forming nanosize mgo coatings on a glass surface

OPTICAL MATERIAL SCIENCE AND TECHNOLOGY

Forming nanosize MgO coatings on a glass surface

K. V. Dukel’ski and S. K. Evstrop’eva�

Scientific Research and Technological Institute of Optical Material Science, VNTs S. I. Vavilov State OpticalInstitute, St. Petersburg�Submitted March 20, 2009�Opticheski� Zhurnal 77, 58–64 �January 2010�

This article shows that nanosize �10–15 nm thick� MgO protective coatings can be formed onglass surfaces, using aqueous and water-alcohol solutions of magnesium nitrate. The effect of theconcentration and pH of the solutions on the coating structure is investigated.© 2010 Optical Society of America.

INTRODUCTION

Transparent coatings made from magnesium oxide�MgO� are widely used in gas-discharge devices such asplasma panels �for example, Refs. 1 and 2� and fluorescentlamps.3 These coatings are usually deposited by vacuumsputtering,1,4 vapor deposition,1 or high-temperaturepyrolysis.5 These methods provide highly homogeneousMgO coatings on glasses, but their use is often limited foreconomic reasons.

Various liquid-based chemical methods of depositingMgO coatings are described in Refs. 2,6–9. Chemical methodsof depositing coatings are usually preferred because they aresimple and economical. However, to put these advantagesinto practice, it is necessary to correctly choose the startingmaterials and to develop a simple and high-throughputmethod of depositing the coatings.

EXPERIMENTAL PART

Magnesium nitrate was used as the main starting mate-rial for this paper. Magnesium nitrate is an inexpensive andavailable material and has been used as the starting materialfor obtaining nanosize MgO powders by thermal decompo-sition of Mg�NO3�2 at temperatures of T�500 °C.8,9 In thisproject, aqueous and water-alcohol solutions of Mg�NO3�2

were used to obtain MgO coatings.In order to investigate how the pH of film-forming solu-

tions affects the morphology and properties of the coatings, a

TABLE I. Chemical composition and pH of film-formcoatings �transmittance of original glass at �=550 nm

Samplenumber

Components of film-form

Mg�NO3�2·6H2O, g Water, mL

NH4OH solu�1%�, mL

1 1 100 —2 1 100 0.53 1 100 14 1 100 25 1 100 406 1 10 —

45 J. Opt. Technol. 77 �1�, January 2010 1070-9762/2010/0

dilute �1%� solution of ammonium hydroxide was slowlyadded to the initial magnesium nitrate solution, which wasslightly acidic due to partial hydrolysis.

It was shown in Refs. 11 and 12 that adding polyvi-nylpyrrolidone �PVP� can effectively prevent various ce-ramic coatings from cracking. In this paper, PVP additives�molecular mass 1 300 000� were used to improve the homo-geneity of the coatings. The chemical compositions and pHvalues of the film-forming solutions are indicated in Table I.

Samples of alkali silicate glasses with dimensions 40�20�2 mm were used as substrates. The initial Mg�NO3�2

coatings were formed on the glass surfaces by spraying thesolutions or by pulling the glasses from the solutions. Afterthe samples of glasses with coatings were dried, they weresubjected to heat treatment at temperatures of 500–560 °C.

To estimate the homogeneity of the film-forming solu-tions, the mean transmittance of the solutions was deter-mined in the spectral range 450–700 nm by measuring theirtransmission spectra on a Shimadzu UV-2101PC spectropho-tometer, using a standard cell. The transmission spectra ofglass samples with the deposited coatings were determinedon the same device.

The pH values of the film-forming solutions were mea-sured with a Mettler DL67 pH meter.

The morphology of the fabricated coatings was investi-gated on a JEOL JSM 6500 scanning electron microscope.

olutions and transmittance of samples of glasses with0%�.

solutions

pHTransmittance

��=550 nm�, %Ethanol, mL PVP, g

— — 5.1 91.9— — 9.1 91.6— — 9.1 91.4— — 9.2 91.0— — 9.5 89.4

150 0.6 — 92.0

ing s92.

ing

tion

4510045-05$15.00 © 2010 Optical Society of America

Powdered samples of magnesium nitrate were used tostudy the evolution of the coating material during heat treat-ment. These samples �the mass of the sample was about2 mg� were heat treated under conditions identical to theheat-treatment conditions of the glasses with coatings andwere investigated by differential-thermal and thermogravi-metric analyses on a SETARAM device �TG-DTA 92-18�.The samples were heated in air at a rate of 10° /min.

X-ray phase analysis using a Rigaku diffractometer �D/max-Rint 2000� was also employed to investigate the evolu-tion of the structure of the materials.

EXPERIMENTAL RESULTS AND DISCUSSION

In order to optimize the coating-deposition conditions,we investigated how the pH of the film-forming solutionsaffects the homogeneity and structure of the coatings. Figure1 shows how the transmittance and pH of the film-formingsolution depends on the volume of the dilute NH4OH solu-tion added to the magnesium nitrate solution. It can be seenfrom the figure that the transmittance already decreasesslightly when the first drops of ammonium hydroxide areadded, and this is evidence that particles of magnesium hy-droxide are formed. The experiments showed that the homo-geneity and transmission of the coatings essentially dependson the pH of the film-forming solutions.

It is well known that magnesium hydroxide has a rela-tively low solubility in water �the solubility product isPRMg�OH�2

=2�10−11�.13 Accordingly, insoluble particles ofmagnesium hydroxide will form when the Mg2+ and OH−

concentrations in solution reach values that satisfy the in-equality

�Mg2+��OH−�2 � PRMg�OH�2= 2 � 10−11.

For any specific aqueous solution of magnesium nitrate, thisinequality makes it possible to estimate the pH of the solu-tion at which low-solubility particles of Mg�OH�2 will beginto form when an NH4OH solution is added to this solution.For example, the rough estimates obtained by the authors forsuch a pH for solution 1 �Table I� had a value of pH�9.3.This pH of the solution is designated in Fig. 1 by line 3.Even though some variations of the concentrations of thecomponents when the dilute solution of NH4OH was addedwere neglected in these rough estimates, Fig. 1 demonstrates

0 40 80 1205

7

9

89

90

91

92

93

8

6

Volume of NH4OH, mL

Tra

nsm

itta

nce

,%

pH 1

2

3

FIG. 1. Variation of the pH �1� and mean transmittance �2� of a magnesiumnitrate solution �solution 1, Table I� when it is titrated with a dilute �1%�ammonium hydroxide solution.

46 J. Opt. Technol. 77 �1�, January 2010

fairly good agreement between the calculated and experi-mental values of the pH of the solution at which a sharpdecrease of its transmission is observed.

It is clear that the light scattering of turbid solutionsfabricated by adding a significant amount of ammonium hy-droxide solution is relatively high because such solutionscontain large magnesium hydroxide particles or large aggre-gates of such particles. Attempts to fabricate homogeneouscoatings from such solutions were not successful—evenfreshly deposited coatings were inhomogeneous. Accordingto the results of this experiment, for a magnesium nitratesolution whose titration curve is shown in Fig. 1, the mosthomogeneous coatings were formed from solutions that havepH=5.0–9.3.

Figure 2 shows an electron micrograph of the surface ofsuch a coating. It can be seen that the initial salt coatingsformed after drying �but before heat treatment� consist ofmagnesium nitrate crystals that are homogeneous in size�10–15 nm�. These results were confirmed by x-ray analysisdata. Well-resolved peaks characteristic of magnesium nitratewere seen on the diffraction patterns, and calculations usingScherrer’s formula showed that the crystals are 10–15 nmacross. A detailed calculational technique using this methodis given in Ref. 14.

The freshly prepared nitrate coatings contained residualwater and ethanol, which were removed by heat treatment.Heat treatment also resulted in decomposition of the magne-sium nitrate and the formation of MgO coatings. Figure 3demonstrates the results of these differential-thermal andthermogravimetric analyses of powdered materials collectedon the glass surface after the deposited nitrate coatings weredried. Curves 1 in Figs. 3a and 3b correspond to powderedmaterial obtained without using PVP, while curves 2 in Figs.3a and 3b demonstrate the evolution of material that containsPVP. The thermogravimetric-analysis curves of both samplesreflect substantial mass losses, determined by the removal ofresidual water and ethanol �at temperatures �100 °C� andstepwise decomposition of magnesium nitrate at higher tem-peratures. These results completely agree with the data pub-lished earlier.9 According to studies of the thermal evolution

100 nm

FIG. 2. Electron micrograph of a Mg�NO3�2 coating formed on a glasssurface.

46K. V. Dukel’ski and S. K. Evstrop’ev

of magnesium nitrate carried out in Ref. 9, the evolution ofthe material includes dehydration �at T�100 °C� and two-stage decomposition of the magnesium nitrate. The first stageof the decomposition occurs at temperatures of 150–200 °Cwith the formation of intermediate magnesium nitrate, whosetotal decomposition and the formation of magnesium oxideis observed at temperatures around 480 °C. The experimentsalso established that the total decomposition of the salt insamples without PVP additives was observed at temperaturesaround 480 °C.

It should be pointed out that several endothermal effectsare observed when magnesium nitrate undergoes thermal de-composition �curve 1, Fig. 3b�, but that the introduction ofPVP into the composition of the material results in the ap-pearance of several peaks that characterize exothermal ef-fects �Fig. 3b, curve 2�. According to the data of Ref. 12,these peaks can be caused by exothermal reactions of thepyrolysis and oxidation of PVP. A comparison of curves 2 inFigs. 3a and 3b shows that all the exothermal effects areaccompanied by mass losses of the material. The results ofdifferential-thermal and thermogravimetric analyses showthat the complete decomposition of the starting materials andremoval of volatile products is observed when they areheated to about 500 °C.

Figure 4 presents the x-ray diffraction pattern of a pow-der sample fabricated from solution 1 and heat-treated at560 °C for 45 min. Peaks are observed on the diffractionpattern that are characteristic of periclase. Similar resultswere observed earlier9 for MgO powders fabricated from

341218 437 535 634

– 1

– 0.8

– 0.6

– 0.4

– 0.2

0

0.2

Т, °СM

ass

loss

,m

g

12

(а)

341218 437

535 634

– 4

– 8

0

4

∆Т,

rel.

un

its

Т, °С

1

2

(b)

FIG. 3. �a� Data of thermogravimetric analysis of powdered samples ob-tained without using PVP �1� and with PVP �2�. �b� Data of differential-thermal analysis of powdered samples obtained without using PVP �1� andwith PVP �2�.


aqueous solutions of magnesium nitrate and heat-treated at500 °C for 2 h.

According to the Scherrer analysis of the half-width ofthe diffraction peaks, the mean size of the periclase crystalsin the samples obtained by the authors was 17 nm. Thisvalue is close to the size of the magnesium nitrate crystalsformed on a glass surface after freshly deposited salt coat-ings are dried �Fig. 2�. This shows that the decomposition ofmagnesium nitrate crystals and the crystallization of MgOoccur at the nanosize level, with no appreciable growth ofthe particles.

Figure 5 demonstrates electron micrographs at variousmagnifications of a powdered sample fabricated from mag-nesium nitrate solution �solution 1�, Table I and heat-treatedat 560 °C for 45 min. The low-magnification picture showsthat the powdered material consists of relatively large par-ticles, more than several microns across. It is also interestingto point out that the boundaries between the MgO nanocrys-tals are invisible even on a high-resolution electron micro-graph �Fig. 5b�. This is evidence that these nanocrystals arevery close-packed inside large aggregates.

The structure and morphology of the thin films fabri-cated by the authors from magnesium nitrate solutions sig-nificantly differ from those of powdered samples heat-treatedunder the same conditions. Figure 6 shows an electron mi-crograph of MgO coatings deposited on a glass surface andheat-treated at 560 °C for 45 min. It is interesting to pointout that the morphologies of coatings fabricated from acidicsolution 1 �Fig. 6a� and alkaline solution 5 �Fig. 6b� arerather similar. These pictures show that the structure of thefabricated coatings consists of homogeneous spherical par-ticles of MgO 15–20 nm across. This shows that the forma-tion of hydroxyl-containing magnesium compounds that un-dergo partial aggregation �an observable decrease of thetransmittance of a magnesium nitrate solution when NH4OHis added �Fig. 1�� partially changes the spatial distribution ofthe particles �the coatings fabricated from the main solutionswere inhomogeneous� but does not change the size of theMgO nanoparticles in the heat-treated coatings.

The x-ray pattern of a MgO coating obtained from anacidic solution and having a thickness of about 100 nmshowed that there were no peaks characteristic of the crys-

30 40 50 60 700

10

20

30

40

Inte

nsi

ty,

rel.

un

its

2 , deg

1

2(111)

(200)

(220)

(311)

FIG. 4. X-ray patterns of a powdered sample of MgO �1� and of a MgOcoating on glass �2�. The heat treatment was carried out at 560 °C for45 min.


talline phase �Fig. 4, curve 2�. The authors assume that this isexplained by the thinness and amorphous nature of the coat-ing. The difference between the structures of the powderedsample and the coating can be explained by the influence ofthe surface of the glass substrate. In the thin film, the inter-action with the glass surface decreases the free energy of theresulting nanoparticles and can prevent them from being spa-tially displaced, stabilizing their amorphous state. The ob-servable difference in the morphology of the MgO coating�Fig. 6a� and the powdered sample �Fig. 5b� indirectly sup-ports this explanation. We should point out that the stabiliz-ing influence of the substrate on the agglomeration and evo-lution of Al2O3 microparticles in a thin coating during itsheat treatment was discussed earlier.15,16

The experimental results also showed that the coatinghomogeneity, important for many practical applications, canbe significantly enhanced by adding PVP to the starting film-forming solution. Figure 7 shows an electron micrograph ofa MgO coating formed on the surface of alkali silicate glassusing solution 8 �Table I�. It can be seen that the coatingconsists of homogeneous spherical nanoparticles having asize of 15–20 nm. A comparison of the morphologies of thecoatings fabricated from solutions 1 �Fig. 6b� and 8 �Fig. 7�shows that introducing PVP into the composition of the start-ing solution does not change the size of the MgO nanopar-

(а)

1 µm

(b)

100 nm

FIG. 5. Electron micrographs �at different magnifications� of a powderedsample fabricated from a magnesium nitrate solution �solution 1, Table I�and heat-treated at 560 °C for 45 min.


ticles but significantly increases their packing density andimproves the coating homogeneity.

One of the most important requirements on protectivecoatings deposited on the inner surface of the glass compo-nents of gas-discharge devices is that they must be highlytransparent in the visible region. Measurements of the trans-mission spectra showed that glass samples coated with MgOpossessed high transparency in the visible �450–700 nm�.Table I shows transmittance data of samples of the original

(а)

рН=5.1100 nm

(b)

100 nm рН=9.5

FIG. 6. �a� Electron micrograph of the surface of a MgO coating fabricatedfrom solution 1 �Table I� and heat-treated at 560 °C for 45 min. �b� Electronmicrograph of the surface of a MgO coating fabricated from solution 5�Table I� and heat-treated at 560 °C for 45 min.

100 nm

FIG. 7. Electron micrograph of the surface of a MgO coating fabricatedfrom solution 6 �Table I� and heat-treated at 560 °C for 45 min.


glass and of glasses with coatings �about 100 nm thick� at awavelength of 550 nm. It can be seen from the tabular datathat depositing MgO coatings using weakly acidic solutionsmakes virtually no change in the transparency of the glass,with the greatest transparency being possessed by glasssamples with coatings obtained from solutions that containPVP. Some of the observed decrease of the transparency ofthe samples with coatings deposited from solutions charac-terized by pH�7 is explained by the increased light scatter-ing at the inhomogeneous particle aggregates that are presentin such coatings.

CONCLUSIONS

A simple and high-throughput technique has been devel-oped for nanosize magnesium oxide coatings on the surfaceof silicate glasses. The coatings are highly homogeneous andtransparent in the visible part of the spectrum and are basedon the thermal decomposition of magnesium nitrate filmsdeposited from water-alcohol solutions. Experiments showedthat the homogeneity and transparency of the coatings sub-stantially depends on the salt concentration and the pH of thefilm-forming solutions. It has been established that introduc-ing polyvinylpyrrolidone into the composition of the film-forming solutions significantly increases the homogeneity ofthe coatings. It has been experimentally shown that the struc-ture and morphology of the powdered samples of MgO differfrom those of thin nanosize MgO coatings obtained underidentical conditions.

a�Email: [email protected]

1
Q. Yan, G. H. Gries, and N. H. Clausen, “Plasma-display panel with low-

voltage material,” Patent WO/2007/133698 �22.11.2007�; H01J17/49�2006/01�.

2H. S. Jung, J. K. Lee, K. S. Hong, and H. J. Youn, “Ion-induced secondaryelectron emission behavior of sol-gel-derived MgO thin films used forprotective layers in alternating plasma-display panels,” J. Appl. Phys. 92,2855 �2002�.

3K. T. Jung, S. K. Evstropiev, K. Y. Lee, and K. S. Lee, “Development ofnano-sized protective layers for flat fluorescent lamps �FFLs�,” SID �So-ciety of Information Display� 07 Digest, Long Beach, Cal., USA, pp.1844–1847.

4J. K. Kim, E. S. Lee, D. H. Kim, and D. G. Kim, “Ion-beam-inducederosion and humidity effect of MgO protective layer prepared by vacuumarc deposition,” Thin Solid Films 447–448, 95 �2004�.

5A. R. Balkenende, A. A. M. B. Bogaerts, J. J. Scholtz, R. R. M. Tijburg,and H. X. Willems, “Thin MgO layers for effective hopping transport ofelectrons,” Interfaces 50, No. 3–4, 365 �1996�.

6C. Bondoux, P. Prene, P. Belleville, F. Guillet, S. Lambert, B. Minot, andR. Jerisian, “Sol-gel MgO thin films for insulation on SiC,” Mater. Sci.Semicond. Process. 7, No. 4–6, 249 �2004�.

7X. Fu, Z. Song, G. Wu, J. Huang, X. Duo, and C. Lin, “Preparation andcharacterization of MgO thin films by a novel sol-gel method,” J. Sol-GelSci. Technol. 16, 277 �1999�.

8J. S. Park and Y. H. Han, “Preparation of MgO-coated BaTiO3 particlesthrough a surface-induced precipitation method,” Ceram. Int. 32, 673�2006�.

9H. Niu, Q. Yang, K. Tang, and Y. Xie, “A simple solution calcination routeto porous MgO nanoplates,” Microporous Mater. 96, 428 �2006�.

10Y. Shanmugam, F. Y. Lin, T. H. Chang, and C. T. Yeh, “Thermal decom-position of metal nitrates in air and hydrogen environments,” J. Phys.Chem. 107�4�, 1044 �2003�.

11H. Kozuka and M. Kajimura, “Single-step dip coating of crack-freeBaTaO3 films �1 �m thick: effect of polyvinylpyrrolidone on criticalthickness,” J. Am. Ceram. Soc. 83, 1056 �2000�.

12Y. Y. Chen and W. C. J. Wei, “Formation of mullite thin film via a sol-gelprocess with polyvinylpyrrolidone additive,” J. Eur. Ceram. Soc. 21, 2535�2001�.

13R. A. Rabinovich and Z. Ya. Khavin, A Short Chemical Handbook�Khimiya, 1977�, p. 240.

14S. I. Rembeza, T. V. Svistova, E. S. Rembeza, and O. I. Borsyakova,“Microstructure and physical properties of SnO2 thin films,” Semiconduc-tors 35�7�, 796 �2001�.

15O. Guillon, S. Krauß, and J. Rödel, “Influence of thickness on the con-strained sintering of alumina films,” J. Eur. Ceram. Soc. 27, 2623 �2007�.

16R. K. Bordia and A. Jagota, “Crack growth and damage in constrained
sintering films,” J. Am. Ceram. Soc. 76�10�, 2475 �1993�.

forming nanosize mgo coatings on a glass surface

Documents