly eliminate the temperature gradient and instantaneously nucleate for precipitate phase of Mg(OH) 2 which was calcined by two-step method in muffle furnace to obtain uniform distribution and dispersion of nano-MgO particles.

XRD Analysis of Nano-MgO.
The nano-MgO powder prepared by microwave-assisted was compared with conventional heating methods by using X-ray diffraction (XRD) in Figure 3.The 2 values of the diffraction peaks of nano-MgO particles prepared by microwave-assisted method and conventional method were basically the same and the crystals were both cubic crystal structure from Figure 3.According to Scherer formula (1) calculation, the size of nano-MgO prepared by microwave-assisted method was about 12.76 nm, and the traditional one was 22.74 nm.

Influence of Nano-MgO Preparation Method on the Volume Resistivity of Composites.
The nano-MgO loading was fixed at 2 wt%, and nano-MgO prepared by different methods was doped into LDPE.The impact on the volume resistivity of composites was investigated and the results were shown in Table 1.
As can be seen from Table 1, after LDPE doped with MgO nano-MgO was prepared by microwave-assisted method, the volume resistivity of composites was the maximum.The reason may be attribuited to the good dispersion of nano-MgO in LDPE matrix.Nano-MgO can limit the movement of molecular chain effectively, forming a lot of traps, and enhance the ability to capture charge under applying the electric field.Therefore, the volume resistivity of composites was higher than other nano-MgO composites [10,11].

Effect of Nano-MgO by Prepared Methods on the DC
Breakdown Strength of Composite.The nano-MgO loading was fixed as 2 wt%; using nano-MgO by different preparation methods in LDPE matrix, the impact on DC breakdown  strength of the composite was investigated.The results were shown in Table 2.
As can be seen from Table 2, DC breakdown strength of the composites has increased in comparison with LDPE without adding nano-MgO.However, the LDPE blended with MgO prepared by microwave-assisted method has the maximum DC breakdown field strength due to its perfect dispersion.There was the Coulomb blockade effect, which will limit the directional movement of carriers.On the other hand, it will result in LDPE matrix stretched to a certain extent.Stretching causes the lattice of LDPE dislocations which produce more and deeper traps.At the same time, the ability to trap electrons will be enhanced [12].

of Nano-MgO Preparation Methods on Space
Charge in the Composites.Nano-MgO was fixed at 2 wt% by weight.Using of nano-MgO blended with LDPE to form the composite, the effect of the space charge distribution on the composite was investigated.The experimental results were shown in Figure 4.
From Figure 4(a), it can be seen that base LDPE has a large number of homocharges injected near the cathode and anode.Comparing Figure 4(b) with Figures 4(c) and 4(d), composite doped with nano-MgO prepared by microwaveassisted method has the anode and cathode without the homocharges injected.It almost has no space charge in the sample with effective effect to suppress space charge in composite.This may be attributed to the fact that the diameter of nano-MgO prepared by microwave-assisted method was small and well distributed in the composite due to good compatibility with LDPE matrix.It will enhance the charge injection barrier and at the same time reduce the mobility of the charge because of the particle surface charge layer effects.So the effect of space charge suppression was the best [2,3].

Conclusions
In this paper, nano-MgO prepared by microwave-assisted method was compared with ones prepared by conventional methods.The former has smallest particle size and no obvious agglomeration.Electric properties of LDPE doped with the nano-MgO prepared by microwave-assisted method have been greatly improved by contrast with LDPE doped with the nano-MgO prepared by conventional method.

Table 1 :
The volume resistivity of the composites.

Table 2 :
DC breakdown strength of the composites.