7/24/2019 Powder Technology Volume 18 Issue 1 1977 [Doi 10.1016%2F0032-5910%2877%2985006-7] E.S. Palik -- Specific

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Powder Techn o logy . 18 (1977) 45 - 48

@ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

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Specific Surface Area hfeasurements on Ceramic Powders*

E. S. PALIK

General E iec t r i c Company . 1099 Iu mh oe Road . C lewe land . OH 111 IO USA. )

(Received February

15. 1977

SUMMARY

Surface area measurements on ceramic

powders are determined by gas adsorption

using the Brunauer-Emmett-Teller (B-E-T_)

equation. Two methods are described: the

static and the dynamic. Emphasis is placed on

sample preparation prior to adsorption_ Data

are presented which compare the two methods

along with the results of a round robin study

on surface area measurements on an alumina

sample_

In this communication the Brunauer-

Emmett-Teller (B.E.T.) method of surface

area measurement will be briefly reviewed,

some remarks given on the importance of

surface area data to the ceramics industry,

some comparison data by static and conti-

nuous flow methods presented, and finally a

preliminary report issued on an ASTM surface

area round robin _

Briefly, in the static method of surface area

measurement, the amount of adsorbed gas is

usually determined by measuring pressure

differences in a calibrated high-vacuum appa-

ratus. In the dynamic or continuous flow

method, the amount of adsorbed gas is deter-

mined by concentration measurements

utilizing a thermal conductivity detector_

The B.E.T. principle is embodied in the

well-known equation:

P

1

=-+

(C-Up

Y P, -P) v,c v,c PO

where P = equilibrium pressure of the gas,

P,, = saturation pressure of the gas at liquid

Na temperature,

V =

volume of gas adsorbed

*Paper presented at the 8 t h Annual Meeting of the

Fine Particle Society Conference, Chicago, August,

1976.

at equilibrium pressure, Vm = volume of gas

required to form a monolayer, and C =

constant related zo a special value of the heat

of

adsorption:

C=esp

HI -Hz

RT

where H , = heat of adsorption at the mono-

layer point, and HZ = heat of condensation.

This equation should result in a linear plot

of P[V P, -P) against P/PO and the value of

V, can then be calculated from the slope and

intercept_ The equation is capable of describ-

ing type 1, type 2 and type 3 isotherms de-

pending on the C constant_ In general, it has

been found that only type 2 isotherms (i.e.

those with high C values) have well-defined

knee-bends, which are essential for accurate

V, values. For type 2 isotherms, the B_E.T.

equation has been found to be valid generally

between 0.05 and 0.3 relative pressure. The

internal consistency of the equation has been

demonstrated by many measurements on

several solids, and the degree of correspon-

dence between the specific surfaces obtained

with several adsorbates allows confidence to

be placed in the method_

Having obtained the value of V, from the

B-E-T_ plot, the following equation is em-

ployed to calculate SW., the specific surface

area in m*/g for the sample:

S,&. = Nav,

M,M

where SW

= specific surface area in m*/g,

N = Avogadros number, 6.023 X 10 mole-

cules/gram molecule, u = area occupied by

one adsorbate molecule, 16.2 X 10p20 m* for

N2, 19.5 X lo-

20 m2 for Kr, V, = monolayer

capacity in ml, M, = g ra m molecular volume =

22410 ml, and W = sample size in grams.

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The analysis time can be considerably

shortened by using a one-point adsorption

method, particularly if a dynamic system is

used, provided the C value of the sample is

Iarge (>50). Most materials which eshibit a

tvpe 2 adsorption isotherm with nitrogen have

high C values and, therefore, present no prob-

lem with one-point adsorption_ The error in

most cases is less than 5% if C = 50 at a rela-

tive pressure of O-3.

First, some words on the importance of

surface area data to the ceramic industry-

Table 1 lists some of the properties of pow-

ders which can be affected by the estent of

the surface area_

T-ABLE 1

The iuflueuce of surface area on powder properties

I_ Blending of particles

2. Adherence of particles

3. Hiding or covering power of paints

4. Flow of particles - bulk density

5. Pa&kg of partictes (escape of gases)

6. RheoIogical properties (yield value: spherical 25

Fig. l_ hlultipoint vs. single-point B-E-T_

It is essential that the thermal history be

known or proper bake-out conditions be

utilized to prepare the sample for adsorption

if the surface area data are to be meaningful.

Before discussing multipoint static and

single point dynamic B-E-T_ data, the two

methods should be compared. In our labora-

tory both instruments are employed, static

and dynamic, and Fig. 1 shows a correlation

plot involving a variety of samples (A120a,

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TiO,, 5X0,). The correlation coefficient for

the dozen or so analyses involved is 0.99. The

range of surface areas is 0.6 - 270 m/g_ With

such a correlation, it is possible to utilize the

one-point continuous flow method for nearly

all of the samples and thereby save consider-

able analysis time.

An in-house standard surface area sample

is used in our Iaboratory, consisting of a well-

characterized titanium dioside powder.

Table 3 summarizes some of our TiOs surface

area data.

The main thrust of this paper will be to

present data from a round robin carried on

by ASTlM subcommittee C21.07. In this work,

eight different Iaboratories participated, and

a variety of commercial instruments were

TABLE 3

Static us. dynamic B.E.T. data for TiO2 standard

Static

Dynamic

Xccepted value

10.6 + 0.3 m2/g

9.5 * 0.7 m2/g

10.3 m*fg

(mean value of 6 tabs)

TABLE 1

Sample: X120, (pretreated). B.E.T. round robin

4

employed (the round robin study is still in

progress). It would be appropriate to present

the first phase of the work at this time, which

is summarized in Table 4. The mean and

standard deviation are identical for both single

and multipoint data, 1.06 2 0.1 m*/g.

It is evident after examining these data that

once the sample has been pretreated (in the

case of the alumina, pre-firing at 1000 C for

-5 minutes to convert to the e-form), the

specific surface area obtained is essentially

independent of which laboratory performs

the analysis on which instrument under which

bake-out conditions, which adsorbate is used

and whether single or multipoint data are

used. In other words, surface area measurement

is a straightforward determination, not subject

to the usual types of errors encountered in

physical measurements. All this is, of course,

true thus far only for alumina powder which

with nitrogen eshibits a high C value (>50).

For a given sample, knowing the thermal

history or controlling bake-out conditions

and establishing the magnitude of the C value,

a method can be developed to determine

specific surface using any of the many avail-

Laboratory

Instrument Outgassing conditions Xdsorbate

Surface area (mlg)

Temp. (C) Time (h)

Single Multipoint

A b1*2100D 145 17 Kr 1.13

150 17 Kr - 1.13

1112300 150 0.67 N2 0.96. 1.03 -

h12205 150 0.67 Xl- 0.97, 0.96 _

B - 25 16 _

1.15.1.13

C Strohlein 150 1 N, 1.08, 1.07 -

Shell-Sorpt** 200 2 N 2 1.18,1.1-l _

bI*MIC-103 125 17 Nq - o.ss

D Quantasorb*** 150 1 N2 1.03, 1.0-l 1.01, 1.05

Monosorb*** 150 1 N

1.05.

1.04 -

E hlonosorb 150 1 N2 l-OS, 1.10 -

Monosorb 150 1

N2 1.07,1.06

-

F hI*2100D 150 1 Kr _ 1.11

M2100D 150 16 Kr - 1.0-l

G Monosorb 150 1 N2 1.09, 1.11

-

1.05, 1 . 0 7

Quantasorb 150 1 N2 1_01,1_03 1.00.1.02

I-I Quantasorb 150 1

N2 -

1.05, l-02

1.03

Mean and std. dev. = 1.06 * 0.1

*Micromeritica In&r. Corp.

**Perkin-Elmer

***Quantachrome Corp.

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48

able instruments found today that will yield

reproducible and comparable values.

The above round robin study is being es-

tended to silica and other ultra-fine forms of

alumina powders and will be reported at a

later date.

It is concluded from this work that for

samples having a known thermal history, a

high C value, and a surface area >0_5 m*/g,

one can use N, single-point adsorption_ For

low surface areas (